Abundance and origin of fine particulate chloride in continental China

Evaluation of a new real-time source apportionment system of PM2.5 and its implication on rapid aging of vehicle exhaust

Accurate identification and rapid analysis of PM2.5 sources and formation mechanisms are essential to mitigate PM2.5 pollution. However, studies were limited in developing a method to apportion sources to the total PM2.5 mass in real-time. In this study, we developed a real-time source apportionment method based on chemical mass balance (CMB) modeling and a mass-closure PM2.5 composition online monitoring system in Shenzhen, China. Results showed that secondary sulfate, secondary organic aerosol (SOA), vehicle emissions and secondary nitrate were the four major PM2.5 sources during autumn 2019 in Shenzhen, together contributed 76 % of PM2.5 mass. The novel method was verified by comparing with other source apportionment methods, including offline filter analysis, aerosol mass spectrometry, and carbon isotopic analysis. The comparison of these methods showed that the new real-time method obtained results generally consistent with the others, and the differences were interpretable and implicative. SOA and vehicle emissions were the major PM2.5 and OA contributors by all methods. Further investigation on the OA sources indicated that vehicle emissions were not only the main source of primary organic aerosol (POA), but also the main contributor to SOA by rapid aging of the exhaust in the atmosphere. Our results demonstrated the great potential of the new real-time source apportionment method for aerosol pollution control and deep understandings on emission sources.

Refined Sea Salt Markers for Coastal Cities Facilitating Quantification of Aerosol Aging and PM2.5 Apportionment

Sea salt (ss) aerosols in PM2.5 are often quantified through source apportionment by applying sodium (Na+) and chloride (Cl-) as the markers, but both markers can be substantially emitted from anthropogenic sources. In this study, we differentiate ss from nonss (nss) portions of Na+ and Cl- to better apportion PM2.5 in a coastal tropical urban environment. Size-resolved ionic profiles accounting for Cl- depletion of aged ss were applied to 162-day measurements during 2012 and 2018-2019. Results show that the nss (likely anthropogenic) portions, on average, account for 50-80% of total Na+ and Cl- in submicron aerosols (PM1). This corresponds to up to 2.5 μg/m3 of ss in submicron aerosols that can be ∼10 times overestimated if one attributes all Na+ and Cl- in PM1 to ss. Employing the newly speciated ss- and nss-portions of Na+ and Cl- to source apportionment of urban PM2.5 via positive matrix factorization uncovers a new source of transported anthropogenic emissions during the southwest monsoon, contributing to 12-15% of PM2.5. This increases anthropogenic PM2.5 by ≥19% and reduces ss-related PM2.5 by >30%. In addition to demonstrating Cl- depletion (aging) in submicron aerosols and quantifying ssNa+, nssNa+, ssCl-, as well as nssCl- therein, the refined PM2.5 apportionment resolves new insights on PM2.5 of anthropogenic origins in urban environments, useful to facilitate policy making.

Wintertime fine aerosol particles composition and its evolution in two megacities of southern and northern China

Study on fine aerosols composition can help understand the particles formation and is crucial for improving the accuracy of model simulations. Based on field data measured by a Q-ACSM (Quadrupole-Aerosol Chemical Speciation Monitor), we have comprehensively compared the characteristics, evolution, and potential formation mechanisms of the components in NR-PM2.5 during wintertime at two megacities (Beijing and Guangzhou) of southern and northern China. We show that as PM pollution intensifies, the mass fraction of the primary aerosols (e.g., COA, HOA) in PM2.5 in Guangzhou increased, along with a slight decline in proportion of both the secondary organic (SOA) and inorganic (SIA) aerosols; In contrast, in Beijing, the proportion of the SIA ramped up from 28 % to 53 % with the pollution evolution; and the fraction of SOA in total OA also increased due to a substantial increment in the proportion of MO-OOA (from 29 % to 48 %), suggesting a significance of the secondary processes in worsening aerosols pollution in Beijing. Our further analysis demonstrates a leading role of aqueous pathway in the secondary formation of aerosols at the Beijing site, presenting an exponential rising of SIA and SOA with the relative humidity (RH) increase. Compared to Beijing, however, we find that the photochemical oxidation other than aqueous process in Guangzhou plays a more critical role in those secondary aerosols formation. Combined with the Hysplit trajectory model, we identify the high humid conditions in Guangzhou are typically affected by clean marine air masses, explaining the slower response of secondary components to the RH changes. Moreover, the particles in Guangzhou were observed less hygroscopic that is adverse to the aerosol aqueous chemistry. The results provide basis for the precise control of PM pollution in different regions across China and would be helpful in improving model simulations.

Impact and pathway of halogens on atmospheric oxidants in coastal city clusters in the Yangtze River Delta region in China

Halogens (chlorine, bromine, and iodine) are known to profoundly influence atmospheric oxidants (hydroxyl radical (OH), hydroperoxyl radical (HO2), ozone (O3), and nitrate radical (NO3)) in the troposphere and subsequently affecting air quality. However, their impact on atmospheric oxidation and air pollution in coastal areas in China is poorly characterized. In this study, we use the WRF-CMAQ (Weather Research and Forecasting-Community Multiscale Air Quality) model with full halogen chemistry and process analysis to assess the influences and pathways of halogens on atmospheric oxidants in the Yangtze River Delta (YRD) region, a typical coastal city cluster in China. Halogens cause the annual OH radical increase by up to 16.4% and NO3 decrease by up to 45.3%. O3 increases by 2.0% in the YRD but decreases by 3.3% in marine environment. Halogen induced changes in atmospheric oxidants lead to a general increase of atmospheric oxidation capacity by 5.1% (maximum 48.4%). The production rate of OH (POH) in the YRD is enhanced by anthropogenic chlorine through both increased HO2 pathway and hypohalous acid photolysis pathway, while POH over ocean is enhanced by oceanic halogens through converting HO2 into hypohalous acid. Anthropogenic chlorine enhances both O3 and NO3 production (PNO3) rates through influencing their precursors while oceanic halogens reduce PNO3 and directly destroy ozone. Iodine contributed most (on average of 91% in oceanic halogens) in reducing production rates of oxidants. Thus, halogen emissions and potential effects of halogens on air quality need to be considered in air quality policies and regulations in the YRD region.

