Heterogeneous NOx chemistry in the polluted PBL

The significance of heterogeneous mechanisms in controlling gas-phase NOx (NO, NO2) mixing ratios in polluted urban air, especially during nighttime, is not well established. Several recent studies have suggested that carbon soot can provide an effective surface for mediating the inter conversion among several NOy members. However, a number of such reactions reported in the literature have widely varying reaction probabilities and often conflicting pathways. We evaluated several of these reactions and choose the NO2 conversion to HONO on the surface of soot particles for further analysis with a box photochemical model. These calculations show that the conversion of NO2 to HONO on particle surfaces produces a large, measurable signal (up to several parts per billion) in nighttime HONO mixing ratios. Inclusion of this reaction was also shown to have significant impacts on ozone, OH and HO2 in the polluted planetary boundary layer (PBL). The sensitivity of these results to the different reaction rate probabilities (γ) and particle surface areas was also examined. Results are then evaluated to find the combination of γ and surface areas that would mostly likely occur in the PBL within the limitations of the model.