Secondary Organic Formation Research Articles

Patterns in atmospheric carbonaceous aerosols in China: emission estimates and observed concentrations

Abstract. China is experiencing severe carbonaceous aerosol pollution driven mainly by large emissions resulting from intensive use of solid fuels. To gain a better understanding of the levels and trends of carbonaceous aerosol emissions and the resulting ambient concentrations at the national scale, we update an emission inventory of anthropogenic organic carbon (OC) and elemental carbon (EC) and employ existing observational studies to analyze characteristics of these aerosols including temporal, spatial, and size distributions, and the levels and shares of secondary organic carbon (SOC) in total OC. We further use ground observations to test the levels and inter-annual trends of the calculated national and provincial emissions of carbonaceous aerosols, and propose possible improvements in emission estimation for the future. The national OC emissions are estimated to have increased 29 % from 2000 (2127 Gg) to 2012 (2749 Gg) and EC by 37 % (from 1356 to 1857 Gg). The residential, industrial, and transportation sectors contributed an estimated 74–78, 17–21, and 4–6 % of the total emissions of OC, respectively, and 49–55, 30–34, and 14–18 % of EC. Updated emission factors (EFs) based on the most recent local field measurements, particularly for biofuel stoves, led to considerably lower emissions of OC compared to previous inventories. Compiling observational data across the country, higher concentrations of OC and EC are found in northern and inland cities, while higher OC / EC ratios are found in southern sites, due to the joint effects of primary emissions and meteorology. Higher OC / EC ratios are estimated at rural and remote sites compared to urban ones, attributed to more emissions of OC from biofuel use, more biogenic emissions of volatile organic compound (VOC) precursors to SOC, and/or transport of aged aerosols. For most sites, higher concentrations of OC, EC, and SOC are observed in colder seasons, while SOC / OC is reduced, particularly at rural and remote sites, attributed partly to weaker atmospheric oxidation and SOC formation compared to summer. Enhanced SOC formation from oxidization and anthropogenic activities like biomass combustion is judged to have crucial effects on severe haze events characterized by high particle concentrations. Several observational studies indicate an increasing trend in ambient OC / EC (but not in OC or EC individually) from 2000 to 2010, confirming increased atmospheric oxidation of OC across the country. Combining the results of emission estimation and observations, the improvement over prior emission inventories is indicated by inter-annual comparisons and correlation analysis. It is also indicated, however, that the estimated growth in emissions might be faster than observed growth, and that some sources with high primary OC / EC, such as burning of biomass, are still underestimated. Further studies to determine changing EFs over time in the residential sector and to compare to other measurements, such as satellite observations, are thus suggested to improve understanding of the levels and trends of primary carbonaceous aerosol emissions in China.

Carbonaceous aerosols over Pune and Hyderabad (India) and influence of meteorological factors

Sampling of particulate matter with aerodynamic diameter cut off at 2.5 μm (PM2.5) has been carried out over a semi urban site of Pune and an urban site of Hyderabad. Analysis of elemental Carbon (EC) and Organic Carbon (OC) present in PM2.5 was carried out using advanced Desert Research Institute’s (DRI) Thermal/Optical Carbon Analyzer operated on IMPROVE_ A (Interagency Monitoring of Protected Visual Environments_ A) protocol. It is found that average concentration of EC and OC both at Pune and Hyderabad was highest during winter season and lowest during monsoon season. Winter high is mainly controlled by inversion, whereas monsoon low is due to rain-out and wash-out process. OC/EC ratio showed higher variation over Pune compared to that over Hyderabad in different seasons, indicating divergent sources of emission of OC and EC at Pune. Formation of Secondary Organic Carbon (SOC) has also been identified as one of the reasons for wide variation in OC/EC ratio value in different seasons over both the sites.

Characterization of carbonaceous aerosols at Mount Lu in South China: implication for secondary organic carbon formation and long-range transport.

