Fossil Fuel Combustion-Related Emissions Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from (15)N-Stable Isotope in Size-Resolved Aerosol Ammonium

Spatio-Temporal Variations of Multiple Primary Air Pollutants Emissions in Beijing of China, 2006–2015

Air pollution in Beijing, China has attracted continuous worldwide public attention along with the rapid urbanization of the city. By implementing a set of air pollution mitigation measures, the air quality of Beijing has been gradually improved in recent years. In this study, the intrinsic factors leading to air quality improvement in Beijing are studied via a quantitative evaluation of the temporal and spatial changes in emissions of primary air pollutants over the past ten years. Based on detailed activity levels of each economic sector and a localized database containing source and pollutant specific emission factors, an integrated emissions inventory of primary air pollutants discharged from various sources between 2006 and 2015 is established. With the implementation of phased air pollution mitigation measures, and the Clean Air Action Plan, the original coal-dominated energy structure in Beijing has undergone tremendous changes, resulting in the substantial reduction of multiple air pollutants. The total of emissions of six major atmospheric pollutants (PM10, PM2.5, SO2, NOX, VOCs and NH3) in Beijing decreased by 35% in 2015 compared to 2006—this noticeable decrease was well consistent with the declining trend of ambient concentration of criterion air pollutants (SO2, PM10, PM2.5 and NO2) and air quality improvement, thus showing a good correlation between the emission of air pollutants and the outcome of air quality. SO2 emission declined the most, at about 71.7%, which was related to the vigorous promotion of combustion source control, such as the shutdown of coal-fired facilities and domestic stoves and transition to clean energy, like natural gas or electricity. Emissions of PM decreased considerably (by 48%) due to energy structure optimization, industrial structure adjustments, and end-of-pipe PM source control. In general, NOX, NH3, and VOCs decreased relatively slightly, by 25%, 14%, and 2%, respectively, and accordingly, they represented the limiting factors for improving air quality and the key points of air pollution mitigation in Beijing for the future.

Meteorological and chemical impacts on PM2.5 during a haze episode in a heavily polluted basin city of eastern China

Haze formation involves many interacting factors, such as secondary aerosol formation, unfavourable synoptic conditions and regional transport. The interaction between these factors complicates scientific understanding of the mechanism behind haze formation. In this study, we investigated the factors resulting in haze events in Longyou, a city located in a basin in China. Aerosol samples of PM_2.5 were collected for subsequent chemical composition analysis between 11 January and 5 February 2018. The impacts of wind on PM_2.5, SO₂ and NO₂ concentrations were analysed. Besides, the origin of air parcels and potential sources of PM_2.5 were analysed by backward trajectory, potential source contribution function (PSCF) and concentration-weighted trajectories (CWT). Among the water-soluble ions identified, NO₃^- had the highest concentration, with further analysis demonstrating the haze evolution was mainly driven by the reactions involving NO₃^- formation. The dramatic increase of nitrate is mainly due to the homogeneous reaction of nitric acid with ammonia, while sulfate is likely due to heterogeneous reactions of NO₂, SO₂ and NH₃. The average wind speed was less than 2 m/s during the aerosol sampling period, which could be considered as a stagnant state. Pollutants emitted by industrial area located in the northeast Longyou were probably brought to observation sites by continuous wind from northeast and accumulated gradually. Air parcels originating from the northeast of Zhejiang province also had large effects on haze pollution in Longyou. Together, our results showed that rapid secondary aerosol formation and unfavourable synoptic conditions are the main factors resulting in haze pollution in Longyou.

Traffic-related dustfall and NOx, but not NH3, seriously affect nitrogen isotopic compositions in soil and plant tissues near the roadside

Ammonia (NH₃) emissions from traffic have received particular attention in recent years because of their important contributions to the growth of secondary aerosols and the negative effects on urban air quality. However, few studies have been performed on the impacts of traffic NH₃ emissions on adjacent soil and plants. Moreover, doubt remains over whether dry nitrogen (N) deposition still contributes a minor proportion of plant N nutrition compared with wet N deposition in urban road environments. This study investigated the δ¹⁵N values of road dustfall, soil, moss, camphor leaf and camphor bark samples collected along a distance gradient from the road, suggesting that samples collected near the road have significantly more positive δ¹⁵N values than those of remote sites. According to the SIAR model (Stable Isotope Analysis in R) applied to dustfall and moss samples from the roadside, it was found that NH₃ from traffic exhaust (8.8 ± 7.1%) contributed much less than traffic-derived NO₂ (52.2 ± 10.0%) and soil N (39.0 ± 13.8%) to dustfall bulk N; additionally, 68.6% and 31.4% of N in mosses near the roadside could be explained by dry N deposition (only 20.4 ± 12.5% for traffic-derived NH₃) and wet N deposition, respectively. A two-member mixing model was used to analyse the δ¹⁵N in continuously collected mature camphor leaf and camphor bark samples, which revealed a similarity of the δ¹⁵N values of plant-available deposited N to ¹⁵N-enriched traffic-derived NO_x-N. We concluded that a relatively high proportion of N inputs in urban road environments was contributed by traffic-related dustfall and NO_x rather than NH₃. These information provide useful insights into reducing the impacts of traffic exhaust on adjacent ecosystems and can assist policy makers in determining the reconstruction of a monitoring network for N deposition that reaches the road level.