Separating Daily 1 km PM2.5 Inorganic Chemical Composition in China since 2000 via Deep Learning Integrating Ground, Satellite, and Model Data

Fine particulate matter (PM2.5) chemical composition has strong and diverse impacts on the planetary environment, climate, and health. These effects are still not well understood due to limited surface observations and uncertainties in chemical model simulations. We developed a four-dimensional spatiotemporal deep forest (4D-STDF) model to estimate daily PM2.5 chemical composition at a spatial resolution of 1 km in China since 2000 by integrating measurements of PM2.5 species from a high-density observation network, satellite PM2.5 retrievals, atmospheric reanalyses, and model simulations. Cross-validation results illustrate the reliability of sulfate (SO42-), nitrate (NO3-), ammonium (NH4+), and chloride (Cl-) estimates, with high coefficients of determination (CV-R2) with ground-based observations of 0.74, 0.75, 0.71, and 0.66, and average root-mean-square errors (RMSE) of 6.0, 6.6, 4.3, and 2.3 μg/m3, respectively. The three components of secondary inorganic aerosols (SIAs) account for 21% (SO42-), 20% (NO3-), and 14% (NH4+) of the total PM2.5 mass in eastern China; we observed significant reductions in the mass of inorganic components by 40-43% between 2013 and 2020, slowing down since 2018. Comparatively, the ratio of SIA to PM2.5 increased by 7% across eastern China except in Beijing and nearby areas, accelerating in recent years. SO42- has been the dominant SIA component in eastern China, although it was surpassed by NO3- in some areas, e.g., Beijing-Tianjin-Hebei region since 2016. SIA, accounting for nearly half (∼46%) of the PM2.5 mass, drove the explosive formation of winter haze episodes in the North China Plain. A sharp decline in SIA concentrations and an increase in SIA-to-PM2.5 ratios during the COVID-19 lockdown were also revealed, reflecting the enhanced atmospheric oxidation capacity and formation of secondary particles.

Assessing the chemical composition, potential toxicity and cancer risk of airborne fine particulate matter (PM2.5) near a petrochemical industrial area

In the vicinity of a petrochemical industrial region in São Paulo, Brazil, PM_2.5-bound organic carbon (OC), elemental carbon (EC), polycyclic aromatic hydrocarbons (PAHs), nitro-PAHs, oxy-PAHs, hopanes, and inorganic species were evaluated. Oxidative potential (OP), burden (OB), and Alivibrio fischeri bioluminescence inhibition (AFBIA) assays were conducted to determine the potential health effects of exposure to these compounds. The PM_2.5 mean concentration was 32.0±18.2µgm^-3, and benzo (a)pyrene was found to exceed recommended levels by at least four times. Secondary sources and vehicular emissions were indicated by nitro-PAHs, oxy-PAHs, and inorganic species. The OP and OB results revealed that secondary compounds favored antioxidant depletion. The AFBIA results showed that 64% of the samples were toxic. These findings emphasize the need to reduce the exposure risk and take measures to protect human health.

Chemical composition, oxidative potential and identifying the sources of outdoor PM2.5 after the improvement of air quality in Beijing

Air pollution poses a serious threat to human health. The implementation of air pollution prevention and control policies has gradually reduced the level of atmospheric fine particles in Beijing. Exploring the latest characteristics of PM_2.5 has become the key to further improving pollution reduction measures. In the current study, outdoor PM_2.5 samples were collected in the spring and summer of Beijing, and the chemical species, oxidative potential (OP), and sources of PM_2.5 were characterized. The mean PM_2.5 concentration during the entire study period was 41.6 ± 30.9 μg m^-3. Although the PM_2.5 level in summer was lower, its OP level was significantly higher than that in spring. SO₄^2-, NH₄⁺, EC, NO^3-, and OC correlated well with volume-normalized OP (OP_v). Strong positive correlations were found between OP_v and the following elements: Cu, Pb, Zn, Ni, As, Cr, Sn, Cd, Al, and Mn. Seven sources of PM_2.5 were identified, including traffic, soil dust, secondary sulfate, coal and biomass burning, oil combustion, secondary nitrate, and industry. Multiple regression analysis indicated that coal and biomass combustion, industry, and traffic were the main contributors to the OP_v in spring, while secondary sulfate, oil combustion, and industry played a leading role in summer. The source region analysis revealed that different pollution sources were related to specific geographic distributions. In addition to local emission reduction policies, multi-provincial cooperation is necessary to further improve Beijing's air quality and reduce the adverse health effects of PM_2.5.