In order to understand the sources and potential formation processes of atmospheric carbonaceous aerosols in South China, fine particle samples were collected at a high-elevation mountain site--Mount Lu (29°35' N, 115°59' E, 1165 m A.S.L.) during August-September, 2011. Eight carbonaceous fractions from particles were resolved following the IMPROVE thermal/optical reflectance protocol. During the observation campaign, the daily concentrations of PM2.5 at Mount Lu ranged from 7.69 to 116.39 μg/m(3), with an average of 58.76 μg/m(3). The observed average organic carbon (OC) and elemental carbon (EC) concentrations in PM2.5 were 3.78 and 1.28 μg/m(3), respectively. Secondary organic carbon (SOC) concentration, estimated by EC-tracer method, was 2.07 μg/m(3) on average, accounting for 45.0% of the total OC. The enhancement of secondary organic aerosol (SOA) formation was observed during cloud/fog processing, and heterogeneous acid-catalyzed reactions may have contributed to SOA formation as well. Back trajectory analysis indicated that air masses were mainly sourced from southern China during observation period, and this air mass source was featured by highest values of OC and effective carbon ratio (ECR). Relation of carbonaceous species and principal component analysis indicated that multiple sources contributed to the carbonaceous aerosols at Mount Lu.

Evaluation of the ability of the EC tracer method to estimate secondary organic carbon

The elemental carbon (EC) tracer method has often been used to estimate the primary and secondary organic aerosol (OA) fractions using field measurements of organic carbon (OC) and EC. In this observation-based approach, EC is used as a tracer for primary OC (POC), which allows for the estimation of secondary OC (SOC). The accuracy of this approach is evaluated using concentrations generated by PMCAMx, a three-dimensional chemical transport model that simulates the complex processes leading to SOC formation (including evaporation and chemical processing of POC and chemical aging of semivolatile and intermediate volatility organics). The ratio of primary organic to elemental carbon [OC/EC]p is estimated in various locations in the Eastern United States, and is then used to calculate the primary and secondary OC concentrations. To estimate the [OC/EC]p from simulated concentrations, we use both a traditional approach and the high EC edge method, in which only values with the highest EC/OC ratio are used. Both methods perform best on a daily-averaged basis, because of the variability of the [OC/EC]p ratio during the day. The SOC estimated by the EC tracer methods corresponds to the biogenic and anthropogenic SOC formed during the oxidation of volatile organic compounds. On the other hand, the estimated POC corresponds to the sum of the fresh POC, the SOC from oxidation of the evaporated POC and the intermediate volatility organic compounds, and the OC from long-distance transport. With this correspondence, the traditional EC tracer method tends to overpredict primary OC and underpredict secondary OC for the selected urban areas in the eastern United States. The high EC edge method performs better, especially in areas where the primary contribution to OC is smaller. Despite the weaknesses of models like the one used here, the conclusions about the accuracy of observation-based methods like the EC-tracer approach should be relatively robust due to the internal consistency of the data and the approach.

Atmospheric oxidation of isoprene and 1,3-butadiene: influence of aerosol acidity and relative humidity on secondary organic aerosol

Abstract. The effects of acidic seed aerosols on the formation of secondary organic aerosol (SOA) have been examined in a number of previous studies, several of which have observed strong linear correlations between the aerosol acidity (measured as nmol H+ m−3 air sample volume) and the percent change in secondary organic carbon (SOC). The measurements have used several precursor compounds representative of different classes of biogenic hydrocarbons including isoprene, monoterpenes, and sesquiterpenes. To date, isoprene has displayed the most pronounced increase in SOC, although few measurements have been conducted with anthropogenic hydrocarbons. In the present study, we examine several aspects of the effect of aerosol acidity on the secondary organic carbon formation from the photooxidation of 1,3-butadiene, and extend the previous analysis of isoprene. The photooxidation products measured in the absence and presence of acidic sulfate aerosols were generated either through photochemical oxidation of SO2 or by nebulizing mixtures of ammonium sulfate and sulfuric acid into a 14.5 m3 smog chamber system. The results showed that, like isoprene and β-caryophyllene, 1,3-butadiene SOC yields linearly correlate with increasing acidic sulfate aerosol. The observed acid sensitivity of 0.11% SOC increase per nmol m−3 increase in H+ was approximately a factor of 3 less than that measured for isoprene. The results also showed that the aerosol yield decreased with increasing humidity for both isoprene and 1,3-butadiene, although to different degrees. Increasing the absolute humidity from 2 to 12 g m−3 reduced the 1,3-butadiene yield by 45% and the isoprene yield by 85%.

