Unexpected rise of ozone in urban and rural areas, and sulfur dioxide in rural areas during the coronavirus city lockdown in Hangzhou, China: implications for air quality

Ground ozone rise during the 2022 Shanghai lockdown caused by the unfavorable emission reduction ratio of nitrogen oxides and volatile organic compounds

Ground-level ozone (O3) pollution has shifted from a scientific issue to a key focus of governmental action in China. In recent years, the concentration of NO2 in Shanghai has shown a decreasing trend of 3.7% annually, but ozone concentrations have exhibited significant interannual variability, particularly with a noticeable increase in 2022. This study focuses on investigating the mechanisms behind the increase in ozone concentration during the COVID-19 pandemic control period in 2022 in Shanghai, utilizing a combination of ground observation data, observation-based models, and chemical transport models for analysis. The results indicate that during the lockdown period, the MDA8 in Shanghai increased by 17 μg/m3 compared to before, with emission-related factors contributing 65.3%, primarily due to a blanket reduction in VOCs and NOx emissions during the lockdown, with a reduction ratio close to 1:1. However, this reduction ratio and intensity are not sufficiently reasonable to alleviate ozone pollution. Meanwhile, adverse meteorological conditions further exacerbated this effect, contributing 34.7%, with temperature rise having the greatest impact. Results from the chemical transport model show that with the total reduction in NOx and VOCs emissions unchanged, the greater the reduction in VOCs emissions, the better the reduction effect on ozone pollution, reducing MDA8 O3 by approximately 10 μg/m3, especially for the control of reactive compounds such as alkenes, aromatics, and OVOCs. However, if the reduction ratio of NOx is greater than that of VOCs, ozone concentrations may not decrease but instead increase. This indicates that ozone concentration is influenced not only by the intensity of emissions reduction but also by the ratio of emissions reduction between NOx and VOCs. Our study emphasizes the critical role of carefully designed strategies, focusing on controlling the ratio of VOCs to NOx and increasing the intensity of VOCs reduction, to effectively alleviate ozone pollution in urban areas.

Crop residue burning increased during the COVID-19 lockdown: A case study of rural India

The customary practice of crop residue burning (CRB) is a major policy concern across several developing economies because of the associated increase in air pollution and reduction in soil quality. CRB poses a hazard to public health and sustainable farmland management. We collected original survey data from a panel of 400 wheat farmers on CRB choices during April-May of 2019 and 2020 - with the latter coinciding with India's COVID-19 nationwide lockdown. This timeline of events facilitated a unique identification of changes in CRB that are attributable to the lockdown. Several studies find that lockdowns during 2020 had beneficial effects on the environment owing to reduced economic activity. However, our findings indicate that CRB may have unintentionally increased during the lockdown. A binary variable regression framework analyzes the determinants of CRB choices of farmers over two years. We control for farmers' opinions on various socioeconomic aspects of the pandemic lockdown to examine its effects on their CRB decisions. The lockdown significantly increased the likelihood of CRB by up to 12%. Furthermore, farmers who lost agricultural income and those compelled to sell assets during the lockdown were 22% and 19% more inclined, respectively, to choose CRB. Labor mobility ceased during the lockdown and increased the cost of environmentally friendly farmland management; this increased the likelihood of CRB by 6%. This study contributes to a growing literature on the unintentional consequences of pandemic lockdowns.

Estimating morning and evening commute period O3 concentration in Taiwan using a fine spatial-temporal resolution ensemble mixed spatial model with Geo-AI technology

Elevated levels of ground-level ozone (O3) can have harmful effects on health. While previous studies have focused mainly on daily averages and daytime patterns, it's crucial to consider the effects of air pollution during daily commutes, as this can significantly contribute to overall exposure. This study is also the first to employ an ensemble mixed spatial model (EMSM) that integrates multiple machine learning algorithms and predictor variables selected using Shapley Additive exExplanations (SHAP) values to predict spatial-temporal fluctuations in O3 concentrations across the entire island of Taiwan. We utilized geospatial-artificial intelligence (Geo-AI), incorporating kriging, land use regression (LUR), machine learning (random forest (RF), categorical boosting (CatBoost), gradient boosting (GBM), extreme gradient boosting (XGBoost), and light gradient boosting (LightGBM)), and ensemble learning techniques to develop ensemble mixed spatial models (EMSMs) for morning and evening commute periods. The EMSMs were used to estimate long-term spatiotemporal variations of O3 levels, accounting for in-situ measurements, meteorological factors, geospatial predictors, and social and seasonal influences over a 26-year period. Compared to conventional LUR-based approaches, the EMSMs improved performance by 58% for both commute periods, with high explanatory power and an adjusted R2 of 0.91. Internal and external validation procedures and verification of O3 concentrations at the upper percentile ranges (in 1%, 5%, 10%, 15%, 20%, and 25%) and other conditions (including rain, no rain, weekday, weekend, festival, and no festival) have demonstrated that the models are stable and free from overfitting issues. Estimation maps were generated to examine changes in O3 levels before and during the implementation of COVID-19 restrictions. These findings provide accurate variations of O3 levels in commute period with high spatiotemporal resolution of daily and 50m * 50m grid, which can support control pollution efforts and aid in epidemiological studies.

Environmentally persistent free radicals in PM2.5 from a typical Chinese industrial city during COVID-19 lockdown: The unexpected contamination level variation

The outbreak of COVID-19 has caused concerns globally. To reduce the rapid transmission of the virus, strict city lockdown measures were conducted in different regions. China is the country that takes the earliest home-based quarantine for people. Although normal industrial and social activities were suspended, the spread of virus was efficiently controlled. Simultaneously, another merit of the city lockdown measure was noticed, which is the improvement of the air quality. Contamination levels of multiple atmospheric pollutants were decreased. However, in this work, 24 and 14 air fine particulate matter (PM2.5) samples were continuously collected before and during COVID-19 city lockdown in Linfen (a typical heavy industrial city in China), and intriguingly, the unreduced concentration was found for environmentally persistent free radicals (EPFRs) in PM2.5 after normal life suspension. The primary non-stopped coal combustion source and secondary Cu-related atmospheric reaction may have impacts on this phenomenon. The cigarette-based assessment model also indicated possible exposure risks of PM2.5-bound EPFRs during lockdown of Linfen. This study revealed not all the contaminants in the atmosphere had an apparent concentration decrease during city lockdown, suggesting the pollutants with complicated sources and formation mechanisms, like EPFRs in PM2.5, still should not be ignored.

How does COVID-19 lockdown affect air quality: Evidence from Lanzhou, a large city in Northwest China

Coronavirus disease (COVID-19) has disrupted health, economy, and society globally. Thus, many countries, including China, have adopted lockdowns to prevent the epidemic, which has limited human activities while affecting air quality. These affects have received attention from academics, but very few studies have focused on western China, with a lack of comparative studies across lockdown periods. Accordingly, this study examines the effects of lockdowns on air quality and pollution, using the hourly and daily air monitoring data collected from Lanzhou, a large city in Northwest China. The results indicate an overall improvement in air quality during the three lockdowns compared to the average air quality in the recent years, as well as reduced PM2.5, PM10, SO2, NO2, and CO concentrations with different rates and increased O3 concentration. During lockdowns, Lanzhou's "morning peak" of air pollution was alleviated, while the spatial characteristics remained unchanged. Further, ordered multi-classification logistic regression models to explore the mechanisms by which socioeconomic backgrounds and epidemic circumstances influence air quality revealed that the increment in population density significantly aggravated air pollution, while the presence of new cases in Lanzhou, and medium- and high-risk areas in the given district or county both increase the likelihood of air quality improvement in different degrees. These findings contribute to the understanding of the impact of lockdown on air quality, and propose policy suggestions to control air pollution and achieve green development in the post-epidemic era.

A Geo-AI-based ensemble mixed spatial prediction model with fine spatial-temporal resolution for estimating daytime/nighttime/daily average ozone concentrations variations in Taiwan

High levels of ground level ozone (O₃) are associated with detrimental health concerns. Most of the studies only focused on daily average and daytime trends due to the presence of sunlight that initiates its formation. However, atmospheric chemical reactions occur all day, thus, nighttime concentrations should be given equal importance. In this study, geospatial-artificial intelligence (Geo-AI) which combined kriging, land use regression (LUR), machine learning, an ensemble learning, was applied to develop ensemble mixed spatial models (EMSMs) for daily, daytime, and nighttime periods. These models were used to estimate the long-term O₃ spatio-temporal variations using a two-decade worth of in-situ measurements, meteorological parameters, geospatial predictors, and social and season-dependent factors. From the traditional LUR approach, the performance of EMSMs improved by 60% (daytime), 49% (nighttime), and 57% (daily). The resulting daily, daytime, and nighttime EMSMs had a high explanatory power with and adjusted R² of 0.91, 0.91, and 0.88, respectively. Estimation maps were produced to examine the changes before and during the implementation of nationwide COVID-19 restrictions. These results provide accurate estimates and its diurnal variation that will support pollution control measure and epidemiological studies.