Diurnal variations in primary and secondary organic aerosols in an eastern China coastal city: The impact of land-sea breezes

The land-sea breeze circulation significantly impacts the atmospheric transport of organic aerosols in coastal regions. However, the links between organic aerosols and land-sea breezes remain poorly understood. In this study, organic marker compounds for biomass burning, primary biological aerosols, biogenic and anthropogenic secondary organic aerosols (SOA) in fine particles from a coastal city in East China were analysed using gas chromatography-mass spectrometry. Land-sea breeze circulations were identified to explore their potential influence on organic molecular compositions. Organic marker compounds showed obvious diurnal/seasonal patterns. Surprisingly, due to the combined influence of weakened East Asian monsoons and land-sea breezes, all detected organic markers decreased except α/β-pinene SOA markers during land-sea breeze periods in early autumn; whereas, all the organic markers increased except α/β-pinene SOA markers, pollen and plant debris markers during land-sea breeze periods in early spring. Furthermore, the reaction pathway and aging of biogenic SOA were also related to land-sea breezes. During the land-sea breeze periods, the ratios of 2-methylglyceric acid (2-MGA) to 2-methyltetrols increased in early autumn, indicating that more isoprene-derived SOA generated from the high-NO_x (nitrogen oxides) pathway when the land-sea breezes occurred; while the ratios decreased in early spring, this may be related to the chemical transformation of 2-MGA to 2-MGA sulfates. Changes in the ratio of monoterpene SOA markers demonstrate that monoterpene SOA was relatively aged during sea breeze periods, while it was fresher when the land breeze occurred. Although boundary layer height, emissions, gas/particle partitioning, etc. are important reasons for the diurnal variations of organic aerosols, night/day ratios of molecular markers increased obviously when land-sea breezes occurred in both early autumn and early spring. Our results provide new insights into the shift in the chemical composition of organic aerosols over coastal areas that are influenced by land-sea breezes.

Fine particulate pollution driven by nitrate in the moisture urban atmospheric environment in the Pearl River Delta region of south China

To identify potential sources of fine particles (PM_2.5, with aerodynamic diameter (D_a) ≤ 2.5 μm) in urban Dongguan of south China, a comprehensive campaign was carried out in the whole 2019. Hourly PM_2.5 and its dominant chemical components including organic carbon (OC), elemental carbon (EC), water-soluble inorganic ions (WSIIs) and thirteen elements were measured using online instruments. Gaseous pollutants including NH₃, HNO₃, NO₂, NO and O₃ and meteorological parameters were also synchronously measured. PM_2.5 was dominated by carbonaceous aerosols in summer and by WSIIs in the other seasons. PM_2.5 and its dominant chemical components mostly peaked around noon (10:00-14:00 LST). Furthermore, high PM_2.5 levels during the daytime were closely related with the increased NO₃^- levels. The high mass concentrations of NO₃^- in urban Dongguan during the daytime were likely related with regional transport of NO₃^- from suburban Dongguan, which was originated from the reaction between NO₂ and O₃ under the moisture condition during the nighttime. Seven major source factors for PM_2.5 including secondary sulfate, ship emission, traffic emission, secondary nitrate, industrial processes, soil dust and coal combustion were identified by positive matrix factorization (PMF) analysis, which contributed 26 ± 14%, 16 ± 16%, 16 ± 10%, 14 ± 11%, 12 ± 11%, 8 ± 6% and 8 ± 6%, respectively, to annual PM_2.5 mass concentration. Although secondary sulfate contributed much more than secondary nitrate to PM_2.5 on annual basis, the latter exceeded the former source factor when daily PM_2.5 mass concentration was higher than 60 μg m^-3, indicating the critical role nitrate played in PM_2.5 episode events.

Aqueous secondary organic aerosol formation attributed to phenols from biomass burning

Biomass burning emits large quantities of phenols, which readily partition into the atmospheric aqueous phase and subsequently may react to produce aqueous secondary organic aerosol (aqSOA). For the first time, we quantitatively explored the influence of phenols emitted from biomass burning on aqSOA formation in the winter of Beijing. A typical haze episode associated with significant aqSOA formation was captured. During this episode, aqueous-phase processing of biomass burning promoted aqSOA formation was identified. Furthermore, high-resolution mass spectrum analysis provided molecular-level evidence of the phenolic aqSOA tracers. Estimation of aqSOA formation rate (R_aqSOA) with compiled laboratory kinetic data indicated that biomass-burning phenols can efficiently produce aqSOA at midday, with R_aqSOA of 0.42 μg m^-3 h^-1 accounting for 15 % of total aqSOA formation rate. The results highlight that aqSOA formation of phenols contributes the haze pollution. This implies the importance of regional joint control of biomass burning to mitigate the heavy haze.

A clear north-to-south spatial gradience of chloride in marine aerosol in Chinese seas under the influence of East Asian Winter Monsoon

Particulate chloride is a major component of sea salt particles and plays a key role in atmospheric chemistry. Anthropogenic pollutants over the northeastern Asia can be transported to the adjacent seas through the northwest monsoon, which profoundly influences the chloride chemistry over the seas. In this study, spatial distribution of particulate chloride and its sources over the Chinese seas were investigated based on shipboard particle samplings especially online Single Particle Aerosol Mass Spectrometer (SPAMS) over Bohai Sea, North Yellow Sea, and South Yellow Sea (SYS) during a cruise in November 2012. A strong north-to-south (N-S) gradience in marine aerosol composition was found. The Cl^-/Na⁺ ratios in PM_2.5 and single particle composition by SPAMS indicated remarkable chloride enrichment in marine aerosol in the north (Bohai Sea), while depletion in southern SYS. The results of size distribution showed that particulate chloride had higher concentration in coarse particles, while the Cl^-/Na⁺ ratio was much higher in submicron particles. In the north (38-40°N), biomass burning, carbonaceous, and Pb-rich type particles had high fractions in all chloride-containing particles identified by SPAMS (on average 66%). Combining chemical composition with back trajectory, it was found that fine-mode chloride enrichment in the north was mainly due to anthropogenic emission especially coal combustion and biomass burning from northern China. However, the high fine-mode chloride depletion in the south (32-34°N) was probably due to acid replacement by sulfate in aged aerosol during atmospheric transport. Our new findings reveal that marine aerosol in Chinese seas would show a clear N-S pattern of more fresh and anthropogenic enriched particles in the north, but more aged aerosol in the south during the East Asia Winter Monsoon, which provides new insights for the quantitative assessment of anthropogenic impact on marine aerosol and future modeling study.

