Fossil and Nonfossil Sources of Winter Organic Aerosols in the Regional Background Atmosphere of China.

Carbonaceous aerosols (CA) from anthropogenic emissions have been significantly reduced in urban China in recent years. However, the relative contributions of fossil and nonfossil sources to CA in rural and background regions of China remain unclear. In this study, the sources of different carbonaceous fractions in fine aerosols (PM2.5) from five background sites of the China Meteorological Administration Atmosphere Watch Network during the winter of 2019 and 2020 were quantified using radiocarbon (14C) and organic markers. The results showed that nonfossil sources contributed 44-69% to total carbon at these five background sites. Fossil fuel combustion was the predominant source of elemental carbon at all sites (73 ± 12%). Nonfossil sources dominated organic carbon (OC) in these background regions (61 ± 13%), with biomass burning or biogenic-derived secondary organic carbon (SOC) as the most important contributors. However, the relative fossil fuel source to OC in China (39 ± 13%) still exceeds those at other regional/background sites in Asia, Europe, and the USA. SOC dominated the fossil fuel-derived OC, highlighting the impact of regional transport from anthropogenic sources on background aerosol levels. It is therefore imperative to develop and implement aerosol reduction policies and technologies tailored to both the anthropogenic and biogenic emissions to mitigate the environmental and health risks of aerosol pollution across China.

Bayesian Inference Approach to Quantify Primary and Secondary Organic Carbon in Fine Particulate Matter Using Major Species Measurements.

The determination of primary organic carbon (POC) and secondary organic carbon (SOC) in fine particulate matter using ambient measurements is essential in atmospheric chemistry. A novel Bayesian inference (BI) approach is proposed to achieve such quantification using only major component measurement data and tested in two case studies. One case study composes of filter-based daily compositional data made in the Pearl River Delta region, China, during 2012, while the other uses online measurement data recorded at the Dianshan Lake monitoring site in Shanghai in wintertime 2019. Source-specific organic trace measurement data are available in both the cases so that positive matrix factorization (PMF) analysis is performed, where PMF-resolved POC and SOC are used as the best available reference values for model evaluation. Meanwhile, traditional techniques, i.e., minimum ratio value, minimum R squared, and multiple linear regression, are also employed and evaluated. For both the cases, the BI models have shown significant advantages in accurately estimating POC and SOC amounts over conventional methods. Further analysis suggests that using sulfate as the SOC tracer in BI model gives the best model performance. This methodological advance provides an improved and practical tool to derive POC and SOC levels for addressing PM-related environmental impacts.

Comparing sources of carbonaceous aerosols during haze and nonhaze periods in two northern Chinese cities.

Radiocarbon (14C), stable carbon isotope (13C), and levoglucosan in PM2.5 were measured in two northern Chinese cities during haze events and nonhaze periods in January 2019, to ascertain the sources and their differences in carbonaceous aerosols between the two periods. The contribution of primary vehicle emissions (17.8 ± 3.7%) to total carbon in Beijing during that haze event was higher than that of primary coal combustion (7.3 ± 4.2%), and it increased significantly (7.1%) compared to the nonhaze period. The contribution of primary vehicle emissions (4.1 ± 2.8%) was close to that of primary coal combustion (4.3 ± 3.3%) during the haze event in Xi'an, and the contribution of primary vehicle emissions decreased by 5.8% compared to the nonhaze period. Primary biomass burning contributed 21.1 ± 10.5% during the haze event in Beijing and 40.9 ± 6.6% in Xi'an (with an increase of 3.3% compared with the nonhaze period). The contribution of secondary fossil fuel sources to total secondary organic carbon increased by 29.2% during the haze event in Beijing and by 18.4% in Xi'an compared to the nonhaze period. These results indicate that specific management measures for air pollution need to be strengthened in different Chinese cities in the future, that is, controlling vehicle emissions in Beijing and restricting the use of coal and biomass fuels in winter in Xi'an.

Source apportionment of carbonaceous aerosols using hourly data and implications for reducing PM2.5 in the Pearl River Delta region of South China.

