Size-resolved source apportionment of particulate matter in urban Beijing during haze and non-haze episodes

Abstract

Additional size-resolved chemical information is needed before the physicochemical characteristics and sources of airborne particles can be understood; however, this information remains unavailable in most regions of China due to lacking measurement data. In this study, we report observations of various chemical species in size-segregated particle samples that were collected over 1 year in the urban area of Beijing, a megacity that experiences severe haze episodes. In addition to fine particles, high concentrations of coarse particles were measured during the periods of haze. The abundance and chemical compositions of the particles in this study were temporally and spatially variable, with major contributions from organic matter and secondary inorganic aerosols. The contributions of organic matter to the particle mass decreased from 37.9 to 31.2%, and the total contribution of sulfate, nitrate and ammonium increased from 19.1 to 33.9% between non-haze and haze days, respectively. Due to heterogeneous reactions and hygroscopic growth, the peak concentrations of the organic carbon, cadmium and sulfate, nitrate, ammonium, chloride and potassium shifted from 0.43 to 0.65μm on non-haze days to 0.65-1.1μm on haze days. Although the size distributions of lead and thallium were similar during the observation period, their concentrations increased by a factor of more than 1.5 on haze days compared with non-haze days. We observed that sulfate and ammonium, which have a size range of 0.43-0.65μm, sulfate and nitrate, which have a size range of 0.65-1.1μm, calcium, which has a size range of 5.8-9μm, and the meteorological factors of relative humidity and wind speed were responsible for haze pollution when the visibility was less than 10km. Source apportionment using Positive Matrix Factorization showed six PM2.1 sources and seven PM2.1-9 common sources: secondary inorganic aerosol (25.1% for fine particles vs. 9.8% for coarse particles), coal combustion (17.7% vs. 7.8%), biomass burning (11.1% vs. 11.8%), industrial pollution (12.1% vs. 5.1%), road dust (8.4% vs. 10.9%), vehicle emissions (19.6% for fine particles), mineral dust (22.6% for coarse particles) and organic aerosol (23.6% for coarse particles). The contributions of the first four factors and vehicle emissions were higher on haze days than non-haze days, while the reverse is true for road dust and mineral dust. The sources' contribution generally increased as the size decreased, with the exception of mineral dust. However, two peaks were consistently found in the fine and coarse particles. In addition, the sources' contribution varied with the wind direction, with coal and oil combustion products increasing during southern flows. This result suggests that future air pollution control strategies should consider wind patterns, especially during episodes of haze. Furthermore, the findings of this study indicated that the PM2.5-based data set is insufficient for determining source control policies for haze in China and that detailed size-resolved information is needed to characterize the important sources of particulate matter in urban regions and better understand severe haze pollution.