Brown carbon aerosol in two megacities in the Sichuan Basin of southwestern China: Light absorption properties and implications

Optical properties and radiative forcing of carbonaceous aerosols in a valley city under persistent high temperature

Carbonaceous aerosols were collected in the valley city of Baoji city in Northern China in August 2022. The light absorption characteristics and influencing factors of black carbon (BC) and brown carbon (BrC) were analyzed, and their radiative forcing was estimated. The results showed that the light absorption of secondary brown carbon [AbsBrC,sec (370)] was 7.5 ± 2.4 Mm-1, which was 2.5 times that of primary brown carbon [AbsBrC,pri (370), 3.0 ± 1.2 Mm-1]. During the study period, the absorption Ångström exponent (AAE) of aerosol was 1.6, indicating that there was obvious secondary aerosol formation or carbonaceous aerosol aging in the valley city of Baoji. Except for secondary BrC (BrCsec), the light absorption coefficient (Abs) and mass absorption efficiency (MAE) of BC and primary BrC (BrCpri) during the persistent high temperature period (PHT) were higher than those during the normal temperature period (NT) and the precipitation period (PP), which indicated that the light absorption capacity of black carbon and primary brown carbon increased, while the light absorption capacity of secondary brown carbon decreased under persistent high temperature period. Secondary aerosols sulfide (SO42-), nitrate (NO3-) and secondary organic carbon (SOC) are important factors for promoting the light absorption enhancemen of BC and BrCpri and photobleaching of BrCsec during persistent high temperature period. The Principal Component Analysis-Multiple Linear Regression (PCA-MLR) model showed that traffic emissions was the most important source of pollution in Baoji City. Based on this, the secondary source accelerates the aging of BC and BrC, causing changes in light absorption. During PHT, the radiative forcing of BC and BrCpri were enhanced, while the radiative forcing of BrCsec was weakened, but the positive radiative forcing generated by them may aggravate the high-temperature disaster.

An observed nocturnal ozone transport event in the Sichuan Basin, Southwestern China

The ozone (O3) pollution in China drew lots of attention in recent years, and the Sichuan Basin (SCB) was one of the regions confronting worsening O3 pollution problem. Many previous studies have shown that regional transport is an important contributor to O3 pollution. However, very few features of the O3 profile during transport have been reported, especially in the border regions between different administrative divisions. In this study, we conducted tethered balloon soundings in SCB during the summer of 2020 and captured a nocturnal O3 transport event during the campaign. Vertically, the O3 transport occurred in the bottom of the residual layer, between 200 and 500 m above ground level. Horizontally, the transport pathway was directed from southeast to northwest based on the analysis of the wind field and air mass trajectories. The effect of transport in the residual layer on the surface O3 concentration was related to the spatial distribution of O3. For cities with high O3 concentrations in the upwind region, the transport process would bring clean air masses and abate pollution. For downwind lightly polluted cities, the transport process would slow down the decreasing or even increase the surface O3 concentration during the night. We provided observational facts on the profile features of a transboundary O3 transport event between two provincial administrative divisions, which implicated the importance of joint prevention and control measures. However, the sounding parameters were limited and the quantitative analysis was preliminary, more integrated, and thorough studies of this topic were called for in the future.

Spatiotemporal distribution, light absorption characteristics, and source apportionments of black and brown carbon in China

Black carbon (BC) and brown carbon (BrC) are aerosols that absorb light and thereby contribute to climate change. In this study, the light absorption properties and spatiotemporal distributions of equivalent BC (eBC) and BrC aerosols were determined based on continuous measurements of aerosol light absorption from January to August 2017, using a seven-channel aethalometer at 49 sampling sites in China. The source apportionments of BC and BrC were identified using the BC/PM2.5, absorption Ångström exponent, the concentration-weighted trajectory method, and the random forest model. Based on the results, BC was the dominant light absorber, whereas BrC was responsible for a higher proportion of the light absorption in northern compared to southern China. The light absorption of BrC was highest in winter (34.3 Mm-1), followed by spring (19.0 Mm-1) and summer (3.6 Mm-1). The combustion of liquid fuels accounted for over 50 % of the light absorption coefficient of BC in most cities and the importance of carbon monoxide (CO) and nitrogen dioxide (NO2) was over 10 % for BC emitted by liquid fuel combustion, based on the random forest model. The contribution of solid fuel combustion to BC in the north was larger than that in the southern regions as coal combustion and crop residue burning are important emission sources of BC in most northern cities. The contribution of primary BrC to light absorption was high in some northern cities, whereas that of secondary BrC was prevalent in some southern cities. The diurnal variations in secondary BrC were affected by changes in odd oxygen and relative humidity, which promoted the photobleaching of the chromophores and aqueous-phase reactions of secondary BrC.

