Characteristics of PM 2.5 mass concentrations and chemical species in urban and background areas of China: emerging results from the CARE-China network

The Campaign on atmospheric Aerosol REsearch network of China (CARE-China) is a long-term project for the study of the spatiotemporal distributions of physical aerosol characteristics as well as the chemical components and optical properties of aerosols over China. This study presents the first long-term datasets from this project, including three years of observations of online PM 2.5 mass concentrations (2012–2014) and one year of observations of PM 2.5 compositions (2012–2013) from the CARE-China network. The average PM 2.5 concentrations at 20 urban sites is 73.2 μg/m 3 (16.8–126.9 μg/m 3 ), which was three times higher than the average value from the 12 background sites (11.2–46.5 μg/m 3 ). The PM 2.5 concentrations are generally higher in east-central China than in the other parts of the country due to their relative large particulate matter (PM) emissions and the unfavorable meteorological conditions for pollution dispersion. A distinct seasonal variability of the PM 2.5 is observed, with highs in the winter and lows during the summer at urban sites. Inconsistent seasonal trends were observed at the background sites. Bimodal and unimodal diurnal variation patterns were identified at both urban and background sites. The chemical compositions of PM 2.5 at six paired urban and background sites located within the most polluted urban agglomerations and cleanest regions of China were analyzed. The major PM 2.5 constituents across all the urban sites are organic matter (OM, 26.0 %), SO 4 2− (17.7 %), mineral dust (11.8 %), NO 3 − (9.8 %), NH 4 + (6.6 %), elemental carbon (EC) (6.0 %), Cl − (1.2 %) at 45 % RH and residual matter (20.7 %). Similar chemical compositions of PM 2.5 were observed at background sites but were associated with higher fractions of OM (33.2 %) and lower fractions of NO 3 − (8.6 %) and EC (4.1 %). Significant variations of the chemical species were observed among the sites. At the urban sites, the OM ranged from 12.6 μg/m 3 (Lhasa) to 23.3 μg/m 3 (Shenyang), the SO 4 2− ranged from 0.8 μg/m 3 (Lhasa) to 19.7 μg/m 3 (Chongqing), the NO 3 − ranged from 0.5 μg/m 3 (Lhasa) to 11.9 μg/m 3 (Shanghai) and the EC ranged from 1.4 μg/m 3 (Lhasa) to 7.1 μg/m 3 (Guangzhou). The PM 2.5 chemical species at the background sites exhibited larger spatial heterogeneities than those at urban sites, suggesting the different contributions from regional anthropogenic or natural emissions and from the long-range transport to background areas. Notable seasonal variations of PM 2.5 polluted days were observed, especially for the megacities in east-central China, resulting in frequent heavy pollution episodes occurring during the winter. The evolution of the PM 2.5 chemical compositions on polluted days was similar for the urban and nearby background sites, suggesting the significant regional pollution characteristics of the most polluted areas of China. However, the chemical species dominating the evolutions of the heavily polluted events were different in these areas, indicating that unique mitigation measures should be developed for different regions of China. This analysis reveals the spatial and seasonal variabilities of the urban and background aerosol concentrations on a national scale and provides insights into their sources, processes, and lifetimes.