Ambient fine particulate matter (PM2.5) levels in South China have been decreasing in the past decade, but the decreasing rates differed between its major chemical components, e.g., with much small rates for carbonaceous aerosols than for secondary inorganic aerosols. To investigate the sources of carbonaceous aerosols in this region, a comprehensive campaign was carried out in urban Guangzhou in the winter of 2019-2020 using a combination of various instruments. Data generated from this campaign include hourly total carbon (TC), black carbon (BC), criteria air pollutants and meteorological parameters, 4-hourly particle-bound elements, and chemically-resolved daily PM2.5. Similar diurnal patterns were observed for TC, CO and NO2, suggesting TC was very likely related to vehicle exhaust emission. Secondary organic carbon (SOC) estimated using the Minimum R squared (MRS) method accounted for 35 ± 17% of OC, indicating strong atmospheric oxidation capacity. Four major source factors for carbonaceous aerosols were identified by positive matrix factorization (PMF) model, including coal combustion, traffic emissions, soil dust and ship emissions, which accounted for 37 ± 23%, 39 ± 23%, 14 ± 10% and 10 ± 13%, respectively, of TC mass concentration, 38 ± 24%, 38 ± 23%, 14 ± 10% and 10 ± 12%, respectively, of OC mass concentration, and 29 ± 21%, 43 ± 22%, 14 ± 11% and 14 ± 15%, respectively, of EC mass concentration. Among these sources, traffic emission was the most important one, suggesting the necessity for promoting clean energy vehicles and relieving urban traffic congestion.

Assessment of carbonaceous aerosols in suburban Nanjing under air pollution control measures: Insights from long-term measurements.

The concentrations of organic carbon (OC) and elemental carbon (EC) in fine particulate matter (PM2.5) were analyzed using a semicontinuous carbon analyzer to characterize their carbonaceous components at the Nanjing University site from August 2013 to December 2018. OC was divided by the minimum R squared (MRS) method into primary organic carbon (POC) and secondary organic carbon (SOC). The results showed that annual mean POC and EC concentrations declined from 10.00 to 3.62 µg m-3 and from 6.73 to 3.40 µg m-3, respectively, during 2013-2018. The apparent reduction in POC and EC concentrations indicated that the implementation of air pollution control measures helped reduce carbonaceous aerosol pollution. Higher concentrations of POC and EC were recorded during the cold season and lower in the warm season. The annual mean SOC concentrations varied between 4.35 and 3.18 µg m-3 from 2013 to 2018. Elevated SOC was observed during the warm season, most likely attributable to the enhanced photochemical activity at high temperatures. Regarding the diurnal variation, the high concentrations of POC and EC were observed at night and in the morning due to stronger primary emissions and accumulations of pollutants with low boundary-layer heights, while the peak of SOC was observed at approximately noon due to the increases in photochemical activity. Nonparametric wind regression analysis showed the higher concentrations of POC, SOC and EC in the northwesterly, southwesterly to southeasterly, and southwesterly winds with high speeds. Concentration-weighted trajectory (CWT) analysis suggests that the areas with potentially high contributions to POC and EC changed from the north to the western areas of China, and that northern China played an increasingly important role in the SOC concentration of Nanjing. These results demonstrate that controlling emissions from the western and the northern areas in China may further alleviate carbonaceous aerosol pollution in Nanjing.

High rise in carbonaceous aerosols under very low anthropogenic emissions over eastern Himalaya, India: Impact of lockdown for COVID-19 outbreak.

The present study has been conducted to investigate the relative changes of carbonaceous aerosols (CA) over a high altitude Himalayan atmosphere with and without (very low) anthropogenic emissions. Measurements of atmospheric organic (OC) and elemental carbon (EC) were conducted during the lockdown period (April 2020) due to global COVID 19 outbreak and compared with the normal period (April 2019). The interesting, unexpected and surprising observation is that OC, EC and the total CA (TCA) during the lockdown (OC: 12.1 ± 5.5 µg m-3; EC: 2.2 ± 1.1 µg m-3; TCA: 21.5 ± 10 µg m-3) were higher than the normal period (OC: 7.04 ± 2.2 µg m-3; EC: 1.9 ± 0.7 µg m-3; TCA: 13.2 ± 4.1 µg m-3). The higher values for OC/EC ratio too was observed during the lockdown (5.7 ± 0.9) compared to the normal period (4.2 ± 1.1). Much higher surface O3 during the lockdown (due to very low NO) could better promote the formation of secondary OC (SOC) through the photochemical oxidation of biogenic volatile organic compounds (BVOCs) emitted from Himalayan coniferous forest cover. SOC during the lockdown (7.6 ± 3.5 µg m-3) was double of that in normal period (3.8 ± 1.4 µg m-3). Regression analysis between SOC and O3 showed that with the same amount of increase in O3, the SOC formation increased to a larger extent when anthropogenic emissions were very low and biogenic emissions dominate (lockdown) compared to when anthropogenic emissions were high (normal). Concentration weighted trajectory (CWT) analysis showed that the anthropogenic activities over Nepal and forest fire over north-east India were the major long-distant sources of the CA over Darjeeling during the normal period. On the other hand, during lockdown, the major source regions of CA over Darjeeling were regional/local. The findings of the study indicate the immense importance of Himalayan biosphere as a major source of organic carbon.

