Source apportionment and wet scavenging ability of atmospheric black carbon during haze in Northeast China

Seasonal variations in black carbon (BC) pollution characteristics during haze episodes in Benxi city, Liaoning province, were analyzed using year-long measurements of BC, carbon monoxide (CO), and PM2.5. Haze frequencies were recorded to be 0.07, 0.03 and 0.14 in spring, autumn, and winter respectively. Solid fuel contributions increased notably by 7%-8% during haze events compared to clean periods in all seasons. Transitioning from clean to haze periods led to ΔBC/ΔCO increases of 16% in spring and autumn, and 6.8% in winter, while BC/PM2.5 ratios decreased by approximately 33%, 50%, and 24% for spring, autumn, and winter respectively, likely indicating enhanced residential and industrial contributions. These further led to an increase in BC absorption capacities by factors of around 2.2 in spring and autumn, and up to 2.6 in winter during haze periods. Despite liquid fuel sources dominating BC emissions, certain haze episodes (frequency <10%) showed solid fuel contributions of up to 65%, highlighting BC pollution complexity in the region during haze. Backward trajectories analysis revealed local air masses from Liaoning province arrived consistently with the most occurrence of haze events across all seasons, while long-range air masses from Mongolian regions, though with less frequent occurrence during haze periods, significantly elevated BC loadings from solid fuel sources, particularly in spring and autumn due to biomass burning. Despite higher BC wet scavenging rates (WSR) in long-range air masses (0.072 ng m-3 ppbv-1 mm-1) compared to local air masses (0.039 ng m-3 ppbv-1 mm-1), significant BC transport persisted due to limited precipitation along transport pathways, especially during haze periods. These findings provide crucial insights for policymakers, highlighting the need for targeted haze prevention and control strategies focusing on mitigating BC emissions in Northeast China.

Characteristics of atmospheric black carbon and its wet scavenging in Nanning, South China

Based on in-situ measurement of black carbon (BC) and carbon monoxide (CO), the characteristics of BC emissions and wet scavenging were comprehensively investigated in Nanning, South China. The average annual BC concentration was 1.02 ± 0.53 μg m-3 with higher pollution levels during winter. In winter, a higher net BC/CO (ΔBC/ΔCO) ratio of 3.3 ± 0.3 ng m-3 ppb-1 along with an increased absorption Ångström exponent (AAE) and BC mass from biomass burning (BCbb), indicated a significant contribution of biomass burning to BC emissions. However, emissions from the traffic sector consistently exerted a dominant influence throughout the year. Cluster analysis of backward trajectories identified three types of air masses with distinct origins. Cluster #1 originated from Guangxi province and its vicinity, intermittently influencing the sampling site throughout the year with varying effects between winter and summer. This air mass brought in clean sea breeze in summer whereas transported a higher proportion of BCbb to the site during wintertime due to local open biomass burning. Cluster #3 primarily arrived in autumn and winter (October-December) from polluted central China, resulting in substantially high BC mass at the site. Cluster #2 coincided with the period (January-March) when extensive surface open biomass burning events occurred in Southeast Asia (SEA) regions. These BC aerosols in cluster#2 initially rose to higher altitudes above SEA before being regionally transported, but were significantly scavenged by clouds and precipitation during vertical uplift. The remaining BC exhibited a notably lower BC loss rate on relative humidity (RH) of -0.01 ng m-3 ppb-1 %-1 compared to cluster #1 (-0.03) and cluster #3 (-0.06), corresponding to an average BC transport efficiency of 0.85, 0.73, and 0.53, respectively. Nonetheless, air masses in cluster #2 could still transport considerably high BC mass to Nanning due to dry conditions and less wet scavenging along trajectory pathways. These findings provide valuable insights for policymakers and government officials in regulating and mitigating BC pollution in South China.

Contrasting the characteristics, sources, and evolution of organic aerosols between summer and winter in a megacity of China

Organic aerosol (OA) accounts for the largest fraction of fine particles in the ambient atmosphere, however, its formation process remains highly uncertain. Here, with an aim of obtaining insights to OA formation mechanism, we have focused on contrasting the characteristics, sources and evolution of OA of PM_2.5 (particulate matter with an aerodynamic diameter < 2.5 μm) between summer and winter based on two field campaigns of urban Beijing. The results show that secondary OA (SOA) dominates OA mass in PM_2.5 both in summer and winter, accounting for 87 % and 74 %, respectively. This is much higher compared to the mass fraction of ~56 % that observed in PM₁, indicating an important role of SOA in larger size particles. We also show that, the SOA is becoming more dominant in total OA and our observed high SOA proportion is reconciled with its overall upward trends in recent years. The observed mass concentrations of SOA (less oxidized oxygenated OA, LO-OOA; more oxidized OOA, MO-OOA) in winter are about twice higher than those in summer. The Van-Krevelen diagram implies that the addition of carboxylic acid may be a primary oxidation process of OA in summer, while the addition of alcohol/peroxide may play a dominant role in OA evolution in winter. Our observation and analysis illustrate a more efficient conversion from LO-OOA to MO-OOA through aqueous-phase processes in winter of Beijing. While, in summer, cooking OA may be easier to convert to MO-OOA through photochemical process than other OA factors. We further show that the POA is mainly locally emitted, while the origin of SOA is from both regional transport and locally formed. The results of this study may provide policy basis for the precise control of OA pollution and would also help to improve the accuracy of assessing the environmental and climate effects of OA.

