Variation of particle number size distributions and chemical compositions at the urban and downwind regional sites in the Pearl River Delta during summertime pollution episodes
In order to characterize the features of particulate pollution in the Pearl River Delta (PRD) in the summer, continuous measurements of particle number size distributions and chemical compositions were simultaneously performed at Guangzhou urban site (GZ) and Back-garden downwind regional site (BG) in July 2006. Particle number concentration from 20 nm to 10 km at BG was (1.7c0.8)10 super(4) cm super(-3), about 40% lower than that at GZ, (2.9c1.1)10 super(4) cm super(-3) with intensive traffic emissions. The total particle volume concentration at BG was 94c34 km super(3) cm super(-3), similar to that at GZ, 96c43 km super(3) cm super(-3). More 20-100 nm particles, significantly affected by the traffic emissions, were observed at GZ, while 100-660 nm particle number concentrations were similar at both sites as they are more regional. PM sub(2.5) values were also similar at GZ (69c43 kg m super(-3)) and BG (69c58 kg m super(-3)), indicating the fine particulate pollution in the PRD region to be regional. Two kinds of pollution episodes, the accumulation pollution episode and the regional transport pollution episode, were observed. Fine particles over 100 nm dominated both number and volume concentrations of total particles during the late periods of these pollution episodes. Accumulation and secondary transformations are two main reasons for the nighttime accumulation pollution episode. SO sub(4) super(− 2), NO sub(3) super(− ), and NH sub(4) super(+) accounted for about 60% in 100-660 nm particle mass and PM sub(2.5). When south or south-southeast wind prevailed in the PRD region, regional transport of pollutants takes place. Regional transport contributed about 30% to fine particulate pollution at BG during a regional transport case. Secondary transformation played an important role during regional transport, causing higher increase rates of secondary ions in PM sub(1.0) than other species and shifting the peaks of sulfate and ammonium mass size distributions to larger sizes. SO sub(4) super(− 2), NO sub(3) super(− ), and NH sub(4) super(+) accounted for about 70% and 40% of PM sub(1.0) and PM sub(2.5), respectively.