Chemical characteristics and source apportionment of PM2.5 between heavily polluted days and other days in Zhengzhou, China.

PM2.5 samples were collected in Zhengzhou during 3years of observation, and chemical characteristics and source contribution were analyzed. Approximately 96% of the daily PM2.5 concentrations and annual average values exceeded the Chinese National Ambient Air Quality Daily and Annual Standards, indicating serious PM2.5 pollution. The average concentration of water-soluble inorganic ions was 2.4 times higher in heavily polluted days (daily PM2.5 concentrations>250µg/m3 and visibility <3km) than that in other days, with sulfate, nitrate, and ammonium as major ions. According to the ratio of NO3-/SO42-, stationary sources are still the dominant source of PM2.5 and vehicle emission could not be ignored. The ratio of secondary organic carbon to organic carbon indicated that photochemical reactivity in heavily polluted days was more intense than in other days. Crustal elements were the most abundant elements, accounting for more than 60% of 23 elements. Chemical Mass Balance results indicated that the contributions of major sources (i.e., nitrate, sulfate, biomass, carbon and refractory material, coal combustion, soil dust, vehicle, and industry) of PM2.5 were 13%, 16%, 12%, 2%, 14%, 8%, 7%, and 8% in heavily polluted days and 20%, 18%, 9%, 2%, 27%, 14%, 15%, and 9% in other days, respectively. Extensive combustion activities were the main sources of polycyclic aromatic hydrocarbons during the episode (Jan 1-9, 2015) and the total benzo[a]pyrene equivalency concentrations in heavily polluted days present significant health threat. Because of the effect of regional transport, the pollution level of PM2.5 in the study area was aggravated.

Insights into the formation of secondary organic carbon in the summertime in urban Shanghai.

To investigate formation mechanisms of secondary organic carbon (SOC) in Eastern China, measurements were conducted in an urban site in Shanghai in the summer of 2015. A period of high O3 concentrations (daily peak > 120 ppb) was observed, during which daily maximum SOC concentrations exceeding 9.0 µg/(C·m3). Diurnal variations of SOC concentration and SOC/organic carbon (OC) ratio exhibited both daytime and nighttime peaks. The SOC concentrations correlated well with Ox (= O3 + NO2) and relative humidity in the daytime and nighttime, respectively, suggesting that secondary organic aerosol formation in Shanghai is driven by both photochemical production and aqueous phase reactions. Single particle mass spectrometry was used to examine the formation pathways of SOC. Along with the daytime increase of SOC, the number fraction of elemental carbon (EC) particles coated with OC quickly increased from 38.1% to 61.9% in the size range of 250-2000 nm, which was likely due to gas-to-particle partitioning of photochemically generated semi-volatile organic compounds onto EC particles. In the nighttime, particles rich in OC components were highly hygroscopic, and number fraction of these particles correlated well with relative humidity and SOC/OC nocturnal peaks. Meanwhile, as an aqueous-phase SOC tracer, particles that contained oxalate-Fe(III) complex also peaked at night. These observations suggested that aqueous-phase processes had an important contribution to the SOC nighttime formation. The influence of aerosol acidity on SOC formation was studied by both bulk and single particle level measurements, suggesting that the aqueous-phase formation of SOC was enhanced by particle acidity.

[Characteristics and Source Analysis of Carbonaceous Aerosols in PM2.5 in Huaxi District, Guiyang].