Changes in air pollution due to COVID-19 lockdowns in 2020: Limited effect on NO2, PM2.5, and PM10 annual means compared to the new WHO Air Quality Guidelines

Background

Lockdowns have been fundamental to decreasing disease transmission during the COVID-19 pandemic even after vaccines were available. We aimed to evaluate and compare changes in air quality during the first year of the pandemic in different cities around the world, investigate how these changes correlate with changes in mobility, and analyse how lockdowns affected air pollutants' annual means.

Methods

We compared the concentrations of NO₂, PM_2.5, and PM₁₀ in 42 cities around the world in the first months of the pandemic in 2020 to data from 2016-2019 and correlated them with changes in mobility using Human Development Indexes (HDIs). Cities with the highest decreases in air pollutants during this period were evaluated for the whole year 2020. We calculated the annual means for these cities and compared them to the new World Health Organization (WHO) Air Quality Guidelines. A Student's t-test (95% confidence interval) was used to evaluate significant changes.

Results

Highest decreases in NO₂, PM_2.5, and PM₁₀ were between -50 and -70%. Cities evaluated for the whole year 2020 generally showed a recovery in air pollution levels after the initial months of the pandemic, except for London. These changes positively correlated with year-long mobility indexes for NO₂ and PM_2.5 for some cities. The highest reductions in air pollutants' annual means were from -20 to -35%. In general, decreases were higher for NO₂, compared to PM_2.5 and PM₁₀. All analysed cities showed annual means incompliant with the new WHO Air Quality Guidelines for NO₂ of 10 μg/m³, with values 1.7 and 4.3 times higher. For PM_2.5, all cities showed values 1.3 to 7.6 times higher than the WHO Guidelines of 5 μg/m³, except for New Delhi, with a value 18 times higher. For PM₁₀, only New York complied with the new guidelines of 15 μg/m³ and all the other cities were 1.1 to 4.2 times higher, except for New Delhi, which was 11 times higher.

Conclusions

These data show that even during a pandemic that highly affected mobility and economic activities and decreased air pollution around the world, complying with the new WHO Guidelines will demand a global strategical effort in the way we generate energy, move in and around the cities, and manufacture products.

The Long-Term Impact of COVID-19 Lockdowns in Istanbul

The World Health Organization (WHO) have set sustainability development goals to reduce diseases, deaths, and the environmental impact of cities due to air pollution. In Istanbul, although average pollutant concentrations have been on a downward trend in recent years, extreme values and their annual exceedance numbers are high based on the air quality standards of WHO and the EU. Due to COVID-19 lockdowns, statistically significant reductions in emissions were observed for short periods. However, how long the effect of the lockdowns will last is unknown. For this reason, this study aims to investigate the impact of long-term lockdowns on Istanbul's air quality. The restriction period is approximated to the same periods of the previous years to eliminate seasonal effects. A series of paired t-tests (p-value < 0.05) were applied to hourly data from 12 March 2016, until 1 July 2021, when quarantines were completed at 36 air quality monitoring stations in Istanbul. The findings reveal that the average air quality of Istanbul was approximately 17% improved during the long-term lockdowns. Therefore, the restriction-related changes in emission distributions continued in the long-term period of 476 days. However, it is unknown how long this effect will continue, which will be the subject of future studies. Moreover, it was observed that the emission probability density functions changed considerably during the lockdowns compared to the years before. Accordingly, notable decreases were detected in air quality limit exceedances in terms of both excessive pollutant concentrations and frequency of occurrence, respectively, for PM10 (-13% and -13%), PM2.5 (-16% and -30%), and NO2 (-3% and -8%), but not for O3 (+200% and +540%) and SO2 (-10% and +2.5%).

Impacts of condensable particulate matter on atmospheric organic aerosols and fine particulate matter (PM2.5) in China

Condensable particulate matter (CPM) emitted from stationary combustion and mobile sources exhibits high emissions and a large proportion of organic components. However, CPM is not generally measured when conducting emission surveys of PM in most countries, including China. Consequently, previous emission inventories have not included emission rates for CPM. Here, we construct an emission inventory of CPM in China with a focus on organic aerosols (OAs) based on collected CPM emission information. Results show that OA emissions are enhanced twofold after the inclusion of CPM in a new inventory for China for the years 2014 and 2017. Considering organic CPM emissions and model representations of secondary OA (SOA) formation from CPM, a series of sensitivity cases have been simulated here using the three-dimensional Community Multiscale Air Quality (CMAQ) model to estimate the contributions of CPM emissions to atmospheric OA and fine PM (PM2.5, particulate matter with aerodynamic diameter not exceeding 2.5 μm) concentrations in China. Compared with observations at a Beijing site during a haze episode from 14 October to 14 November 2014, estimates of the temporal average primary OA (POA) and SOA concentrations were greatly improved after including the CPM effects. These scenarios demonstrated the significant contributions of CPM emissions from stationary combustion and mobile sources to the POA (51 %-85 %), SOA (42 %-58 %), and total OA concentrations (45 %-75 %). Furthermore, the contributions of CPM emissions to total OA concentrations were demonstrated over the 2 major cities and 26 other cities of the Beijing-Tianjin-Hebei region (hereafter referred to as the "BTH2 + 26 cities") in December 2018, with average contributions of up to 49 %, 53 %, 54 %, and 50 % for Handan, Shijiazhuang, Xingtai, and Dezhou, respectively. Correspondingly, the inclusion of CPM emissions also narrowed the gap between simulated and observed PM2.5 concentrations over the BTH2 + 26 cities. These results improve the simulation performance of atmospheric OA and PM2.5 and may also provide important implications for the sources of OA.

Quantifying changes in ambient NOx, O3 and PM10 concentrations in Austria during the COVID-19 related lockdown in spring 2020

During spring 2020, unprecedented changes in local and regional emissions have occurred around the globe due to governmental restrictions associated with COVID-19. Many European countries including Austria issued partial curfews or stay-at-home order policies, which have impacted ambient air quality through reductions in non-essential transportation and energy consumption of industrial sites and work places. Here, we analyse the effect of these measures on ambient concentrations of nitrogen oxides (NOx), ozone (O3) and particulate matter (PM10) during the first nationwide lockdown in Austria (16.03.2020 to 14.04.2020). To ensure a robust analysis, the Austrian domain is divided into four individual subsectors contingent on regional climate. For air quality analysis a novel method is applied for filtering days with comparable weather conditions during the 2020 lockdown and spring 2017 to 2019. In general, our analysis shows decreasing pollutant concentrations, although in magnitude dependent on pollutant and regional subdomain. Largest reductions are found for NOx reaching up to -68% at traffic sites reflecting the substantial decrease in non-essential transport. Changes in the O3 concentrations at background sites show a rather weak response to NOx declines varying between roughly -18 to +8% for both the median and the upper tail of the distribution. Occasional site level increases in O3 concentrations can be attributed to comparably weak titration during night-time. PM10 concentrations show the smallest response among air pollutants, attributable to manifold precursor sources not affected by the lockdown measures. However, our analysis indicates also a shift of PM10 distributions at traffic sites closer to distributions observed at background sites.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11869-022-01232-w.

Disentangling drivers of air pollutant and health risk changes during the COVID-19 lockdown in China

The COVID-19 restrictions in 2020 have led to distinct variations in NO2 and O3 concentrations in China. Here, the different drivers of anthropogenic emission changes, including the effects of the Chinese New Year (CNY), China's 2018-2020 Clean Air Plan (CAP), and the COVID-19 lockdown and their impact on NO2 and O3 are isolated by using a combined model-measurement approach. In addition, the contribution of prevailing meteorological conditions to the concentration changes was evaluated by applying a machine-learning method. The resulting impact on the multi-pollutant Health-based Air Quality Index (HAQI) is quantified. The results show that the CNY reduces NO2 concentrations on average by 26.7% each year, while the COVID-lockdown measures have led to an additional 11.6% reduction in 2020, and the CAP over 2018-2020 to a reduction in NO2 by 15.7%. On the other hand, meteorological conditions from 23 January to March 7, 2020 led to increase in NO2 of 7.8%. Neglecting the CAP and meteorological drivers thus leads to an overestimate and underestimate of the effect of the COVID-lockdown on NO2 reductions, respectively. For O3 the opposite behavior is found, with changes of +23.3%, +21.0%, +4.9%, and -0.9% for CNY, COVID-lockdown, CAP, and meteorology effects, respectively. The total effects of these drivers show a drastic reduction in multi-air pollutant-related health risk across China, with meteorology affecting particularly the Northeast of China adversely. Importantly, the CAP's contribution highlights the effectiveness of the Chinese government's air-quality regulations on NO2 reduction.

