The relationship between the intensified heat waves and deteriorated summertime ozone pollution in the Beijing-Tianjin-Hebei region, China, during 2013-2017

Summertime ozone (O₃) pollution has frequently occurred in the Beijing-Tianjin-Hebei (BTH) region, China, since 2013, resulting in detrimental impacts on human health and ecosystems. The contribution of weather shifts to O₃ concentration variability owing to climate change remains elusive. By combining regional air chemistry model simulations with near-surface observations, we found that anthropogenic emission changes contributed to approximately 23% of the increase in maximum daily 8-h average O₃ concentrations in the BTH region in June-July-August (JJA) 2017 (compared with that in 2013). With respect to the weather shift influence, the frequencies, durations, and magnitudes of O₃ exceedance were consistent with those of the heat wave events in the BTH region during JJA in 2013-2017. Intensified heat waves are a significant driver for worsening O₃ pollution. In particular, the prolonged duration of heat waves creates consecutive adverse weather conditions that cause O₃ accumulation and severe O₃ pollution. Our results suggest that the variability in extreme summer heat is closely related to the occurrence of high O₃ concentrations, which is a significant driver of deteriorating O₃ pollution.

Exploring the link between ozone pollution and stratospheric intrusion under the influence of tropical cyclone Ampil

Tropical cyclones (TCs) are synoptic-scale systems with a diameter of up to 2000 km, and may persist for several days to weeks. They can significantly affect the atmospheric conditions and ozone (O₃) concentrations in coastal areas. They also induce stratospheric intrusion (SI, the injection of stratospheric O₃ into the troposphere) by changing the height of the tropopause. Focusing on the Yangtze River Delta (YRD) region, a highly urbanized coastal area with severe O₃ pollution, we systematically analyze the characteristics of O₃ pollution and SIs as well as their connections under the influence of tropical cyclone Ampil. Based on surface observations, 120-h backward trajectories and ERA5 reanalysis meteorological data, the urban O₃ pollution affected by tropical cyclone Ampil mainly resulted from active photochemical reactions inside the boundary layer and poor diffusion conditions characterized by weak winds. SIs induced by tropical cyclone Ampil were important for the upper and middle troposphere, where O₃ concentration could increase up to 180 ppbv. But they hardly reached the ground over the YRD. Therefore, there was no direct connection between O₃ pollution and SIs. However, the location of SIs can predict urban O₃ pollution. SIs moved with tropical cyclone Ampil but appeared on the northwest side of Ampil, usually 500-1000 km away from tropical cyclone Ampil center. At 200 hPa, they corresponded to a high-PV (>2 PVU) air mass rich in O₃ (>200 ppbv). Below this stratospheric PV signature, urban O₃ concentration was usually high.

Characterizing the distinct modulation of future emissions on summer ozone concentrations between urban and rural areas over China

Previous studies on ozone pollution primarily focus on the characterization of ozone on a large regional scale, yet much less attention has been paid towards the contrast between urban and surrounding suburban-rural areas. As anthropogenic emissions are projected to decrease in the coming decades, the evolutions of ozone concentrations over urban and suburban-rural areas are compared using the Community Multiscale Air Quality (CMAQ) model coupled with the Weather Research and Forecasting (WRF). The top 25 city clusters are firstly identified across China based on the amount of NO_x emissions and population size. At present, the averages of maximum daily 8-h (MDA8) ozone concentrations over the suburban-rural areas (65.74 ppbv) among these city clusters are higher than the corresponding urban areas (62.00 ppbv). The projections in 2050s depend on the scenarios. Under the Shared Socioeconomic Pathways (SSPs) scenarios such as SSP1-2.6, SSP2-4.5 and SSP5-8.5 indicative of primary decreases in anthropogenic emissions, the mean MDA8 ozone concentrations in suburban-rural areas decrease more than urban areas. In contrast, the gap of higher MDA8 ozone concentrations in suburban-rural than urban areas increases under SSP3-7.0 concomitant with increase in anthropogenic emissions. The strongest response to emission changes is found in SSP1-2.6, in which anthropogenic emission reduction leads to lower MDA8 ozone concentrations in the suburban-rural than urban areas, reversing the present state of higher ozone concentrations in the suburban-rural areas. The feature abovementioned regarding changes of mean MDA8 ozone concentrations is largely reproduced in the response of ozone exceedance. The finding highlights different efficacies of anthropogenic emissions in urban from suburban-rural areas. In a region like China where the population density is much higher in urban than suburban-rural regions, the benefit of ozone reduction due to large emission reductions (e.g., SSP1-2.6) is diminished to some extent because the effect is larger in suburban-rural areas.