Global Emissions of Hydrogen Chloride and Particulate Chloride from Continental Sources

Gaseous and particulate chlorine species play an important role in modulating tropospheric oxidation capacity, aerosol water uptake, visibility degradation, and human health. The lack of recent global continental chlorine emissions has hindered modeling studies of the role of chlorine in the atmosphere. Here, we develop a comprehensive global emission inventory of gaseous HCl and particulate Cl- (pCl), including 35 sources categorized in six source sectors based on published up-to-date activity data and emission factors. These emissions are gridded at a spatial resolution of 0.1° × 0.1° for the years 1960 to 2014. The estimated emissions of HCl and pCl in 2014 are 2354 (1661-3201) and 2321 (930-3264) Gg Cl a-1, respectively. Emissions of HCl are mostly from open waste burning (38%), open biomass burning (19%), energy (19%), and residential (13%) sectors, and the major sources classified by fuel type are combustion of waste (43%), biomass (32%), and coal (25%). Emissions of pCl are mostly from biofuel (29%) and open biomass burning processes (44%). The sectoral and spatial distributions of HCl and pCl emissions are very heterogeneous along the study period, and the temporal trends are mainly driven by the changes in emission factors, energy intensity, economy, and population.

Photodissociation of particulate nitrate as a source of daytime tropospheric Cl2

Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl₂), a Cl precursor, in the polar atmosphere, its sources in other regions are still poorly understood. Here we report the daytime Cl₂ concentrations of up to 1 ppbv observed in a coastal area of Hong Kong, revealing a large daytime source of Cl₂ (2.7 pptv s⁻¹ at noon). Field and laboratory experiments indicate that photodissociation of particulate nitrate by sunlight under acidic conditions (pH < 3.0) can activate chloride and account for the observed daytime Cl₂ production. The high Cl₂ concentrations significantly increased atmospheric oxidation. Given the ubiquitous existence of chloride, nitrate, and acidic aerosols, we propose that nitrate photolysis is a significant daytime chlorine source globally. This so far unaccounted for source of chlorine can have substantial impacts on atmospheric chemistry.

This study unravels an important daytime Cl₂ source in the extra-polar atmosphere and shows that photolysis of particle nitrate at high acidity produced unprecedented levels of Cl₂, boosting the oxidative power and air pollutants like O₃.

An unexpected large continental source of reactive bromine and chlorine with significant impact on wintertime air quality

Halogen atoms affect the budget of ozone and the fate of pollutants such as hydrocarbons and mercury. Yet their sources and significances in polluted continental regions are poorly understood. Here we report the observation of unprecedented levels (averaging at 60 parts per trillion) of bromine chloride (BrCl) at a mid-latitude site in North China during winter. Widespread coal burning in rural households and a photo-assisted process were the primary source of BrCl and other bromine gases. BrCl contributed about 55% of both bromine and chlorine atoms. The halogen atoms increased the abundance of 'conventional' tropospheric oxidants (OH, HO₂ and RO₂) by 26%-73%, and enhanced oxidation of hydrocarbon by nearly a factor of two and the net ozone production by 55%. Our study reveals the significant role of reactive halogen in winter atmospheric chemistry and the deterioration of air quality in continental regions where uncontrolled coal combustion is prevalent.

Impacts of chlorine chemistry and anthropogenic emissions on secondary pollutants in the Yangtze river delta region

Multiphase chemistry of chlorine is coupled into a 3D regional air quality model (CMAQv5.0.1) to investigate the impacts on the atmospheric oxidation capacity, ozone (O₃), as well as fine particulate matter (PM_2.5) and its major components over the Yangtze River Delta (YRD) region. The developed model has significantly improved the simulated hydrochloric acid (HCl), particulate chloride (PCl), and hydroxyl (OH) and hydroperoxyl (HO₂) radicals. O₃ is enhanced in the high chlorine emission regions by up to 4% and depleted in the rest of the region. PM_2.5 is enhanced by 2-6%, mostly due to the increases in PCl, ammonium, organic aerosols, and sulfate. Nitrate exhibits inhomogeneous variations, by up to 8% increase in Shanghai and 2-5% decrease in most of the domain. Radicals show different responses to the inclusion of the multiphase chlorine chemistry during the daytime and nighttime. Both OH and HO₂ are increased throughout the day, while nitrate radicals (NO₃) and organic peroxy radicals (RO₂) show an opposite pattern during the daytime and nighttime. Higher HCl and PCl emissions can further enhance the atmospheric oxidation capacity, O₃, and PM_2.5. Therefore, the anthropogenic chlorine emission inventory must be carefully evaluated and constrained.