Seasonal Variation of PM2.5 Components Observed in an Industrial Area of Chiba Prefecture, Japan

In order to survey the seasonal variation of the chemical composition of particulate matter of 2.5 μm or less (PM 2.5 ), PM 2.5 was sampled from 8 February 2013 to 31 March 2014 in an industrial area of Chiba Prefecture, Japan. Chemical measurements of the sample included: ionic components (Na+, NH 4+ , Ca 2+ , Mg 2+ , K + , Cl - , NO 3- and SO₄ 2- ), carbonaceous components - organic carbon (OC) and elemental carbon (EC), and water-soluble organic carbon (WSOC). Also, secondary organic carbon (SOC) was measured based using the EC tracer method, and char-EC and soot-EC were calculated from the analytical results. The data obtained were interpreted in terms of temporal variation. Of the overall mean value of PM 2.5 mass concentration obtained during the study period, ionic components, OC and EC accounted for 45.3%, 19.7%, and 8.0%, respectively. NO 3- showed a unique seasonal distribution pattern due to a dependence on temperature and absolute humidity. It was estimated that an approximate temperature of 14℃ and absolute humidity of 7 g/m³ were critical for the reversible reaction of NH₄NO₃ (p) ≒ NH₃ (g)+HNO₃(g). The amount of OC and EC contributing to the monthly PM 2.5 mass concentration was higher in autumn and winter compared to spring and summer. This result could be attributed to the impact of burning biomass, since WSOC and the ratio of char-EC/soot-EC showed a similar pattern during the corresponding period. From the comparison of monthly WSOC/OC values, a maximum ratio of 83% was obtained in August (summer). The WSOC and estimated SOC levels derived from the EC tracer method correlated (R=0.77) in summer. The high occurrence of WSOC during summer was mainly due to the formation of SOC by photochemical reactions. Through long-term observation of PM 2.5 chemical components, we established that the degree to which the abovementioned factors influence PM 2.5 composition, fluctuates with seasonal changes.

Industrial sources of primary and secondary organic aerosols in two urban environments in Spain.

In urban areas, primary and secondary organic aerosols are typically considered to originate from vehicular traffic emissions. However, industrial emissions within or in the vicinity of urban areas may also be significant contributors to carbonaceous aerosol concentrations. This hypothesis was tested and validated in two urban areas in Spain. The observed unusual dominance of organic carbon (OC) over elemental carbon (EC), the analysis of the variability of OC, EC and OC/EC and their correlation with transport patterns suggested the presence of OC sources associated with industrial activities. A methodology based on chemical speciation of particulate matter (PM) followed by the application of receptor modelling techniques allowed for the identification of the specific industrial sources of OC, which were linked to primary OC emissions from a grain drying plant (cereal) and to secondary OC formation from paper production activities (paper mills), as well as from urban sources and biogenic emissions. This work presents an integrated approach to identifying and characterizing of industrial sources of carbonaceous aerosols in urban areas, aiming to improve the scarce body of literature currently available on this topic.

Fossil vs. non-fossil sources of fine carbonaceous aerosols in four Chinese cities during the extreme winter haze episode of 2013

Abstract. During winter 2013, extremely high concentrations (i.e., 4–20 times higher than the World Health Organization guideline) of PM2.5 (particulate matter with an aerodynamic diameter < 2.5 μm) mass concentrations (24 h samples) were found in four major cities in China including Xi'an, Beijing, Shanghai and Guangzhou. Statistical analysis of a combined data set from elemental carbon (EC), organic carbon (OC), 14C and biomass-burning marker measurements using Latin hypercube sampling allowed a quantitative source apportionment of carbonaceous aerosols. Based on 14C measurements of EC fractions (six samples each city), we found that fossil emissions from coal combustion and vehicle exhaust dominated EC with a mean contribution of 75 ± 8% across all sites. The remaining 25 ± 8% was exclusively attributed to biomass combustion, consistent with the measurements of biomass-burning markers such as anhydrosugars (levoglucosan and mannosan) and water-soluble potassium (K+). With a combination of the levoglucosan-to-mannosan and levoglucosan-to-K+ ratios, the major source of biomass burning in winter in China is suggested to be combustion of crop residues. The contribution of fossil sources to OC was highest in Beijing (58 ± 5%) and decreased from Shanghai (49 ± 2%) to Xi'an (38 ± 3%) and Guangzhou (35 ± 7%). Generally, a larger fraction of fossil OC was from secondary origins than primary sources for all sites. Non-fossil sources accounted on average for 55 ± 10 and 48 ± 9% of OC and total carbon (TC), respectively, which suggests that non-fossil emissions were very important contributors of urban carbonaceous aerosols in China. The primary biomass-burning emissions accounted for 40 ± 8, 48 ± 18, 53 ± 4 and 65 ± 26% of non-fossil OC for Xi'an, Beijing, Shanghai and Guangzhou, respectively. Other non-fossil sources excluding primary biomass burning were mainly attributed to formation of secondary organic carbon (SOC) from non-fossil precursors such as biomass-burning emissions. For each site, we also compared samples from moderately to heavily polluted days according to particulate matter mass. Despite a significant increase of the absolute mass concentrations of primary emissions from both fossil and non-fossil sources during the heavily polluted events, their relative contribution to TC was even decreased, whereas the portion of SOC was consistently increased at all sites. This observation indicates that SOC was an important fraction in the increment of carbonaceous aerosols during the haze episode in China.