Understanding the spatiotemporal variability and trends of surface ozone over India

With rising anthropogenic activities, surface ozone levels have increased across different parts of the world including India. Previous studies have shown that surface ozone shows distinct characteristics across India but these results are based on isolated locations and any comprehensive and spatiotemporally consistent study about surface ozone variability lacks thus far. Keeping these facts in mind, we utilize ground-based observations and reanalysis datasets to investigate spatiotemporal variations of surface ozone and its linkages with meteorology and precursors over Indian region. A validation exercise shows that the Copernicus Atmosphere Monitoring Service Reanalysis (CAMSRA) reasonably compares against the ground-based observations showing better correlations (> 0.7) over southern regions and relatively lesser (> 0.5) correlations over northern and eastern regions. We have further quantified this agreement in terms of range, mean absolute error (MAE), and root mean square error (RMSE). A time series analysis shows that the CAMSRA captures seasonal variations irrespective of location. Spatial distribution of surface ozone shows higher (lower) concentrations of about 40-60 ppb (15-20 ppb) during pre-monsoon (monsoon) months over northern and western parts and peninsular India. A prominent increase during May is noted over the northern region, especially over the Indo-Gangetic Plains (IGP). These seasonal variations are linked to solar radiation (SR), temperature, low-level circulation, and boundary layer height (BLH). CAMSRA-based surface ozone shows increasing trends across all four regions (north, east, west, and south India) and also India as a whole (0.069 ppb year^-1, p = 0.001) with highest trends over the eastern region. Furthermore, principal component analysis (PCA) reveals that the first (second) mode shows a high percentage variance explained, ranging between 30 and 50% (10-20%). The corresponding PC-1 time series exhibits a notable increase in the surface ozone over south and central India, which corroborates the trend obtained through the area averaged time series. The second mode (PC-2) indicates prominent interannual variability over the IGP (southern India) in the pre-monsoon (post-monsoon). During the monsoon season, an interesting dipole pattern is noticeable, which closely resembles the active and break spell patterns of the Indian summer monsoon. Further, we quantify the weightage of precursors and meteorological parameters on surface ozone concentrations. The analysis suggests that PC1 of surface ozone is strongly influenced by CO and NOx (the precursors) while meteorology seems to dominate the PC2 during the pre-monsoon season. Overall, the results indicate that changes in the precursors or meteorological conditions have significant influences on the surface ozone concentrations across India.

A radical shift in air pollution

[Figure: see text].

Seasonal Variations of Fine Particulate Matter and Mortality Rate in Seoul, Korea with a Focus on the Short-Term Impact of Meteorological Extremes on Human Health

Rapid industrialization of Korea’s economy has brought with it environmental pollution that threatens human health. Among various other pollutants, ambient fine particulate matter known to endanger human health often exceeds air quality standards in Seoul, South Korea’s capital. The goal of this research is to find the impact of meteorological extremes and particle levels on human health. The analysis was conducted using hourly air pollutant concentrations, meteorological variables, and the daily mortality from cerebrovascular disease. Results show that the effect of fine particulate matter on mortality from cerebrovascular disease was more noticeable during meteorological extremes. The linkage between extreme weather conditions and mortality was more apparent in winter than in summer. Comprehensive studies of various causes of diseases should be continued to more accurately analyze the effects of fine particulate matter on human health and meteorological extremes, and to further minimize the public health impact of air pollution and meteorological conditions.

Increasing surface ozone and enhanced secondary organic carbon formation at a city junction site: An epitome of the Yangtze River Delta, China (2014-2017)

