A Cloud Chemistry Model: Simulation of Cloud Water Acidity and Its Dependence on Droplet Size

Abstract

The purpose of this study is to investigate the possible chemical pathways determining the acidity and chemical composition of cloud water and the role played by individual cloud droplets. A kinetic cloud chemistry model (CCM) involving the aqueous phase chemistry of sulfur species and aerosol loadings is developed for a modeling approach. The solution chemistry involves SO2, HNO3, HCl, NH3, CO2, O3 and H2O2 gases and the oxidation of S(IV) by O3 and H2O2. The scavenging of acidic (H2SO4), neutral ((NH2)2SO4 and NH4NO3), maritime (NaCl and KCl), and continental (CaCO3 and MgCO3) aerosols are included in CCM. A scheme is developed to investigate the dependence of the acidity and chemical composition in cloud droplets upon their sizes. Using the Khrgian-Mazin droplet size distribution, the model simulations show that contrary to expectations, smaller droplets have higher pH values although the sulfate ion concentrations in them are much higher than in larger droplets. This seems to result from the loadings of carbonate aerosols. If the latter is excluded, the pH value for the smallest droplet can be even below 2.0. The variation of pH with size for larger droplets is not significant. The acidity in smaller droplets is most sensitive to the mass loading of aerosol particles. The dilution effect in larger droplets is clearly seen. The model results are compared with direct measurements made in clouds at Mt. Mitchell, NC. The data on cloud water acidity with a temporal resolution of about 10 min are available. Cloud droplet size distribution was simultaneously measured using Forward Scattering Spectrometer Probe (FSSP). A case study comparing the model results with the observations is presented and the dependence of the chemical composition of cloud water upon droplet size is analyzed. A qualitative agreement between the model predictions and direct observations is found.

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