Isotopic advances in understanding reactive nitrogen deposition and atmospheric processing

Recent advances in stable isotope measurements now allow for detailed investigations of the sources, transformations, and deposition of reactive nitrogen (N) species. Stable isotopes show promise as a complementary tool for apportioning emissions sources that contribute to deposition and also for developing a more robust understanding of the transformations that can influence these isotope ratios. Methodological advances have facilitated the unprecedented examination of the isotopic composition of reactive N species in the atmosphere and in precipitation including nitrogen oxides (NOx = nitric oxide (NO) + nitrogen dioxide (NO2)), atmospheric nitrate (NO3-), nitric acid (HNO3), ammonia (NH3), and ammonium (NH4+). This isotopic information provides new insight into the mechanisms of transformation and cycling of reactive N in the atmosphere and moreover helps resolve the contribution of multiple NOx and NH3 emission sources to deposition across landscapes, regions, and continents. Here, we highlight the current state of knowledge regarding the isotopic ratios of NOx and NH3 emission sources and chemical alterations of isotopic ratios during atmospheric transformations. We also highlight illustrative examples where isotopic approaches are used and review recent methodological advances. While these highlights are not an exhaustive review of the literature, we hope they provide a glimpse of the potential for these methods to help resolve knowledge gaps regarding total N deposition to Earth surfaces. We conclude with promising opportunities for future research in the short-, medium-, and long-term.

Seasonal pattern of ammonium 15N natural abundance in precipitation at a rural forested site and implications for NH3 source partitioning

Excess ammonia (NH₃) emissions and deposition can have negative effects on air quality and terrestrial ecosystems. Identifying NH₃ sources is a critical step for effectively reducing NH₃ emissions, which are generally unregulated around the world. Stable nitrogen isotopes (δ¹⁵N) of ammonium (NH₄⁺) in precipitation have been directly used to partition NH₃ sources. However, nitrogen isotope fractionation during atmospheric processes from NH₃ sources to sinks has been previously overlooked. Here we measured δ¹⁵NNH₄⁺ in precipitation on a daily basis at a rural forested site in Northeast China over three years to examine its seasonal pattern and attempt to constrain the NH₃ sources. We found that the NH₄⁺ concentrations in precipitation ranged from 5 to 1265 μM, and NH₄⁺ accounted for 65% of the inorganic nitrogen deposition (20.0 kg N ha^-1 yr^-1) over the study period. The δ¹⁵N values of NH₄⁺ fluctuated from -24.6 to +16.2‰ (average -6.5‰) and showed a repeatable seasonal pattern with higher values in summer (average -2.3‰) than in winter (average -16.4‰), which could not be explained by only the seasonal changes in the NH₃ sources. Our results suggest that in addition to the NH₃ sources, isotope equilibrium fractionation contributed to the seasonal pattern of δ¹⁵NNH₄⁺ in precipitation, and thus, nitrogen isotope fractionation should be considered when partitioning NH₃ sources based on δ¹⁵NNH₄⁺ in precipitation.

Source signatures from combined isotopic analyses of PM2.5 carbonaceous and nitrogen aerosols at the peri-urban Taehwa Research Forest, South Korea in summer and fall

Isotopes are essential tools to apportion major sources of aerosols. We measured the radiocarbon, stable carbon, and stable nitrogen isotopic composition of PM_2.5 at Taehwa Research Forest (TRF) near Seoul Metropolitan Area (SMA) during August-October 2014. PM_2.5, TC, and TN concentrations were 19.4 ± 10.1 μg m^-3, 2.6 ± 0.8 μg C m^-3, and 1.4 ± 1.4 μg N m^-3, respectively. The δ¹³C of TC and the δ¹⁵N of TN were - 25.4 ± 0.7‰ and 14.6 ± 3.8‰, respectively. EC was dominated by fossil-fuel sources with F_ff (EC) of 78 ± 7%. In contrast, contemporary sources were dominant for TC with F_c (TC) of 76 ± 7%, revealing the significant contribution of contemporary sources to OC during the growing season. The isotopic signature carries more detailed information on sources depending on air mass trajectories. The urban influence was dominant under stagnant condition, which was in reasonable agreement with the estimated δ¹⁵N of NH₄⁺. The low δ¹⁵N (7.0 ± 0.2‰) with high TN concentration was apparent in air masses from Shandong province, indicating fossil fuel combustion as major emission source. In contrast, the high δ¹⁵N (16.1 ± 3.2‰) with enhanced TC/TN ratio reveals the impact of biomass burning in the air transported from the far eastern border region of China and Russia. Our findings highlight that the multi-isotopic composition is a useful tool to identify emission sources and to trace regional sources of carbonaceous and nitrogen aerosols.

