Abstract
Daily fine particulate matter (PM2.5) and precipitation samples were collected simultaneously at an urban site in southwest China in four segregated months in 2015 for measuring major water-soluble inorganic ions (WSIIs). Online hourly concentrations of PM10 and PM2.5 were also monitored, which showed annual mean concentrations of 67.8 and 41.6 µg m–3, respectively. PM2.5 showed the highest concentration in winter and lowest in summer. The annual mean concentration of the total WSIIs was 20.3 µg m–3, accounting for about 48.7% of PM2.5. Among the total WSIIs in ambient PM2.5, SO42– was the predominant component (49.7%), followed by NH4+ (24.1%) and NO3– (21.4%). NH4+ and SO42– were the two most abundant ions in precipitation, followed by Ca2+ and NO3–. Seasonal patterns of the major inorganic ions in precipitation were similar to those in PM2.5, with the highest concentration in winter and lowest in summer. The mean scavenging ratios were 454, 445, 364, 456, and 394 for SO42–, NO3–, NH4+, Cl–, and K+, and 116, 353, and 18 for gas SO2, HNO3, and NH3, respectively. The higher scavenging ratios of particulate ions than their gaseous precursors suggest the higher contributions of particles than gases to the total wet deposition.
Highlights
Wet scavenging efficiently removes particulate matter and their gaseous precursors from the atmosphere (Wang et al, 2018a)
On 21st July with the rain duration of 0.8 h and rain intensity of 0.13 mm h–1, PM2.5 and PM10 concentrations decreased by 39% and 48% during rain, respectively. We found that this phenomenon was caused by the increased PM2.5 and PM10 concentrations during the rush hours in the evening, and when the rush hours ended after 21:00 local time, the air pollutants concentrations began to decrease
PM2.5 and PM10 concentrations were generally decreased after occurrence of moderate or heavy rain event, but this was not always the case after light rain event due to too many other factors affecting the ambient concentration
Summary
Wet scavenging efficiently removes particulate matter and their gaseous precursors from the atmosphere (Wang et al, 2018a). In-cloud (rainout) and below-cloud (washout) scavenging are the two components of wet scavenging processes for air pollutants. The relative contributions of rainout and washout to the total wet deposition could be estimated based on the sequential precipitation sampling measurements (Aikawa and Hiraki, 2009). Using this method, Xu et al (2017) estimated that washout contributed about 56%, 61%, and 47% to total wet scavenging of SO42–, NO3–, and NH4+ respectively, in Beijing, and Ge et al (2016) obtained 88%, 92%, and 84% in Dalian and 56%, 50%, and 46% in Dandong in North China. Washout accounted for about two-thirds of total wet scavenging for NO3– and half for SO42– in Japan using the same method (Aikawa et al, 2014; Kajino and Aikawa, 2015)
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