Observations of atmospheric CO2 and CO based on in-situ and ground-based remote sensing measurements at Hefei site, China

The characteristics of long time series of CO₂ and CO surface concentrations, tropospheric and total column dry-air mole fractions (DMF) from May 2015 to December 2019 were investigated. Both CO₂ and CO show different seasonality for the three datasets. The annual increasing trend of CO₂ is similar for all three datasets. However, the annual decreasing trend of CO for surface concentration is high compared to the other two measurements, mainly due to the improved combustion efficiency from power generation in recent years. The correlation between the tropospheric and total atmospheric CO₂ and CO is higher than that between the surface concentration and tropospheric CO₂ and CO. This is because the tropospheric and total atmospheric results both have common vertical profiles for CO₂ and CO respective mole fractions that were observed in troposphere. Furthermore, the enhancement ratios of CO₂ to CO derived from the three datasets during the period from 2016 to 2019 were compared. The ratio of ∆CO₂ to ∆CO has an obvious increase with altitude each year, which means that the combustion efficiencies obtained from the three datasets are different. All ratios for the three datasets showed a slight increasing trend in recent years, which is attributed to increased combustion efficiency due to governmental measures for energy savings and emission reductions.

Characterization of Regional Combustion Efficiency using ΔXCO: ΔXCO2 Observed by a Portable Fourier-Transform Spectrometer at an Urban Site in Beijing

Measurements of column-averaged dry-air mole fractions of carbon dioxide and carbon monoxide, CO₂ (XCO₂) and CO (XCO), were performed throughout 2019 at an urban site in Beijing using a compact Fourier Transform Spectrometer (FTS) EM27/SUN. This data set is used to assess the characteristics of combustion-related CO₂ emissions of urban Beijing by analyzing the correlated daily anomalies of XCO and XCO₂ (e.g., ΔXCO and ΔXCO₂). The EM27/SUN measurements were calibrated to a 125HR-FTS at the Xianghe station by an extra EM27/SUN instrument transferred between two sites. The ratio of ΔXCO over ΔXCO₂ (ΔXCO:ΔXCO₂) is used to estimate the combustion efficiency in the Beijing region. A high correlation coefficient (0.86) between ΔXCO and ΔXCO₂ is observed. The CO:CO₂ emission ratio estimated from inventories is higher than the observed ΔXCO:ΔXCO₂ (10.46 ± 0.11 ppb ppm^-1) by 42.54%-101.15%, indicating an underestimation in combustion efficiency in the inventories. Daily ΔXCO:ΔXCO₂ are influenced by transportation governed by weather conditions, except for days in summer when the correlation is low due to the terrestrial biotic activity. By convolving the column footprint [ppm (µmol m^-2 s^-1)^-1] generated by the Weather Research and Forecasting-X-Stochastic Time-Inverted Lagrangian Transport models (WRF-X-STILT) with two fossil-fuel emission inventories (the Multi-resolution Emission Inventory for China (MEIC) and the Peking University (PKU) inventory), the observed enhancements of CO₂ and CO were used to evaluate the regional emissions. The CO₂ emissions appear to be underestimated by 11% and 49% for the MEIC and PKU inventories, respectively, while CO emissions were overestimated by MEIC (30%) and PKU (35%) in the Beijing area.

Atmospheric characteristics and population exposure assessment of black carbon at a regional representative site in the Yangtze River Delta region, China based on the five-year monitoring

To investigate the multi-year atmospheric characteristic and population exposure level of black carbon (BC) in the Yangtze River Delta (YRD) region, China, about five years of ambient BC measurement was performed at Dianshan Lake (DSL) regional Supersite from February 2014 to February 2019. BC concentration at DSL was 1.39 ± 1.15 μg m^-3, which was at low to medium level compared to other areas in the world, and annual BC level was decreased by an average of 45.4% from 2014 to 2018. The absorption Ångström exponent (AAE) value was 1.30 ± 0.173, indicating the predominant contribution of liquid fuel sources such as traffic exhaust to BC. Meanwhile, AAE and BC values both showed the winter-high and summer-low temporal patterns, which highlighted the increasing contribution from solid fuels to BC in winter. Moreover, diurnal characteristics of BC, AAE, carbon monoxide and nitrogen oxide demonstrated the dominance of traffic sources for BC. The average estimated daily intakes (EDIs) of BC through inhalation for 17 population age subgroups were 0.0177-0.0811 μg kg^-1 day^-1, which the highest EDIs for male and female were both observed in infants (9 months ~ < 1 year). Male generally taken higher exposure level of BC compared to female. This work could improve the insights for atmospheric characteristic and population exposure level of BC, and potentially facilitate the development of abatement policies of BC in YRD region, China.