Seasonal variation of urban carbonaceous aerosols in a typical city Nanjing in Yangtze River Delta, China

The Yangtze River Delta (YRD) is one of the regions with the most dynamic economy and severe atmospheric pollution in China. In order to characterize the particle features, especially the carbonaceous component in the YRD, particulate matter smaller than 2.5 μm (PM2.5) and 10 μm (PM10) samples in each season were collected in urban Nanjing, a typical city that locates in the west part of the YRD. The organic carbon (OC) and elemental carbon (EC) was differentiated using the thermal optical reflectance method. The average concentrations of PM2.5, OC and EC during the study periods were observed to be 117.6, 13.8, and 5.3 μg/m3 respectively, with all the highest levels in winter. The mass fraction of the Total carbonaceous aerosol (TCA) in PM2.5 was estimated at 23% on average, lower than those reported for other cities in the YRD. The OC and EC correlated well in all the seasons, especially in spring and winter, implying that OC and EC were attributed to common emission sources. Good correlation was observed between OC and estimated K+ from biomass burning in the harvest season in autumn and summer, indicating biomass burning a significant source of carbonaceous aerosols. This could also be confirmed by the lower fraction of OC3 + OC4 in OC during autumn and summer. The secondary organic carbon (SOC) estimated by EC-tracer method was the highest in winter (7.3 μg/m3) followed by autumn (6.7 μg/m3), summer (3.7 μg/m3) and spring (2.0 μg/m3). However, the SOC/OC in winter was not as high as that in summer and autumn, implying the high concentration of OC in winter was probably due to the stable weather but not mainly caused by SOC formation. The high SOC/OC ratio in summer was attributed to stronger oxidation, which could be suggested by higher sulfur oxidation ratio (SOR).

Characterization of Particulate Matter and Carbonaceous Aerosol over Two Urban Environments in Northern India

Monitoring and simultaneous sampling of Particulate matter (PM10 and PM2.5) was carried out for the first time over two urban sites in Northern India (Jabalpur and Udaipur) during December 2010–November 2012 (up to August 2012 over Udaipur). The samples of PM2.5 were analyzed for elemental carbon (EC) and organic carbon (OC) using advanced DRI Thermal optical carbon Analyzer. The monthly mean PM10 values were as high as 149 ± 44 µg m–3 over Jabalpur (JBL) and 171 ± 42.2 µg m–3 over Udaipur (UDPR). PM2.5 mass over JBL varied between 25–79 µg m–3 and over UDPR between 24–82 µg m–3. The monthly mean OC concentration varied from 12.5 ± 7.3 µg m–3 to 28.4 ± 10.7 µg m–3 over JBL and from 7.8 ± 2.9 to 39.7 ± 11.6 µg m–3 over UDPR. The variation of monthly mean EC concentration was from 3.9–10.3 µg m–3 over JBL and from 3–10.9 µg m–3 over UDPR. The contribution of TC to PM2.5 was in the range of 31–75% over JBL and 30–83% over UDPR. The EC showed higher concentration in winter and minimal values in monsoon. The OC/EC ratio showed low variation over JBL compared to that over UDPR suggesting spectrum of sources responsible for EC and OC components over UDPR. Formation of secondary organic carbon (SOC) was also recognized as a potential component altering OC/EC ratio. Up on extracting the sub fractions, it is found that OC2 and OC3 are the major component over both the sites contributing up to 51% to the total OC in different months. EC1 (EC component derived at 580°C) was found to be the major EC component contributing up to 79% over the sites.