Anthropogenic emissions of atomic chlorine precursors in the Yangtze River Delta region, China

Chlorine radical plays an important role in the formation of ozone and secondary aerosols in the troposphere. It is hence important to develop comprehensive emissions inventory of chlorine precursors in order to enhance our understanding of the role of chlorine chemistry in ozone and secondary pollution issues. Based on a bottom-up methodology, this study presents a comprehensive emission inventory for major atomic chlorine precursors in the Yangtze River Delta (YRD) region of China for the year 2017. Four primary chlorine precursors are considered in this study: hydrogen chloride (HCl), fine particulate chloride (Cl^-) (Cl^- in PM_2.5), chlorine gas (Cl₂), and hypochlorous acid (HClO) with emissions estimated for twelve source categories. The total emissions of these four species in the YRD region are estimated to be 20,424 t, 15,719 t, 1556 and 9331 t, respectively. The emissions of HCl are substantial, with major emissions from biomass burning and coal combustion, together accounting for 68% of the total HCl emissions. Fine particulate Cl^- is mainly emitted from industrial processing, biomass burning and waste incineration. The emissions of Cl₂ and HClO are mainly associated with usage of chlorine-containing disinfectants, for example, water treatment, wastewater treatment, and swimming pools. Emissions of each chlorine precursor are spatially allocated based on the characteristics of individual source category. This study provides important basic dataset for further studies with respect to the effects of chlorine chemistry on the formation of air pollution complex in the YRD region.

Characteristics and sources of hourly elements in PM10 and PM2.5 during wintertime in Beijing

Characteristics and sources of ambient particle elements in urban Beijing were studied by hourly observations in two size fractions (PM₁₀ and PM_2.5) during November and December 2017 using an online multi-element analyzer. The reconstructed oxide concentrations of 24 elements (from Al to Pb) comprise an appreciable fraction of PM₁₀ and PM_2.5, accounting for 37% and 17%, respectively on average. We demonstrate the benefit of using high-time-resolution chemical speciation data in achieving robust source apportionment of the total elemental PM₁₀ (PM_10el) and elemental PM_2.5 (PM_2.5el) mass using positive matrix factorization (PMF). Biomass burning, coal combustion, secondary sulfate, industry, non-exhaust traffic and dust were identified in both size fractions (with varying relative concentrations), which accounted on average for 4%, 12%, 5%, 2%, 14%, and 63%, respectively to the total PM_10el, and 14%, 35%, 21%, 6%, 12% and 12%, respectively to the total PM_2.5el. Biomass burning and coal combustion exhibited higher concentrations during haze episodes of the heating season. In contrast, secondary sulfate and industry contributed more to haze episodes during the non-heating season. The fractional contribution of dust was mostly high during clean days, while the fractional non-exhaust traffic emission contribution was similar throughout the measurement period. The non-exhaust traffic emissions contributed locally, while the remaining sources were dominated by neighboring areas. Furthermore, trajectory analysis showed that the origin of the industrial sources roughly agreed with the locations of the main point sources. Overall, this work provides detailed information on the characteristics of the elements during different haze events during heating and non-heating seasons.

Effects of Anthropogenic Chlorine on PM2.5 and Ozone Air Quality in China

China has large anthropogenic chlorine emissions from agricultural fires, residential biofuel, waste incineration, coal combustion, and industrial processes. Here we quantify the effects of chlorine on fine particulate matter (PM_2.5) and ozone air quality across China by using the GEOS-Chem chemical transport model with comprehensive anthropogenic emissions and detailed representation of gas-phase and heterogeneous chlorine chemistry. Comparison of the model to observed ClNO₂, HCl, and particulate Cl^- concentrations shows that reactive chlorine in China is mainly anthropogenic, unlike in other continental regions where it is mostly of marine origin. The model is successful in reproducing observed concentrations and their distributions, lending confidence in the anthropogenic chlorine emission estimates and the resulting chemistry. We find that anthropogenic chlorine emissions increase total inorganic PM_2.5 by as much as 3.2 μg m^-3 on an annual mean basis through the formation of ammonium chloride, partly compensated by a decrease of nitrate because ClNO₂ formation competes with N₂O₅ hydrolysis. Annual mean MDA8 surface ozone increases by up to 1.9 ppb, mainly from ClNO₂ chemistry, while reactivities of volatile organic compounds increase (by up to 48% for ethane). We find that a sufficient representation of chlorine chemistry in air quality models can be obtained from consideration of HCl/Cl^- thermodynamics and ClNO₂ chemistry, because other more complicated aspects of chlorine chemistry have a relatively minor effect.

Multiphase Photochemistry of Iron-Chloride Containing Particles as a Source of Aqueous Chlorine Radicals and Its Effect on Sulfate Production

Photolysis of iron chlorides is a well-known photolytic source of Cl^• in environmental waters. However, the role of particulate chlorine radicals (Cl^• and Cl₂^•-) in their multiphase oxidative potential has been much less explored. Herein, we examine the effect of Cl^•/Cl₂^•- produced from photolysis of particulate iron chlorides on atmospheric multiphase oxidation. As a model system, experiments on multiphase oxidation of SO₂ by Cl^•/Cl₂^•- were performed. Fast sulfate production from SO₂ oxidation was observed with reactive uptake coefficients of ∼10^-5, comparable to the values necessary for explaining the observations in the haze events in China. The experimental and modeling results found a good positive correlation between the uptake coefficient, γ_SO2, and the Cl^• production rate, d[Cl^•]/dt, as γ_SO2 = 5.3 × 10^-6 × log(d[Cl^•]/dt) + 4.9 × 10^-5. When commonly found particulate dicarboxylic acids (oxalic acid or malonic acid) were added, sulfate production was delayed due to the competition of Fe³⁺ between chloride and the dicarboxylic acid for its complexation at the initial stage. After the delay, comparable sulfate production was observed. The present study highlights the importance of photochemistry of particulate iron chlorides in multiphase oxidation processes in the atmosphere.