One-Year Real-Time Measurement of Black Carbon in the Rural Area of Qingdao, Northeastern China: Seasonal Variations, Meteorological Effects, and the COVID-19 Case Analysis

In this paper, we report the results obtained from one year of real-time measurement (i.e., from December 2019 to November 2020) of atmospheric black carbon (BC) under a rural environment in Qingdao of Northeastern China. The annual average concentration of BC was 1.92 ± 1.89 μg m−3. The highest average concentration of BC was observed in winter (3.65 ± 2.66 μg m−3), followed by fall (1.73 ± 1.33 μg m−3), spring (1.53 ± 1.33 μg m−3), and summer (0.83 ± 0.56 μg m−3). A clear weekend effect was observed in winter, which was characterized by higher BC concentration (4.60 ± 2.86 μg m−3) during the weekend rather than that (3.22 ± 2.45 μg m−3) during weekdays. The influence of meteorological parameters, including surface horizontal wind speed, boundary layer height (BLH), and precipitation, on BC, was investigated. In particular, such BLH influence presented evidently seasonal dependence, while there was no significant seasonality for horizontal wind speed. These may reflect different roles of atmospheric vertical dilution on affecting BC in different seasons. The △BC/△CO ratio decreased with the increase of precipitation, indicative of the influence of below-cloud wet removal of BC, especially during summertime where rainfall events more frequently occurred than any of other seasons. The bivariate-polar-plot analysis showed that the high BC concentrations were mainly associated with low wind speed in all seasons, highlighting an important BC source originated from local emissions. By using concentration-weighted trajectory analysis, it was found that regional transports, especially from northeastern in winter, could not be negligible for contributing to BC pollution in rural Qingdao. In the coronavirus disease 2019 (COVID−19) case analysis, we observed an obvious increase in the BC/NO2 ratio during the COVID-19 lockdown, supporting the significant non-traffic source sector (such as residential coal combustion) for BC in rural Qingdao.

Variation in black carbon concentration and aerosol optical properties in Beijing: Role of emission control and meteorological transport variability

Black carbon (BC), which is a by-product with incomplete combustion of carbonaceous materials, can be used as an indicator of combustion emissions and is an important climate forcer. In this study, a spatial-temporal synthesis of BC aerosols and the affecting factors was conducted in urban Beijing. As observed, BC showed a spatial pattern with high concentration in south and low in north. BC concentration evidently decreased by approximately 61% between 2005 and 2017. From 2015 to 2017, the mass ratio of BC/PM_2.5 dropped by 28%, which suggested a more efficient effect of control measures to BC than PM_2.5. The BC/CO ratio dropped by 22%, which indicated the decreasing emission from fossil fuel sources. With regard to BC loading, the spectral dependence of absorption aerosol exhibited significant seasonal variations. High absorption Ångström exponent (α) was observed during heating season, which reflected the increasing contribution of brown carbon (BrC) to light absorption. Backward trajectory analysis showed that the levels of BC and PM_2.5 were high in Cluster-South and Cluster-West. BrC absorption was high in Cluster-West, Cluster-Northwest and Cluster-Northeast, due to the biomass and coal burning for domestic heating and aging processes on a regional scale. The effects of emission control and transport variability on pollutant variation were estimated on the basis of the cluster analysis. Results indicated that the effect of emission reduction was the major reason for the decrease of BC from 2015 to 2017, which resulted in a 34% reduction of BC concentration. Meanwhile, transport variability caused a 15% reduction.

Chemical characterization of submicron aerosol in summertime Beijing: A case study in southern suburbs in 2018

Atmospheric particulate matters have a crucial impact on climate change, visibility and human health. In this study, a detailed characteristic of summertime PM₁ was real-time measured in south suburb of Beijing from 16^th August to 16^th September 2018. Averaged PM₁ concentration of 24.1 ± 18.0 μg m^-3 was observed, consisting of OM (50.8%), SO₄^2- (16.0%), BC (13.2%), NO₃^- (10.2%), NH₄⁺ (9.2%), and Cl^- (0.6%). There was an accumulation mode with a peak diameter of ∼500 nm for all the species (except BC), and OM was additionally characterized by a smaller mode of <100 nm. Elemental analysis of OM showed that the diurnal variations of H/C, O/C, N/C, and S/C were correlated to the photochemical and aqueous-phase process. Four organic factors including one hydrocarbon-like (HOA) and three oxygenated (LO-OOA, SV-OOA and MO-OOA) organic aerosol factors were identified by positive matrix factorization (PMF) analysis. The contributions of these factors varied with PM₁ concentration and their average values were 31%, 30%, 14%, and 25%, respectively. Contribution of HOA was RH-independent but decreased with the increasing PM₁ concentration, while OOA factors were a combined result of RH and O_x (=O₃+NO₂), revealing the important role of photochemical and aqueous-phase process in OA evolution. The contribution of SV-OOA with the highest S/C increased significantly with RH, indicating a certain number of S-containing organics. Our results also showed that secondary OA was the dominant species, as well as increased with the pollution level, implicating VOCs and NO_x should be controlled to relieve the secondary OA pollution.