Characterizing Particulate Pollutants in an Enclosed Museum in Shanghai, China

The present study reported simultaneous measurements of particle number concentration (PNC), particle mass concentration, water soluble organic carbon (WSOC), organic carbon (OC), elemental carbon (EC), particulate morphology and main elemental properties. The measurements were conducted in a museum located in the downtown area of Shanghai. The impacts of storey, particle size and decorative flooring on particle characteristics inside museum were investigated. Size fractionated PNC in display halls belonging to the same storey exhibited similar daily variations. Particles within the range of 0.3-0.5 µm were identified as the most significant contributors to the overall size distribution measured, as ambient fine particles penetrated through the mechanical ventilation system. Multi-lognormal fitting OC concentrations can be assigned to the presence of tourists in the museum or originate from secondary OC formation. The latter can be estimated by comparing the relationship between OC and EC, as well as OC and WSOC.

Size distribution of carbonaceous aerosols at a high-altitude site on the central Tibetan Plateau (Nam Co Station, 4730 m a.s.l.)

The chemical composition and size distribution characteristics of atmospheric aerosols have important effects on the environment, human health and climate change. In this paper, we study the size distribution of carbonaceous aerosols at the remote and pristine site, Nam Co Monitoring and Research Station for Multisphere Interactions, in the inland Tibetan Plateau (TP) based on collected size-segregated aerosols during 2012. The samples were quantified using the thermal/optical (TOR) method. The overall average concentrations of OC and EC in TSP, PM9.0, PM2.1, and PM1.0 were 4.61μgm−3 and 0.19μgm−3, 4.52μgm−3 and 0.18μgm−3, 2.72μgm−3 and 0.11μgm−3, and 2.11μgm−3 and 0.09μgm−3, respectively. Generally, the highest concentration of OC and EC in different aerosol size occurred during winter. The low level of EC indicated that direct anthropogenic disturbances in the interior of the TP still remain insignificant.The size distributions of OC and EC concentrations presented bimodal variations. In winter, pre-monsoon, monsoon, and post-monsoon seasons, the peaks for OC were in droplet mode (0.43–0.65μm) and coarse mode (4.7–5.8μm); while in the monsoon period, the coarse mode shifted to a smaller size bin (3.3–4.7μm). The coarse mode may be due to dust particles while the droplet mode may be due to the growth process of particles. For EC, the peaks variations in coarse mode were as same as OC, while the other peaks were complicated: the peaks during winter, pre-monsoon, and monsoon seasons exhibited in droplet mode (1.1–2.1μm, 0.65–1.1μm, and 0.43–0.65μm, respectively), and in post-monsoon period, the peak located in condensation mode. The highest peak concentrations for OC and EC occurred in winter and the pre-monsoon period, while the lowest peak values in the monsoon and post-monsoon periods, respectively. The size distribution variations may be caused by deposition, gas/particles exchange, hygroscopic growth, external mixing, and secondary organic carbon formation. OC/EC ratios in aerosols over the TP mostly exhibit high values, which emphasizes the importance of OC over this region.

Diurnal cycle of fossil and nonfossil carbon using radiocarbon analyses during CalNex

AbstractRadiocarbon (14C) analysis is a unique tool to distinguish fossil/nonfossil sources of carbonaceous aerosols. We present 14C measurements of organic carbon (OC) and total carbon (TC) on highly time resolved filters (3–4 h, typically 12 h or longer have been reported) from 7 days collected during California Research at the Nexus of Air Quality and Climate Change (CalNex) 2010 in Pasadena. Average nonfossil contributions of 58% ± 15% and 51% ± 15% were found for OC and TC, respectively. Results indicate that nonfossil carbon is a major constituent of the background aerosol, evidenced by its nearly constant concentration (2–3 μgC m−3). Cooking is estimated to contribute at least 25% to nonfossil OC, underlining the importance of urban nonfossil OC sources. In contrast, fossil OC concentrations have prominent and consistent diurnal profiles, with significant afternoon enhancements (~3 μgC m−3), following the arrival of the western Los Angeles (LA) basin plume with the sea breeze. A corresponding increase in semivolatile oxygenated OC and organic vehicular emission markers and their photochemical reaction products occurs. This suggests that the increasing OC is mostly from fresh anthropogenic secondary OC (SOC) from mainly fossil precursors formed in the western LA basin plume. We note that in several European cities where the diesel passenger car fraction is higher, SOC is 20% less fossil, despite 2–3 times higher elemental carbon concentrations, suggesting that SOC formation from gasoline emissions most likely dominates over diesel in the LA basin. This would have significant implications for our understanding of the on‐road vehicle contribution to ambient aerosols and merits further study.

