Derek J. Straub

N-nitrosodimethylamine occurrence, formation and cycling in clouds and fogs.

The occurrence, source, and sink processes of N-nitrosodimethylamine (NDMA) have been explored by means of combined laboratory, field, and model studies. Observations have shown the occurrence of NDMA in fogs and clouds at substantial concentrations (7.5-397 ng L(-1)). Laboratory studies were conducted to investigate the formation of NDMA from nitrous acid and dimethylamine in the homogeneous aqueous phase. While NDMA was produced in the aqueous phase, the low yields (<1%) observed could not explain observational concentrations. Therefore gaseous formation of NDMA with partitioning to droplets likely dominates aqueous NDMA formation. Box-model calculations confirmed the predominant contributions from gas phase formation followed by partitioning into the cloud droplets. Measurements and model calculations showed that while NDMA is eventually photolyzed, it might persist in the atmosphere for hours after sunrise and before sunset since the photolysis in the aqueous phase might be much less efficient than in the gas phase.

The influence of chemical heterogeneity among cloud drop populations on processing of chemical species in winter clouds

Abstract Drop size-resolved measurements of winter cloud composition in the Rocky Mountains of northern Colorado revealed significant variations of cloud drop pH, ion (SO 4 2− , NO 3 − , NH 4 + , and Ca 2+ ) concentrations, and concentrations of trace metal catalysts (Fe and Mn) with drop size. Simultaneous measurements of snow chemical composition and the degree of cloud drop capture by snow crystals (riming) revealed a positive correlation between snow composition and the extent of ice crystal riming in two of four cases studied. Observations indicate that the size-dependent chemical composition of the clouds tends to enhance aqueous phase sulfate production rates when the primary oxidant is ozone. Enrichment of accumulation mode aerosol species in small cloud drops, which are inefficiently scavenged by ice crystals, appears to cause small (typically less than 15%) reductions in the efficiency with which these species are scavenged by precipitation during accretional ice crystal growth.