Carbonaceous aerosol, as an important component of atmospheric aerosol, has a significant impact on atmospheric environmental quality, human health, and global climate change. To investigate the characteristics and sources of carbonaceous aerosol in atmospheric fine particulate matter (PM2.5) in Huaxi District of Guiyang, an in-situ observational study was conducted during different seasons in 2020, and the carbonaceous components of PM2.5 were measured using a thermal-optical carbon analyzer (DRI Model 2015). The results of the study showed that the average concentrations of PM2.5, total carbonaceous aerosol (TCA), organic carbon (OC), secondary organic carbon (SOC), and elemental carbon (EC) concentrations during the observation period were (39.7±22.3), (14.1±7.2), (7.6±3.9), (4.4±2.6), and (2.0±1.0) µg·m-3, respectively, and the mean value of OC/EC was (3.9±0.8). ρ(PM2.5), ρ(TCA), ρ(OC), ρ(SOC), and ρ(EC) showed a seasonal variation pattern with the highest in winter [(52.6±28.6), (17.0±9.6), (9.1±5.2), (6.1±3.9), and (2.4±1.2) µg·m-3, respectively] and the lowest in summer [(25.1±7.1), (11.6±3.6), (6.3±1.9), (3.7±1.2), and (1.6±0.6) µg·m-3, respectively]. The seasonal variation in OC/EC showed summer (4.2±0.8) > winter (3.8±0.9) > autumn (3.8±0.5) > spring (3.7±0.9), indicating the presence of SOC generation in all seasons in Huaxi District. SOC showed a significant correlation with OC (R2 =0.9), and the SOC concentration tended to increase with the increase in atmospheric oxidation. OC showed a good correlation with EC in all seasons, with the highest in autumn (R2 =0.9) and lower correlations in the other three seasons (R2 ranged from 0.74 to 0.75), indicating a common source. According to OC/EC ratio range, it was preliminarily determined that carbonaceous aerosol came from vehicle exhaust emissions, coal burning emissions, and biomass combustion emissions. In order to further quantify the contribution of major emission sources to carbonaceous aerosol, the results of this study using PMF to analyze the sources of carbonaceous aerosol showed that the main sources of carbonaceous aerosol in Huaxi District of Guiyang were coal combustion sources (29.3%), motor vehicle emission sources (21.5%), and biomass combustion sources (49.2%).

[Characteristics and Source Apportionment of Carbonaceous Aerosols in the Typical Urban Areas in Chongqing During Winter].

To investigate the characteristics, source apportionment, and potential source areas of carbonaceous aerosols in Chongqing during winter, PM2.5 samples were collected from January 2021 to February 2021 in the urban areas of Wanzhou (WZ), Yubei (YB), and Shuangqiao (SQ). The results showed that the average mass concentrations of PM2.5, OC, and EC in SQ were (72.6 ±33.3), (18.2 ±8.2), and (4.4 ±1.7) µg·m-3, respectively, higher than those in WZ[(67.2 ±30.3), (17.2 ±7.4), and (5.1 ±2.4) µg·m-3] and YB[(63.4 ±25.7), (15.4 ±6.3), and (4.2 ±1.9) µg·m-3]. Compared with that during the clear period, the concentration and fraction of EC in total carbon increased by 103.0% and 8.1%, respectively, in WZ compared to that in other areas during pollution period, whereas the OC/EC ratio was decreased significantly (-10.5%), indicating that the primary emission of carbonaceous aerosols increased significantly during the pollution period. The average mass concentrations of secondary organic carbon (SOC) in SQ and YB were (7.7 ±4.8) µg·m-3 and (6.9 ±2.8) µg·m-3 significantly higher, respectively, than that in WZ[(4.5 ±1.9) µg·m-3] during the campaign. This indicated that the secondary transformation had a greater influence on the carbonaceous aerosols in SQ and YB than that in WZ. Furthermore, in contrast to that in WZ, the ratios of SOC/OC were increased with the increase in PM2.5 concentrations, and significant correlations between SOC concentration and aerosol water content, NO2 concentration, and the value of NOR were observed in SQ and YB (P < 0.01), indicating that the increasing of carbonaceous aerosol concentrations might be mainly driven by the SOC with -NO2 groups produced by aqueous chemical reactions during winter in SQ and YB. The positive definite matrix factor (PMF) results in these urban areas showed that the contribution of biomass/coal combustion source in WZ (47.4%) was significantly higher than that in YB (34.2%) and SQ (38.1%), whereas the gasoline motor vehicle emission and secondary transformation impacts were more significant in YB. The results of the concentration weighted trajectory (CWT) showed that the potential sources of carbonaceous aerosols were mainly the local and northeastern parts of these urban areas (such as Changshou).

