Volume uptake of carbonyls during diffusional ice crystal growth

Abstract

Carbonyls are highly relevant atmospheric constituents that influence tropospheric photochemistry and oxidative capacity. They can be removed from the upper troposphere via ice phase deposition scavenging. The gas-volume uptake coefficients for 14 different carbonyl compounds were determined using a flowtube apparatus. Ice crystals were grown from vapor deposition in the presence of gas phase carbonyls at −20, −30, and −40 °C. Using van't Hoff analysis, the entropy and enthalpy of uptake were determined. An inverse relationship between uptake coefficients and temperature was observed for all species except methyl vinyl ketone. A linear correlation between ΔS and ΔH arose which was statistically validated and determined with 99 % confidence to not be a statistical artifact. This compensation behavior could be an indication of a surface liquid layer or quasi-liquid layer behavior involved in the uptake process and could also indicate a single dominant influence on a compound's uptake. The most significant physicochemical properties correlated with uptake were identified to be vapor pressure and molar mass, which indicate that smaller compounds with higher vapor pressures are more readily taken into the ice phase. The volume uptake coefficients observed here are below the 10 mol m−3 Pa−1 threshold given by Crutzen and Lawrence (2000) to be considered a substantial atmospheric removal process.