Inorganic ion chemistry of local particulate matter in a populated city of North China at light, medium, and severe pollution levels

Twenty-six pairs of PM_2.5 and PM₁₀ samples were collected during haze episodes in Zhengzhou (113°28' E, 34°37' N), a highly populated city in North China. The samples were used to examine the inorganic ion chemistry of particulate matter (PM) of local origin at light (PM_2.5 < 60 μg m^-3 and PM₁₀ < 135 μg m^-3), medium (PM_2.5: 60-170 μg m^-3 and PM₁₀: 135-325 μg m^-3), and severe (PM_2.5 > 170 μg m^-3 and PM₁₀ > 325 μg m^-3) pollution levels. At the light and severe pollution levels, the increase of PM₁₀ was accounted for by the increase of PM_2.5, and the variation of PM_10-2.5 was small. In contrast, the increase of PM₁₀ at the medium pollution level was caused by the increase in both PM_2.5 and PM_10-2.5. Sulfate (SO₄^2-), nitrate (NO₃^-), ammonium (NH₄⁺), and chloride in the form of ammonium chloride (Cl^-_S) accounted for 47.8% and 60.3% of the PM_2.5 mass at the light and severe levels, respectively. These values indicate a large contribution of secondary inorganic species to the PM_2.5 growth. As the pollution level changed from light to medium, the contribution of SO₄^2- to the growth of PM_2.5 decreased from 49.0% to 15.1%, while those of NO₃^- and Cl^-_S increased from 25.1% and 0.6% to 32.5% and 2.8%, respectively, indicating the substantial production of nitrate and chloride. At the severe level, the contribution of SO₄^2- was 30.1%, while those of NO₃^- and Cl^-_S were 5.9% and 0.5%, respectively, suggesting a hindering effect of sulfate on the production of nitrate and chloride. These results indicate that the production of secondary species with the increase of PM_2.5 was dominated by sulfate-associated conversions at the light and severe pollution levels and was substantially influenced by nitrate- and chloride-associated conversions at the medium pollution level. The estimation of carbonate presence in the PM indicates that part of the carbonate in coarse particles (PM_10-2.5) of crustal origin enhanced sulfate production via heterogeneous surface reactions. Quantification of the contribution of primary and secondary species to PM_2.5 showed that it was dominated by both primary and secondary particles at the light pollution level, and it was mainly composed of secondary species at the severe pollution level.

Ammonia Emission Measurements for Light-Duty Gasoline Vehicles in China and Implications for Emission Modeling

Motor vehicle ammonia (NH₃) emissions have attracted increasing attention for their potential to form secondary aerosols in urban atmospheres. However, vehicle NH₃ emission factors (EFs) remain largely unknown due to a lack of measurements. Thus, we conducted detailed measurements of NH₃ emissions from 18 Euro 2 to Euro 5 light-duty gasoline vehicles (LDGVs) in Shanghai, China. The distance- and fuel-based NH₃ EFs average 29.2 ± 24.1 mg·km^-1 and 0.49 ± 0.41 g·kg^-1, respectively. The average NH₃-to-CO₂ ratio is 0.41 ± 0.34 ppbv·ppmv^-1. The measurements reveal that NH₃ emissions from LDGVs are strongly correlated with both vehicle specific power (VSP) and the modified combustion efficiency (MCE); these relationships were used to predict LDGV NH₃ EFs via a newly developed model. The predicted LDGV NH₃ EFs under urban and highway driving cycles are 23.3 mg·km^-1 and 84.5 mg·km^-1, respectively, which are consistent with field measurements. The NH₃ EF has decreased by 32% in average since the implementation of vehicle emission control policies in China five years ago. The model presented herein more accurately predicts LDGV NH₃ emissions, contributing substantially to the compilation of NH₃ emission inventories and prediction of future motor vehicle emissions in China.

