A chamber investigation of nitric acid-soot aerosol chemistry at 298 k

Long-pathlength infrared absorption spectroscopy was used to investigate ni tric acid-soot aerosol chemistry at 298 K and 0.5% relative humidity. Exper iments were performed by introducing nitric acid vapor (P-HNO3 similar to 3 Pa, P-total similar to 40 kPa) into a teflon-coated chamber and initiating acquisition of infrared spectra at 3 minute time intervals. After 36 minut es of data collection, soot powder was rapidly expanded into nitric acid co ntained in the chamber to generate a soot-HNO3 aerosol. Infrared spectra co llected before, and after, soot introduction to the chamber were used to ch aracterize chamber wall reaction processes and soot aerosol chemistry, resp ectively. Three soot types were investigated (Degussa FW2, Cabot Monarch 10 00, and crystalline graphite), each yielding similar chemistry. Upon soot i ntroduction to the chamber both HNO3 uptake and NO2 production occurred, wi th the molar ratio of HNO3 uptake to NO2 production varying from 1.2 to 2.9 for the three soot types studied. Unreacted HNO3 was present at the conclu sion of each of the aerosol experiments, indicating incomplete conversion o f HNO3 into NO2. This observation suggested that 'active' sites at the soot surface responsible for the reduction of HNO3 are not regenerated (i.e., f ormed) in the reaction process. In essence, a titration occurred between th ese active sites and HNO3. The NO2 concentrations produced, the soot mass c oncentrations used, and the BET measured specific surface area of the powde rs allowed computation of the surface density of active sites of similar to 4.0 x 10(-18) m(2)/active site (describing all three powders studied). Thi s is the first reported measurement of surface density of active sites for nitric acid chemistry on soot. Since atmospheric heterogeneous reactions th at exhibit surface deactivation may, in principle, affect trace gas concent ration, we perform an assessment in this regard.