Decrease in ambient volatile organic compounds during the COVID-19 lockdown period in the Pearl River Delta region, south China

During the COVID-19 lockdown, ambient ozone levels are widely reported to show much smaller decreases or even dramatical increases under substantially reduced precursor NOx levels, yet changes in ambient precursor volatile organic compounds (VOCs) have been scarcely reported during the COVID-19 lockdown, which is an opportunity to examine the impacts of dramatically changing anthropogenic emissions on ambient VOC levels in megacities where ozone formation is largely VOC-limited. In this study, ambient VOCs were monitored online at an urban site in Guangzhou in the Pearl River Delta region before, during, and after the COVID-19 lockdown. The average total mixing ratios of VOCs became 19.1% lower during the lockdown than before, and those of alkanes, alkenes and aromatics decreased by 19.0%, 24.8% and 38.2%, respectively. The levels of light alkanes (C < 6) decreased by only 13.0%, while those of higher alkanes (C ≥ 6) decreased by 67.8% during the lockdown. Disappeared peak VOC levels in morning rush hours and the drop in toluene to benzene ratios during the lockdown suggested significant reductions in vehicle exhaust and industrial solvent emissions. Source apportioning by positive matrix factorization model revealed that reductions in industrial emissions, diesel exhaust (on-road diesel vehicles and off-road diesel engines) and gasoline-related emissions could account for 48.9%, 42.2% and 8.8%, respectively, of the decreased VOC levels during the lockdown. Moreover, the reduction in industrial emissions could explain 56.0% and 70.0% of the reductions in ambient levels of reactive alkenes and aromatics, respectively. An average increase in O₃-1 h by 17% and a decrease in the daily maximum 8-h average ozone by 11% under an average decrease in NOx by 57.0% and a decrease in VOCs by 19.1% during the lockdown demonstrated that controlling emissions of precursors VOCs and NOx to prevent ambient O₃ pollution in megacities such as Guangzhou remains a highly challenging task.

Investigation of PM2.5 pollution during COVID-19 pandemic in Guangzhou, China

The COVID-19 pandemic has raised awareness about various environmental issues, including PM2.5 pollution. Here, PM2.5 pollution during the COVID-19 lockdown was traced and analyzed to clarify the sources and factors influencing PM2.5 in Guangzhou, with an emphasis on heavy pollution. The lockdown led to large reductions in industrial and traffic emissions, which significantly reduced PM2.5 concentrations in Guangzhou. Interestingly, the trend of PM2.5 concentrations was not consistent with traffic and industrial emissions, as minimum concentrations were observed in the fourth period (3/01-3/31, 22.45 μg/m3) of the lockdown. However, the concentrations of other gaseous pollutants, e.g., SO2, NO2 and CO, were correlated with industrial and traffic emissions, and the lowest values were noticed in the second period (1/24-2/03) of the lockdown. Meteorological correlation analysis revealed that the decreased PM2.5 concentrations during COVID-19 can be mainly attributed to decreased industrial and traffic emissions rather than meteorological conditions. When meteorological factors were included in the PM2.5 composition and backward trajectory analyses, we found that long-distance transportation and secondary pollution offset the reduction of primary emissions in the second and third stages of the pandemic. Notably, industrial PM2.5 emissions from western, southern and southeastern Guangzhou play an important role in the formation of heavy pollution events. Our results not only verify the importance of controlling traffic and industrial emissions, but also provide targets for further improvements in PM2.5 pollution.

Modelling the Effect of COVID-19 Lockdown on Air Pollution in Makkah Saudi Arabia with a Supervised Machine Learning Approach

To reduce the spread of COVID-19, lockdowns were implemented in almost every single country in the world including Saudi Arabia. In this paper, the effect of COVID-19 lockdown on O3, NO2, and PM10 in Makkah was analysed using air quality and meteorology data from five sites. Two approaches were employed: (a) comparing raw measured concentrations for the lockdown period in 2019 and 2020; and (b) comparing weather-corrected concentrations estimated by the machine learning approach with observed concentrations during the lockdown period. According to the first approach, the average levels of PM10 and NO2 decreased by 12% and 58.66%, respectively, whereas the levels of O3 increased by 68.67%. According to the second approach, O3 levels increased by 21.96%, while the levels of NO2 and PM10 decreased by 13.40% and 9.66%, respectively. The machine learning approach after removing the effect of changes in weather conditions demonstrated relatively less reductions in the levels of NO2 and PM10 and a smaller increase in the levels of O3. This showed the importance of adjusting air pollutant levels for meteorological conditions. O3 levels increased due to its inverse correlation with NO2, which decreased during the lockdown period.

Spatio-Temporal Heterogeneity of the Relationships Between PM2.5 and Its Determinants: A Case Study of Chinese Cities in Winter of 2020

Fine particulate matter (PM2.5) poses threat to human health in China, particularly in winter. The pandemic of coronavirus disease 2019 (COVID-19) led to a series of strict control measures in Chinese cities, resulting in a short-term significant improvement in air quality. This is a perfect case to explore driving factors affecting the PM2.5 distributions in Chinese cities, thus helping form better policies for future PM2.5 mitigation. Based on panel data of 332 cities, we analyzed the function of natural and anthropogenic factors to PM2.5 pollution by applying the geographically and temporally weighted regression (GTWR) model. We found that the PM2.5 concentration of 84.3% of cities decreased after lockdown. Spatially, in the winter of 2020, cities with high PM2.5 concentrations were mainly distributed in Northeast China, the North China Plain and the Tarim Basin. Higher temperature, wind speed and relative humidity were easier to promote haze pollution in northwest of the country, where enhanced surface pressure decreased PM2.5 concentrations. Furthermore, the intensity of trip activities (ITAs) had a significant positive effect on PM2.5 pollution in Northwest and Central China. The number of daily pollutant operating vents of key polluting enterprises in the industrial sector (VOI) in northern cities was positively correlated with the PM2.5 concentration; inversely, the number of daily pollutant operating vents of key polluting enterprises in the power sector (VOP) imposed a negative effect on the PM2.5 concentration in these regions. This work provides some implications for regional air quality improvement policies of Chinese cities in wintertime.

The impact of lockdown on nitrogen dioxide (NO2) over Central Asian countries during the COVID-19 pandemic

Nitrogen dioxide (NO₂) is one of the main air pollutants, formed due to both natural and anthropogenic processes, which has a significant negative impact on human health. The COVID-19 pandemic has prompted countries to take various measures, including social distancing or stay-at-home orders. This study analyzes the impact of COVID-19 lockdown measures on nitrogen dioxide (NO₂) changes in Central Asian countries. Data from TROPOspheric Monitoring Instrument (TROPOMI) on the Sentinel-5 Precursor satellite, as well as meteorological data, make it possible to assess changes in NO₂ concentration in countries and major cities in the region. In particular, the obtained satellite data show a decreased tropospheric column of NO₂. Its decrease during the lockdown (March 19-April 14) ranged from - 5.1% (Tajikistan) to - 11.6% (Turkmenistan). Based on the obtained results, it can be concluded that limitations in anthropogenic activities have led to improvements in air quality. The possible influence of meteorology is not assessed in this study, and the implied uncertainties cannot be quantified. In this way, the level of air pollution is expected to decrease as long as partial or complete lockdown continues.

Less COVID-19 deaths in southern and insular Italy explained by forest bathing, Mediterranean environment, and antiviral plant volatile organic compounds

The coronavirus disease 2019 (COVID-19) is causing major sanitary and socioeconomic issues, yet some locations are less impacted than others. While densely populated areas are likely to favor viral transmission, we hypothesize that other environmental factors could explain lower cases in some areas. We studied COVID-19 impact and population statistics in highly forested Mediterranean Italian regions versus some northern regions where the amount of trees per capita is much lower. We also evaluated the affinity of Mediterranean plant-emitted volatile organic compounds (VOCs) isoprene, α-pinene, linalool and limonene for COVID-19 protein targets by molecular docking modeling. Results show that while mean death number increased about 4 times from 2020 to 2021, the percentage of deaths per population (0.06-0.10%) was lower in the greener Mediterranean regions such as Sardinia, Calabria and Basilica versus northern regions with low forest coverage, such as Lombardy (0.33%) and Emilia Romagna (0.29%). Data also show that the pandemic severity cannot be explained solely by population density. Modeling reveals that plant organic compounds could bind and interfere with the complex formed by the receptor binding domain of the coronavirus spike protein with the human cell receptor. Overall, our findings are likely explained by sea proximity and mild climate, Mediterranean diet and the abundance of non-deciduous Mediterranean plants which emit immunomodulatory and antiviral compounds. Potential implications include 'forest bathing' as a therapeutic practice, designing nasal sprays containing plant volatile organic compounds, and preserving and increasing forest coverage.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10311-021-01309-5.

