Observational and modeling studies of chemical species concentrations as a function of raindrop size

The Guttalgor method has been used to determine the chemical species concentrations in size-selected raindrops in nine rain events at Hong Kong from 1999 to 2001. The curve (concentration against raindrop radius) patterns for all the species are similar but depend on the starting time of sampling within a rain event. In these plots, the maximum concentration occurs at the same range of droplet radius, irrespective of the species, and this indicates the importance of coalescence and breakup processes. The maximum is located at a smaller droplet radius than was found in previous studies in Germany. All results show almost constant concentrations with size for large raindrops, and these indicate the in-cloud contributions. The pH of raindrops of similar size is linearly correlated with a function of the sulfate, nitrate, acetate, formate, calcium and ammonium ion species concentrations. Within a single raindrop, chloride depletion is not significant, and sulfate, ammonium and hydrogen ions are found in ratios compatible with the precursor solid-phase mixture of ammonium sulfate and ammonium bisulphate. When simulated by a below-cloud model, good agreement between the modeled and measured sodium and sulfate concentrations has been found. Below-cloud sulfur dioxide scavenging contributes at most 60% of the sulfate concentration in a single raindrop.