HNO₃/NO_x; ratio in the remote troposphere during MLOPEX 2: Evidence for Nitric Acid Reduction on Carbonaceous Aerosols?

The [HNO₃]/[NO_x] ratio is generally over-estimated by a factor of 5-10 in photochemical models in comparison to tropospheric measurements. In this study, the heterogeneous reduction of HNO₃ into NO on carbonaceous 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 NO_X decreases the [HNO₃]/[NO_X] ratio close to observed values. The concomitant increase in modeled NO_X concentration is also in better agreement with the observations, and has substantial implications for the ozone budget in the remote atmosphere. Large uncertainties in the estimate of black carbon surface area and of accomodation coefficients preclude definitive conclusions until more detailed measurements are carried out.