Scavenging of acidic gases (HCOOH, CH3COOH, HNO3, HCl, and SO2) and ammonia in mixed liquid-solid water clouds at the Puy de Dome mountain (France)

In order to study scavenging processes of chemical species in mixed phase c louds, in-cloud field measurements were conducted in December 1997 at the P uy de Dome mountain (center of France, 1465 m above sea level). Soluble spe cies including NH4+, Cl-, NO3-, SO4-, HCOO-, CH3COO-, and C2O4- present in the different phases (supercooled water droplets, rimed snowflakes, interst itial gases, and aerosols) of cold clouds have been investigated. Conducted in parallel to microphysical studies of clouds (liquid water and ice conte nts, and size distribution of hydrometeors), these chemical investigations allow us to examine the partitioning of strong (HNO3 and HCl) and weak (SO2 , HCOOH, and CH3COOH) acids as well as ammonia between interstitial air and the condensed phases (liquid and solid water particles) in mixed clouds pr esent during winter at midlatitude regions. From that, we discuss the proce sses by which these key atmospheric species are taken up from the gas phase by the condensed phases (liquid and ice) in these cold clouds. We examine several factors which are of importance in driving the final composition of cloud ice. They include the partitioning of species between gaseous and su percooled liquid phases, the amount of rimed ice collected by snowflakes, a nd the retention of gas during shock freezing of supercooled droplets onto ice particles. Strong acids (HCl and HNO3) as well as NH,, being sufficient ly soluble in water, are mainly partitioned into supercooled water droplets . Furthermore, being subsaturated in liquid droplets, these species are wel l retained in rimed ice. For these species, riming is found to be the Main process driving the final composition of snowflakes, direct incorporation f rom the gas :phase during growth of snowflakes remaining insignificant beca use of low concentrations in the gas phase. For light carboxylic acids the riming process mainly determines the composition of the snowflakes, but an additional. Significant contribution by gas incorporation during the growth of snowflakes cannot he excluded. SO2 is also present at significant level s in the interstitial air and is poorly retained in ice during riming of su percooled water droplets. However, hydroxymethanesulfonate (HMSA) was likel y present in supercooled liquid droplets, making it difficult to evaluate b y which mechanism S(IV) (i.e., HMSA plus SO2) has been incorporated into sn owflakes.