Carbonaceous species in PM2.5 in the coastal urban agglomeration in the Western Taiwan Strait Region, China

The carbonaceous species and potassium in PM2.5 were measured in the coastal urban agglomeration in the Western Taiwan Strait Region (WTSR), China in November 2010, January, April and August 2011, with the objectives to understand the spatiotemporal distribution of carbonaceous aerosols and to estimate the contribution from biomass burning and non-biomass burning to carbonaceous aerosols in this region. The average concentrations of organic carbon (OC) and elemental carbon (EC) in PM2.5 at the urban sites in the WTSR were 12.7±4.3μgm−3 and 2.3±0.9μgm−3, respectively, with the maxima in January/April and minima in August for most sites. It was found that the OC concentrations on no-rain days were higher than those on rainy days due to precipitation scavenging. Total carbonaceous aerosol (OC×1.6+EC) made up 32.1±15.6% of PM2.5. High OC/EC ratios (average 5.8) indicated the formation of secondary organic carbon (SOC), which contributed higher to the OC in January than that in August. Strong correlations were observed between biomass-burning potassium (K+bb) and OC (R2=0.56, p<0.01) and EC (R2=0.54, p<0.01) in November. It was roughly estimated that 42.1±15.1% of total carbon (TC) in November were attributed to biomass burning, which suggested that non-biomass burning was the main source of carbonaceous materials in aerosols in the coastal urban agglomeration in the WTSR.

Formation of secondary organic carbon and long-range transport of carbonaceous aerosols at Mount Heng in South China

To understand the sources and formation processes of atmospheric carbonaceous aerosols in rural and mountainous areas of South China, an intensive measurement campaign was conducted at the summit of Mount Heng (27°18′N, 112°42′E, 1269 m asl) during the spring of 2009. The observed average concentrations of organic carbon (OC) and elemental carbon (EC) were 3.01 ± 2.2 and 0.54 ± 0.3 μg m−3, respectively. The total carbonaceous aerosols (TCA) averagely contributed to 20.7% of PM2.5. High OC/EC ratios (range: 1.6–10.4; average: 5.2 ± 1.8) were observed, suggesting the transport and secondary origins of the carbonaceous aerosols at Mount Heng. The amount of secondary organic carbon (SOC) was estimated using the EC-tracer method and accounted for 53.9% of the total OC on average. Good correlations were found between SOC and droplet-mode sulfate, and oxalate and droplet-mode sulfate, indicating the occurrence of in-cloud secondary organic aerosol (SOA) formation at Mount Heng. The concentrations of SOC and water soluble organic acids also displayed positive relationship with aerosol acidity, suggesting the enhancement of SOC formation by acid-catalyzed heterogeneous reactions. Backward trajectory analysis revealed that the observed aerosols at Mount Heng were predominantly associated with the air masses from the Pearl River Delta region (PRD) and Eastern China, which brought significant amounts of anthropogenic pollutants to the site. In addition, strong signals of biomass burning (elevated carbonaceous concentrations, OC/EC ratios and K+ concentrations) were observed in air masses from Southeast Asia, demonstrating that the biomass burning emissions in Southeast Asia could reach the boundary layer of South China and affect the air quality there during the spring season.