Carbonaceous fraction in PM2.5 of six Latin American cities: Seasonal variations, sources and secondary organic carbon contribution.

Latin American (LatAm) cities are grappling with elevated levels of gaseous and particulate pollutants, which are having detrimental effects on both the local ecosystem and human health. Of particular concern are aerosols with smaller diameters (lower or equal to 2.5 µm, PM2.5), known for their ability to penetrate deep into the respiratory system. While measurements in the region are increasing, they remain limited. This study addresses this gap by presenting the results of a comprehensive, year-long PM2.5 monitoring campaign conducted in six LatAm cities: Buenos Aires, São Paulo, Medellín, San José, Quito and Ciudad de México. Despite all six monitoring sites being urban, they exhibited significant variations in PM2.5 levels, as well as in the content and seasonal behavior of elemental carbon (EC) and organic carbon (OC). Estimations of secondary organic carbon (SOC) using the EC-tracer method revealed a notable SOC relevance across all cities: secondary organic aerosols (SOA) accounted in average for between 19 % to 48 % of the total carbonaceous matter. Source attribution, conducted through the Positive Matrix Factorization (PMF) model, highlights substantial contributions from gasoline and diesel traffic emissions (29 % to 49 % of total carbon, TC), regional biomass burning (21 % to 27 %), and SOA (20 % to 38 %) in all cities, with similar chemical signatures. Additionally, industrial emissions were significant in two cities (Medellín and San José), while two others experienced minor impacts from construction machinery at nearby sites (Buenos Aires and Quito). This comparative analysis underscores the importance of considering not only the thermal optical EC/OC fractions as tracers of sources but also the OC/EC ratio of the PMF factors. This dual approach not only adds depth to the research but also suggests varied methodologies for addressing this crucial environmental concern. This study lays the groundwork for future investigations into the factors influencing the content and seasonality of SOA in the region.

Stark seasonal contrast of fine aerosol levels, composition, formation mechanism, and characteristics in a polluted megacity.

In this study, we investigated the temporal variation of organic and inorganic aerosol with its optical properties in Mumbai (India), an urban coastal region. Mean PM2.5 concentrations during the sampling period were 175 µg/m3 (winter) and 90 µg/m3 (summer). During winter, the average concentrations of organic (OC), elemental (EC), and water-soluble organic carbon (WSOC) were three times higher than in summer. Secondary organic carbon (SOC) contribution in OC was higher in summer (78%) than in winter (53%), and strong solar radiation in summer likely caused this outcome. Aerosols were slightly acidic in both seasons, with an average pH of 5.7 (winter) and 6.0 (summer). A correlation was observed between SOC and the acidity of particles in summer (R2 = 0.6), indicating some amount of acid-catalysed SOC formation. In both seasons, the sulphate oxidation ratio (SOR) was higher than the nitrate oxidation ratio (NOR), which may reflect a preference for SO2 oxidation over NO2 or the difference in partitioning ammonium nitrate into ammonium sulphate under high RH. The dominant mechanism of SOC formation (gas vs aqueous phase oxidation) also showed seasonal variation. In winter, a relatively steep reduced major axis (RMA) slope of O3/CO suggests gas phase oxidation was the dominant mechanism of SOC production. Winter has more BrC fraction than summer, indicating higher absorbing aerosols, though the efficiency of absorbing the light was higher in summer. To assess the radiative forcing of PM2.5 on a local scale, an effective carbon ratio (ECR) was computed. The findings pointed to a local radiative heating impact caused by PM2.5. The spectral slope ratio and MAE at 250 to 300 nm ratio (E2/E3) revealed a higher abundance of high molecular weight species in WSOC during summer than in winter.

Spatiotemporal distribution and influencing factors of secondary organic aerosols in the summer atmosphere from the Bering Sea to the western North Pacific.