Stable oxygen isotope constraints on nitrate formation in Beijing in springtime

Rapid accumulation of aerosol nitrate (NO₃^-) contributes to haze pollution; however, studies quantifying NO₃^- formation mechanisms remain scarce. To explore aerosol nitrate formation pathways, total suspended particulate (TSP) samples were collected in Beijing during the spring of 2013, and the concentration of NO₃^- and δ¹⁸O- NO₃^- value were analyzed. The NO₃^- concentrations on polluted days (PD) were higher than those on non-polluted days (NPD). Furthermore, higher δ¹⁸O- NO₃^- values were observed on PD (86.8 ± 8.1‰) as compared with NPD (73.7 ± 11.0‰) suggest that more nitrate was produced by pathways with relative high δ¹⁸O-HNO₃ values during PD. Based on the calculated δ¹⁸O-HNO₃ values from different formation pathways and the observed δ¹⁸O- NO₃^- values, the possible fractional contributions of HNO₃ formed via various pathways to TSP NO₃^- were estimated using the Bayesian isotope mixing model. The δ¹⁸O- NO₃^- constrained calculations suggest that the pathways of N₂O₅ + H₂O/Cl^-, NO₃ + VOCs, and ClNO₃ + H₂O possibly contributed 53%-89% to nitrate production during PD. During NPD, the NO₂ + OH pathway produced 37%-69% of the NO₃^-. Using the δ¹⁸O- NO₃^- value combined with the isotope mixing model is a promising approach for exploring NO₃^- formation pathways.

Investigating the impacts of coal-fired power plants on ambient PM2.5 by a combination of a chemical transport model and receptor model

Aimed at evaluating the impacts of coal-fired power plants on urban air quality and human health, a one-month intensive observation campaign was conducted in a typical polluted city located in the 2 + 26 city cluster (Beijing, Tianjin and 26 other cities) of the North China Plain in December 2017. The observation results illustrated that the coal-fired power plant in this city increased the monthly average fine particulate matter (PM_2.5) concentration by ~5% at the city scale. The impacts differed under various diffusion conditions. A three-dimensional nested air quality condition model (the Nested Air Quality Perdition Model System or NAQPMS) with source apportionment was employed to analyze the impacts. The results indicated that power plants had the largest effect on regional air quality during the severe-pollution period, while any influence could be ignored during periods with excellent dissipation under robust winds. PM_2.5 contributed by the power plant mainly occurred below 150 m, diffused 100 km away, and reached a level of approximately 5 μg m^-3 during the light-pollution period. During the accumulation period, the plume reached a height of 500 m, diffused to the downwind area approximately 100 km away within half a day, and contributed at most 40 μg m^-3 to PM_2.5. The affected area expanded to 250 km during the severe-pollution period, and the contribution to PM_2.5 was at least 10 μg m^-3 at different distances. The affected height reached approximately 500 m, with PM_2.5 exceeding 10 μg m^-3, mainly constrained below 150 m. Overall, regional integrated control strategies should be implemented for the power plants in the 2 + 26 city cluster during pollution episodes to further improve air quality.

Links between airborne microbiome, meteorology, and chemical composition in northwestern Turkey

The composition of atmospheric aerosols is dynamic and influenced by their emission sources, organic and inorganic composition, transport pathways, chemical and physical processes, microorganisms' content and more. Characterization of such factors can improve the ability to evaluate air quality and health risks under different atmospheric scenarios. Here we investigate the microbial composition of the atmospheric particulate matter (<10 μm; PM₁₀), sampled in Bolu, Turkey, and the linkage to the chemical composition changes, and different environmental factors. We show distinct differences between aerosol composition of different sources and air-mass transport patterns, sampled in July-August 2017 and in February 2018. The summer samples had a typical northern component air mass trajectories and higher local wind speed. They were characterized by high PM₁₀ levels, marine and mineral dust tracers and high relative abundance of Ascomycota, suggesting long-range transport of the particles from remote sources. In contrast, samples collected in February were characterized by a dominant contribution of southern air masses, and low wind speed. They had low PM₁₀ values, higher relative abundance of antibiotic resistance genes and anthropogenic ions related to local industries and farming, suggesting a dominance of local sources. With the microbiome analyses reported here for the first time for this region, we show good agreement between airborne microbial composition, aerosol mass load, chemistry, and meteorology. These results allow better air quality evaluation and prediction capabilities.

Potential Effect of Halogens on Atmospheric Oxidation and Air Quality in China

Air pollution has been a hazard in China over recent decades threatening the health of half a billion people. Much effort has been devoted to mitigating air pollution in China leading to a significant reduction in primary pollutants emissions from 2013 to 2017, while a continuously worsening trend of surface ozone (O₃, a secondary pollutant and greenhouse gas) was observed over the same period. Atmospheric oxidation, dominated by daytime reactions involving hydroxyl radicals (OH), is the critical process to convert freshly-emitted compounds into secondary pollutants, and is underestimated in current models of China's air pollution. Halogens (chlorine, bromine, and iodine) are known to profoundly influence oxidation chemistry in the marine environment; however, their impact on atmospheric oxidation and air pollution in China is unknown. In the present study, we report for the first time that halogens substantially enhance the total atmospheric oxidation capacity in polluted areas of China, typically 10% to 20% (up to 87% in winter) and mainly by significantly increasing OH level. The enhanced oxidation along the coast is driven by oceanic emissions of bromine and iodine, and that over the inland areas by anthropogenic emission of chlorine. The extent and seasonality of halogen impact are largely explained by the dynamics of Asian monsoon, location and intensity of halogen emissions, and O₃ formation regime. The omission of halogen emissions and chemistry may lead to significant errors in historical re-assessments and future projections of the evolution of atmospheric oxidation in polluted regions.