Changes in black carbon and PM2.5 in Tokyo in 2003-2017

Black carbon (BC) particles cause adverse health effects and contribute to the heating of the atmosphere by absorbing visible solar radiation. Efforts have been made to reduce BC emissions, especially in urban areas; however, long-term measurements of BC mass concentration (M_BC) are very limited in Japan. We report M_BC measurements conducted in Tokyo from 2003 to 2017, showing that M_BC decreased by a factor of 3 from 2003 to 2010 and was stable from 2010 to 2017. Fine particulate concentrations (PM_2.5) decreased by a much smaller factor during 2003-2010. The diurnal variations of BC size distributions suggest that the BC in Tokyo originates mainly from local sources, even after 2010. Our three-dimensional model calculations show that BC from the Asian continent contributes a small portion (about 20%) of the annual average M_BC in the Kanto region of Japan, which includes Tokyo. This indicates that continued reduction of BC emissions inside Japan should be effective in further decreasing M_BC.

Characteristics of fine particle explosive growth events in Beijing, China: Seasonal variation, chemical evolution pattern and formation mechanism

Fine particle explosive growth (FPEG) events are frequently observed in heavy haze episodes in Beijing, the characteristics and formation mechanism of which remain not fully understood. In this study, a five year (2013-2017) online observation was conducted in Beijing and the chemical evolution pattern of FPEG events was analyzed to understand its formation mechanism. A total of 132 FPEG events were identified, and steadily decreased from 39 events in 2013 to 19 events in 2017. More than 70% of the FPEG events occurred in winter and autumn, which coincides with adverse weather conditions and enhanced primary emissions. Organic matter (OM) was the dominated components (~30%) in PM_2.5, but it only accounted for 10% of total FPEG events as a driven factor, because its contribution usually decreased when the FPEG events developed. In contrast, the secondary inorganic species were the dominated driven factors, and sulfate-driven events accounted >50%. During the period of 2013-2017, the contribution from regional sources decreased significantly mainly due to the reduction of emissions from regional sources, while the contribution from local sources remained largely unchanged, indicating that the local secondary transformation played a leading role in promoting the FPEG events. The low nitrogen oxidation rates (NOR, 0.12 ± 0.07) and the weak increase trend of NOR with elevated RH were observed, indicating the formation of which might be promoted by the homogenous reaction between HNO₃ and NH₃. In contrast, a significant increase in sulfur oxidation rate (SOR, 0.50 ± 0.19) was observed when RH > 50%, suggesting enhanced heterogeneous oxidation of SO₂ in FPEG events. In addition, our analysis suggest the S (IV) heterogeneous oxidation rates in FPEG events depend mainly on the aerosol liquid water content (ALWC) in addition to the aerosol acidity. This study provides observational evidence for understanding the formation mechanism of FPEG events in Beijing.

The carbonaceous aerosol levels still remain a challenge in the Beijing-Tianjin-Hebei region of China: Insights from continuous high temporal resolution measurements in multiple cities

Carbonaceous aerosols in high emission areas attract worldwide attention of the scientific community and the public due to their adverse impacts on the environment, human health and climate. However, long-term continuous hourly measurements are scarce on the regional scale. In this study, a one-year hourly measurement (from December 1, 2016 to November 30, 2017) of organic carbon (OC) and elemental carbon (EC) in airborne fine particles was performed using semi-continuous OC/EC analyzers in Beijing, Tianjin, Shijiazhuang and Tangshan in the Beijing-Tianjin-Hebei (BTH) region in China, which is one of high emission areas in China, even in the world. Marked spatiotemporal variations were observed. The highest concentrations of OC (22.8 ± 30.6 μg/m³) and EC (5.4 ± 6.5 μg/m³) occurred in Shijiangzhuang while the lowest concentrations of OC (11.0 ± 10.7 μg/m³) and EC (3.1 ± 3.6 μg/m³) were obtained in Beijing and Tianjin, respectively. Pronounced monthly, seasonal and diurnal variations of OC and EC were recorded. Compared to published data from the past two decades for the BTH region, our OC and EC levels were lower, implying some effect of recent measures for improving the air quality. Significant correlations of OC versus EC (p < 0.001) were found throughout the study period with high slopes and correlation coefficients in winter, but low slopes and correlation coefficients in summer. The estimated secondary OC (SOC), based on the minimum R squared (MRS) method, represented 29%, 47%, 38% and 48% of the OC for Beijing, Tianjin, Shijiazhuang and Tangshan, respectively. These percentages are larger than previous ones obtained for the BTH region in the past decade. There were obvious differences in the potential source regions of OC and EC among the four cities. Obvious prominent potential source areas of OC and EC were observed for Beijing, which were mainly located in the central and western areas of Inner Mongolia and even extended to the Mongolian regions, which is different from the findings in previous studies. For all sites, adjacent areas of the main provinces in northern China were found to be important potential source areas.

