Heterogeneous chemistry in the troposphere: The nitric acid 'renoxification'

The current observed discrepancy between the field and modeled NO x/HNO3 ratios suggests that chemistry that may affect the oxidative capacity of the atmosphere remains unidentified. We studied several heterogeneous reactions involving HNO3 and various atmospheric species (NO, CO, CH4 and SO2) proposed to reconcile these ratios.; The BET surface area of several atmospherically available natural and anthropogenic surfaces was determined to evaluate the potential role in heterogeneous chemistry. The BET surface area of these surfaces was 3 to 5 orders of magnitude higher than the geometric surface area of the samples. Silica was chosen as the proxy surface of atmospherically available surfaces, and its interaction with water using a combination of FTIR and BET theory was studied. Hydroxylated silica absorbs ∼1.6 monolayers of water under ambient conditions (296 K, ∼50% RH).; Using transmission FTIR, we monitored the reaction of surface-adsorbed HNO3 with gaseous CO, SO2, CH4 and NO. No reaction between HNO3 and CO, CH4 or SO2 was observed. Upper limits to the reaction probabilities (gammarxn ) were derived: ≤10-10 for CO and SO 2, and ≤10-12 for CH4. Therefore, these reactions are not expected to participate in "renoxification" in the boundary layer. However, the reaction of HNO3(ads) with NO does occur, producing gaseous NO2, with a lower limit for the reaction probability of gammaNO > (7 +/- 1) x 10-8 (2s) when only the surface area covered by HNO 3 was used. Molecular HNO3 was shown to be the reactive species instead of NO3-. This chemistry requires the presence of a thin water film on the surface. Recent studies by Kleffmann et al. (2004) reported to have an upper limit for the reaction probability for the HNO3-NO reaction of gamma NO→NO2 < 2.5 x 10-9. However, it is not clear whether the HNO3 was dissociated or the molecular form under their experimental conditions.; The HNO3-NO reaction could be a significant means of "renoxification" of HNO3 on surfaces. Therefore, the chemistry elucidated by these experiments may help to resolve some discrepancies between model-predicted ozone and field observations in polluted urban atmospheres.; The heterogeneous hydrolysis of NO2 under dark and light (400 ≤ gamma ≤ 500 nm) conditions, and pre-treating the silica surface with HNO3, as an important pathway for the production of HNO3(ads) was investigated. HNO3 remains surface-adsorbed, and hence available to further react with atmospheric species, such as NO. In the presence of light, the decay of HNO3(ads) was observed. Additionally, NO2 hydrolysis has been shown to slow down when occurring on acidified surfaces.