Concentration Variability of Water-Soluble Ions during the Acceptable and Exceeded Pollution in an Industrial Region

This study investigates the chemical composition of water-soluble inorganic ions at eight localities situated in the Moravian-Silesian Region (the Czech Republic) at the border with Poland. Water-soluble inorganic ions were monitored in the winter period of 2018 (January, 11 days and February, 5 days). The set was divided into two periods: the acceptable period (the 24-h concentration of PM₁₀ < 50 µg/m³) and the period with exceeded pollution (PM₁₀ ˃ 50 µg/m³). Air quality in the Moravian-Silesian Region and Upper Silesia is among the most polluted in Europe, especially in the winter season when the concentration of PM₁₀ is repeatedly exceeded. The information on the occurrence and behaviour of water-soluble inorganic ions in the air during the smog episodes in Europe is insufficient. The concentrations of water-soluble ions (chlorides, sulphates, nitrates, ammonium ions, potassium) during the exceeded period are higher by two to three times compared with the acceptable period. The major anions for both acceptable period and exceeded pollution are nitrates. During the period of exceeded pollution, percentages of water-soluble ions in PM₁₀ decrease while percentages of carbonaceous matter and insoluble particles (fly ash) increase.

Enhanced Sulfate Production by Nitrate Photolysis in the Presence of Halide Ions in Atmospheric Particles

Heterogeneous oxidation of SO₂ is an effective production pathway of sulfate in the atmosphere. We recently reported a novel pathway for the heterogeneous oxidation of SO₂ by in-particle oxidants (OH, NO₂, and NO₂^-/HNO₂) produced from particulate nitrate photolysis (Environ. Sci. Technol. 2019, 53, 8757-8766). Particulate nitrate is often found to coexist with chloride and other halide ions, especially in aged sea-salt aerosols and combustion aerosols. Reactive uptake experiments of SO₂ with UV-irradiated nitrate particles showed that sulfate production rates were enhanced by a factor of 1.4, 1.3, and 2.0 in the presence of Cl^-, Br^-, and I^-, respectively, compared to those in the absence of halide ions. The larger sulfate production was attributed to enhanced nitrate photolysis promoted by the increased incomplete solvation of nitrate at the air-particle interface due to the presence of surface-active halide ions. Modeling results based on the experimental data showed that the nitrate photolysis rate constants increased by a factor of 2.0, 1.7, and 3.7 in the presence of Cl^-, Br^-, and I^-, respectively. A linear relation was found between the nitrate photolysis rate constant, j_NO3-, and the initial molar ratio of Cl^- to NO₃^-, [Cl^-]₀/[NO₃^-]₀, as j_NO3- = 9.7 × 10^-5[Cl^-]₀/[NO₃^-]₀ + 1.9 × 10^-5 at [Cl^-]₀/[NO₃^-]₀ below 0.2. The present study demonstrates that the presence of halide ions enhances sulfate production produced during particulate nitrate photolysis and provides insights into the enhanced formation of in-particle oxidants that may increase atmospheric oxidative capacity.

Positive matrix factorization on source apportionment for typical pollutants in different environmental media: a review

A bibliometric analysis of published papers with the key words "positive matrix factorization" and "source apportionment" in 'Web of Science', reveals that more than 1000 papers are associated with this research and that approximately 50% of these were produced in Asia. As a receptor-based model, positive matrix factorization (PMF) has been widely used for source apportionment of various environmental pollutants, such as persistent organic pollutants (POPs), heavy metals, volatile organic compounds (VOCs) as well as inorganic cations and anions in the last decade. In this review, based on the papers mainly from 2008 to 2018 that focused on source apportionment of pollutants in different environmental media, we provide a comparison and summary of the source categories of typical environmental pollutants, with a special focus on polycyclic aromatic hydrocarbons (PAHs), apportioned using PMF. Based on the statistical average, coal combustion and vehicular emission, are shown to be the two most common sources of PAHs, and contribute much more to emissions than other sources, such as biomass burning, biogenic sources and waste incineration. Heavy metals were mainly from agricultural activities, industrial and vehicular emissions and mining activities. Quantitative source apportionment on pollutants such as VOCs and particulate matter were also apportioned, showing a prominent contribution from fossil-fuel combustion. We conclude that, aside from natural sources, abatement strategies should be focused on changes in energy structure and industrial activities, especially in China. Source apportionment of typical POPs including polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs), halogenated flame retardants (HFRs) and perfluorinated compounds (PFCs) is less comprehensive and further study is required.

Spatial Distributions and Sources of Inorganic Chlorine in PM2.5 across China in Winter

Chlorine is an important atmospheric photochemical oxidant, but few studies have focused on atmospheric chlorine. In this study, PM2.5 samples were collected from urban and rural sites across China in January 2018, and concentrations of Cl− and other water-soluble ions in PM2.5 were analyzed. The size-segregated aerosol Cl− data measured across Chinese cities by other studies were compiled for comparison. The observed data demonstrated that the Cl− concentrations of PM2.5 in northern cities (5.0 ± 3.7 µg/m3) were higher than those in central (1.9 ± 1.2 µg/m3) and southern cities (0.84 ± 0.54 µg/m3), suggesting substantial chlorine emissions in northern cities during winter. The concentrations of Cl− in aerosol were significantly higher in urban regions (0.11–26.7 µg/m3) compared to than in rural regions (0.03–0.61 µg/m3) across China during winter, implying strong anthropogenic chlorine emission in cities. Based on the mole ratios of Cl−/Na+, Cl−/K+ and Cl−/ SO 4 2 − and the PMF model, Cl− in northern and central cities was mainly sourced from the coal combustion and biomass burning, but in southern cities, Cl− in PM2.5 was mainly affected by the equilibrium between gas-phase HCl and particulate Cl−. The size-segregated statistical data demonstrated that particulate Cl− had a bimodal pattern, and more Cl− was distributed in the fine model than that in the coarse mode in winter, with the opposite pattern was observed in summer. This may be attributed to both sources of atmospheric Cl− and Cl− involved in chemical processes. This study reports the concentrations of aerosol Cl− on a national scale, and provides important information for modeling the global atmospheric reactive chlorine distribution and the effects of chlorine on atmospheric photochemistry.