To better understand the formation process of biogenic and anthropogenic secondary organic aerosols (BSOA and ASOA) in the marine atmosphere under the background of global warming, aerosol samples were collected over three summers (i.e., 2014, 2016 and 2018) from the Bering Sea (BS) to the western North Pacific (WNP). The results showed that temporally, atmospheric concentrations of isoprene-derived SOA (SOAI) tracers were the lowest in 2014 regardless of the marine region, while atmospheric concentrations of monoterpenes-derived SOA (SOAM) tracers in this year were the highest and the aerosols were more aged than those in the other two years. In comparison, the concentrations of ß-caryophyllene-derived and toluene-derived SOA (SOAC and SOAA) tracers were relatively low overall. Spatially, the concentrations of SOA tracers were significantly higher over the WNP than over the BS, with SOA tracers over the BS mainly coming from marine sources, while the WNP was strongly influenced by terrestrial inputs. In particular, for land-influenced samples from the WNP, NOx-channel products of SOAI were more dependent on O3 and SO2 relative to HO2-channel product, and the high atmospheric oxidation capacity and SO2 could promote the formation of later-generation SOAM products. The extent of terrestrial influence was further quantified using a principal component analysis (PCA)-generalized additive model (GAM), which showed that terrestrial emissions explained more than half of the BSOA tracers' concentrations and contributed almost all of the ASOA tracer. In addition, the assessment of secondary organic carbon (SOC) highlighted the key role of anthropogenic activities in organic carbon levels in offshore areas. Our study revealed significant contributions of terrestrial natural and anthropogenic sources to different SOA over the WNP, and these relevant findings help improve knowledge about SOA in the marine atmosphere.

Comparison and implications of the carbonaceous fractions under different environments in polluted central plains in China: Insight from the lockdown of COVID-19 outbreak.

Before and during the COVID-19 outbreak in the heated winter season of 2019, the carbonaceous fractions including organic carbon (OC), elemental carbon (EC), OC1-4, and EC1-5 were investigated between normal (November 1, 2019, to January 24, 2020) and lockdown (January 25, to February 29, 2020) periods in polluted regions of northern Henan Province. In comparison to urban site, four rural sites showed higher concentrations of carbonaceous components, especially secondary OC (SOC); the concentration of SOC in rural sites was 1.5-3.4 times that in the urban site. During the lockdown period, SOC in urban site decreased slightly, while it increased significantly in rural sites. NO2 has a significant effect on SOC generation, particularly in normal period when NO2 concentrations were high. Nevertheless, NO2 significantly decreased, and the elevated O3 (increased by 103-138%) contributed considerably to the generation of SOC during lockdown. Relative humidity (RH) promoted SOC production when RH was below 60%, but SOC was negatively correlated or uncorrelated with RH when RH exceeded 60%. Additionally, RH has a more pronounced effect on SOC during lockdown. The contribution of gasoline vehicle emissions decreases significantly in both urban and rural sites (3-12%) due to the significant reduction of anthropogenic activities during lockdown, although the urban site remained with the biggest contributions (37%). These results provide innovative insights into the variations in carbonaceous aerosols and SOC generation during the unique time when anthropogenic sources were significantly reduced and illustrate the differences in pollution characteristics and sources of carbonaceous fractions in different environments.

Evolution of organic carbon during COVID-19 lockdown period: Possible contribution of nocturnal chemistry.

Carbonaceous aerosol is one of the main components of atmospheric particulate matter, which is of great significance due to its role in climate change, earth's radiation balance, visibility, and human health. In this work, carbonaceous aerosols were measured in Shijiazhuang and Beijing using the OC/EC analyzer from December 1, 2019 to March 15, 2020, which covered the Coronavirus Disease 2019 (COVID-19) pandemic. The observed results show that the gas-phase pollutants, such as NO, NO2, and aerosol-phase pollutants (Primary Organic Compounds, POC) from anthropogenic emissions, were significantly reduced during the lockdown period due to limited human activities in North China Plain (NCP). However, the atmospheric oxidation capacity (Ox/CO) shows a significantly increase during the lockdown period. Meanwhile, additional sources of nighttime Secondary Organic Carbon (SOC), Secondary Organic Aerosol (SOA), and babs, BrC(370 nm) are observed and ascribed to the nocturnal chemistry related to NO3 radical. The Potential Source Contribution Function (PSCF) analysis indicates that the southeast areas of the NCP region contributed more to the SOC during the lockdown period than the normal period. Our results highlight the importance of regional nocturnal chemistry in SOA formation.

Tracer-based characterization of fine carbonaceous aerosol in Beijing during a strict emission control period.