Two-year continuous measurements of carbonaceous aerosols in urban Beijing, China: Temporal variations, characteristics and source analyses

Organic carbon (OC) and elemental carbon (EC) in the PM_2.5 of urban Beijing were measured hourly with a semi-continuous thermal-optical analyzer from Jan 1, 2013 to Dec 31, 2014. The annual average OC and EC concentrations in Beijing were 17.0 ± 12.4 and 3.4 ± 2.0 μg/m³ for 2013, and 16.8 ± 14.5 and 3.5 ± 2.9 μg/m³ for 2014. It is obvious that the annual average concentrations of OC and EC in 2014 were not less than those in 2013 while the annual average PM_2.5 concentration (89.4 μg/m³) in 2014 was slightly reduced as compared to that (96.9 μg/m³) in 2013. Strong seasonality of the OC and EC concentrations were found with high values during the heating seasons and low values during the non-heating seasons. The diurnal cycles of OC and EC characterized by higher values at night and in the morning were caused by primary emissions, secondary transformation and stable meteorological condition. Due to increasing photochemical activity, the OC peaks were observed at approximately noon. No clear weekend effects were observed. Interestingly, in the early mornings on weekends in the autumn and winter, the OC and EC concentrations were close to or higher than those on weekdays. Our data also indicate that high OC and EC concentrations were closely associated with their potential source areas which were determined based on the potential source contribution function analysis. High potential source areas were identified and were mainly located in the south of Beijing and the plain of northern China. A much denser source region was recorded in the winter than in the other seasons, indicating that local and regional transport over regional scales are the most important. These results demonstrate that both regional transport from the southern regions and local accumulation could lead to the enhancements of OC and EC and likely contribute to the severe haze pollution in Beijing.

Source apportionment of black carbon during winter in Beijing

Black carbon (BC) in PM_2.5 was measured at an urban site in Beijing during winter 2015 using an aethalometer. The characteristics and sources of BC during pollution episodes and clean days were analyzed. The average hourly mass concentration of BC during the study period was 5.31±6.27μg/m³. BC was highly correlated with PM_2.5 (R²=0.80), with its concentration ranging from 0.17μg/m³ in clean days to 35.33μg/m³ in haze days. Source apportionment results showed that the average contribution of liquid fuel source (e.g., vehicle emission) to BC was around 50% in clean days. While during the pollution episodes, solid fuel sources including coal combustion and biomass burning were the predominant sources, accounting for 61-83% of BC. Specific source tracers suggested that coal combustion and biomass burning dominated in different pollution episodes. Ratios of BC/PM_2.5 and BC/CO as well as source tracers provided further supportive evidences for the source apportionment results. Our findings suggest that it is more important to control solid fuel sources such as coal combustion for BC abatement in Beijing during haze days, while liquid fuel source (e.g., vehicle emission) plays a relatively more important role in clean days compared to pollution episodes.

Insights into the characteristics and sources of primary and secondary organic carbon: High time resolution observation in urban Shanghai

There is growing evidence suggesting that organic aerosols play an important role in the evolution of severe haze episodes. However, long-term investigations of the different characteristics of carbonaceous aerosols during haze and non-haze days are insufficient. In this work, hourly measurements of organic carbon (OC) and elemental carbon (EC) in PM_2.5 were conducted in Shanghai, a megacity in Eastern China, over the course of a year from July 2013 to June 2014. Both OC and EC exhibited a bimodal diel pattern and were highly dependent on the wind speed and direction. The concentration-weighted trajectory (CWT) analysis illustrated that primary OC (POC) and EC were largely associated with regional and long-range transport. Secondary OC (SOC) formation was the strongest during the harvest season owing to significant biomass burning emissions from the adjacent Yangtze River Delta and farther agricultural regions. Compared to OC (6.7 μg m^-3) and EC (2.0 μg m^-3) in the non-haze days, higher levels of both OC (15.6 μg m^-3) and EC (7.7 μg m^-3) were observed in the haze days as expected, but with lower OC/EC ratios in the haze days (2.4) than in non-haze days (4.6). The proportion of POC and EC in PM_2.5 remained relatively constant as a function of PM_2.5 mass loadings, while that of SOC significantly decreased on the highly polluted days. It is concluded that the haze pollution in urban Shanghai was influenced more by the primary emissions (POC and EC), while the role of SOC in triggering haze was limited.

