Coupling quasi-spectral microphysics with multiphase chemistry: a case study of a polluted air mass at the top of the Puy de Dôme mountain (France)

An explicit multiphase chemistry model (Atm. Environ. 34 (29/30) (2000) 5015) has been coupled with quasi-spectral microphysics, based upon Berry and Reinhardtʹs parameterizations (1974a, b). This coupled model has been initialized with polluted conditions as observed at the Puy de Dôme mountain in the center of France and for a maritime cloud. The presence of clouds results in two effects on multiphase chemistry: a direct effect through mass transfer, solubility and reactivity, and an indirect effect through microphysical transfer from cloud water into rainwater and redistribution of reactive soluble species among interstitial air, cloud droplets and raindrops. Results demonstrate that microphysical processes are necessary to sketch out the complex, nonlinear multiphase chemistry in a real cloud. In addition to the direct exchange through mass transfer, incorporation of reactive oxidants such as HOx in droplets can arise and consequently make those species no longer available for reacting in the gas-phase. Moreover, microphysical coalescence conversions favor NOx destruction and enhance the chemical nitric acid production. Coalescence of cloud drops to form rain transfers dissolved species into drops that are undersaturated compared to Henryʹs law equilibrium. The rain becomes a reservoir for these species, allowing aqueous chemistry to produce more nitric acid than would be possible without the presence of rain. Finally, for the different cloud types, the fate of those intermediate and reactive species is investigated, looking at their budget in clear sky situation versus cloudy and/or rainy situations.