Strict emission controls were implemented in Beijing and the surrounding regions in the North China Plain to guarantee good air quality during the 2014 Asia-Pacific Economic Cooperation (APEC) summit. Thus, the APEC period provides a good opportunity to study the sources and formation processes of atmospheric organic aerosol. Here, fine particles (PM2.5, particulate matter with a diameter of 2.5 µm or less) collected in urban Beijing before and during the APEC period were analyzed for molecular tracers of primary and secondary organic aerosol (SOA). The primary organic carbon (POC) and secondary organic carbon (SOC) were also reconstructed using a tracer-based method. The concentrations of biogenic SOA tracers ranged from 1.09 to 34.5 ng m-3 (mean 10.3 ± 8.51 ng m-3). Monoterpene oxidation products were the largest contributor to biogenic SOA, followed by isoprene- and sesquiterpene-derived SOA. The concentrations of biogenic SOA tracers decreased by 50 % during the APEC, which was largely attributed to the implementation of emission controls by the Chinese government. The increasing mass fractions of biogenic SOA tracers from isoprene and sesquiterpene during the pollution episodes implied that their photooxidation processes contributed to the poor air quality in urban Beijing. The reconstructed biogenic and anthropogenic SOC and POC concentrations were 89.6 ± 96.8 ng m-3, 570 ± 611 ng m-3, and 2.49 ± 2.08 µg m-3, respectively, accounting for 21.9 ± 11.4 % of OC in total. Biomass-burning derived OC was the largest contributor to carbonaceous aerosol over the North China Plain. By comparing the results before and during the APEC, the emission controls effectively mitigated about 34 % of the estimated OC and were more effective at reducing SOC than POC. This suggests that the reduction of the primary organic aerosol loading is harder than SOA over the North China Plain.

Impact of COVID-19 lockdown on carbonaceous aerosols in a polluted city: Composition characterization, source apportionment, influence factors of secondary formation.

Carbonaceous fractions throughout the normal period and lockdown period (LP) before and during COVID-19 outbreak were analyzed in a polluted city, Zhengzhou, China. During LP, fine particulate matters, elemental carbon (EC), and secondary organic aerosol (SOC) concentrations fell significantly (29%, 32% and 21%), whereas organic carbon (OC) only decreased by 4%. Furthermore, the mean OC/EC ratio increased (from 3.8 to 5.4) and the EC fractions declined dramatically, indicating a reduction in vehicle emission contribution. The fact that OC1-3, EC, and EC1 had good correlations suggested that OC1-3 emanated from primary emissions. OC4 was partly from secondary generation, and increased correlations of OC4 with OC1-3 during LP indicated a decrease in the share of SOC. SOC was more impacted by NO2 throughout the research phase, thereby the concentrations were lower during LP when NO2 levels were lower. SOC and relative humidity (RH) were found to be positively associated only when RH was below 80% and 60% during the normal period (NP) and LP, respectively. SOC, Coal combustion, gasoline vehicles, biomass burning, diesel vehicles were identified as major sources by the Positive Matrix Factorization (PMF) model. Contribution of SOC apportioned by PMF was 3.4 and 3.0 µg/m3, comparable to the calculated findings (3.8 and 3.0 µg/m3) during the two periods. During LP, contributions from gasoline vehicles dropped the most, from 47% to 37% and from 7.1 to 4.3 µg/m3, contribution of biomass burning and diesel vehicles fell by 3% (0.6 µg/m3) and 1% (0.4 µg/m3), and coal combustion concentrations remained nearly constant. The findings of this study highlight the immense importance of anthropogenic source reduction in carbonaceous component variations and SOC generation, and provide significant insight into the temporal variations and sources of carbonaceous fractions in polluted cities.

Molecular characteristics, sources and influencing factors of isoprene and monoterpenes secondary organic aerosol tracers in the marine atmosphere over the Arctic Ocean.

Biogenic secondary organic aerosols (BSOA) are important components of the remote marine atmosphere. However, the response of BSOA changes to sea ice reduction over the Arctic Ocean remains unclear. Here we investigated isoprene and monoterpenes secondary organic aerosol (SOAI and SOAM) tracers in three years of summer aerosol samples collected from the Arctic Ocean atmosphere. The results indicated that methyltetrols were the most abundant SOAI tracers, while the main oxidation products of monoterpenes varied over the years owing to different aerosol aging. The results of the principal component analysis (PCA)-generalized additive model (GAM) combined with correlation analysis suggested that SOAI tracers were mainly generated by the oxidation of isoprene from marine emissions, while SOAM tracers were probably more influenced by terrestrial transport. Estimation of secondary organic carbon (SOC) indicated that monoterpenes oxidation contributed more than isoprene and that sea ice changes had a relatively small effect on biogenic SOC concentration levels. Our study quantified the contribution of influencing factors to the atmospheric concentration of BSOA tracers in the Arctic Ocean, and showed that there were differences in the sources of precursors for different BSOA. Hence, our findings have contributed to a better understanding of the characteristics, sources and formation of SOA in the atmosphere of the Arctic Ocean.