Vertical characteristics of carbonaceous species and their source contributions in a Chinese mega city

In this work, two new chemical mass balance (CMB) models were applied to estimate the possible source contributions to organic carbon (OC) and the secondary OC (SOC) concentrations in PM10 at 10, 40, 120 and 220 m heights on a meteorological tower in Tianjin. The OC and EC (elemental carbon) were simultaneously measured at the four heights from 24 August to 12 September 2009. The vertical characteristics of OC and EC showed a general decreasing trend with the increasing height. The SOC concentrations were then estimated by the minimum OC/EC ratio method, the NCPCRCMB model and the CMB-Iteration method. The results suggest increasing trends for SOC/TOC and SOC/PM10 up the tower. Additionally, wind rose maps and hourly 72-h back trajectory cluster maps were combined with the source apportionment results to discuss the vertical source contributions to OC, indicating that the percentage contributions of long-range transport sources (such as soil dust and SOC sources) increased with height. SOC formation and transportation by wind affected OC more strongly than each primary source at greater heights. Finally, the levels and sources of the excess OC (the difference between OC concentrations at lower heights to that at 220 m) are discussed. The results suggest that it is more effectively managed for reducing OC to control vehicle and coal sources at the ground level in Tianjin. Information about the more significant sources of excess OC can be used for the development of effective control strategies.

Estimation of seasonal diurnal variations in primary and secondary organic carbon concentrations in the urban atmosphere: EC tracer and multiple regression approaches

In order to investigate seasonal and diurnal variation of primary organic carbon (POC) and secondary organic carbon (SOC) concentrations in a megacity, hourly measurements of particulate and gaseous pollutants were carried out in Seoul from January to December 2010. The EC Tracer Method (ECTM) and the Multiple Regression Method (MRM) have been used to estimate seasonal and diurnal concentrations of POC and SOC concentrations. Annual mean SOC concentrations estimated by ECTM (SOCECTM) and MRM (SOCMRM) accounted for 14.61 and 17.21% of TOC concentrations, respectively. Seasonal patterns in SOCMRM were comparable to those of SOCECTM, but the annual average SOCMRM was about 15% greater than that of SOCECTM. In spring, however, a large discrepancy was observed between SOCECTM and SOCMRM, which is thought to be due to a high ozone concentration and primary TOC/EC ratio. Regarding the annual mean diurnal characteristics, POC concentration showed peaks around 10:00 and 00:00 local time that were also observed in diurnal variations of TOC and EC concentrations. Annual mean SOC concentration, however, showed peaks at around 15:00. In the morning over all seasons, we found discrepancies between SOCECTM and SOCMRM due to overestimated SOCECTM concentration. The diurnal variations in SOC concentrations were found to have seasonal characteristics. The diurnal pattern of SOC concentration in spring was similar to that in autumn, and SOC concentrations in all seasons with the exception of winter showed a peak at around 15:00. In summer, however, the SOC concentration peak at around 15:00 was greater by 70%, 81%, and 54% than the peaks seen in spring, autumn, and winter, respectively, which could be explained by the high ozone concentration and strong UV radiation in summer. From 10:00 to 15:00 in summer, the average increase rates in SOCECTM and SOCMRM were 0.39 and 0.24 μg m−3 h−1, respectively. In winter, negligible diurnal variations of estimated SOC concentrations demonstrate that SOC formation is less active than in other seasons. The high concentration level of mean SOC in winter could be attributed to a low mixing height or stagnant atmospheric condition.

Carbonaceous aerosols in China: top-down constraints on primary sources and estimation of secondary contribution

Abstract. We simulated elemental carbon (EC) and organic carbon (OC) aerosols in China and compared model results to surface measurements at Chinese rural and background sites, with the goal of deriving "top-down" emission estimates of EC and OC, as well as better quantifying the secondary sources of OC. We included in the model state-of-the-science Chinese "bottom-up" emission inventories for EC (1.92 TgC yr−1) and OC (3.95 TgC yr−1), as well as updated secondary OC formation pathways. The average simulated annual mean EC concentration at rural and background sites was 1.1 μgC m−3, 56% lower than the observed 2.5 μgC m−3. The average simulated annual mean OC concentration at rural and background sites was 3.4 μgC m−3, 76% lower than the observed 14 μgC m−3. Multiple regression to fit surface monthly mean EC observations at rural and background sites yielded the best estimate of Chinese EC source of 3.05 ± 0.78 TgC yr−1. Based on the top-down EC emission estimate and observed seasonal primary OC/EC ratios, we estimated Chinese OC emissions to be 6.67 ± 1.30 TgC yr−1. Using these top-down estimates, the simulated average annual mean EC concentration at rural and background sites was significantly improved to 1.9 μgC m−3. However, the model still significantly underestimated observed OC in all seasons (simulated average annual mean OC at rural and background sites was 5.4 μgC m−3), with little skill in capturing the spatiotemporal variability. Secondary formation accounts for 21% of Chinese annual mean surface OC in the model, with isoprene being the most important precursor. In summer, as high as 62% of the observed surface OC may be due to secondary formation in eastern China. Our analysis points to four shortcomings in the current bottom-up inventories of Chinese carbonaceous aerosols: (1) the anthropogenic source is underestimated on a national scale, particularly for OC; (2) the spatiotemporal distributions of emissions are misrepresented; (3) there is a missing source in western China, likely associated with the use of biofuels or other low-quality fuels for heating; and (4) sources in fall are not well represented, either because the seasonal shifting of emissions and/or secondary formation are poorly captured or because specific fall emission events are missing. In addition, secondary production of OC in China is severely underestimated. More regional measurements with better spatiotemporal coverage are needed to resolve these shortcomings.

