Chlorine loss from Puerto Rican and San Francisco Bay area marine aerosols

Instrumental neutron activation measurements of Cl and Na in aerosols collected by Andersen cascade impactors from Puerto Rico and the San Francisco Bay area were used to calculate Cl loss from the total aerosol mass and loss as a function of particle size. Calculated Cl losses from Puerto Rican samples ranged from 0.18 to 0.27 μg/m3, representing a 7–25% loss of Cl ascribed to the marine aerosol component. The Cl losses for the bay area marine aerosol ranged from 0.24 to 2.78 μg/m3, or from 6% to nearly 90% of the total particulate marine Cl. In all samples, increasing loss of Cl with decreasing particle size was observed. A linear correlation exists between measured gaseous NO2 concentrations and total Cl loss in the bay area samples, indicating a 0.06 μmole/m3 Cl loss per μmole/m3 NO2. This suggests that the Cl loss is controlled by the attachment of HNO3 vapor to the sea-salt droplets, which lowers the pH, causing the release of HCl to the atmosphere. The size dependency of Cl loss could thus be explained by a size-dependent gain of H+ resulting from the accretion of HNO3 vapor on the marine aerosol. A diffusion model based on the accumulation of HNO3 necessary to balance the loss of Cl as HCl yields an average time for completion of gas-particle reactions of 19 min, which is in agreement with previous studies; however, the Cl/Na ratio variation with particle size suggests that chemical equilibrium is not reached. Background levels of gaseous NO2 and SO2 may account for the narrow range of Cl loss observed in the less polluted Puerto Rican atmosphere.