Fast screening compositions of PM2.5 by ATR-FTIR: Comparison with results from IC andOC/EC analyzers

Chemical speciation of fine particles or PM_2.5 collected on filters is still a costly and time-consuming task. In this study, filter-based PM_2.5 samples were collected during November-December 2013 at four sites in Guangzhou, and the major components were fast screened (~7min per filter sample) by Attenuated Total Reflectance (ATR)-Fourier Transform Infrared Spectroscopic (FTIR) in comparison with that measured by Organic carbon/Element carbon (OC/EC) analyzer and Ion Chromatography (IC). The concentrations of nitrate, ammonium, sulfate, primary organic carbon (POC) and secondary organic carbon (SOC) measured by OC/EC and IC analyzers were better correlated with their infrared absorption peak heights at 1320cm^-1 for nitrate, 1435, 3045 and 3215cm^-1 for ammonium, 615cm^-1 for sulfate, 690, 760 and 890cm^-1 for POC and 1640 and 1660cm^-1 for SOC respectively, during polluted days (PM_2.5>75μg/m³) than during clean days (PM_2.5≤75μg/m³). With the evolution of a haze episode during our field campaign, the concentrations of the major PM_2.5 components displayed consistent variations with their infrared absorption peak heights, suggesting ATR-FTIR could be a fast and useful technique to characterize filter-based PM_2.5 compositions particularly during pollution events although cautions should be taken when PM_2.5 levels are low. Notably, elevated PM_2.5 mass concentrations occurred with enhanced ratios of [NO₃^-]/[SO₄^2-] and [NH₄⁺]/[SO₄^2-], implying that nitrogenous components play vital roles in the PM_2.5 pollution events in the study region.

Isotopic evidence for enhanced fossil fuel sources of aerosol ammonium in the urban atmosphere

The sources of aerosol ammonium (NH₄⁺) are of interest because of the potential of NH₄⁺ to impact the Earth's radiative balance, as well as human health and biological diversity. Isotopic source apportionment of aerosol NH₄⁺ is challenging in the urban atmosphere, which has excess ammonia (NH₃) and where nitrogen isotopic fractionation commonly occurs. Based on year-round isotopic measurements in urban Beijing, we show the source dependence of the isotopic abundance of aerosol NH₄⁺, with isotopically light (-33.8‰) and heavy (0 to +12.0‰) NH₄⁺ associated with strong northerly winds and sustained southerly winds, respectively. On an annual basis, 37-52% of the initial NH₃ concentrations in urban Beijing arises from fossil fuel emissions, which are episodically enhanced by air mass stagnation preceding the passage of cold fronts. These results provide strong evidence for the contribution of non-agricultural sources to NH₃ in urban regions and suggest that priority should be given to controlling these emissions for haze regulation. This study presents a carefully executed application of existing stable nitrogen isotope measurement and mass-balance techniques to a very important problem: understanding source contributions to atmospheric NH₃ in Beijing. This question is crucial to informing environmental policy on reducing particulate matter concentrations, which are some of the highest in the world. However, the isotopic source attribution results presented here still involve a number of uncertain assumptions and they are limited by the incomplete set of chemical and isotopic measurements of gas NH₃ and aerosol NH₄⁺. Further field work and lab experiments are required to adequately characterize endmember isotopic signatures and the subsequent isotopic fractionation process under different air pollution and meteorological conditions.

Identifying Ammonia Hotspots in China Using a National Observation Network

The limited availability of ammonia (NH₃) measurements is currently a barrier to understanding the vital role of NH₃ in secondary aerosol formation during haze pollution events and prevents a full assessment of the atmospheric deposition of reactive nitrogen. The observational gaps motivated us to design this study to investigate the spatial distributions and seasonal variations in atmospheric NH₃ on a national scale in China. On the basis of a 1-year observational campaign at 53 sites with uniform protocols, we confirm that abundant concentrations of NH₃ [1 to 23.9 μg m^-3] were identified in typical agricultural regions, especially over the North China Plain (NCP). The spatial pattern of the NH₃ surface concentration was generally similar to those of the satellite column concentrations as well as a bottom-up agriculture NH₃ emission inventory. However, the observed NH₃ concentrations at urban and desert sites were comparable with those from agricultural sites and 2-3 times those of mountainous/forest/grassland/waterbody sites. We also found that NH₃ deposition fluxes at urban sites account for only half of the emissions in the NCP, suggesting the transport of urban NH₃ emissions to downwind areas. This finding provides policy makers with insights into the potential mitigation of nonagricultural NH₃ sources in developed regions.