The characteristics and origins of carbonaceous aerosol at a rural site of PRD in summer of 2006

Abstract. Both organic carbon (OC) and elemental carbon (EC) were measured during PRIDE-PRD 2006 summer campaign by using a semi-continuous thermal-optical carbon analyzer at a rural site, Back Garden (BG), which is located 50 km to the northwest of Guangzhou City. Together with the online EC/OC analyzer, various kinds of instruments related to aerosol chemical properties were employed here, which provided a good opportunity to check data quality. The concentrations of OC correlated well with the mass of organic matter (OM) and water soluble organic carbon (WSOC), implying the reliability of the data measured in this campaign. The average OC concentrations in fine particle for three typical periods during the campaign (local emission influence, typhoon and precipitation and normal days) were 28.1 μgC m−3, 4.0 μgC m−3 and 5.7 μgC m−3, respectively; and EC were 11.6 μgC m−3, 1.8 μgC m−3, and 3.3 μgC m−3, respectively. The diurnal patterns of EC and OC during the campaign were higher at night and in early morning than daytime, which was probably caused by the primary emission and accumulation in the occurrence of low boundary layer. Compared with the constant diurnal enhancement ratios of EC, the enhancement ratio of OC (OC versus (CO-CObackground)) kept in a relative high level in the afternoon, with a similar diurnal profile to oxygenated organic aerosol (OOA), due to the strong photochemical formation of OC. Here, a modified EC tracer method was used to estimate the formation of secondary organic carbon (SOC). These results showed that the average SOC concentration (normal days) at BG site was about 2.0 ± 2.3 μgC m−3, and the SOC fraction in OC could reach up to 80% with the average of 47%. The modified approach in this study proved to be effective and reliable for SOC estimation based on good correlations between estimated SOC versus OOA or WSOC, and estimated POC versus hydrocarbon-like organic aerosol (HOA).

One-year measurement of organic and elemental carbon in size-segregated atmospheric aerosol at a coastal and suburban site in Southeast China

To understand the influence of the urbanization process on the air quality in the urban neighbourhood area, the size distribution and seasonal variations of elemental and organic carbon in aerosols were studied at a coastal and suburban site in Xiamen City, China. A total of 87 samples were obtained during the one-year measurement campaign from June 2009 to May 2010. The results indicated that 79.3 ± 3.2% of the organic carbon (OC) and 88.3 ± 1.7% of the elemental carbon (EC) were associated with fine particles (PM(2.5)), which consist of 32.0 ± 8.3% of the total carbonaceous aerosol (TCA). The concentrations of the OC and EC in PM(2.5) were 17.8 ± 11.2 and 3.8 ± 1.9 μg m(-3), respectively, and high concentrations were usually observed when the wind direction was northeast (NE). High OC/EC ratios (average 5.1) in PM(2.5) indicated the formation of secondary organic carbon (SOC), which contributed 60.0% to the OC and 11.0% to the particulate matter. At this site, SOC had a significant negative correlation with the temperature (R(2) = 0.42), and a favorable meteorological condition for SOC formation was found in the wintertime. The OC/EC ratios increased with particle size, while the fractions of the carbonaceous aerosols to particulate matter decreased. OC, EC and SOC concentrations and OC/EC ratios followed the same seasonal pattern of winter > spring > autumn > summer, which mainly resulted from the various origins of the air masses in different seasons. This study indicates the requirement for mitigating the pollution of carbonaceous aerosol at this coastal and suburban area in Xiamen City.