This study aims to understand the characteristics of surface ozone (O₃), search for factors affecting the variations in its concentration, and estimate its impacts on the secondary organic carbon (SOC) levels and atmospheric oxidation capacities in the Yangtze River Delta (YRD). Four years of continuous observations (2014-2017) of the surface O₃, organic carbon, elemental carbon, nitrogen oxides, PM_2.5 and meteorological factors along with three years of measurements (2015-2017) of the concentrations of 56 volatile organic compounds were conducted at a rural site. Our measurements showed that the total number of O₃ pollution days more than doubled over the four-year period, from 28 days in 2014 to 76 days in 2017. The annual mean of the maximum daily 8-h average O₃ concentration during the months with the strongest solar radiation (July-September) showed a 6.8% growth rate, from 124.5 (2014) to 149.8 μg m^-3 (2017). Regional transport was shown to be the dominant contributor to the high level of O₃ based on a process analysis of the O₃ variation using the Weather Research and Forecasting-Community Multiscale Air Quality model for this site. The simulation results indicated that the city junction site served well as an epitome of the regional background of the YRD. We also found that the level of SOC, which is a major component of PM_2.5 that results from atmospheric oxidizing processes, gradually increased with the increase in the surface O₃ level, even though the overall PM_2.5 concentration significantly decreased each year. There was an increasingly strong correlation between SOC and O_x (O₃ + nitrogen dioxide) during both the daytime and night-time from 2014 to 2017 when the highest annual O₃ concentration was observed. These findings imply that the atmospheric oxidation capacity increased and likely contributed to the SOC production in the YRD during 2014-2017.

Atmospheric Lifetimes of Halogenated Hydrocarbons: Improved Estimations from an Analysis of Modeling Results

Detailed results of computer modeling of halocarbon removal rates from the atmosphere are analyzed to find simple correlations useful for improving estimations of the atmospheric lifetimes of industrial chemicals based on the rate constants for their reactions with OH and O(¹D) and their UV absorption spectra. Ths analysis is limited to relatively long-lived chemicals that are well mixed in the troposphere.

Triple oxygen isotopic evidence for atmospheric nitrate and its application in source identification for river systems in the Qinghai-Tibetan Plateau

Nitrate source identification in river systems is important for water quality management. Recently, the oxygen isotopic anomaly of nitrate in atmospheric deposition (Δ¹⁷O_atm) is used to identify unprocessed atmospheric nitrate in river systems to reduce the uncertainty caused by the wide range of δ¹⁸O. In high-elevation regions, such as the Qinghai-Tibetan Plateau (QTP) featured with lower temperature and pressure as well as strong radiation, the Δ¹⁷O_atm might be different from that in low-elevation regions, but no relevant studies have been reported. In this work, Δ¹⁷O_atm in the QTP was studied, and the fingerprints of nitrate isotopes in synthetic fertilizer, livestock manure, domestic sewage, and soil organic nitrogen (SON) were identified and used to quantify various source contributions to riverine nitrate in the Yellow River and Changjiang River source regions located in the QTP during 2016-2017. The results showed that the average of Δ¹⁷O_atm in the QTP was 16.4‰, lower than the range (19-30‰) reported for the low-elevation regions. The possible mechanism is decreased O₃ as well as increased hydroxyl and peroxy radical levels in the troposphere caused by the climate condition and ozone valley in the QTP will affect the production pathways of atmospheric nitrate. By combining the sewage discharge data with the output results of the SIAR (stable isotope analysis in R) model based on the stable isotope data, manure was determined to be one of the major sources to riverine nitrate for both rivers. The contributions of various sources to riverine nitrate were 47 ± 10% for manure, 30 ± 5% for SON, 10 ± 4% for atmospheric precipitation, 9 ± 2% for synthetic fertilizer, and 4 ± 0% for sewage in the Yellow River source region. This study indicates that the unique atmospheric conditions in the QTP have led to a lower Δ¹⁷O_atm value, and atmospheric source makes a considerable contribution to riverine nitrate in the QTP.

Pressure-Dependent Kinetics of the Reaction between CH3OO and OH Focusing on the Product Yield of Methyltrioxide (CH3OOOH)

The reaction kinetics of methyl peroxy radical (CH₃OO) and hydroxyl radical (OH) has attracted an increasing level of interest in the past decade, while the branching yields of various product channels are still under debate. In this work, a comprehensive theoretical effort was made to investigate the branching yield of the stabilized methyltrioxide (CH₃OOOH, TRIOX) adduct, which has recently been a research focus. Our computed branching ratio of TRIOX at 298 K and 760 Torr is ∼0.04, in agreement with the result of multiplexed photoionization mass spectrometry. We show that the large branching yield obtained in an early theoretical study mainly originated from the collision-induced strong stabilization presented in their simulation. Our findings clarify the controversial product yield results for this important species in recent studies. The computed rate constants over wide temperature and pressure ranges allow better integration of this reaction into global atmospheric models and low-temperature combustion kinetic models.

