Characteristics of PM_2.5 speciation in representative megacities and across China

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 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 [NO₃⁻]/[SO₄²⁻] 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 [NO₃⁻]/[SO₄²⁻] ratio exhibited steadily increasing trends, implying that the characteristic of "complex atmospheric pollution" and the contribution from mobile sources were being enhanced.