Wet-only deposition of atmospheric inorganic nitrogen and associated isotopic characteristics in a typical mountain area, southwestern China

To quantify and compare atmospheric nitrogen (N) deposition and its N isotopic ratio are critical for constraining N sources and effective reduction of reactive N emissions. In this study, a total of 223 rainwater samples were collected by wet-only auto-samplers, and wet-only deposition and isotopic composition (δ¹⁵N) of reduced (NH₄⁺-N) and oxidized (NO₃^--N) N were measured at three typical mountain sites, including an urban (Wanzhou, WZ), a town (Gaoyang, GY) and a rural (Dade, DD) site in Chongqing, southwestern China in 2016. The wet-only inorganic N deposition (DIN, sum of NO₃^--N and NH₄⁺-N) were 17.50, 8.63 and 12.16kgNha^-1yr^-1 at WZ, GY and DD site, respectively. Annual δ¹⁵N-NH₄⁺ values of rainwaters were negative at the urban site (-3.12±3.21‰, WZ) and positive at both town and rural site (0.65±12.51‰, GY; 2.16±6.11‰, DD). Annual δ¹⁵N-NO₃^- values, on the contrary, were positive at the urban site (0.33±7.87‰, WZ) and negative at both town and rural site (-5.59±2.20‰, GY; -0.39±8.89‰, DD). These results reveal the urban site was wet-only DIN hotspot and had a different N source compared with the town-rural site in the mountain area. Moreover, precipitation DIN had a potentially negative risk on both aquatic and forest ecosystems.

Wet nitrogen deposition across the urban-intensive agricultural-rural transect of a small urban area in southwest China

Understanding of the spatial and temporal variation of the flux of atmospheric nitrogen (N) deposition is essential for assessment of its impact on ecosystems. However, little attention has been paid to the variability of N deposition across urban-intensive agricultural-rural transects. A continuous 2-year observational study (from January 2015 to December 2016) was conducted to determine wet N deposition across the urban-intensive agricultural-rural transect of a small urban area in southwest China. Significantly spatial and temporal variations were found in the research area. Along the urban-intensive agricultural-rural transect, the TN and NH₄⁺-N deposition first increased and then decreased, and the NO₃^--N and dissolved organic N (DON) deposition decreased continuously. Wet N deposition was mainly affected by the districts of agro-facilities, roads and build up lands. Wet NH₄⁺-N deposition had non-seasonal emission sources including industrial emissions and urban excretory wastes in urban districts and seasonal emission sources such as fertilizer and manure volatilization in the other districts. However, wet NO₃^--N deposition had seasonal emission sources such as industrial emissions and fireworks in urban district and non-seasonal emission sources such as transportation in the other districts. Deposition of DON was likely to have had similar sources to NO₃^--N deposition in rural district, and high-temperature-dependent sources in urban and intensive agricultural districts. Considering the annual wet TN deposition in the intensive agricultural district was about 11.1% of the annual N fertilizer input, N fertilizer rates of crops should be reduced in this region to avoid the excessive application, and the risk of N emissions to the environment.

Wet deposition and sources of inorganic nitrogen in the Three Gorges Reservoir Region, China

Precipitation samples were collected at five rural and one urban sites in the Three Gorges Reservoir Region (TGR), China from March 2014 to February 2016. The inorganic reactive nitrogen (Nr) contents were analysed to investigate their wet deposition flux, budget, and sources in the area. Annual Nr wet deposition varied from 7.1 to 23.4 kg N ha^-1 yr^-1 over the six sites during the two-year study campaign. The six-site average Nr wet deposition flux was 17.1 and 11.7 kg N ha^-1 yr^-1 in 2014 and 2015, respectively, with 71% from NH₄⁺ and 29% from NO₃^-. Dry deposition flux was estimated using the inferential method, which combined the measured ambient concentrations and modelled dry deposition velocities. The total (dry + wet) Nr deposition fluxes were estimated to be 21.4 kg N ha^-1 yr^-1 in 2014 and 16.0 kg N ha^-1 yr^-1 in 2015 at rural sites, and 31.4 and 25.3 kg N ha^-1 yr^-1 at the urban site. Annual average volume weighted mean (VWM) concentrations in precipitation at all the six sites differed little for NO₃^- but up to a factor of 2.0 for NH₄⁺ with the highest value at the urban site. Industrial emissions, agricultural emissions, soil dust, and biomass burning were identified as potential sources of the major inorganic ions in precipitation using factor analysis and correlation analysis. Conditional probability function (CPF) analysis indicated that the urban site was predominantly affected by industrial emissions from a power plant, cement manufactory, and salt chemical facility located ∼13 km southeast of the sampling site.

Influence of flue gas desulfurization (FGD) installations on emission characteristics of PM2.5 from coal-fired power plants equipped with selective catalytic reduction (SCR)

Flue gas desulfurization (FGD) and selective catalytic reduction (SCR) technologies have been widely used to control the emissions of sulphur dioxide (SO₂) and nitrogen oxides (NO_X) from coal-fired power plants (CFPPs). Field measurements of emission characteristics of four conventional CFPPs indicated a significant increase in particulate ionic species, increasing PM_2.5 emission with FGD and SCR installations. The mean concentrations of PM_2.5 from all CFPPs tested were 3.79 ± 1.37 mg/m³ and 5.02 ± 1.73 mg/m³ at the FGD inlet and outlet, respectively, and the corresponding contributions of ionic species were 19.1 ± 7.7% and 38.2 ± 7.8%, respectively. The FGD was found to enhance the conversion of NH₃ slip from the SCR to NH₄⁺ in the PM_2.5, together with the conversion of SO₂ to SO₄^2-, and increased the primary NH₄⁺ and SO₄^2- aerosol emissions by approximately 18.9 and 4.2 times, respectively. This adverse effect should be considered when updating the emission inventory of CFPPs and should draw the attention of policy-makers for future air pollution control.

