The mass absorption efficiency (MAE) of elemental carbon (EC) in Beijing was quantified using a thermal-optical carbon analyzer. The MAE measured at 632 nm was 8.45±1.71 and 9.41±1.92 m² g⁻¹ during winter and summer respectively. The daily variation of MAE was found to coincide with the abundance of organic carbon (OC), especially the OC to EC ratio, perhaps due to the enhancement by coating with organic aerosol (especially secondary organic aerosol, SOA) or the artifacts resulting from the redistribution of liquid-like organic particles during the filter-based absorption measurements. Using a converting approach that accounts for the discrepancy caused by measurements methods of both light absorption and EC concentration, previously published MAE values were converted to the equivalent-MAE, which is the estimated value if using the same measurement methods as used in this study. The equivalent-MAE was found to be much lower in the regions heavily impacted by biomass burning (e.g., below 2.7 m² g⁻¹ for two Indian cities). Results from source samples (including diesel exhaust samples and biomass smoke samples) also demonstrated that emissions from biomass burning would decrease the MAE of EC. Moreover, optical properties of water-soluble organic carbon (WSOC) in Beijing were presented. Light absorption by WSOC exhibited strong wavelength (λ) dependence such that absorption varied approximately as λ^−7, which was characteristic of the brown carbon spectra. The MAE of WSOC (measured at 365 nm) was 1.79±0.24 and 0.71±0.20 m² g⁻¹ during winter and summer respectively. The large discrepancy between the MAE of WSOC during winter and summer was attributed to the difference in the precursors of SOA such that anthropogenic volatile organic compounds (AVOCs) should be more important as the precursors of SOA in winter. The MAE of WSOC in Beijing was much higher than results from the southeastern United States which were obtained using the same method as used in this study, perhaps due to the stronger emissions of biomass burning in China

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Abstract. Based on PM 2.5 chemical data sets from literature and from our surface observations, chemical species and reconstructed speciation of PM 2.5 in representative Chinese megacities and across China were compared to draw insights into the characteristics of PM 2.5 speciation. PM 2.5 mass concentrations and speciation compositions varied substantially over geographical regions in China. Near six-fold variations in average PM 2.5 concentrations (34.0-193.4 µg m −3 ) across China were found with high PM 2.5 levels (>100 µg m −3 ) appearing in the cities in the northern and western regions and low levels (<40 µg m −3 ) in the remote forest area (Changbai Mountain) and in Hong Kong. The percentages of the sum of sulfate, nitrate and ammonium, organic matter, crustal material, and elemental carbon in PM 2.5 mass ranged 7.1-57 %, 17.7-53 %, 7.1-43 %, and 1.3-12.8 %, respectively. At both urban and rural sites in the eastern region, the sum of sulfate, nitrate and ammonia typically constituted much higher fractions (40-57 %) of PM 2.5 mass, indicative of more local formation/production and regional transport of the secondary aerosols, thus more intensive characteristic of "complex atmospheric pollution" compared to the western region. Organic matter had significant contribution to PM 2.5 over all the sites. Organic matter plus sulfate, nitrate, and ammonia accounted for 53-90 % of PM 2.5 mass across China. PM 2.5 speciation across China was also characterized by high content of crustal material, which was usually at more than ∼10 µg m −3 level or shared ∼ 10 % of PM 2.5 mass in urban Correspondence to: K. He (hekb@tsinghua.edu.cn) areas, due to transported desert dust and locally induced dust. In four representative megacities (i.e. Beijing, Chongqing, Shanghai, and Guangzhou), PM 2.5 mass and major components (except sulfate) were at higher levels than those in US continental east by one order of magnitude. Distinct differences in nitrate and sulfate levels and their mass ratio [NO 4 ] imply that mobile sources are likely more important in Guangzhou, whereas in Chongqing it is stationary (coal combustion) sources. The observed intra-city variations in PM 2.5 mass and speciation indicate that both local emissions and regional transportation contributed significantly to high fine particle loadings in Beijing, while local contribution likely played a predominant role in Chongqing. During the ten-year period from 1999 through 2008 in urban Beijing, both the sum of sulfate, nitrate, and ammonia and [NO ] ratio exhibited steadily increasing trends, implying that the characteristic of "complex atmospheric pollution" and the contribution from mobile sources were being enhanced.

Biomass burning, the largest global source of elemental carbon (EC) and primary organic carbon (OC), is strongly associated with many subjects of great scientific concern, such as secondary organic aerosol and brown carbon which exert important effects on the environment and on climate in particular. This study investigated the relationships between levoglucosan and other biomass burning tracers (i.e., water soluble potassium and mannosan) based on both ambient samples collected in Beijing and source samples. Compared with North America and Europe, Beijing was characterized by high ambient levoglucosan concentrations and low winter to summer ratios of levoglucosan, indicating significant impact of biomass burning activities throughout the year in Beijing. Comparison of levoglucosan and water soluble potassium (K⁺) levels suggested that it was acceptable to use K⁺ as a biomass burning tracer during summer in Beijing, while the contribution of fireworks to K⁺ could be significant during winter. Moreover, the levoglucosan to K⁺ ratio was found to be lower during the typical summer period (0.21 ± 0.16) compared with the typical winter period (0.51 ± 0.15). Levoglucosan correlated strongly with mannosan (R² = 0.97) throughout the winter and the levoglucosan to mannosan ratio averaged 9.49 ± 1.63, whereas levoglucosan and mannosan exhibited relatively weak correlation (R² = 0.73) during the typical summer period when the levoglucosan to mannosan ratio averaged 12.65 ± 3.38. Results from positive matrix factorization (PMF) model analysis showed that about 50% of the OC and EC in Beijing were associated with biomass burning processes. In addition, a new source identification method was developed based on the comparison of the levoglucosan to K⁺ ratio and the levoglucosan to mannosan ratio among different types of biomass. Using this method, the major source of biomass burning aerosol in Beijing was suggested to be the combustion of crop residuals, while the contribution from softwood burning was also non-negligible, especially in winter