Links between chronic exposure to outdoor air pollution and cardiovascular diseases: a review

Acute exposure to air pollution is associated with an increasing risk of death and cardiovascular disorders. Nonetheless, the impact of chronic exposure to air pollution on the circulatory system is still debated. Here, we review the links of chronic exposure to outdoor air pollution with mortality and most common cardiovascular diseases, in particular during the coronavirus disease 2019 event (COVID-19). We found that recent studies provide robust evidence for a causal effect of chronic exposure to air pollution and cardiovascular mortality. In terms of mortality, the strongest relationship was noted for fine particulate matter, nitrogen dioxide, and ozone. There is also increasing evidence showing that exposure to air pollution, mainly fine particulate matter and nitrogen dioxide, is associated with the development of atherosclerosis, hypertension, stroke, and heart failure. However, available scientific evidence is not strong enough to support associations with cardiac arrhythmias and coagulation disturbances. Noteworthy, for some pollutants, the risk of negative health effects is high for concentrations lower than the limit values recommended by the European Union and Word Health Organization. Efforts to diminish exposure to air pollution and to design optimal methods of air pollution reduction should be urgently intensified and supported by effective legislation and interdisciplinary cooperation.

Air quality index variation before and after the onset of COVID-19 pandemic: a comprehensive study on 87 capital, industrial and polluted cities of the world

BACKGROUND

Coronavirus disease 2019 (COVID-19) pandemic provided an opportunity for the environment to reduce ambient pollution despite the economic, social and health disruption to the world. The purpose of this study was to investigate the changes in the air quality indexes (AQI) in industrial, densely populated and capital cities in different countries of the world before and after 2020. In this ecological study, we used AQI obtained from the free available databases such as the World Air Quality Index (WAQI). Bivariate correlation analysis was used to explore the correlations between meteorological and AQI variables. Mean differences (standard deviation: SD) of AQI parameters of different years were tested using paired-sample t-test or Wilcoxon signed-rank test as appropriate. Multivariable linear regression analysis was conducted to recognize meteorological variables affecting the AQI parameters.

RESULTS

AQI-PM2.5, AQI-PM10 and AQI-NO2 changes were significantly higher before and after 2020, simultaneously with COVID-19 restrictions in different cities of the world. The overall changes of AQI-PM2.5, AQI-PM10 and AQI-NO2 in 2020 were - 7.36%, - 17.52% and - 20.54% compared to 2019. On the other hand, these results became reversed in 2021 (+ 4.25%, + 9.08% and + 7.48%). In general, the temperature and relative humidity were inversely correlated with AQI-PM2.5, AQI-PM10 and AQI-NO2. Also, after adjusting for other meteorological factors, the relative humidity was inversely associated with AQI-PM2.5, AQI-PM10 and AQI-NO2 (β = - 1.55, β = - 0.88 and β = - 0.10, P < 0.01, respectively).

CONCLUSIONS

The results indicated that air quality generally improved for all pollutants except carbon monoxide and ozone in 2020; however, changes in 2021 have been reversed, which may be due to the reduction of some countries' restrictions. Although this quality improvement was temporary, it is an important result for planning to control environmental pollutants.

Comparison of the characterization of allergenic protein 3 (Pla a3) released from Platanus pollen grains collected in Shanghai during the spring of 2019 and 2020

Due to the COVID-19 pandemic in early 2020, large-scale industrial production has been stagnant and reduced, the urban air quality has been greatly improved. It provided an excellent opportunity to explore the effects of air pollutants on the sensitization of pollen allergen proteins in the environment. Platanus pollen grains sampled in the spring of 2019 and 2020 were used for detailed characterization and analysis. Scanning electron microscopy, Fourier transform infrared, X-ray spectroscopy (XPS), trypan blue staining, and western blot analysis were employed to characterize Platanus pollen protein released from pollen grains. Our data showed that the viability of the pollen grains in 2019 was lower compared that in 2020, and the pollen grains collected in 2019 had a higher absorption peak of protein functional groups. The XPS spectra assay result demonstrated that the binding energy of the high-resolution components had not variation on the surface of pollen grains, but relative content of nitrogen and peptide chain in the pollen grains sampled in 2019 were higher than in 2020. These results suggested that more protein in the pollen grains was released onto the surface of pollen grains. In addition, western blot assay showed that the expression of Pla a3 protein in pollen grains sampled in 2019 was significantly higher than that in 2020, revealing that air pollutants could enhance the expression of Pla a3 proteins in Platanus pollen.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10453-021-09731-6.

Opposite impact of emission reduction during the COVID-19 lockdown period on the surface concentrations of PM2.5 and O3 in Wuhan, China

To prevent the spread of the COVID-19 epidemic, the Chinese megacity Wuhan has taken emergent lockdown measures starting on January 23, 2020. This provided a natural experiment to investigate the response of air quality to such emission reductions. Here, we decoupled the influence of meteorological and non-meteorological factors on main air pollutants using generalized additive models (GAMs), driven by data from the China National Environmental Monitoring Center (CNEMC) network. During the lockdown period (Jan. 23 - Apr. 8, 2020), PM2.5, PM10, NO2, SO2, and CO concentrations decreased significantly by 45 %, 49 %, 56 %, 39 %, and 18 % compared with the corresponding period in 2015-2019, with contributions by S(meteos) of 15 %, 17 %, 13 %, 10 %, and 6 %. This indicates an emission reduction of NOx at least 43 %. However, O3 increased by 43 % with a contribution by S(meteos) of 6 %. In spite of the reduced volatile organic compound (VOC) emissions by 30 % during the strict lockdown period (Jan. 23 - Feb. 14, 2020), which likely reduced the production of O3, O3 concentrations increased due to a weakening of the titration effect of NO. Our results suggest that conventional emission reduction (NOx reduction only) measures may not be sufficient to reduce (or even lead to an increase of) surface O3 concentrations, even if reaching the limit, and VOC-specific measures should also be taken.

Non-stop industries were the main source of air pollution during the 2020 coronavirus lockdown in the North China Plain

Despite large decreases of emissions of air pollution during the coronavirus disease 2019 (COVID-19) lockdown in 2020, an unexpected regional severe haze has still occurred over the North China Plain. To clarify the origin of this pollution, we studied air concentrations of fine particulate matter (PM2.5), NO2, O3, PM10, SO2, and CO in Beijing, Hengshui and Baoding during the lockdown period from January 24 to 29, 2020. Variations of PM2.5 composition in inorganic ions, elemental carbon and organic matter were also investigated. The HYSPLIT model was used to calculate backward trajectories and concentration weighted trajectories. Results of the cluster trajectory analysis and model simulations show that the severe haze was caused mainly by the emissions of northeastern non-stopping industries located in Inner Mongolia, Liaoning, Hebei, and Tianjin. In Beijing, Hengshui and Baoding, the mixing layer heights were about 30% lower and the maximum relative humidity was 83% higher than the annual averages, and the average wind speeds were lower than 1.5 m s-1. The concentrations of NO3 -, SO4 2-, NH4 +, organics and K+ were the main components of PM2.5 in Beijing and Hengshui, while organics, K+, NO3 -, SO4 2-, and NH4 + were the main components of PM2.5 in Baoding. Contrary to previous reports suggesting a southerly transport of air pollution, we found that northeast transport caused the haze formation.

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The online version contains supplementary material available at 10.1007/s10311-021-01314-8.

Impact of PM2.5 concentration, weather and population on COVID-19 morbidity and mortality in Baghdad and Kuwait cities

The coronavirus (COVID-19) pandemic is a global health crisis and biggest challenge facing the world. Station measurements of fine particulate matter (PM2.5) concentration in Baghdad and Kuwait during the period January–July 2020 are analyzed as well as assessment of correlation between PM2.5, weather conditions (air temperature, relative humidity, wind speed), population density and COVID-19 morbidity and mortality. A significant improvement (decrease) has observed during total and partial curfew in PM2.5 at Baghdad by 35%, 12.4%, respectively, from PM2.5 mean during the study period that is less than the WHO recommended PM2.5 level especially in total curfew. This decrease in PM2.5 pollution and people’s mobility in Baghdad at total and partial curfew contributed to decrease injuries and mortality. PM2.5 during total and partial curfew in Kuwait country witnessed increasing by 38.4% and decreasing by 22.3% from the PM2.5 mean, respectively, but still higher than WHO standard level. This increase in PM2.5 at total curfew was related to burning accidents in the oil wells which caused increasing in PM2.5 pollutant and then an increase in number of injuries and mortality during that time. In general during all study period our research found that PM2.5 and wind speed exhibit weak relation with COVID-19 morbidity and mortality but strong relation with increasing temperature and decreasing humidity. The high population density had a good association with increasing daily new cases, mortality due to COVID-19 pandemic. Thus, these factors may be taken into consideration in policy development for the control and prevention of new chains of the Coronavirus pandemic.

