Long-term real-time measurements of aerosol particle composition in Beijing, China: seasonal variations, meteorological effects, and source analysis

摘要

High concentrations of fine particles (PM_2.5) are frequently observedduring all seasons in Beijing, China, leading to severe air pollution andhuman health problems in this megacity. In this study, we conductedreal-time measurements of non-refractory submicron aerosol (NR-PM₁)species (sulfate, nitrate, ammonium, chloride, and organics) in Beijingusing an Aerodyne Aerosol Chemical Speciation Monitor for 1 year, from July2011 to June 2012. This is the first long-term, highly time-resolved(~ 15 min) measurement of fine particle composition in China.The seasonal average (±1σ) mass concentration of NR-PM₁ranged from 52 (±49) μg m?3 in the spring season to 62(±49) μg m?3 in the summer season, with organics beingthe major fraction (40–51 %), followed by nitrate (17–25 %) andsulfate (12–17 %). Organics and chloride showed pronounced seasonalvariations, with much higher concentrations in winter than in the otherseasons, due to enhanced coal combustion emissions. Although the seasonalvariations of secondary inorganic aerosol (SIA, i.e., sulfate + nitrate +ammonium) concentrations were not significant, higher contributions of SIAwere observed in summer (57–61 %) than in winter (43–46 %), indicatingthat secondary aerosol production is a more important process than primaryemissions in summer. Organics presented pronounced diurnal cycles that weresimilar among all seasons, whereas the diurnal variations of nitrate weremainly due to the competition between photochemical production andgas–particle partitioning. Our data also indicate that high concentrationsof NR-PM₁ (> 60 μg m−3) are usually associatedwith high ambient relative humidity (RH) (> 50 %) and thatsevere particulate pollution is characterized by different aerosolcomposition in different seasons. All NR-PM₁ species showed evidentconcentration gradients as a function of wind direction, generally withhigher values associated with wind from the south, southeast or east. Thiswas consistent with their higher potential as source areas, as determined bypotential source contribution function analysis. A common high potentialsource area, located to the southwest of Beijing along the TaihangMountains, was observed during all seasons except winter, when smallersource areas were found. These results demonstrate a high potential impactof regional transport from surrounding regions on the formation of severehaze pollution in Beijing.