Characteristics of PM2.5 mass concentrations and chemical species in urban and background areas of China: emerging results from the CARE-China network
Authors: Liu Zirui , Gao Wenkang , Yu Yangchun , Hu Bo , Xin Jinyuan , Sun Yang , Wang Lili , Wang Gehui , Bi Xinhui , Zhang Guohua , Xu Honghui , Cong Zhiyuan , He Jun , Xu Jingsha , Wang Yuesi 
Published: June 2018

Abstract. The “Campaign on Atmospheric Aerosol Research” network of China
(CARE-China) is a long-term project for the study of the spatio-temporal
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 data sets from this project, including 3 years of
observations of online PM2.5 mass concentrations (2012–2014) and 1
year of observations of PM2.5 compositions (2012–2013) from the
CARE-China network. The average PM2.5 concentration at 20 urban sites
is 73.2 µg m−3 (16.8–126.9 µg m−3), which was 3 times
higher than the average value from the 12 background sites (11.2–46.5 µg m−3).
The PM2.5 concentrations are generally higher in
east-central China than in the other parts of the country due to their
relatively large particulate matter (PM) emissions and the unfavourable
meteorological conditions for pollution dispersion. A distinct seasonal
variability in PM2.5 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
PM2.5 were analysed at six paired urban and background sites located within the most
polluted urban agglomerations – North China Plain (NCP), Yangtze River delta
(YRD), Pearl River delta (PRD), North-east China region (NECR), South-west China region (SWCR) – and the cleanest region of China – the Tibetan Autonomous Region
(TAR). The major PM2.5 constituents across all the
urban sites are organic matter (OM, 26.0 %), SO42- (17.7 %),
mineral dust (11.8 %), NO3- (9.8 %), NH4+ (6.6 %),
elemental carbon (EC) (6.0 %), Cl− (1.2 %) at 45 % RH and
unaccounted matter (20.7 %). Similar chemical compositions of PM2.5
were observed at background sites but were associated with higher fractions
of OM (33.2 %) and lower fractions of NO3- (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 SO42- ranged from
0.8 µg m−3 (Lhasa) to 19.7 µg m−3 (Chongqing), the NO3-
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 PM2.5 chemical species at the background sites
exhibited larger spatial heterogeneities than those at urban sites,
suggesting different contributions from regional anthropogenic or natural
emissions and from long-range transport to background areas. Notable
seasonal variations of PM2.5-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 PM2.5
chemical compositions on polluted days was consistent for the urban and
nearby background sites, where the sum of sulfate, nitrate and ammonia
typically constituted much higher fractions (31–57 %) of PM2.5 mass,
suggesting fine-particle pollution in the most polluted areas of China
assumes a regional tendency, and the importance of addressing the emission
reduction of secondary aerosol precursors including SO2 and NOx.
Furthermore, distinct differences in the evolution of
[NO3-]/[SO42-] ratio and OC∕EC ratio on polluted days imply
that mobile sources and stationary (coal combustion) sources are likely more
important in Guangzhou and Shenyang, respectively, whereas in Beijing it is
mobile emission and residential sources. As for Chongqing, the higher
oxidation capacity than the other three cities suggested it should pay more
attention to the emission reduction of secondary aerosol precursors. 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.