Long-term spatiotemporal evolution and coordinated control of air pollutants in a typical mega-mountain city of Cheng-Yu region under the "dual carbon" goal

Clarifying the spatiotemporal distribution and impact mechanism of pollution is the prerequisite for megacities to formulate relevant air pollution prevention and control measures and achieve carbon neutrality goals. Chongqing is one of the dual-core key megacities in Cheng-Yu region and as a typical mountain-city in China, environmental problems are complex and sensitive. This research aims to investigate the exceeding standard levels and spatio-temporal evolution of criteria pollutants between 2014 and 2020. The results indicated that PM10, PM2.5, CO and SO2 were decreased significantly by 45.91%, 52.86%, 38.89% and 66.67%, respectively. Conversely, the concentration of pollutant O3 present a fluctuating growth and found a "seesaw" phenomenon between it and PM. Furthermore, PM and O3 are highest in winter and summer, respectively. SO2, NO2, CO, and PM showed a "U-shaped", and O3 showed an inverted "U-shaped" seasonal variation. PM and O3 concentrations are still far behind the WHO, 2021AQGs standards. Significant spatial heterogeneity was observed in air pollution distribution. These results are of great significance for Chongqing to achieve "double control and double reduction" of PM2.5 and O3 pollution, and formulate a regional carbon peaking roadmap under climate coordination. Besides, it can provide an important platform for exploring air pollution in typical terrain around the world and provide references for related epidemiological research.Implications: Chongqing is one of the dual-core key megacities in Cheng-Yu region and as a typical mountain city, environmental problems are complex and sensitive. Under the background of the "14th Five-Year Plan", the construction of the "Cheng-Yu Dual-City Economic Circle" and the "Dual-Carbon" goal, this article comprehensively discussed the annual and seasonal excess levels and spatiotemporal evolution of pollutants under the multiple policy and the newest international standards (WHO,2021AQG) backgrounds from 2014 to 2020 in Chongqing. Furthermore, suggestions and measures related to the collaborative management of pollutants were discussed. Finally, limitations and recommendations were also put forward.Clarifying the spatiotemporal distribution and impact mechanism of pollution is the prerequisite for cities to formulate relevant air pollution control measures and achieve carbon neutrality goals. This study is of great significance for Chongqing to achieve "double control and double reduction" of PM2.5 and O3 pollution, study and formulate a regional carbon peaking roadmap under climate coordination and an action plan for sustained improvement of air quality.In addition, this research can advanced our understanding of air pollution in complex terrain. Furthermore, it also promote the construction of the China national strategic Cheng-Yu economic circle and build a beautiful west. Moreover, it provides scientific insights for local policymakers to guide smart urban planning, industrial layout, energy structure, and transportation planning to improve air quality throughout the Cheng-Yu region. Finally, this is also conducive to future scientific research in other regions of China, and even megacities with complex terrain in the world.

Long-Term Air Pollution Characteristics and Multi-scale Meteorological Factor Variability Analysis of Mega-mountain Cities in the Chengdu-Chongqing Economic Circle

Currently, air quality has become central to global environmental policymaking. As a typical mountain megacity in the Cheng-Yu region, the air pollution in Chongqing is unique and sensitive. This study aims to comprehensively investigate the long-term annual, seasonal, and monthly variation characteristics of six major pollutants and seven meteorological parameters. The emission distribution of major pollutants is also discussed. The relationship between pollutants and the multi-scale meteorological conditions was explored. The results indicate that particulate matter (PM), SO2 and NO2 showed a "U-shaped" variation, while O3 showed an "inverted U-shaped" seasonal variation. Industrial emissions accounted for 81.84%, 58% and 80.10% of the total SO2, NOx and dust pollution emissions, respectively. The correlation between PM2.5 and PM10 was strong (R = 0.98). In addition, PM only showed a significant negative correlation with O3. On the contrary, PM showed a significant positive correlation with other gaseous pollutants (SO2, NO2, CO). O3 is only negatively correlated with relative humidity and atmospheric pressure. These findings provide an accurate and effective countermeasure for the coordinated management of air pollution in Cheng-Yu region and the formulation of the regional carbon peaking roadmap. Furthermore, it can improve the prediction accuracy of air pollution under multi-scale meteorological factors, promote effective emission reduction paths and policies in the region, and provide references for related epidemiological research.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s11270-023-06279-8.