100 Years of Progress in Gas-Phase Atmospheric Chemistry Research

Remarkable progress has occurred over the last 100 years in our understanding of atmospheric chemical composition, stratospheric and tropospheric chemistry, urban air pollution, acid rain, and the formation of airborne particles from gas-phase chemistry. Much of this progress was associated with the developing understanding of the formation and role of ozone and of the oxides of nitrogen, NO and NO2, in the stratosphere and troposphere. The chemistry of the stratosphere, emerging from the pioneering work of Chapman in 1931, was followed by the discovery of catalytic ozone cycles, ozone destruction by chlorofluorocarbons, and the polar ozone holes, work honored by the 1995 Nobel Prize in Chemistry awarded to Crutzen, Rowland, and Molina. Foundations for the modern understanding of tropospheric chemistry were laid in the 1950s and 1960s, stimulated by the eye-stinging smog in Los Angeles. The importance of the hydroxyl (OH) radical and its relationship to the oxides of nitrogen (NO and NO2) emerged. The chemical processes leading to acid rain were elucidated. The atmosphere contains an immense number of gas-phase organic compounds, a result of emissions from plants and animals, natural and anthropogenic combustion processes, emissions from oceans, and from the atmospheric oxidation of organics emitted into the atmosphere. Organic atmospheric particulate matter arises largely as gas-phase organic compounds undergo oxidation to yield low-volatility products that condense into the particle phase. A hundred years ago, quantitative theories of chemical reaction rates were nonexistent. Today, comprehensive computer codes are available for performing detailed calculations of chemical reaction rates and mechanisms for atmospheric reactions. Understanding the future role of atmospheric chemistry in climate change and, in turn, the impact of climate change on atmospheric chemistry, will be critical to developing effective policies to protect the planet.

Cascading effects of elevated ozone on wheat rhizosphere microbial communities depend on temperature and cultivar sensitivity

Tropospheric ozone (O₃) concentrations have now reached levels that can potentially affect crop production in several regions of the world. The interacting effects of the elevated O₃ and temperature on plants are still unclear and their consequences on the rhizosphere microbial communities never studied yet. Here, we conducted a 3-week fumigation experiment on two cultivars of wheat with different tolerance to O₃ (Premio and Soissons) at two temperatures (20 °C and 30 °C). The impacts of O₃ were measured on plants physiology, rhizosphere chemical environment and microbial communities. Globally, most of the results showed that elevated O₃ effects were more pronounced at 20 °C than 30 °C, especially on the most O_3-sensitive cultivar (Soissons). Elevated O₃ reduced significantly plant root biomass (up to -37% for Soissons) compared to non-fumigated plants. A decrease in the dissolved organic matter with a relative increase of aromatic compounds concentration was also observed under elevated O₃, suggesting quantitative and qualitative impacts on roots exudation. While bacterial abundance was negatively affected by O₃ plant stress, fungal abundance was found to be stimulated (up to 12 fold compared to non-fumigated plants for Soissons at 20 °C). These changes were accompanied by modifications of the genetic structures and metabolic profiles, with a relative increase of amino acids catabolism. This fully controlled laboratory experiment showed that the effects of elevated O₃ on soil microbial communities i) are plant-mediated and depend on the cultivar sensitivity, ii) decrease in warming condition, iii) increase the fungi to bacteria ratio and iv) alter both the genetic structure and the metabolic activities. This study highlights the importance of considering interactive effects between pollutants and climate changes on plant-microbe relationship to better inform models and improve predictions of future states of agroecosystems.

The reaction of methyl peroxy and hydroxyl radicals as a major source of atmospheric methanol

Methyl peroxy, a key radical in tropospheric chemistry, was recently shown to react with the hydroxyl radical at an unexpectedly high rate. Here, the molecular reaction mechanisms are elucidated using high-level quantum chemical methodologies and statistical rate theory. Formation of activated methylhydrotrioxide, followed by dissociation into methoxy and hydroperoxy radicals, is found to be the main reaction pathway, whereas methylhydrotrioxide stabilization and methanol formation (from activated and stabilized methylhydrotrioxide) are viable minor channels. Criegee intermediate formation is found to be negligible. Given the theoretical uncertainties, useful constraints on the yields are provided by atmospheric methanol measurements. Using a global chemistry-transport model, we show that the only explanation for the high observed methanol abundances over remote oceans is the title reaction with an overall methanol yield of ∼30%, consistent with the theoretical estimates given their uncertainties. This makes the title reaction a major methanol source (115 Tg per year), comparable to global terrestrial emissions.