Combining Positive Matrix Factorization and Radiocarbon Measurements for Source Apportionment of PM2.5 from a National Background Site in North China

To explore the utility of combining positive matrix factorization (PMF) with radiocarbon (¹⁴C) measurements for source apportionment, we applied PM_2.5 data collected for 14 months at a national background station in North China to PMF models. The solutions were compared to ¹⁴C results of four seasonally averaged samples and three outlier samples. Comparing the most readily interpretable PMF solutions and ¹⁴C results revealed that PMF modeling was well able to capture the source patterns of PM_2.5 with two and three irrelevant source classifications for the seasonal and outlier samples. The contribution of sources that could not be classified as either fossil or non-fossil sources in the PMF solution, and the errors between the modeled and measured concentrations weakened the effectiveness of the comparison. Based on these two factors, we developed an index for selecting the most suitable ¹⁴C measurement samples for combining with the PMF model. Then we examined the potential for coupling PMF modeling and ¹⁴C data with a constrained PMF run using the ¹⁴C data as a priori information. The restricted run could provide a more reliable solution; however, the PMF model must provide a flexible dialog to input the priori restrictions for executing the constraint simulation.

Contributions of local and regional anthropogenic sources of metals in PM2.5 at an urban site in northern France

PM_2.5 have been related to various adverse health effects, mainly due to their ability to penetrate deeply and to convey harmful chemical components, such as metals inside the body. In this work, PM_2.5 were sampled at Saint-Omer, a medium-sized city located in northern France, in March-April 2011 and analyzed for their total carbon, water-soluble ions, major and trace elements. More specifically, the origin of 15 selected elements was examined using different tools including enrichment factors, conditional bivariate probability function (CBPF) representations, diagnostic ratios and receptor modelling. The results indicated that PM_2.5 metal composition is affected by both emissions of a local glassmaking factory and an integrated steelworks located at a distance of 35 km from the sampling site. For the first time, diagnostic ratios were proposed for the glassmaking activity. Therefore, metals in PM_2.5 could be attributed to the following anthropogenic sources: (i) local glassmaking industry for Sn, As, Cu and Cr, (ii) distant integrated steelworks for Ag, Fe, Cd, Mn, Rb and Pb, (iii) heavy fuel oil combustion for Ni, V and Co and (iv) non-exhaust traffic for Zn, Pb, Mn, Sb, and Cu. The impact of such sources on metal concentrations in PM_2.5 was assessed using a constrained receptor model. Despite their low participation to PM_2.5 concentration (2.7%), the latter sources were found as the main contributors (80%) to the overall concentration levels of the 15 selected elements in PM_2.5.

Multiple Sulfur Isotope Constraints on Sources and Formation Processes of Sulfate in Beijing PM2.5 Aerosol

Recently air pollution is seriously threatening the health of millions of people in China. The multiple sulfur isotopic composition of sulfate in PM_2.5 samples collected in Beijing is used to better constrain potential sources and formation processes of sulfate aerosol. The Δ³³S values of sulfate in PM_2.5 show a pronounced seasonality with positive values in spring, summer and autumn and negative values in winter. Positive Δ³³S anomalies are interpreted to result from SO₂ photolysis with self-shielding, and may reflect air mass transport between the troposphere and the stratosphere. The negative Δ³³S signature (-0.300‰ < Δ³³S < 0‰) in winter is possibly related to incomplete combustion of coal in residential stoves during the heating season, implying that sulfur dioxide released from residential stoves in more rural areas is an important contributor to atmospheric sulfate. However, negative Δ³³S anomalies (-0.664‰ < Δ³³S ← 0.300‰) in winter and positive Δ³³S anomalies (0.300‰ < Δ³³S < 0.480‰) in spring, summer, and autumn suggest sulfur isotopic equilibrium on an annual time frame, which may provide an implication for the absence of mass-independent fractionation of sulfur isotopes (S-MIF) in younger sediments. Results obtained here reveal that reducing the usage of coal and improving the heating system in rural areas will be important for efficiently decreasing the emissions of sulfur in China and beyond.