Airborne black carbon variations during the COVID-19 lockdown in the Yangtze River Delta megacities suggest actions to curb global warming

Airborne black carbon is a strong warming component of the atmosphere. Therefore, curbing black carbon emissions should slow down global warming. The 2019 coronavirus pandemic (COVID-19) is a unique opportunity for studying the response of black carbon to the varied human activities, in particular due to lockdown policies. Actually, there is few knowledge on the variations of black carbon in China during lockdowns. Here, we studied the concentrations of particulate matter (PM2.5) and black carbon before, during, and after the lockdown in nine sites of the Yangtze River Delta in Eastern China. Results show 40-60% reduction of PM2.5 and 40-50% reduction of black carbon during the lockdown. The classical bimodal peaks of black carbon in the morning and evening rush hours were highly weakened, indicating the substantial decrease of traffic activities. Contributions from fossil fuels combustion to black carbon decreased about 5-10% during the lockdown. Spatial correlation analysis indicated the clustering of the multi-site black carbon concentrations in the Yangtze River Delta during the lockdown. Overall, control of emissions from traffic and industrial activities should be efficient to curb black carbon levels in the frame of a 'green public transit system' for mega-city clusters such as the Yangtze River Delta.

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The online version contains supplementary material available at 10.1007/s10311-021-01327-3.

Effect of COVID-19-Related Lockdown οn Hospital Admissions for Asthma and COPD Exacerbations: Associations with Air Pollution and Patient Characteristics

We conducted a retrospective observational study to assess the hospitalization rates for acute exacerbations of asthma and COPD (chronic obstructive pulmonary disease) during the first imposed lockdown in Athens, Greece. Patient characteristics and the concentration of eight air pollutants [namely, NO (nitrogen monoxide), NO2 (nitrogen dioxide), CO (carbon monoxide), PM2.5 (particulate matter 2.5), PM10 (particulate matter 10), O3 (ozone), SO2 (sulfur dioxide) and benzene] were considered. A total of 153 consecutive hospital admissions were studied. Reduced admissions occurred in the Lockdown period compared to the Pre-lockdown 2020 (p < 0.001) or the Control 2019 (p = 0.007) period. Furthermore, the concentration of 6/8 air pollutants positively correlated with weekly hospital admissions in 2020 and significantly decreased during the lockdown. Finally, admitted patients for asthma exacerbation during the lockdown were younger (p = 0.046) and less frequently presented respiratory failure (p = 0.038), whereas patients with COPD presented higher blood eosinophil percentage (p = 0.017) and count (p = 0.012). Overall, admissions for asthma and COPD exacerbations decreased during the lockdown. This might be partially explained by reduction of air pollution during this period while medical care avoidance behavior, especially among elderly patients cannot be excluded. Our findings aid in understanding the untold impact of the pandemic on diseases beyond COVID-19, focusing on patients with obstructive diseases.

The impact of successive COVID-19 lockdowns on people mobility, lockdown efficiency, and municipal solid waste

The COVID-19 pandemic has induced many issues for all societal sectors, in particular in the production and disposal of municipal solid waste. This may be because successive easing and reimposing of lockdown measures have deeply changed people's movements, consumers' behaviors and waste management. Previous studies have focused on the short-term effects of lockdowns on waste changes, yet there is little knowledge on waste variations during successive lockdowns and unlocking of various lockdown intensities. Moreover, the efficiency of lockdown and its relation to people's mobility in different countries are still not clear. Here, we studied the variations of amount and composition of municipal solid waste before the pandemic in 2019 and during the pandemic in 2020-2021 in USA, Brazil, Canada, UK, France and Italy. We used a stringency index and a composite mobility index to assess the lockdown intensity and people's movements. Results show that the mobility index sharply decreased with lockdown intensity, and enforcing measures were more efficient in France and Italy. Compared to 2019, prolonged lockdowns caused larger decreases in the quantity of commercial and construction wastes versus household waste. The initial implementation of lockdown or unlocking measures promoted inhabitants' consumption, generally leading to the increased waste amount, by about 9% for Trento and 12% for Montreal at the beginning of lockdown, respectively. Moreover, larger variations in the waste amount in Trento, from - 25.3 to 9.8%, were in line with higher lockdown intensity compared to those in Montreal, from - 9.5 to 12.7%, affected by people's mobility, consumers' behaviors and waste management.

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The online version contains supplementary material available at 10.1007/s10311-021-01290-z.

A Critical Review on Removal of Gaseous Pollutants Using Sulfate Radical-based Advanced Oxidation Technologies

Excessive emissions of gaseous pollutants such as SO₂, NO_x, heavy metals (Hg, As, etc.), H₂S, VOCs, etc. have triggered a series of environmental pollution incidents. Sulfate radical (SO₄•^-)-based advanced oxidation technologies (AOTs) are one of the most promising gaseous pollutants removal technologies because they can not only produce active free radicals with strong oxidation ability to simultaneously degrade most of gaseous pollutants, but also their reaction processes are environmentally friendly. However, so far, the special review focusing on gaseous pollutants removal using SO₄•^--based AOTs is not reported. This review reports the latest advances in removal of gaseous pollutants (e.g., SO₂, NO_x, Hg, As, H₂S, and VOCs) using SO₄•^--based AOTs. The performance, mechanism, active species identification and advantages/disadvantages of these removal technologies using SO₄•^--based AOTs are reviewed. The existing challenges and further research suggestions are also commented. Results show that SO₄•^--based AOTs possess good development potential in gaseous pollutant control field due to simple reagent transportation and storage, low product post-treatment requirements and strong degradation ability of refractory pollutants. Each SO₄•^--based AOT possesses its own advantages and disadvantages in terms of removal performance, cost, reliability, and product post-treatment. Low free radical yield, poor removal capacity, unclear removal mechanism/contribution of active species, unreliable technology and high cost are still the main problems in this field. The combined use of multiactivation technologies is one of the promising strategies to overcome these defects since it may make up for the shortcomings of independent technology. In order to improve free radical yield and pollutant removal capacity, enhancement of mass transfer and optimization design of reactor are critical issues. Comprehensive consideration of catalytic materials, removal chemistry, mass transfer and reactor is the promising route to solve these problems. In order to clarify removal mechanism, it is essential to select suitable free radical sacrificial agents, probes and spin trapping agents, which possess high selectivity for target specie, high solubility in water, and little effect on activity of catalyst itself and mass transfer/diffusion parameters. In order to further reduce investment and operating costs, it is necessary to carry out the related studies on simultaneous removal of more gaseous pollutants.

COVID-19 and energy: Influence mechanisms and research methodologies

Considering the important role of energy in modern society, it is imperative to study the current situation and future development of energy under the influence of COVID-19. This paper identifies the current research hotspots, proposes future research directions accordingly, and summarizes the methodologies via a bibliometric analysis. Five research hotspots include COVID-19 and the changes of energy consumption, COVID-19 and the fluctuation of the energy market, COVID-19 and the development of renewable energy, COVID-19 and climate impacts caused by energy consumption, and COVID-19 and the energy policy. According to the influence mechanism of COVID-19 on each hotspot, the pandemic has exerted short-term influencs on energy consumption, energy price, and air pollution. Meanwhile, the pandemic could have a far-reaching impact on the renewable energy sector, climate, and energy policy. In addition, the main methodologies are reviewed, revealing that regression analysis and scenario analysis are commonly used as the quantitative and qualitative methods, respectively. Moreover, given the nonlinear relations between the pandemic and energy, an artificial neural networks model is used to enhance the prediction efficiency of energy demand and price. Finally, policy implications for obtaining clean, low-carbon, safe, and efficient energy in the context of COVID-19 are proposed.

How do pollutants change post-pandemic? Evidence from changes in five key pollutants in nine Chinese cities most affected by the COVID-19

Under the COVID-19 global pandemic, China has weakened the large-scale spread of the epidemic through lockdown and other measures. At the same time, with the recovery of social production activities, China has become the only country which achieves positive growth in 2020 in the major economies. It entered the post pandemic period. These measures improved the local environmental quality. However, whether this improvement can be sustained is also a problem that needs to be solved. So, this study investigated the changes of five air pollutants (PM2.5, PM10, NO2, SO2, and CO) in the nine cities most severely affected by the pandemic in China during the lockdown and post pandemic period. We emphasized that when analyzing the changes of environmental quality during the epidemic, we must consider not only the impact of the day and short-term changesbut also the cumulative lag effect and sustainable development. Through a combination of qualitative and quantitative methods, it is found that the concentration of pollutants decreased significantly during the lockdown compared to the situation before the epidemic. PM10 and NO2 are falling most, which downs 39% and 46% respectively. During the lockdown period, the pollutant concentrations response to the pandemic has a lag of 3-7 days. More specifically, in the cities related to single pollutants, the impact on the pollutant shows a significant correlation when the measures are delayed for seven days. In the cities that are related to multiple pollutants, the correlation is usually highest in 3-5 days. This means that the impact of policy measures on the environment lasted for 3-5 days. Besides, Wuhan, Jingmen and Jingzhou have seen the most obvious improvement. However, this improvement did not last. In the post pandemic period, the pollutants rebounded, the growth rates of PM10 and NO2 reached 44% and 87% in September. When compared with the changes of pollutants concentration in the same period from 2017 to 2019, the decline rate has also been significantly slower, even higher than the average concentration of previous years. The research not only contributes to China's economic "green recovery" plan during the post epidemic period, but also provides references for environmental governance during economic recovery in other countries.

