Estimating the impact of natural and anthropogenic emissions on cloud chemistry: Part I. Sulfur cycle

In order to estimate the anthropogenic influence of gas and aerosol emissions from the Petroleum Industry in maritime zones with clouds of small vertical extent, a numerical 1D Eulerian cloud-chemical model with detailed microphysics was developed. The model simulates the evolution of the raindrop spectrum by activation, condensation, coalescence, breakup and the scavenging process of sulfur dioxide (SO 2). Three distribution functions were defined in the model, one for cloud condensation nuclei (CCN), with a maximum of 64 categories from 0.0041 to 5.93 μm; one for small drops up to 1 μm, and another for drops up to 4096 μm with 73 categories. The initial radius of the activated droplets was parameterized according to Kogan [J. Atmos. Sci. 48 (1991) 1160]. Diffusion of SO 2 was calculated by a new approach introducing a quasi-analytical solution of the diffusion equation. The sensitivity of the cloud and precipitation development to chemical composition and solubility was tested, and an earlier development of precipitation in the low solubility case was observed. The pH spectral evolution was studied, not only during the development stage, but also during the mature and precipitation stage. More acidic droplets were obtained at the large end of the spectrum during the development and mature stage, while in undersaturated areas below cloud base, more acidic droplets were obtained at the small end of the spectrum. The influence of the multimodality of drop size distributions in the obtained pH was analyzed, with more acidic droplets (originated at upper levels) at cloud base during the mature stage of the cloud. The pH distributions for incloud levels and at the earth's surface were estimated for cases of low SO 2 (1 ppb) and high (50 ppb) background concentrations, with qualitatively similar behaviour, but with more acidic droplets in the 50 ppb case, as expected.