Temporal and spatial variation in major ion chemistry and source identification of secondary inorganic aerosols in Northern Zhejiang Province, China

To investigate the seasonal and spatial variations of ion chemistry of fine particles in Northern Zhejiang Province (NZP), China, one year-long field sampling was conducted at four representative sites (two urban, one suburb, and one rural sites) in both cities of Hangzhou and Ningbo from December 2014 to November 2015. Twelve water soluble inorganic ions (WSII) were characterized in this comprehensive study. The annual average of PM_2.5 concentration in NZP as overall was 66.2 ± 37.7 μg m^-3, and urban sites in NZP were observed with more severe PM_2.5 pollution than the suburban and rural sites. The annual average concentration of total WSII at four sampling sites in NZP was 29.1 ± 19.9 μg m^-3, dominated by SO₄^2- (10.3 μg m^-3), and followed by NO₃^- (8.9 μg m^-3), NH₄⁺ (6.6 μg m^-3), Cl^- (1.3 μg m^-3) and K⁺ (0.7 μg m^-3). Among all cations, NH₄⁺ was the predominant neutralizing ion with the highest neutralization factor (NF), while the remaining cations showed limited neutralization capacity. The highest and lowest sulfur oxidation ratio (SOR) values in this region were found in summer and winter, respectively; while the seasonal patterns for nitrogen oxidation ratio (NOR) were opposite to that of SOR. Principal component analysis (PCA) showed that the significant sources of WSII in NZP were industrial emissions, biomass burning, and formation of secondary inorganic aerosols. In addition, contribution from transboundary transport of polluted aerosols was also confirmed from the assessment through air mass backward trajectory analysis.

First Assessment of NOx Sources at a Regional Background Site in North China Using Isotopic Analysis Linked with Modeling

Nitrogen oxides (NO_x, including NO and NO₂) play an important role in the formation of atmospheric particles. Thus, NO_x emission reduction is critical for improving air quality, especially in severely air-polluted regions (e.g., North China). In this study, the source of NO_x was investigated by the isotopic composition (δ¹⁵N) of particulate nitrate (p-NO₃^-) at Beihuangcheng Island (BH), a regional background site in North China. It was found that the δ¹⁵N-NO₃^- (n = 120) values varied between -1.7‰ and +24.0‰ and the δ¹⁸O-NO₃^- values ranged from 49.4‰ to 103.9‰. On the basis of the Bayesian mixing model, 27.78 ± 8.89%, 36.53 ± 6.66%, 22.01 ± 6.92%, and 13.68 ± 3.16% of annual NO_x could be attributed to biomass burning, coal combustion, mobile sources, and biogenic soil emissions, respectively. Seasonally, the four sources were similar in spring and fall. Biogenic soil emissions were augmented in summer in association with the hot and rainy weather. Coal combustion increased significantly in winter with other sources showing an obvious decline. This study confirmed that isotope-modeling by δ¹⁵N-NO₃^- is a promising tool for partitioning NO_x sources and provides guidance to policymakers with regard to options for NO_x reduction in North China.

Improvement of a Global High-Resolution Ammonia Emission Inventory for Combustion and Industrial Sources with New Data from the Residential and Transportation Sectors

There is increasing evidence indicating the critical role of ammonia (NH₃) in the formation of secondary aerosols. Therefore, high quality NH₃ emission inventory is important for modeling particulate matter in the atmosphere. Unfortunately, without directly measured emission factors (EFs) in developing countries, using data from developed countries could result in an underestimation of these emissions. A series of newly reported EFs for China provide an opportunity to update the NH₃ emission inventory. In addition, a recently released fuel consumption data product has allowed for a multisource, high-resolution inventory to be assembled. In this study, an improved global NH₃ emission inventory for combustion and industrial sources with high sectorial (70 sources), spatial (0.1° × 0.1°), and temporal (monthly) resolutions was compiled for the years 1960 to 2013. The estimated emissions from transportation (1.59 Tg) sectors in 2010 was 2.2 times higher than those of previous reports. The spatial variation of the emissions was associated with population, gross domestic production, and temperature. Unlike other major air pollutants, NH₃ emissions continue to increase, even in developed countries, which is likely caused by an increased use of biomass fuel in the residential sector. The emissions density of NH₃ in urban areas is an order of magnitude higher than in rural areas.