Enhanced nitrate contribution to light extinction during haze pollution in Chengdu: Insights based on an improved multiple linear regression model

In recent years, the annual mean concentration of PM_2.5 has decreased in Chengdu, China; however, atmospheric visibility has not improved accordingly. Low-visibility events occurred even when the PM_2.5 mass concentrations were below the national ambient air quality secondary standard (daily mean concentration, 75 μg/m³). In this study, the non-linear relationship between PM_2.5 and visibility was analyzed under different NO₃^- mass fractions in PM_2.5 based on 2-year field observation data. The results indicated that NO₃^- formation contributed to particulate pollution events and reduced atmospheric visibility. Multiple linear regression was used to propose a localized reconstruction equation for the light-scattering coefficient. According to the maximum likelihood estimation method and log-transformed residuals, the mass scattering coefficients (MSEs) of organic matter (OM), NH₄NO₃, and (NH₄)₂SO₄ in Chengdu were 7.42, 3.83, and 3.80, respectively. OM and NH₄NO₃ contributed to more than 50% of the light-extinction coefficient (b_ext). NH₄NO₃ was the main pollutant causing the substantial increase in b_ext. Chengdu has a high relative humidity (annual mean 70%), and under such conditions, the contribution of NH₄NO₃ to b_ext was considerably enhanced through hygroscopic growth and heterogeneous reactions. This study estimated the localized MSEs of OM, NH₄NO₃, and (NH₄)₂SO₄ in Chengdu and emphasized that effective control measures to reduce nitrate and its precursors could simultaneously ameliorate air quality and visibility in humid regions with poor atmospheric visibility.

Vehicle exhausts contribute high near-UV absorption through carbonaceous aerosol during winter in a fast-growing city of Sichuan Basin, China

Carbonaceous aerosols pose significant climatic impact, however, their sources and respective contribution to light absorption vary and remain poorly understood. In this work, filter-based PM_2.5 samples were collected in winter of 2021 at three urban sites in Yibin, a fast-growing city in the south of Sichuan Basin, China. The composition characteristics of PM_2.5, light absorption and source of carbonaceous aerosol were analyzed. The city-wide average concentration of PM_2.5 was 87.4 ± 31.0 μg/m³ in winter. Carbonaceous aerosol was the most abundant species, accounting for 42.5% of the total PM_2.5. Source apportionment results showed that vehicular emission was the main source of PM_2.5 during winter, contributing 34.6% to PM_2.5. The light absorption of black carbon (BC) and brown carbon (BrC) were derived from a simplified two-component model. We apportioned the light absorption of carbonaceous aerosols to BC and BrC using the Least Squares Linear Regression with optimal angstrom absorption exponent of BC (AAE_BC). The average absorption of BC and BrC at 405 nm were 51.6 ± 21.5 Mm^-1 and 17.7 ± 8.0 Mm^-1, respectively, with mean AAE_BC = 0.82 ± 0.02. The contribution of BrC to the absorption of carbonaceous reached 26.1% at 405 nm. Based on the PM_2.5 source apportionment and the mass absorption cross-section (MAC) value of BrC at 405 nm, vehicle emission was found to be the dominant source of BrC in winter, contributing up to 56.4%. Therefore, vehicle emissions mitigation should be the primary and an effective way to improve atmospheric visibility in this fast-developing city.

Deconvolving light absorption properties and influencing factors of carbonaceous aerosol in Shanghai

Black carbon (BC) and brown carbon (BrC) have intensive impacts on atmospheric visibility and global climate change. In this study, PM_2.5 samples were collected at Pudong (PD) and Qingpu (QP) of Shanghai in 2017, and characterized typical organic molecular tracers by gas chromatography-mass spectrometer. The light absorption (Abs) of carbonaceous aerosol and water-soluble organic matter was analyzed by a multi-wavelength thermal/optical carbon analyzer and a long-range ultraviolet-visible spectrophotometer. An improved two-component model integrated with both optical and chemical fingerprints of carbonaceous aerosol was applied to analyze the Abs of BC, water-soluble organic carbon (WSOC) and water-insoluble organic carbon (WISOC), with which the potential influencing factors including emission source and atmospheric aging were investigated. Results indicated that BrC contributed 19% at PD and 16% at QP of the total light absorption of the carbonaceous aerosol at 405 nm wavelength. Meanwhile, Abs_WSOC(405)/Abs_BrC(405) showed significant seasonal variations (27-50%) at both sites. Positive matrix factorization (PMF) analysis showed that vehicle emissions (60-61%) and biomass combustion (38-39%) were the major contributors to Abs_BC(405), while biomass burning (34-40%), nitrate-relevant secondary processes (22-23%), vehicle emissions (18-19%) and biogenic SOA (13-19%) were major contributors to Abs_WSOC(405). Hybrid combustion source (94-96%) had a predominant contribution to Abs_WISOC(405). Statistical analysis showed that biomass burning had a great impact on the enhancement of Abs_WISOC. Absorption Ångström exponent (AAE) and mass absorption efficiency (MAE) of each factor (source) using PMF analysis indicated that WSOC from combustion sources had higher AAE_{WSOC(350-550)} values (8.11 and 8.29 for coal and biomass burning, respectively) and MAE_WSOC(365) values (0.63-0.99) compared to other sources. Atmospheric aging process can lower the MAE_WSOC(365) value (0.24-0.52). Overall, our study facilitates a better understanding of the relationships among source, optical properties, and atmospheric transformation processes of the carbonaceous aerosols in Shanghai.