Highlighting the compound risk of COVID-19 and environmental pollutants using geospatial technology

The new COVID-19 coronavirus disease has emerged as a global threat and not just to human health but also the global economy. Due to the pandemic, most countries affected have therefore imposed periods of full or partial lockdowns to restrict community transmission. This has had the welcome but unexpected side effect that existing levels of atmospheric pollutants, particularly in cities, have temporarily declined. As found by several authors, air quality can inherently exacerbate the risks linked to respiratory diseases, including COVID-19. In this study, we explore patterns of air pollution for ten of the most affected countries in the world, in the context of the 2020 development of the COVID-19 pandemic. We find that the concentrations of some of the principal atmospheric pollutants were temporarily reduced during the extensive lockdowns in the spring. Secondly, we show that the seasonality of the atmospheric pollutants is not significantly affected by these temporary changes, indicating that observed variations in COVID-19 conditions are likely to be linked to air quality. On this background, we confirm that air pollution may be a good predictor for the local and national severity of COVID-19 infections.

COVID-19 transmission, vulnerability, persistence and nanotherapy: a review

End 2019, the zoonotic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), named COVID-19 for coronavirus disease 2019, is the third adaptation of a contagious virus following the severe acute respiratory syndrome coronavirus in 2002, SARS-CoV, and the Middle East respiratory syndrome virus in 2012, MERS-CoV. COVID-19 is highly infectious and virulent compared to previous outbreaks. We review sources, contagious routes, preventive measures, pandemic, outbreak, epidemiology of SARS-CoV, MERS-CoV and SARS-CoV-2 from 2002 to 2020 using a Medline search. We discuss the chronology of the three coronaviruses, the vulnerability of healthcare workers, coronaviruses on surface and in wastewater, diagnostics and cures, and measures to prevent spreading.

COVID-19 and the emerging research trends in environmental studies: a bibliometric evaluation

The ongoing pandemic of the coronavirus disease 2019 (COVID-19) is a global health emergency. Thousands of articles have been published to tackle this crisis. Here, a bibliometric study of the publications in environmental studies has been conducted to identify the emerging research trends in this field in the era of COVID-19. Bibliometric analysis serves as a useful tool to evaluate research productivity and scholarly trends in a field. For this, publications were searched in nine environment-related subject categories indexed in Science Citation Index Expanded (SCI-EXPANDED) database of the Web of Science Core Collection. A bibliometric evaluation of 495 relevant documents was performed to identify various essential research indicators, including the type of the publication, the most prominent journals, subject categories, authors, institutions, and the countries, that contributed significantly to this theme. Major focus of this bibliometric study is to illustrate the potential research hotspots emerged during this pandemic. It has been found that significant amount of research has been conducted for the assessment of environmental quality and its contribution in environmental transmission of COVID-19. In addition to its positive impacts on environment, COVID-19 has contributed indirectly in worsening many environmental threats such as increased exposure to disinfectants and antimicrobials, poor solid waste management, and food insecurity. Researchers have also been focusing on the strategies for the planning of post-COVID-19 cities and buildings and to protect the ecology. This bibliometric study allowed the visualization of research agenda in the field of environmental studies during this pandemic.

Geospatial analysis of COVID-19 lockdown effects on air quality in the South and Southeast Asian region

The COVID-19 pandemic, induced by the novel Coronavirus worldwide outbreak, is causing countries to introduce different types of lockdown measures to curb the contagion. The implementation of strict lockdown policies has had unprecedented impacts on air quality globally. This study is an attempt to assess the effects of COVID-19 induced lockdown measures on air quality in both regional, country, and city scales in the South and Southeast Asian region using open-source satellite-based data and software frameworks. We performed a systematic review of the national lockdown measures of 19 countries of the study area based on publicly available materials. We considered two temporal settings over a period of 66 days to assess and compare the effects of lockdown measures on air quality levels between standard business as usual and current situation COVID-19 lockdown. Results showed that compared to the same period of 2019, atmospheric NO2, SO2, PM2.5, and CO levels decreased by an average of 24.16%, 19.51%, 20.25%, and 6.88%, respectively during the lockdown, while O3 increased by a maximum of 4.52%. Among the 19 studied cities, Dhaka, Kathmandu, Jakarta, and Hanoi experienced the highest reduction of NO2 (40%-47%) during the lockdown period compared to the corresponding period of 2019. The methodological framework applied in this study can be used and extended to future research in the similar domain such as understanding long-term effects of COVID-19 mitigation measures on the atmospheric pollution at continental-scale or assessing the effects of the domestic emissions during the stay-at-home; a standard and effective COVID-19 lockdown measure applied in most of the countries.

Air pollution and critical air pollutant assessment during and after COVID-19 lockdowns: Evidence from pandemic hotspots in China, the Republic of Korea, Japan, and India

The COVID-19 virus outbreak has been declared a "global pandemic". Therefore, "lockdown" was issued in affected countries to control the spread of the virus. To assess air pollution during and after lockdowns, this study selected pandemic hotspots in China (Wuhan), Japan (Tokyo), the Republic of Korea (Daegu), and India (Mumbai) and compared the Air Quality Index (AQI) in these areas for the past three years. The results indicated that air pollution levels were positively correlated with a reduction in pollutant levels during and after lockdowns in these cities. In Tokyo, low levels of air pollution, no significant change in the distribution of "good" and "moderate" days was observed during lockdown. In Daegu, mid-level air pollution, the percentage of "unhealthy" days (AQI>100) markedly reduced during lockdown; however, this reverted after lockdown was lifted. In Wuhan and Mumbai, high air pollution levels, the percentage of unhealthy days remarkably decreased during lockdown and continued to reduce after lockdown. It was found that PM2.5 was the critical pollutant for all cities because its sub-AQI was the largest of the six pollutant species for the majority of days. In addition, PM10 dominated the overall AQI for 2.2-9.6% of the period in Wuhan and Mumbai, and its sub-AQI reduced during lockdown. The mean sub-AQI for NO2, SO2, CO, and O3 was within the "good" category for all cities. In conclusion, the lockdown policy reduced air pollution in general and this reduction was more significant for regions with high air pollution levels.

Pseudoviruses for the assessment of coronavirus disinfection by ozone

The COVID-19 pandemic has severely impacted public health worldwide. Evidence of SARS-CoV-2 transmission via aerosols and surfaces has highlighted the need for efficient indoor disinfection methods. For instance, the use of ozone gas as a safe and potent disinfectant against SARS-CoV-2 virus is of particular interest. Here we tested the use of pseudoviruses as a model for evaluating ozone disinfection of the coronavirus at ozone concentrations of 30, 100, and 1000 ppmv. Results show that ozone disinfection rate of pseudoviruses was similar to that of coronavirus 229E (HuCoV-229E) at short contact times, below 30 min. Viral infection decreased by 95% following ozone exposure for 20 min at 1000 ppmv, 30 min at 100 ppmv and about 40 min at 30 ppmv. This findings mean that ozone is a powerful disinfectant toward the enveloped pseudovirus even at low ozone exposure. We also showed that viral disinfection occurs on various contaminated surfaces, with a positive association between disinfection and surface hydrophilicity. Infected surfaces made of aluminum alloy, for example, were better disinfected with ozone as compared to brass, copper, and nickel surfaces. Lastly, we demonstrate the advantage of ozone over liquid disinfectants by showing similar viral disinfection on top, side, bottom, and interior surfaces. Overall, our study demonstrates the potential use of ozone gas disinfection to combat the COVID-19 outbreak.

Role of indoor aerosols for COVID-19 viral transmission: a review

The relationship between outdoor atmospheric pollution by particulate matter and the morbidity and mortality of coronavirus disease 2019 (COVID-19) infections was recently disclosed, yet the role of indoor aerosols is poorly known . Since people spend most of their time indoor, indoor aerosols are closer to human occupants than outdoors, thus favoring airborne transmission of COVID-19. Therefore, here we review the characteristics of aerosol particles emitted from indoor sources, and how exposure to particles affects human respiratory infections and transport of airborne pathogens. We found that tobacco smoking, cooking, vacuum cleaning, laser printing, burning candles, mosquito coils and incenses generate large quantities of particles, mostly in the ultrafine range below 100 nm. These tiny particles stay airborne, are deposited in the deeper regions of human airways and are difficult to be removed by the respiratory system. As a consequence, adverse effects can be induced by inhaled aerosol particles via oxidative stress and inflammation. Early epidemiological evidence and animal studies have revealed the adverse effects of particle exposure in respiratory infections. In particular, inhaled particles can impair human respiratory systems and immune functions, and induce the upregulation of angiotensin-converting enzyme 2, thus inducing higher vulnerability to COVID-19 infection. Moreover, co-production of inflammation mediators by COVID-19 infection and particle exposure magnifies the cytokine storm and aggravates symptoms in patients. We also discuss the role of indoor aerosol particles as virus carriers. Although many hypotheses were proposed, there is still few knowledge on interactions between aerosol articles and virus-laden droplets or droplet nuclei.