Long-term trend of chemical composition of atmospheric precipitation at a regional background station in Northern China

Understanding the trend of chemical composition of precipitation is of great importance for air pollution control strategies in Northern China. A comprehensive study on the long-term chemical compositions of precipitation was carried out from 2003 to 2014 at the Shangdianzi (SDZ) regional background station in northern China. All samples were analyzed for pH, electrical conductivity and major ions (F^-, Cl^-, NO₃^-, SO₄^2-, NH₄⁺, Mg²⁺, Ca²⁺, K⁺ and Na⁺). The average pH during this period was 4.53±0.35, which is considerably lower than those reported in other background stations in China (Linan, Waliguan and Longfengshan). NH₄⁺, SO₄^2-, Ca²⁺ and NO₃^- were the dominant ions in precipitation, with concentrations (volume-weighted mean) of 212.99μeqL^-1, 200.20μeqL^-1, 116.88μeqL^-1 and 98.56μeqL^-1, respectively. The ion concentrations at SDZ were much higher than those of other background stations and megacities in China. A significantly increasing trend was observed for NO₃^- (7.26%year^-1), and a decreasing trend was observed for SO₄^2-/NO₃^-, suggesting that the precipitation of SDZ evolved from a sulfuric acid type to a mixed type dominated by both sulfuric and nitric acid. The source identification indicated that SO₄^2-, NO₃^-, NH₄⁺ and F^- were dominated by secondary sources, Mg²⁺, Ca²⁺ and Na⁺ mostly originated from natural sources, and K⁺ and Cl^- probably associated with anthropogenic sources. Long-range transport of air masses could influence the acidity, electrical conductivity and ion concentrations of precipitation at SDZ. The higher acidity and ion concentrations mainly occurred in the southerly and westerly trajectory pathways and partially in northwest pathways. Anthropogenic pollutants and crustal sources along these pathways were significant contributors to the chemical composition of precipitation in SDZ.

Ambient concentration and dry deposition of major inorganic nitrogen species at two urban sites in Sichuan Basin, China

To assess pollution levels of major inorganic nitrogen species and their atmospheric deposition input to sensitive ecosystems in Sichuan Basin, southwest China, ambient concentrations of oxidized (NO_y ∼ NO₂, HNO₃, NO₃^-) and reduced (NH_x = NH₃, NH₄⁺) nitrogen species were collected at two urban sites during four one-month periods, each in a different season from July 2014 to April 2015. Estimated annual mean concentration of NO_y was 20.3 and 13.5 μg N m^-3 in Chengdu and Wanzhou, respectively, and NH_x was 16.9 and 13.6 μg N m^-3, respectively. Back trajectory cluster analysis indicated that high levels of NO_y and NH_x in Chengdu were mainly caused by local emissions while those in Wanzhou were caused by both the local emissions and long-range transport of pollutants. On annual basis, NO₂ contributed the most to NO_y, followed by NO₃^- and HNO₃, accounting for 87.5%, 10.5% and 2.0%, respectively, of NO_y in Chengdu, and 91.4%, 6.9% and 1.7%, respectively, in Wanzhou. NH₃ was the predominant contributor to NH_x, contributing 65.6% and 72.2% in Chengdu and Wanzhou, respectively. Dry deposition fluxes were estimated using the inferential method with measured ambient concentrations and modelled dry deposition velocities. The total inorganic nitrogen dry deposition flux was estimated to be 21.4 and 8.5 kg N ha^-1 yr^-1, with 44.3% and 41.4% from NO_y in Chengdu and Wanzhou, respectively. NO₂ and NH₃ each contributed about 80% of NO_y and NH_x dry deposition, respectively. Wet deposition was only collected in Wanzhou, where the annual wet deposition of NO₃^- and NH₄⁺ was 4.5 and 15.7 kg N ha^-1 yr^-1, respectively. The total wet plus dry deposition was 28.7 kg N ha^-1 yr^-1 in Wanzhou with 72.2% from reduced nitrogen. Therefore, controlling NH₃ emissions from agricultural, traffic, waste containers and sewage system sources would be effective to reduce the total nitrogen deposition in the Sichuan Basin area.

Persistent sulfate formation from London Fog to Chinese haze

Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atmospheric models consistently underpredict sulfate levels under diverse environmental conditions. From atmospheric measurements in two Chinese megacities and complementary laboratory experiments, we show that the aqueous oxidation of SO₂ by NO₂ is key to efficient sulfate formation but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH₃ neutralization or under cloud conditions. Under polluted environments, this SO₂ oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH₃ and NO₂ control measures. In addition to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate production mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.

Airborne particulate matter pollution in urban China: a chemical mixture perspective from sources to impacts

Rapid urban and industrial development has resulted in severe air-pollution problems in developing countries such as China, especially in highly industrialized and populous urban clusters. Dissecting the complex mixtures of airborne particulate matter (PM) has been a key scientific focus in the last two decades, leading to significant advances in understanding physicochemical compositions for comprehensive source apportionment. However, identifying causative components with an attributable link to population-based health outcomes remains a huge challenge. The microbiome, an integral dimension of the PM mixture, is an unexplored frontier in terms of identities and functions in atmospheric processes and human health. In this review, we identify the major gaps in addressing these issues, and recommend a holistic framework for evaluating the sources, processes and impacts of atmospheric PM pollution. Such an approach and the knowledge generated will facilitate the formulation of regulatory measures to control PM pollution in China and elsewhere.