Ambient particulate matter and carbon monoxide at an urban site of India: Influence of anthropogenic emissions and dust storms

Continuous measurements of PM_2.5, PM₁₀ and CO were conducted at an urban site of Udaipur in India from April 2011 to March 2012. The annual mean concentrations of PM_2.5, PM₁₀ and CO were 42 ± 17 μg m^-3, 114 ± 31 μg m^-3 and 343 ± 136 ppbv, respectively. Concentrations of both particulate and CO showed high values during winter/pre-monsoon (dry) period and lowest in the monsoon season (wet). Local anthropogenic emission and long-range transport from open biomass burning sources along with favourable synoptic meteorology led to elevated levels of pollutants in the dry season. However, higher values of PM₁₀/PM_2.5 ratio during pre-monsoon season were caused by the episodes of dust storm. In the monsoon season, flow of cleaner air, rainfall and negligible emissions from biomass burning resulted in the lowest levels of pollutants. The concentrations of PM_2.5, PM₁₀ and CO showed highest values during morning and evening rush hours, while lowest in the afternoon hours. In winter season, reductions of PM_2.5, CO and PM₁₀ during weekends were highest of 15%, 13% and 9%, respectively. In each season, the highest PM_2.5/PM₁₀ ratio coincided with the highest concentrations of pollutants (CO and NO_X) indicating predominant emissions from anthropogenic sources. Exceptionally high concentrations of PM₁₀ during the episode of dust storm were due to transport from the Arabian Peninsula and Thar Desert. Up to ∼32% enhancements of PM₁₀ were observed during strong dust storms. Relatively low levels of O₃ and NO_x during the storm periods indicate the role of heterogeneous removal.

Significant cooling effect on the surface due to soot particles over Brahmaputra River Valley region, India: An impact on regional climate

Black carbon (BC) is an important atmospheric aerosol constituent that affects the climate by absorbing (directly) the sunlight and modifying cloud characteristics (indirectly). Here, we present first time yearlong measurements of BC and carbon monoxide (CO) from an urban location of Guwahati located in the Brahmaputra River valley (BRV) in the northeast region of India from 1st July 2013 to 30th June 2014. Daily BC concentrations varied within the range of 2.86 to 11.56μgm(-3) with an annual average of 7.17±1.89μgm(-3), while, CO varied from 0.19 to 1.20ppm with a mean value of 0.51±0.19ppm during the study period. The concentrations of BC (8.37μgm(-3)) and CO (0.67ppm) were ~39% and ~55% higher during the dry months (October to March) than the wet months (April to September) suggesting that seasonal changes in meteorology and emission sources play an important role in controlling these species. The seasonal ΔBC/ΔCO ratios were highest (lowest) in the pre-monsoon (winter) 18.1±1.4μgm(-3)ppmv(-1) (12.6±2.2μgm(-3)ppmv(-1)) which indicate the combustion of biofuel/biomass as well as direct emissions from fossil fuel during the pre-monsoon season. The annual BC emission was estimated to be 2.72Gg in and around Guwahati which is about 44% lower than the mega city 'Delhi' (4.86Gg). During the study period, the annual mean radiative forcing (RF) at the top of the atmosphere (TOA) for clear skies of BC was +9.5Wm(-2), however, the RF value at the surface (SFC) was -21.1Wm(-2) which indicates the net warming and cooling effects, respectively. The highest RF at SFC was in the month of April (-30Wm(-2)) which is coincident with the highest BC mass level. The BC atmospheric radiative forcing (ARF) was +30.16 (annual mean) Wm(-2) varying from +23.1 to +43.8Wm(-2). The annual mean atmospheric heating rate (AHR) due to the BC aerosols was 0.86Kday(-1) indicates the enhancement in radiation effect over the study region. The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) captured the seasonal cycle of observed BC fairly well but underestimated the observed BC during the month of May-August. Model results show that BC at Guwahati is controlled mainly by anthropogenic emissions except during the pre-monsoon season when open biomass burning also makes a similar contribution.

Source appointment of fine particle number and volume concentration during severe haze pollution in Beijing in January 2013

Extreme haze episodes repeatedly shrouded Beijing during the winter of 2012-2013, causing major environmental and health problems. To better understand these extreme events, particle number size distribution (PNSD) and particle chemical composition (PCC) data collected in an intensive winter campaign in an urban site of Beijing were used to investigate the sources of ambient fine particles. Positive matrix factorization (PMF) analysis resolved a total of eight factors: two traffic factors, combustion factors, secondary aerosol, two accumulation mode aerosol factors, road dust, and long-range transported (LRT) dust. Traffic emissions (54%) and combustion aerosol (27%) were found to be the most important sources for particle number concentration, whereas combustion aerosol (33%) and accumulation mode aerosol (37%) dominated particle volume concentrations. Chemical compositions and sources of fine particles changed dynamically in the haze episodes. An enhanced role of secondary inorganic species was observed in the formation of haze pollution. Regional transport played an important role for high particles, contribution of which was on average up to 24-49% during the haze episodes. Secondary aerosols from urban background presented the largest contributions (45%) for the rapid increase of fine particles in the severest haze episode. In addition, the invasion of LRT dust aerosols further elevated the fine particles during the extreme haze episode. Our results showed a clear impact of regional transport on the local air pollution, suggesting the importance of regional-scale emission control measures in the local air quality management of Beijing.

