SIMULATIONS OF CONDENSATION AND CLOUD CONDENSATION NUCLEI FROM BIOGENIC SO2 IN THE REMOTE MARINE BOUNDARY-LAYER

An aerosol dynamics model is presented describing the formation and growth of H2SO4-H2O droplets in the homogeneous gas phase. The model describes the evolution in time of the particle size distribution and hence predicts the number densities of the detectable fraction of the aerosol (CN) as well as the number of cloud condensation nuclei (CCN) active at various supersaturations. The model is applied to CN and CCN formation from biogenic SO2 in the remote marine boundary layer (MBL). In the calculations the MBL is characterized by the following average conditions: [SO2] < 30 pptv, [OH]max = 2 x 10(6) molecules cm-3, the relative humidity is 80%, T = 10-degrees-C, the MBL height is 1000 m. The calculations suggest that under these conditions, biogenic SO2 Will commonly lead to the observed levels of CN but not of CCN active at 2% supersaturation; in order to obtain realistic levels of both CN and CCN (2%), more than 3 days of undisturbed homogeneous gas-phase conditions are required. A number of alternative scenarios and mechanisms are therefore discussed that might facilitate the formation of CCN(2%) in the MBL. Of those, two are likely to be relevant: (1) the formation of new particles outside the MBL, with subsequent transport to the MBL, and (2) the occurrence of heterogeneous SO2 --> SO4 conversion on newly formed H2SO4-H2O droplets. Inclusion of cloud-induced and ion-induced nucleation in the model does not favor CCN formation.