Determination of the personal, indoor and outdoor exposure levels of inorganic gaseous pollutants in different microenvironments in an industrial city

We measured SO2, NO2 and O3 concentrations during the summer and winter in Kocaeli, Turkey. The sampling was carried out indoors and outdoors at homes, schools and offices. Personal samplers were also used to determine personal exposures to these pollutants. High NO2 and SO2 concentrations were observed in outdoor samples collected close to locations characterized by heavy urban traffic. Concentrations of O3, on the other hand, were higher in rural areas around the city due to ozone distillation. For both sampling periods, the concentrations of outdoor SO2 and O3 were higher than for indoor and personal samples; however, the NO2 concentrations were higher in indoor and personal samples, indicating that outdoor sources significantly contribute to indoor SO2 and O3 levels and that indoor NO2 concentrations are primarily modulated by sources within buildings. Seasonal variations in pollutant concentrations showed statistically significant differences. Indoor and outdoor concentrations of NO2 and SO2 measured in the winter were higher than the levels measured in the summer; O3 concentrations, on the other hand, exhibited the opposite trend. Active-to-passive concentration ratios for NO2, SO2 and O3 were 0.99, 1.08 and 1.16, respectively; the corresponding outdoor ratios were 0.95, 0.99 and 1.00.

Wintertime peroxyacetyl nitrate (PAN) in the megacity Beijing: role of photochemical and meteorological processes

Previous measurements of peroxyacetyl nitrate (PAN) in Asian megacities were scarce and mainly conducted for relative short periods in summer. Here, we present and analyze the measurements of PAN, O3, NO(x), etc., made at an urban site (CMA) in Beijing from 25 January to 22 March 2010. The hourly concentration of PAN averaged 0.70 x 10(-9) mol/mol (0.23 x 10(-9) -3.51 x 10(-9) mol/mol) and was well correlated with that of NO2 but not O3, indicating that the variations of the winter concentrations of PAN and 03 in urban Beijing are decoupled with each other. Wind conditions and transport of air masses exert very significant impacts on O3, PAN, and other species. Air masses arriving at the site originated either from the boundary layer over the highly polluted N-S-W sector or from the free troposphere over the W-N sector. The descending free-tropospheric air was rich in O3, with an average PAN/O3 ratio smaller than 0.031, while the boundary layer air over the polluted sector contained higher levels of PAN and primary pollutants, with an average PAN/O3 ratio of 0.11. These facts related with transport conditions can well explain the observed PAN-O3 decoupling. Photochemical production is important to PAN in the winter over Beijing. The concentration of the peroxyacetyl (PA) radical was estimated to be in the range of 0.0014 x 10(-12) -0.0042 x 10(-12) mol/mol. The contributions of the formation reaction and thermal decomposition to PAN's variation were calculated and found to be significant even in the colder period in air over Beijing, with the production exceeding the decomposition.

Toward stand-off open-path measurements of NO and NO(2) in the sub-parts per million meter range using quantum cascade lasers (QCLs) in the intra-pulse absorption mode

Two thermoelectrically cooled mid-infrared distributed feedback quantum cascade lasers operated in pulsed mode have been used for the quasi-simultaneous determination of NO and NO2 in the sub-parts per million meter (sub-ppm-m) range. Using a beam splitter, the beams of the two lasers were combined and sent to a retro-reflector. The returned light was recorded with a thermoelectrically cooled mercury cadmium telluride detector with a rise time of 4 ns. Alternate operation of the lasers with pulse lengths of 300 ns and a repetition rate of 66 kHz allowed quasi-simultaneous measurements. During each pulse the laser temperature increased, causing a thermal chirp of the laser line of up to 1.3 cm(-1). These laser chirps were sufficient to scan rotational bands of NO centered at 1902 cm(-1) and NO2 located at 1632 cm(-1). In that way an absorption spectrum could be recorded from a single laser pulse. Currently achieved limits of detection are 600 parts per billion meter (ppb-m) for NO and 260 ppb-m for NO2 using signal averaging over 1 min. This work presents the first steps toward a portable stand-off, open-path instrument that uses thermoelectrically cooled detector and lasers.