Spectral characteristics of dissolved organic carbon (DOC) derived from biomass pyrolysis: Biochar-derived DOC versus smoke-derived DOC, and their differences from natural DOC

Biochar-derived dissolved organic carbon (BDOC) and smoke-derived dissolved organic carbon (SDOC) are two different biomass-pyrogenic DOCs. They inevitably enter soil and water, then potentially pose different impacts on the chemistry of these media. This study systemically investigated the emissions and spectral characteristics of BDOC and SDOC as well as their differences from natural DOC. The results showed that the emission of SDOC was 1-3 orders of magnitude greater than that of BDOC after biomass pyrolysis. UV-vis spectra indicated that BDOC had higher aromaticity and molecular weight as well as lower polarity than SDOC. The two-dimensional correlation infrared spectrum (2D-PCIS) matrix indicated that BDOC contained more chemical groups with stronger temperature-dependence than SDOC. Fluorescence EEM-PARAFAC analysis showed that BDOC was dominated by macromolecular humic-like substances, while SDOC was primarily composed of small molecules of aromatic protein/polyphenols-like compounds. The fluorescence indicators including humification index (HIX) (0.08-0.76) and biological index (BIX) (1.18-1.72) of SDOC were significantly different from those of BDOC (HIX: 1.64-12.68, and BIX: 0.17-1.62). The higher BIX and more small molecules of aromatic protein/polyphenols-like compounds indicated SDOC had potentially higher bioavailability and turnover rate in the environment than BDOC. Furthermore, the UV-vis spectral indicator (S_275-295) and fluorescence spectral indicators (HIX, and BIX) of BDOC were equivalent to those of natural DOC, whereas these indicators of SDOC were significantly different from those of natural DOC. This study demonstrated that BDOC and SDOC had significantly different components and properties and they might present different environmental behaviors and effects.

Characteristics of Environmentally Persistent Free Radicals in PM2.5 and the Influence of Air Pollutants in Shihezi, Northwestern China

Environmentally persistent free radicals (EPFRs) are a kind of hazardous substance that exist stably in the atmosphere for a long time. EPFRs combined with fine particulate matter (PM_2.5) can enter the human respiratory tract through respiration, causing oxidative stress and DNA damage, and they are also closely related to lung cancer. In this study, the inhalation risk for EPFRs in PM_2.5 and factors influencing this risk were assessed using the equivalent number of cigarette tar EPFRs. The daily inhalation exposure for EPFRs in PM_2.5 was estimated to be equivalent to 0.66–8.40 cigarette tar EPFRs per day. The concentration level and species characteristics were investigated using electron paramagnetic resonance spectroscopy. The concentration of EPFRs in the study ranged from 1.353–4.653 × 10¹³ spins/g, and the types of EPFRs were mainly oxygen- or carbon-centered semiquinone-type radicals. Our study showed that there is a strong correlation between the concentrations of EPFRs and conventional pollutants, except for sulfur dioxide. The major factors influencing EPFR concentration in the atmosphere were temperature and wind speed; the higher the temperature and wind speed, the lower the concentration of EPFRs. The findings of this study provide an important basis for further research on the formation mechanism and health effects of EPFRs.