Pseudoviruses for the assessment of coronavirus disinfection by ozone

The COVID-19 pandemic has severely impacted public health worldwide. Evidence of SARS-CoV-2 transmission via aerosols and surfaces has highlighted the need for efficient indoor disinfection methods. For instance, the use of ozone gas as a safe and potent disinfectant against SARS-CoV-2 virus is of particular interest. Here we tested the use of pseudoviruses as a model for evaluating ozone disinfection of the coronavirus at ozone concentrations of 30, 100, and 1000 ppmv. Results show that ozone disinfection rate of pseudoviruses was similar to that of coronavirus 229E (HuCoV-229E) at short contact times, below 30 min. Viral infection decreased by 95% following ozone exposure for 20 min at 1000 ppmv, 30 min at 100 ppmv and about 40 min at 30 ppmv. This findings mean that ozone is a powerful disinfectant toward the enveloped pseudovirus even at low ozone exposure. We also showed that viral disinfection occurs on various contaminated surfaces, with a positive association between disinfection and surface hydrophilicity. Infected surfaces made of aluminum alloy, for example, were better disinfected with ozone as compared to brass, copper, and nickel surfaces. Lastly, we demonstrate the advantage of ozone over liquid disinfectants by showing similar viral disinfection on top, side, bottom, and interior surfaces. Overall, our study demonstrates the potential use of ozone gas disinfection to combat the COVID-19 outbreak.

Total OH Reactivity of Emissions from Humans: In Situ Measurement and Budget Analysis

Humans are a potent, mobile source of various volatile organic compounds (VOCs) in indoor environments. Such direct anthropogenic emissions are gaining importance, as those from furnishings and building materials have become better regulated and energy efficient homes may reduce ventilation. While previous studies have characterized human emissions in indoor environments, the question remains whether VOCs remain unidentified by current measuring techniques. In this study conducted in a climate chamber occupied by four people, the total OH reactivity of air was quantified, together with multiple VOCs measured by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) and fast gas chromatography-mass spectrometry (fast-GC-MS). Whole-body, breath, and dermal emissions were assessed. The comparison of directly measured OH reactivity and that of the summed reactivity of individually measured species revealed no significant shortfall. Ozone exposure (37 ppb) was found to have little influence on breath OH reactivity but enhanced dermal OH reactivity significantly. Without ozone, the whole-body OH reactivity was dominated by breath emissions, mostly isoprene (76%). With ozone present, OH reactivity nearly doubled, with the increase being mainly caused by dermal emissions of mostly carbonyl compounds (57%). No significant difference in total OH reactivity was observed for different age groups (teenagers/young adults/seniors) without ozone. With ozone present, the total OH reactivity decreased slightly with increasing age.

Lower COVID-19 mortality in Italian forested areas suggests immunoprotection by Mediterranean plants

The COVID-19 pandemic has induced dramatic effects on the population of the industrialized north of Italy, whereas it has not heavily affected inhabitants of the southern regions. This might be explained in part by human exposure to high levels of fine particulate matter (PM) in the air of northern Italy, thus exacerbating the mortality. Since trees mitigate air pollution by intercepting PM onto plant surfaces and bolster the human immune system by emitting bioactive volatile organic compounds (VOCs), we hypothesize a protective role of evergreen forested areas in southern Italy. We compared the mortality rate  due to COVID-19, the death number, the positivity rate and the forest coverage per capita in various Italian regions. Hectares of forest per capita and prevalence of deciduous versus evergreen forestal species were also estimated. In silico docking studies of potentially protective compounds found in Laurus nobilis L., a typical Mediterranean plant, were performed to search for potential antivirals. We found that the pandemic's severity was generally lower in southern regions, especially those with more than 0.3 hectares of forest per capita. The lowest mortality rates were found in southern Italy, mainly in regions like Molise (0.007%) and Basilicata (0.005%) where the forest per capita ratio is higher than 0.5 Ha/person. Our findings suggest that evergreen Mediterranean forests and shrubland plants could have protected the southern population by emission of immuno-modulating VOCs and provision of dietary sources of bioactive compounds. Moreover, in silico studies revealed a potential anti-COVID-19 activity in laurusides, which are unexplored glycosides from bay laurel. Overall, our results highlight the importance of nature conservation and applications to the search for natural antivirals.

Open defecation and squat toilets, an overlooked risk of fecal transmission of COVID-19 and other pathogens in developing communities

The novel coronavirus disease COVID-19 has infected over 46 million people in 219 countries and territories. Following evidence of viral loadings and infectivity of feces of infected individuals, public health authorities have suggested to take precautions on the transmission of COVID-19 via fecal-associated routes. Recent discussions on fecal transmission of COVID-19 have mainly focused on municipal sewage. Yet, a widely neglected aspect in containing the virus is that a major part of the population in developing regions do not have access to private, clean sanitary facilities. Therefore, we hypothesize that open defecation and the prevalent use of squat toilets are additional risk factors in those communities. Here, we review fecal transmission of COVID-19, the practices of open defecation, and the resultant routes of transmission of fecal pathogens. Also, we highlight the open design of common squat toilets and the potential exposure to fecal droplets and residues. We observed that at least 20 countries reporting more than 10,000 confirmed infections have 5-26% of their population practicing open defecation. We illustrate the potential routes of transmission of COVID-19 and other fecal pathogens via human feces in communities practicing open defecation. Here, poor hand hygiene, contaminated shoes and objects, mechanical vectors, and outdoor human activities can all contribute to fecal transmission. Other risk factors include squat pans with lidless designs and open flushing mechanisms, in-cubicle open waste bins, and the lack of water-sealing U-traps in squat toilets.

Electrostatic fine particles emitted from laser printers as potential vectors for airborne transmission of COVID-19

The COVID-19 pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has infected more than 14 million people globally. Recently, airborne transmission has been postulated to be a major contributor to the spread of the novel coronavirus, especially in enclosed public spaces. While many studies have demonstrated positive correlations between atmospheric pollutants and SARS-CoV-2 infection, the impact of indoor air pollutants on airborne transmission has been largely overlooked. In particular, laser printers are a primary source of particle emission that increases the concentrations of particulate matter in indoor atmosphere by releasing substantial quantities of electrostatic fine particles, at rates comparable with tobacco smoking and incense burning. We hypothesized that particles emitted from laser printers present a potential risk factor for the transmission of SARS-CoV-2 in offices and other indoor environments with high user occupancy. To test this hypothesis, we reviewed recent knowledge on the characteristics of particles emitted by laser printing, including their emission rates and accumulation in indoor air, electrostatic charges, localized emission and subsequent particle diffusion in relation to the human breathing zone. We then discuss the potential impact on the transmission of SAR-CoV-2 in indoor spaces. We found that emission rates from laser printers ranged from 10⁸ to 10¹² particles min^-1, and these fine particles typically remain suspended for prolonged periods in indoor air. Electrostatic charges carried by these particles can reach 260-379 e per particle, thus enhancing their surface adsorption and deposition in human airways. Localized emission by laser printers and subsequent diffusion highly increase particle concentrations near the human breathing zone.

Response of surface ozone concentration to emission reduction and meteorology during the COVID‐19 lockdown in Europe

The lockdown period (March–May 2020) during the COVID‐19 pandemic in Europe led to a reduction in the anthropogenic emissions of primary pollutants. For three‐quarters of over 1,100 available monitoring stations, the average nitrogen dioxide (NO₂) concentrations decreased by at least 2.7 μg·m⁻³ (or 25%) compared with the average concentrations recorded during the same period of the previous seven years. This reduction was not specific to urban or rural areas because the relative reduction was of similar magnitude in both areas. The ozone (O₃) response differed spatially, with positive anomalies in Northern Europe and negative anomalies in Southwestern Europe. Reduced cloudiness and related enhanced radiation in Northern Europe played a significant role in the increase of surface O₃ concentrations by shifting the photochemical partitioning between NO₂ and O₃ toward more O₃. The level of total oxidant (O_x = O₃ + NO₂) remained unchanged, except in Southwestern Europe where it decreased. Several episodes lasting a few days of a high level of total oxidants were observed in Northern Europe. The results illustrate the complexity of the atmospheric response to the unprecedented reduction in the emission of primary pollutants.