Air Mass Trajectories to Estimate the “Most Likely” Areas to Be Affected by the Release of Hazardous Materials in the Atmosphere—Feasibility Study

Countries continuously review and improve their Emergency Preparedness and Response (EP&R) arrangements and capabilities to take agile and rapid actions with the intent of minimizing health, environmental and economic impacts of potential harmful releases into the atmosphere. One of the specific topics within the EP&R field is the estimation of the areas that might be affected. A proposal is presented to estimate the spatial distribution of the released material. The methodology combines the computation of air mass trajectories and the elaboration of density maps from the corresponding end-point positions. To this purpose, density maps are created in a three-way procedure; first, forward trajectories are calculated from a certain location and for a long period of time, e.g., a decade; second, the selected end-point positions are aggregated in a density field by applying the kernel density estimation method, and then the density field is visualized. The final product reports the areas with the longest residence time of air masses, and hence, the areas “most likely” to be affected and where the deposit may be substantial. The usefulness of this method is evaluated taking as reference a ten-year period (2007–2016) and against two different radioactive release scenarios, such as the Chernobyl accident and the Algeciras release. While far from being fully comprehensive, as only meteorological data are used, the performance of this method is reasonably efficient, and hence, it is a desirable alternative to estimating those areas potentially affected by a substantial deposit following the releases of a harmful material in the atmosphere.

Spatial differences in ambient coarse and fine particles in the Monterrey metropolitan area, Mexico: Implications for source contribution

The ambient air of the Monterrey Metropolitan Area (MMA) in Mexico frequently exhibits high levels of PM₁₀ and PM_2.5. However, no information exists on the chemical composition of coarse particles (PM_c = PM₁₀ - PM_2.5). A monitoring campaign was conducted during the summer of 2015, during which 24-hr average PM₁₀ and PM_2.5 samples were collected using high-volume filter-based instruments to chemically characterize the fine and coarse fractions of the PM. The collected samples were analyzed for anions (Cl^-, NO₃^-, SO₄^2-), cations (Na⁺, NH₄⁺, K⁺), organic carbon (OC), elemental carbon (EC), and 35 trace elements (Al to Pb). During the campaign, the average PM_2.5 concentrations did not showed significance differences among sampling sites, whereas the average PM_c concentrations did. In addition, the PM_c accounted for 75% to 90% of the PM₁₀ across the MMA. The average contribution of the main chemical species to the total mass indicated that geological material including Ca, Fe, Si, and Al (45%) and sulfates (11%) were the principal components of PM_c, whereas sulfates (54%) and organic matter (30%) were the principal components of PM_2.5. The OC-to-EC ratio for PM_c ranged from 4.4 to 13, whereas that for PM_2.5 ranged from 3.97 to 6.08. The estimated contribution of Secondary Organic Aerosol (SOA) to the total mass of organic aerosol in PM_2.5 was estimated to be around 70-80%; for PM_c, the contribution was lower (20-50%). The enrichment factors (EF) for most of the trace elements exhibited high values for PM_2.5 (EF: 10-1000) and low values for PM_c (EF: 1-10). Given the high contribution of crustal elements and the high values of EFs, PM_c is heavily influenced by soil resuspension and PM_2.5 by anthropogenic sources. Finally, the airborne particles found in the eastern region of the MMA were chemically distinguishable from those in its western region. Implications: Concentration and chemical composition patterns of fine and coarse particles can vary significantly across the MMA. Public policy solutions have to be built based on these observations. There is clear evidence that the spatial variations in the MMA's coarse fractions are influenced by clearly recognizable primary emission sources, while fine particles exhibit a homogeneous concentration field and a clear spatial pattern of increasing secondary contributions. Important reductions in the coarse fraction can come from primary particles' emission controls; for fine particles, control of gaseous precursors-particularly sulfur-containing species and organic compounds-should be considered.

Characteristics of atmospheric PM2.5 composition during the implementation of stringent pollution control measures in shanghai for the 2016 G20 summit

To reduce air pollution within a 300 km radius from Hangzhou (the capital city of Zhejiang Province in East China) for the 2016 G20 summit (9/4-9/5), the 14-day (8/24-9/6) stringent pollution control measures were implemented in Shanghai. Changes in atmospheric concentrations during the same 14-day period from 2014 to 2016 were examined at two Supersites, i.e., urban Pudong site (PD) and Dianshan Lake regional site (DSL). Up to 50% reductions were found for PM_2.5, with 13.1% and 9.7% reductions for SO₂ and NO₂, respectively. No apparent improvements were found for 8-h average O₃ concentrations. Large reductions were also found for SO₄^2- (51.4%), NO₃^- (68.8%), and NH₄⁺ (84.4%), on average. Elevated coefficient of divergence values (0.52-0.56) suggested that pollutant sources differed at the two sites. Biomass burning, resuspended dust, combustion, iron and steel industry, sea salt, secondary aerosol, and vehicle exhaust were identified at the DSL site by Positive Matrix Factorization (PMF). Secondary aerosol and vehicle exhaust accounted for 45.7% of PM_2.5 mass, followed 11.2%-13.7% each by industry, resuspended dust, and coal and oil combustion.