Long-term variations in aerosol optical properties, types, and radiative forcing in the Sichuan Basin, Southwest China

Long-term variations in aerosol optical properties, types, and radiative forcing over the Sichuan Basin (SCB) and surrounding regions in Southwest China were investigated based on two-decade data (2001-2020) from the Moderate Resolution Imaging Spectroradiometer, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, and the Santa Barbara DISORT Atmospheric Radiative Transfer model. The results showed that the aerosol optical depth (AOD_550nm) in the SCB, a major polluted region in Southwest China, experienced an increasing tendency at a rate of +0.052 yr^-1 during 2001-2006; thereafter, it decreased speedy up from -0.020 to -0.058 yr^-1 over recent years, whereas the interannual variation in Ångström exponent (AE_470-660nm) presented a persistently increasing trend during 2001-2020, with a rate of +0.014 yr^-1. An improved atmospheric environment but an enhanced fine particle contribution to regional aerosols in the SCB was observed. Over the polluted SCB region, the dominant aerosol types were biomass burning/urban industrial and mixed-type aerosols with the proportions of 80.7%-87.5% in regional aerosols, with a higher frequency of clean aerosols in recent years, reflecting an effect of controlling anthropogenic emission in the SCB owing to governmental regulation. By contrast, few changes were observed in the aerosol types and amounts in the eastern Tibetan Plateau (ETP), where clean continental aerosols dominate with high proportion of 93.7% in the clean atmospheric environment. A significant decline in polluted anthropogenic aerosols was observed below 3 km over the SCB, resulting in the regional aerosol extinction coefficients at 532 nm (EC_532nm) were declined by -0.22 km^-1 from 2013 to 2020. Notably, the decreases in aerosol radiative forcing within the atmosphere were found in the SCB and the adjacent northern Yunnan-Guizhou Plateau (NYGP) and ETP, with -41.6%, -33.7%, and -13.6%, respectively during 2013-2020. This indicates that such an attenuated aerosol heating rate in the atmosphere, caused by aerosol variation, could alter the atmospheric thermal structure over the SCB and surrounding areas for regional changes of environment and climate in recent years.

Carbonaceous aerosols in urban Chongqing, China: Seasonal variation, source apportionment, and long-range transport

Seventy-seven PM2.5 samples were collected at an urban site (Chongqing University Campus A) in October 2015 (autumn), December 2015 (winter), March 2016 (spring), and August 2016 (summer). These samples were analysed for organic carbon (OC), elemental carbon (EC), and their associated char, soot, 16 PAHs, and 28 n-alkanes to trace sources, and atmospheric transport pathways. The annual average of OC, EC, char, soot, ΣPAHs, and Σn-alkanes were 20.75 μg/m³, 6.18 μg/m³, 5.43 μg/m³, 0.75 μg/m³, 38.29 ng/m³, and 328.69 ng/m³, respectively. OC, ΣPAHs, and Σn-alkane concentrations were highest in winter and lowest in summer. EC, char, and soot concentrations were highest in autumn and lowest in winter. Source apportionment via positive matrix factorization (PMF) indicated that coal/biomass combustion-natural gas emissions (23.8%) and motor vehicle exhaust (20.2%) were the two major sources, followed by diesel and petroleum residue (21.1%), natural biogenic sources (17.7%), and evaporative/petrogenic sources (17.2%). The highest source contributor in autumn and winter was evaporative/petrogenic sources (30.6%) and natural biogenic sources (34.5%), respectively, whereas diesel engine emission contributed the most in spring and summer (32.1% and 38.0%, respectively). Potential source contribution function (PSCF) analysis identified southeastern Sichuan and northwestern Chongqing as the major potential sources of these pollutants. These datasets provide critical information for policymakers to establish abatement strategies for the reduction of carbonaceous pollutant emissions and improve air quality in Chongqing and other similar urban centres across China.

Enhancing effect of NO2 on the formation of light-absorbing secondary organic aerosols from toluene photooxidation

Aromatic hydrocarbons are one of the major precursors of atmospheric brown carbon (BrC) and both abundantly co-exist with NO_x in the urban atmosphere especially in winter haze period. However, the impact of NOx on the formation of BrC derived from aromatic hydrocarbons is still not fully understood. In this study, the yield and light absorption of secondary organic aerosols (SOA) from toluene photooxidation under various nitrogen oxides (NO₂) levels were investigated by using a 5 m³ photooxidation smog chamber. A trend of increase at first and then decrease in the SOA yield with an increasing NO₂ concentration was observed. The acid-catalyzed heterogeneous reactions lead to the increase of SOA yield in the low-NO₂ regime. The formation of low-volatility species might be suppressed at high-NO₂ conditions is responsible for the decreased SOA yield. In contrast, light absorption and mass absorption coefficient (MAC) of the toluene-derived SOA continuously increased with the increasing NO₂ concentrations. HR-ToF-AMS results showed that nitrogen-containing organic compounds (NOCs) are the main species that lead to the increase of the SOA light absorption. The ratio of CHN family to the total NOCs, which are derived from the nitro compounds, also increased dominantly with the increasing NO₂ levels and accounted for more than half of the total NOCs when the NO₂ concentration increased to 495 ppbv, indicating that nitro compounds rather than organic nitrates are the major light-absorbing species and preferably formed in the toluene oxidation process.