Optical and thermal characteristics of carbonaceous aerosols measured at an urban site in Gwangju, Korea, in the winter of 2011

Carbonaceous components (organic carbon [OC] and elemental carbon [EC]) and optical properties (light absorption and scattering) of fine particulate matter (aerodynamic diameter <2.5 μm; PM2.5) were simultaneously measured at an urban site in Gwangju, Korea, during the winter of 2011. OC was further classified into OC1, OC2, OC3, and OC4, based on a temperature protocol using a Sunset OC/EC analyzer. The average OC and EC concentrations were 5.0 ± 2.5 and 1.7 ± 0.9 μg C m(-3), respectively. The average single-scattering albedo (SSA) at a wavelength of 550 nm was 0.58 ± 0.11, suggesting that the aerosols observed in the winter of 2011 had a local warming effect in this area. During the whole sampling period, "stagnant PM" and "long-range transport PM" events were identified. The light absorption coefficient (babs) was higher during the stagnant PM event than during the long-range transport PM event due to the existence of abundant light-absorbing OC during the stagnant PM event. In particular, the OC2 and OC3 concentrations were higher during the stagnant PM event than those during the long-range transport event, suggesting that OC2 and OC3 might be more related to the light-absorbing OC. The light scattering coefficient (bscat) was similar between the events. On average, the mass absorption efficiency attributed to EC (σEC) was 9.6 m(2) g(-1), whereas the efficiency attributed to OC (σOC) was 1.8 m(2) g(-1) at λ = 550 nm. Furthermore, the σEC is comparable among the PM event days, but the σOC for the stagnant PM event was significantly higher than that for the long-range transport PM event (1.7 vs. 0.5).

IMPLICATIONS

Optical and thermal properties of carbonaceous aerosol were measured at Gwangju, and carbonaceous aerosol concentration and optical property varied between "stagnant PM" and "long-range transport PM" events. More abundant light absorbing OC was observed during the stagnant PM event.

Characterization and sources of black carbon in PM(2.5) at a site close to a roadway in Gwangju, Korea, during winter

Continuous measurements of black carbon (BC) concentrations in PM2.5 were conducted using a single-wavelength aethalometer (@880 nm, Magee Sci., AE16) at a site close to a roadway (∼70 m from roadside) in Gwangju, Korea, during winter (December-February) to investigate the characteristics and sources of BC particles. The BC concentrations ranked in the order of January > December > February, probably due to lower boundary layer height, ambient temperature, and wind speed during January. Diurnal patterns in BC and carbon monoxide (CO) levels exhibited peak concentrations during the morning and evening hours coinciding with rush-hour traffic, with a strong correlation (R(2)) ranging from 0.52 (December) to 0.87 (January). It was found that wind speed was an important factor controlling BC concentrations at the site. Very high BC concentrations, up to ∼18.0 μg m(-3), were observed at wind speeds < 1.5 m s(-1). The BC concentrations acquired under weak wind conditions are highly correlated with CO with ΔBC/ΔCO (the slope of BC and CO correlation) of 0.0063 (R(2) = 0.55, p < 0.01) and 0.0065 (R(2) = 0.59, p < 0.01) μg m(-3) ppbv(-1) during day and night, respectively, suggesting no significant difference in the fraction of diesel vehicles between the daytime and nighttime periods. Two BC episodes, "A" and "B", were classified based on BC, PM2.5, and secondary SO4(2-) concentrations, and discussed to investigate the difference in the evolution of the BC observed. Episode "A" was associated with high BC and low PM2.5 and SO4(2-) concentrations, while episode "B" was associated with high concentrations of BC, PM2.5, and SO4(2-). Based on the temporal profiles of BC, NO, and NOx concentrations, CO/NOx ratio, and potential source contribution function map for BC, the BC observed during episode "A" was mostly attributed to locally produced emissions (e.g., traffic). However, the BC during episode "B" was influenced by long-range transport of air masses from China, as well as the local emissions.