Characteristics of fine carbonaceous aerosols in Wuhai, a resource-based city in Northern China:Insights from energy efficiency and population density.

As one of the predominant compositions of PM2.5 (particulate matter with aerodynamic diameter ≤2.5 µm), carbonaceous aerosols not only have adverse effects on air quality, but also can affect climate change. Although there are extensive recent studies on carbonaceous aerosols, comprehensive studies on their socioeconomic influencing factors in a resource-based city are relatively limited. In this study, the spatial-temporal variations of organic carbon (OC), elemental carbon (EC), and secondary organic carbon (SOC) were investigated in January, July, and October in 2015 and April in 2016 in Wuhai and its surrounding areas. The population distribution and industry layout have led to the uneven spatial-temporal distribution of carbonaceous aerosols. The concentrations of carbonaceous aerosols were higher in winter due to the unfavorable meteorology and the increased emissions from heating. The SOC is a significant contributor to OC in the cold season (52.0% for January). Primary carbonaceous aerosols pollution is higher in the industrial sites of resource-based cities, whereas the SOC makes a significant contribution in the residential sites. The results of backward-trajectory and concentration-weighted trajectory analysis suggest that the local emissions and short-range atmospheric transport from nearby areas have a significant impact on PM2.5 and carbonaceous aerosols. A strong correlation between population density and OC/EC ratio was found, indicating that the megacities with high population density have a higher SOC contribution than the resource-based cities. Resource-based cities are characterized by high level of primary OC emissions, whereas cities with high energy efficiency have a more significant SOC contribution. These results provide a more comprehensive understanding of carbonaceous aerosols in a resource-based city.

[Changes in Carbonaceous Aerosol in the Northern Suburbs of Nanjing from 2015 to 2019].

Carbonaceous aerosol is an important component of atmospheric fine particles that has an important impact on air quality, human health, and climate change. In order to explore the long-term changes in carbonaceous aerosol under the background of emission reduction, this study measured the mass concentrations of organic carbon (OC) and elemental carbon (EC) of PM2.5, which collected in the northern suburbs of Nanjing for five years (December 17, 2014 to January 5, 2020). The results showed that the five-year average ρ(OC) and ρ(EC) were (10.2±5.3) µg·m-3 and (1.6±1.1) µg·m-3, accounting for 31.1% and 5.2% of PM2.5, respectively. OC and EC concentrations were both high in winter and low in summer. According to the nonparametric Mann-Kendall test and Sen's slope, the mass concentrations of OC and PM2.5 decreased significantly[OC:P<0.0001, -0.79 µg·(m3·a)-1, -0.29%·a-1; PM2.5:P<0.0001, -4.59 µg·(m3·a)-1, -1.58%·a-1]. Although EC had an upward trend, the significance and range of change were not obvious[P=0.02, 0.05 µg·(m3·a)-1, 0.02%·a-1]. OC and EC decreased significantly during winter from 2014 to 2019[OC:P<0.0001, -2.05 µg·(m3·a)-1, -0.74%·a-1; EC:P=0.001, -0.15 µg·(m3·a)-1, -0.05%·a-1], and the decline was more obvious than the whole. The correlation between OC and EC showed that the sources in winter and summer were more complex than those in spring and autumn. According to the characteristic ratio of OC and EC, the contribution of coal combustion and biomass burning decreased from 2015 to 2019, whereas the impact of industrial sources and vehicle emissions became more significant. Corresponding to this was the obvious decline in OC and the slight recovery of EC. The OC/EC ratio was over 2.0, indicating that there was secondary pollution in the study area. Further calculation revealed that the variation in SOC was consistent with that in OC, showing a significant decrease[P<0.0001, -0.47 µg·(m3·a)-1, -0.17%·a-1]. The average mass concentration of SOC was (5.0±3.5) µg·m-3, accounting for 49.2% of OC. These changes indicate clear effects of the prevention and control of air pollution in Nanjing in recent years. Furthermore, future control can focus on the emissions of VOCs to reduce secondary pollution.