Rain-aerosol coupling in the tropical atmosphere of Southeast Asia: Distribution and scavenging ratios of major ionic species

Both aerosol and rainwater samples were collected and analyzed for ionic species at a coastal site in Southeast Asia over a period of 9 months (January-September 2006) covering different monsoons. In general, the occurrence and distribution of ionic species showed a distinct seasonal variation in response to changes in air mass origins. Real-time physical characterization of aerosol particles during rain events showed changes in particle number distributions which were used to assess particle removal processes associated with precipitation, or scavenging. The mean scavenging coefficients for particles in the range 10-500 nm and 500-10 μm were 7. 0∈×∈10^-5∈±∈2. 8∈×∈10^-5 s^-1 and 1. 9∈×∈10^-4∈±∈1. 6∈×∈10^-5 s^-1, respectively. A critical analysis of the scavenging coefficients obtained from this study suggested that the wet removal of aerosol particles was greatly influenced by rain intensity, and was particle size-dependent as well. The scavenging ratios, another parameter used to characterize particle removal processes by precipitation, for NH₄ ⁺, Cl^-, SO₄ ^2-, and NO₃ ^- were found to be higher than those of Na ⁺, K⁺, and Ca²⁺ of oceanic and crustal origins. This enrichment implied that gaseous species NH₃, HCl, and HNO ₃ could also be washed out readily. These additional sources of ions in precipitation presumably counter-balanced the dilution effect caused by high total precipitation volume in the marine and tropical area.