Uptake of nitric acid by sub-micron-sized ice particles

The uptake of gas phase nitric acid by half-micron-diameter ice crystals has been studied at 230 K by measuring the nitrate content of ice particles which have been exposed to 5x10(-6) torr of nitric acid in a low temperature flow tube. A cold NaOH-coated denuder is used to distinguish gas-phase nitric acid from adsorbed nitric acid. Ice particle diameters were determined by fitting measured aerosol infrared extinction spectra to spectra calculated via Mie theory, and their number density is measured directly with a CN counter. Under conditions in which the surface is saturated and not all the gas-phase nitric acid adsorbs, the measured uptakes are 1.2x10(14) molecules/cm(2) where the surface area is the geometric area of the particles. Within experimental uncertainties, this surface coverage is the same as that measured on thin films of ice formed by freezing liquid water. These results are the first quantitative study of the nitric acid uptake capacity of ice particles, and they provide additional support to the suggestion that ice and snow provide a route for the efficient scavenging of nitric acid from the atmosphere.