Sources of primary and secondary organic aerosol and their diurnal variations

PM(2.5), as one of the criteria pollutants regulated in the U.S. and other countries due to its adverse health impacts, contains more than hundreds of organic pollutants with different sources and formation mechanisms. Daytime and nighttime PM2.5 samples from the August Mini-Intensive Gas and Aerosol Campaign (AMIGAS) in the southeastern U.S. were collected during summer 2008 at one urban site and one rural site, and were analyzed for organic carbon (OC), elemental carbon (EC), water soluble organic carbon (WSOC), and various individual organic compounds including some important tracers for carbonaceous aerosol sources by gas chromatography-mass spectrometry. Most samples exhibited higher daytime OC concentration, while higher nighttime OC was found in a few events at the urban site. Sources, formation mechanisms and composition of organic aerosol are complicated and results of this study showed that it exhibited distinct diurnal variations. With detailed organic tracer information, sources contributing to particulate OC were identified: higher nighttime OC concentration occurring in several occasions was mainly contributed by the increasing primary emissions at night, especially diesel exhaust and biomass burning; whereas sources responsible for higher daytime OC concentration included secondary organic aerosol (SOA) formation (e.g., cis-pinonic acid and non-biomass burning WSOC) together with traffic emissions especially gasoline engine exhaust. Primary tracers from combustion related sources such as EC, polycyclic aromatic hydrocarbons, and hopanes and steranes were significantly higher at the urban site with an urban to rural ratio between 5 and 8. However, this urban-rural difference for secondary components was less significant, indicating a relatively homogeneous distribution of SOA spatially. We found cholesterol concentrations, a typical tracer for meat cooking, were consistently higher at the rural site especially during the daytime, suggesting the likely additional sources for this tracer at rural site and that it should be used with caution as meat cooking tracer in rural areas in the future.

Chemical characteristics of fine particulate matters measured during severe winter haze events in Ulaanbaatar, Mongolia

In order to investigate the chemical characteristics of atmospheric aerosol measured during a severe winter haze event, 12-hr PM2.5 (particulate matter with an aerodynamic diameter < or = 2.5 microm) samples were collected at an urban site in Ulaanbaatar Mongolia, from January 9 to February 17, 2008. On average, 12-hr PM2.5 mass concentration was 105.1 +/- 34.9 microg/m3. Low PM2.5 mass concentrations were measured when low pressure developed over central Mongolia. The 12-hr average organic mass by carbon (OMC) varied from 6.4 to 132.3 microg/m3, with a mean of 54.9 +/- 25.4 microg/m3, whereas elemental carbon (EC) concentration ranged from 0.1 to 3.6 microgC/m3, with a mean of 1.5 +/- 0.8 microgC/m3. Ammonium sulfate was found to be the most abundant water-soluble ionic component in Ulaanbaatar during the sampling period, with an average concentration of 11.3 +/- 5.0 microg/m3. In order to characterize the effect of air mass pathway on fine particulate matter characteristics, 5-day back-trajectory analysis was conducted, using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. The haze level was classified into three categories, based on the 5-day air mass back trajectories, as Stagnant (ST), Continental (CT), and Low Pressure (LP) cases. PM2.5 mass concentration during the Stagnant condition was approximately 2.5 times higher than that during the Low Pressure condition, mainly due to increased pollutant concentration of OMC and secondary ammonium sulfate.

Time-resolved measurements of PM2.5 carbonaceous aerosols at Gosan, Korea

In order to better understand the characteristics of atmospheric carbonaceous aerosol at a background site in Northeast Asia, semicontinuous organic carbon (OC) and elemental carbon (EC), and time-resolved water-soluble organic carbon (WSOC) were measured by a Sunset OC/ EC and a PILS-TOC (particle-into-liquid sampler coupled with an online total organic carbon) analyzer, respectively, at the Gosan supersite on Jeju Island, Korea, in the summer (May 28-June 17) and fall (August 24-September 30) of 2009. Hourly average OC concentration varied in the range of approximately 0.87-28.38 microgC m-3, with a mean of 4.07+/- 2.60 microgC m-3, while the hourly average EC concentration ranged approximately from 0.04 to 8.19 .microgC m-3, with a mean of 1.35 +/- 0.71 microgC m-3, from May 28 to June 17, 2009. During the fall season, OC varied in the approximate range 0.9-9.6 microgC m-3, with a mean of 2.30 +/-0.80 microgC m-3, whereas EC ranged approximately from 0.01 to 5.40 microgC m-3, with a mean of 0.66 +/- 0.38 microgC m-3. Average contributions of EC to TC and WSOC to OC were 26.0% +/- 9.7% and 20.6% +/-7.4%, and 37.6% +/- 23.5% and 57.2% +/- 22.2% during summer and fall seasons, respectively. As expected, clear diurnal variation of WSOC/OC was found in summer, varying from 0.22 during the nighttime up to 0.72 during the daytime, mainly due to the photo-oxidation process. In order to investigate the effect of air mass pathway on the characteristics of carbonaceous aerosol, 5-day back-trajectory analysis was conducted using the HYSPLIT model. The air mass pathways were classified into four types: Continental (CC), Marine (M), East Sea (ES) and Korean Peninsula (KP). The highest OC/EC ratio of 3.63 was observed when air mass originated from the Continental area (CC). The lowest OC/EC ratio of 0.79 was measured when air mass originated from the Marine area (M). A high OC concentration was occasionally observed at Gosan due to local biomass burning activities. The contribution of secondary OC to total OC varied approximately between 8.4% and 32.2% and depended on air mass type.