Exploring the variation of black and brown carbon during COVID-19 lockdown in megacity Wuhan and its surrounding cities, China

Absorbing carbonaceous aerosols, i.e. black and brown carbon (BC and BrC), affected heavily on climate change, regional air quality and human health. The nationwide lockdown measures in 2020 were performed to against the COVID-19 outbreak, which could provide an important opportunity to understand their variations on light absorption, concentrations, sources and formation mechanism of carbonaceous aerosols. The BC concentration in Wuhan megacity (WH) was 1.9 μg m^-3 during lockdown, which was 24% lower than those in the medium-sized cities and 26% higher than those in small city; in addition, 39% and 16-23% reductions occurred compared with the same periods in 2019 in WH and other cities, respectively. Fossil fuels from vehicles and industries were the major contributors to BC; and compared with other periods, minimum contribution (64-86%) mainly from fossil fuel to BC occurred during the lockdown in all cities. Secondary BrC (BrCsec) played a major role in the BrC light absorption, accounting for 65-77% in WH during different periods. BrCsec was promoted under high humidity, and decreased through the photobleaching of chromophores under higher Ox. Generally, the lockdown measures reduced the BC concentrations significantly; however, the variation of BrCsec was slight.

Emissions and light absorption of carbonaceous aerosols from on-road vehicles in an urban tunnel in south China

With changing numbers, compositions, emission standards and fuel quality of on-road vehicles, it is imperative to accordingly characterize and update vehicular emissions of carbonaceous aerosols for better understanding their health and climatic effects. In this study, a 7-day field campaign was conducted in 2019 in a busy urban tunnel (>30,000 vehicles day^-1) in south China with filter-based aerosol samples collected every 2 h at both the inlet and the outlet for measuring carbonaceous aerosols and their light absorbing properties. Observed fleet average emission factor (EF) of total carbon (TC) was 13.4 ± 8.3 mg veh^-1 km^-1, and 17.4 ± 11.3 mg veh^-1 km^-1 if electric and LPG-driven vehicles were excluded; and fleet average EF of organic carbon (OC) and elemental carbon (EC) was 8.5 ± 6.6 and 4.9 ± 2.6 mg veh^-1 km^-1 (11.0 ± 8.8 and 6.3 ± 3.6 mg veh^-1 km^-1 if excluding electric and LPG vehicles), respectively. Regression analysis revealed an average TC-EF of 319.8 mg veh^-1 km^-1 for diesel vehicles and 2.1 mg veh^-1 km^-1 for gasoline vehicles, and although diesel vehicles only shared ~4% in the fleet compositions, they still dominate on-road vehicular carbonaceous aerosol emissions due to their over 150 times higher average TC-EF than gasoline vehicles. Filter-based light absorption measurement demonstrated that on average brown carbon (BrC) could account for 19.1% of the total carbonaceous light absorption at 405 nm, and the average mass absorption efficiency of EC at 635 nm and that of OC at 405 nm were 5.2 m² g^-1 C and 1.0 m² g^-1 C, respectively.

Links between the optical properties and chemical compositions of brown carbon chromophores in different environments: Contributions and formation of functionalized aromatic compounds

Links between the optical properties and chemical compositions of brown carbon (BrC) are poorly understood because of the complexity of BrC chromophores. We conducted field studies simultaneously at both vehicle-influenced site and biomass burning-affected site in China in polluted winter. The chemical compositions and light absorption values of functionalized aromatic compounds, including phenyl aldehyde, phenyl acid, and nitroaromatic compounds, were measured. P-phthalic acid, nitrophenols and nitrocatechols were dominant BrC species, accounting for over 50% of the concentration of identified chromophores. Nitrophenols and nitrocatechols contributed more than 50% of the identified BrC absorbance between 300 and 400 nm. Oxidation of biomass burning-related products (e.g., pyrocatechol and methylcatechols) and anthropogenic volatile organic compounds (e.g., benzene and toluene) generated similar BrC chromophores, implying that these functionalized aromatic compounds play an important role in both environments. Compared with the biomass burning-affected site (22%), functionalized aromatic compounds at vehicle-influenced site accounted for a higher percentage of BrC absorption (25%). This research improves our understanding of the links between optical properties and composition of BrC, and the difference between BrC chromophores from BB-influenced area and vehicle-affected area under polluted atmospheric conditions.

