HNO3 NOX RATIO IN THE REMOTE TROPOSPHERE DURING MLOPEX-2 - EVIDENCE FOR NITRIC-ACID REDUCTION ON CARBONACEOUS AEROSOLS/

The [HNO3]/[NOx] ratio is generally overestimated by a factor of 5-10 in photochemical models in comparison ta tropospheric measurements. In this study, the heterogeneous reduction of HNO3 into NO on carbonaceo us aerosols [Lary et al., 1996] has been introduced in a photochemical box-model on the basis of black carbon mass densities measured during MLOPEX 2. This recycling to NOx decreases the [HNO3]/[NOx] ratio clos e to observed values. The concomitant increase in modeled NOx concentr ation is also in better agreement with the observations, and has subst antial implications for the ozone budget in the remote atmosphere. Lar ge uncertainties in the estimate of black carbon surface area and of a ccomodation coefficients preclude definitive conclusions until more de tailed measurements are carried out.