High resolution mapping of nighttime light and air pollutants during the COVID-19 lockdown in Wuhan

The novel coronavirus (COVID-19) has induced unprecedented improvements of air quality due to drastic shrinking of human activities during the pandemic lockdown in 2020. While declines of most air pollutants have been globally evidenced in most cities worldwide, there is few detailed spatial knowledge at local scale. Therefore, we present here a high resolution mapping of the 2018-2020 evolution of human activities and air pollutants in Wuhan. Human activities were assessed by nighttime light radiance. We measured the air quality index (AQI) as the maximum value among air quality sub-indices of SO₂, NO₂, CO, O₃ and particulate matter. We also compared mean monthly pollutant concentration during January-April in 2018, 2019 and 2020. Mapping results show that variations of nighttime light radiance were heterogenous at local scale, showing both rises and declines in the same district. The radiance decreased in eight districts located mostly in the city center, as a result of lower human activity, but the radiance increased in the five surrounding districts, as a consequence of people staying at home. AQI was low during lockdown, averaging at 57, but showed strong daily variations with a slight pollution around February 5 with AQI rising to 126. During this pollution event, particulate matter, SO₂, NO₂ and CO levels were positively correlated, suggesting common sources, but were not correlated with ozone; and particulate matter, SO₂, NO₂ and CO decreased with relative humidity, suggesting removal by precipitation. Comparison of 2020 data with previous years shows that particulate matter and NO₂ were highly reduced, CO was less reduced due to ongoing power industries, SO₂ first declined then increased to exceed 2018-19 values due to coal combustion, and ozone levels was more abundant due both to less NO_x pollution and the weekend effect.

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The online version contains supplementary material available at 10.1007/s10311-021-01222-x.

The impact of COVID-19 lockdown on the air quality of Eastern Province, Saudi Arabia

Since the identification of the COVID-19 outbreak in Wuhan, China, in December 2019, the death toll from the direct infection by COVID-19 has exceeded 775,000, and more than 21 million cases have been reported to the World Health Organization (WHO) around the world. It is strongly believed that its impact might be worsened by poor outdoor and indoor air qualities, particularly on older adults. The nationwide lockdown measures were imposed between March 23 and June 20, 2020, to stop the spread of COVID-19 pandemic in the Kingdom of Saudi Arabia (KSA). In this work, the possible effects of the lockdown on the air quality were investigated using meteorological and air quality datasets obtained from eight monitoring stations covering the Eastern Province of the KSA. The studied air pollutants include carbon monoxide (CO), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), ozone (O₃), and inhalable particulate matter (PM₁₀). The NO₂ was found to be the marker pollutant responding best to the lockdown measures since its concentrations decreased at all sites during- and post-lockdown periods and ranged between 12-86% and 14-81%, respectively. Compared with pre-lockdown period, the Eastern Province also experienced significant concentration reductions at varying rates for PM₁₀ (21-70%), CO (5.8-55%), and SO₂ (8.7-30%), while O₃ concentrations showed increasing rates ranging between 6.3 and 45%. The consequences of these reductions were reflected in easing the outdoor air quality, which might reduce the impact of COVID-19 pandemic, especially on elderly and sensitive groups.

Impact of the COVID-19 pandemic and control measures on air quality and aerosol light absorption in Southwestern China

China has been performing nationwide social lockdown by releasing the Level 1 response to major public health emergencies (RMPHE) to struggle against the COVID-19 (SARS-CoV-2) outbreak since late January 2020. During the Level 1 RMPHE, social production and public transport were maintained at minimal levels, and residents stayed in and worked from home. The universal impact of anthropogenic activities on air pollution can be evaluated by comparing it with air quality under such extreme conditions. We investigated the concentration of both gaseous and particulate pollutants and aerosol light absorption at different levels of (RMPHE) in an urban area of southwestern China. During the lockdown, PM2.5, PM10, SO2, NOx, and BC decreased by 30-50%, compared to the pre-Level 1 RMPHE period. Meanwhile, the decrease of NOx caused the rise of O3 by up to 2.3 times due to the volatile organic compounds (VOCs) limitation. The aerosol light absorption coefficient at multiple wavelengths decreased by 50%, and AAE decreased by 20% during the Level 1 RMPHE. BrC played essential roles in light absorption after the RMPHE was announced, accounting for 54.0% of the aerosol absorption coefficient at 370 nm. Moreover, the lockdown down-weighted the fraction of fossil fuel in BC concentrations to 0.43 (minima). This study characterizes air pollution at the most basic level and can provide policymakers with references for the "baseline."

Ambient air quality of a less industrialized region of India (Kerala) during the COVID-19 lockdown

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Air quality of major cities of Kerala State (India) during the COVID-19 lockdown.

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Substantial reduction of NO₂ (-48 %) NO_x (-53 % to -90 %) and CO (-24 % to -67 %).

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Significant decrease in PM_2.5 (-24 % to -47 %) and PM₁₀ (-17 % to -20 %) levels.

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Increase of O₃ levels in Northern Kerala during lockdown period.

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Indications of multiple sources (other than traffic and industry) of air pollution.

This study assesses the effect of lockdown, due to the coronavirus disease (COVID-19) pandemic, on the concentration of different air pollutants and overall air quality of a less industrialized region (Kerala) of India. We analysed data from four ambient air quality stations over three years (January to May, 2018–2020) with pairwise comparisons, trend analysis, etc. Results indicated unprecedented reduction in the concentration of the air pollutants: nitrogen dioxide, NO₂ (-48 %), oxides of nitrogen, NO_x (-53 % to -90 %), carbon monoxide, CO (-24 % to -67 %) and the particulate matter (-24 % to -47 % for particulate matter with a diameter of less than 2.5 μm, PM_2.5; -17 % to -20 % for particulate matter with a diameter of less than 10 μm, PM₁₀), as well as the reduction of the National Air Quality Index (NAQI). These reductions indicate an overall improvement of the ambient air quality due to restrictions on transportation, construction, and the industrial sectors during lockdown, even in an area considered less industrial. Despite the general decreasing trend of the concentration of various air pollutants from January to May, suggesting seasonal influences, the trend was intensified in the year 2020 due to the added effect of the lockdown measures. Comparison of the results with those from larger and more industrialized cities suggests that the effects of lockdown are more variable, and focused on the levels of gaseous pollutants. Findings from this study demonstrate the far-reaching effects and implications of the COVID-19 lockdown on ambient air quality, even on less industrialized and less urbanized regions.

Air pollution by NO2 and PM2.5 explains COVID-19 infection severity by overexpression of angiotensin-converting enzyme 2 in respiratory cells: a review

Many major cities that witnessed heavy air pollution by nitrogen dioxide (NO2) and particulate matter (PM) have experienced a high rate of infection and severity of the coronavirus disease pandemic (COVID-19). This phenomenon could be explained by the overexpression of the angiotensin converting enzyme 2 (ACE-2) on epithelial cell surfaces of the respiratory tract. Indeed, ACE-2 is a receptor for coronaviruses including the severe acute respiratory syndrome coronavirus 1 and 2 (SARS-CoV), and ACE-2 is overexpressed under chronic exposure to air pollution such as NO2 and PM2.5. In this review, we explain that ACE-2 acts as the sole receptor for the attachment of the SARS-CoV-2 via its spike protein. The fact that respiratory and vascular epithelial cells express ACE-2 has been previously observed during the 2003 epidemic of the SARS-CoV-1 in China, and during the 2012 Middle East respiratory syndrome in Saudi Arabia. High ACE-2 expression in respiratory epithelial cells under air pollution explains the positive correlation between the severity in COVID-19 patients and elevated air pollution, notably high NO2 and PM2.5 levels. Specific areas in India, China, Italy, Russia, Chile and Qatar that experience heavy air pollution also show high rates of COVID-19 infection and severity. Overall, we demonstrate a link between NO2 emissions, PM2.5 levels, ACE-2 expression and COVID-19 infection severity. Therefore, air pollution should be reduced in places where confirmed cases of COVID-19 are unexpectedly high.

The impact of COVID-19 lockdown on the air quality of Eastern Province, Saudi Arabia

Since the identification of the COVID-19 outbreak in Wuhan, China, in December 2019, the death toll from the direct infection by COVID-19 has exceeded 775,000, and more than 21 million cases have been reported to the World Health Organization (WHO) around the world. It is strongly believed that its impact might be worsened by poor outdoor and indoor air qualities, particularly on older adults. The nationwide lockdown measures were imposed between March 23 and June 20, 2020, to stop the spread of COVID-19 pandemic in the Kingdom of Saudi Arabia (KSA). In this work, the possible effects of the lockdown on the air quality were investigated using meteorological and air quality datasets obtained from eight monitoring stations covering the Eastern Province of the KSA. The studied air pollutants include carbon monoxide (CO), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), ozone (O₃), and inhalable particulate matter (PM₁₀). The NO₂ was found to be the marker pollutant responding best to the lockdown measures since its concentrations decreased at all sites during- and post-lockdown periods and ranged between 12-86% and 14-81%, respectively. Compared with pre-lockdown period, the Eastern Province also experienced significant concentration reductions at varying rates for PM₁₀ (21-70%), CO (5.8-55%), and SO₂ (8.7-30%), while O₃ concentrations showed increasing rates ranging between 6.3 and 45%. The consequences of these reductions were reflected in easing the outdoor air quality, which might reduce the impact of COVID-19 pandemic, especially on elderly and sensitive groups.