PM1 chemical composition and light absorption properties in urban and rural areas within Sichuan Basin, southwest China

Sichuan Basin is encircled by high mountains and plateaus with the heights ranging from 1 km to 3 km, and is one of the most polluted regions in China. However, the dominant chemical species and light absorption properties of aerosol particles is still not clear in rural areas. Chemical composition in PM₁ (airborne particulate matter with an aerodynamic diameter less than 1 μm) and light-absorbing properties were determined in Chengdu (urban) and Sanbacun (rural) in western Sichuan Basin (WSB), Southwest China. Carbonaceous aerosols and secondary inorganic ions (NH₄⁺, NO₃^- and SO₄^2-) dominate PM₁ pollution, contributing more than 85% to PM₁ mass at WSB. The mean concentrations of organic and elemental carbon (OC, EC), K⁺ and Cl^- are 19.69 μg m^-3, 8.00 μg m^-3, 1.32 μg m^-3, 1.16 μg m^-3 at the rural site, which are 26.2%, 65.3%, 34.7% and 48.7% higher than those at the urban site, respectively. BrC (brown carbon) light absorption coefficient at 405 nm is 63.90 ± 27.81 M m^-1 at the rural site, contributing more than half of total absorption, which is about five times higher than that at urban site (10.43 ± 4.74 M m^-1). Compared with secondary OC, rural BrC light absorption more depends on primary OC from biomass and coal burning. The rural MAE_BrC (BrC mass absorption efficiency) at 405 nm ranges from 0.6 to 5.1 m² g^-1 with mean value of 3.5 ± 0.8 m² g^-1, which is about three times higher than the urban site.

Effect of common inorganic anions on iron-catalyzed secondary brown carbon formation from guaiacol

Brown carbon (BrC) is the important component of aerosol with strong UV-visible absorbance. However, the formation of BrC is still elusive. Inorganic anions, e.g., Cl^-, NO₃^- and SO₄^2-, exist ubiquitously in the atmosphere, while their effects on the formation of BrC are poorly understood. In this study, we have systematically investigated the effects of pH (1, 2 and 3), inorganic anion (Cl^-, NO₃^- and SO₄^2-) and ionic strength (0.1, 0.5 and 1.0 M) on BrC generation process by measuring the optical, aggregation and product properties. Our results clearly show that the three factors strongly affect the BrC formation by influencing the oxidation activity and the complexation capability of different Fe(III) species. Marcus theory was used in this research to calculate the oxidation activity of different Fe(III) species. Among all the species of Fe(III), FeOH²⁺ is the most reactive form in the BrC formation reaction. Furthermore, the aggregation process of BrC was also studied, which is affected by different anions due to their different concentration and hydrability, and SO₄^2- exhibits the highest efficiency to induce the aggregation of BrC. This study will deepen our understanding about the natural formation of BrC under environmentally relevant conditions, and be beneficial for controlling the production of atmospheric particulates and the subsequent health effects.

Machine Learning on the COVID-19 Pandemic, Human Mobility and Air Quality: A Review

The ongoing COVID-19 global pandemic is touching every facet of human lives (e.g., public health, education, economy, transportation, and the environment). This novel pandemic and non-pharmaceutical interventions of lockdown and confinement implemented citywide, regionally or nationally are affecting virus transmission, people's travel patterns, and air quality. Many studies have been conducted to predict the diffusion of the COVID-19 disease, assess the impacts of the pandemic on human mobility and on air quality, and assess the impacts of lockdown measures on viral spread with a range of Machine Learning (ML) techniques. This literature review aims to analyze the results from past research to understand the interactions among the COVID-19 pandemic, lockdown measures, human mobility, and air quality. The critical review of prior studies indicates that urban form, people's socioeconomic and physical conditions, social cohesion, and social distancing measures significantly affect human mobility and COVID-19 viral transmission. During the COVID-19 pandemic, many people are inclined to use private transportation for necessary travel to mitigate coronavirus-related health problems. This review study also noticed that COVID-19 related lockdown measures significantly improve air quality by reducing the concentration of air pollutants, which in turn improves the COVID-19 situation by reducing respiratory-related sickness and deaths. It is argued that ML is a powerful, effective, and robust analytic paradigm to handle complex and wicked problems such as a global pandemic. This study also explores the spatio-temporal aspects of lockdown and confinement measures on coronavirus diffusion, human mobility, and air quality. Additionally, we discuss policy implications, which will be helpful for policy makers to take prompt actions to moderate the severity of the pandemic and improve urban environments by adopting data-driven analytic methods.

Overviewing the air quality models on air pollution in Sichuan Basin, China

Most developing countries in the world face the common challenges of reducing air pollution and advancing the process of sustainable development, especially in China. Air pollution research is a complex system and one of the main methods is through numerical simulation. The air quality model is an important technical method, it allows researchers to better analyze air pollutants in different regions. In addition, the SCB is a high-humidity and foggy area, and the concentration of atmospheric pollutants is always high. However, research on this region, one of the four most polluted regions in China, is still lacking. Reviewing the application of air quality models in the SCB air pollution has not been reported thoroughly. To fill these gaps, this review provides a comprehensive narration about i) The status of air pollution in SCB; ii) The application of air quality models in SCB; iii) The problems and application prospects of air quality models in the research of air pollution. This paper may provide a theoretical reference for the prevention and control of air pollution in the SCB and other heavily polluted areas in China and give some¹inspirations for air pollution forecast in other countries with complex terrain.

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."

Changes in light absorption by brown carbon in soot particles due to heterogeneous ozone aging in a smog chamber

Light absorption by brown carbon (BrC) is dynamic due to atmospheric aging processes, leading to complex and poorly constrained effects on photochemistry and climate. In this study, a smog chamber was used to simulate the heterogeneous ozone (O₃) aging of soot particles. Twelve aging times and seven O₃ concentrations were set to investigate the effects of aging degree on BrC light absorption. The results showed that light absorption by BrC was enhanced after O₃ aging, but followed a non-monotonic trend with an initial increase and subsequent decrease. An aging time of 60 min and O₃ concentration of 1.2 ppm were optimal for enhancing BrC absorption, where the contribution of BrC to total absorption and the contribution of BrC relative to black carbon absorption at 370 nm of ozonized soot were 23.0 ± 1.8% and 30.0 ± 3.0%, respectively, much greater than those of fresh soot (8.1 ± 1.1% and 8.8 ± 1.3%, respectively). The absorption Ångström exponent (AAE) and delta C (ΔC) of ozonized soot at 60 min ranged from 1.18 ± 0.01 to 1.31 ± 0.03 and from 13.5 ± 7.0 to 24.3 ± 13.5 μg m^-3, respectively, and were greater than those of fresh soot (1.12 ± 0.02 and 8.0 ± 0.8 μg m^-3), but also showed non-monotonic trends, suggesting the formation of BrC during O₃ aging. Comparative results indicated that AAE might be a better BrC indicator for soot than ΔC. The non-monotonic trend was tentatively explained by changes in organic carbon, oxygenated functional groups and conjugated structures, as well as polycyclic aromatic hydrocarbon (PAH) degradation and oxygenated PAH formation. The relative intensities of oxidative formation and degradation of chromophores may determine BrC evolution during O₃ aging. This study will be useful for clarifying BrC evolution in the atmosphere and estimating its radiative forcing.

Characteristics and potential exposure risks of environmentally persistent free radicals in PM2.5 in the three gorges reservoir area, Southwestern China

Environmentally persistent free radicals (EPFRs) are a novel class of hazardous substances that can exist stably in airborne particles for a period ranging from days to weeks and are potentially toxic to human health. Electron paramagnetic resonance spectroscopy (EPR) was used to characterize particulate EPFRs in Wanzhou in the Three Gorges Reservoir area in 2017. During the whole of 2017, the average concentration of particulate EPFRs was 7.0 × 10¹³ ± 1.7 × 10¹³ spins/m³. The seasonal concentration of EPFRs in PM_2.5 showed a trend of autumn > winter > spring > summer. The maxima and minima of EPFRs occurred in spring with concentrations of 2.1 × 10¹⁴ spins/m³ and 9.4 × 10¹² spins/m³ respectively. The EPFRs in PM_2.5 were mainly carbon-centered radicals with adjacent oxygen atoms. Significant positive correlations were found between EPFRs and SO₄^2-, NO₃^- and NH₄⁺ (r > 0.55, n = 111), indicating that EPFRs are associated with secondary sources. The atmospheric processing of particles from coal combustion, traffic, and agriculture were important sources of EPFRs. They were also particularly well correlated with K⁺ and Cl^- in winter, suggesting that EPFRs may also be derived from wintertime biomass burning emissions. The amount of inhalable EPFRs in Wanzhou was equivalent to the range of 2.3-6.8 cigarettes per capita per day. This study provides evidence of the potential health risks of EPFRs in PM_2.5, and references for air pollution control in the Three Gorges Reservoir area.