Carbonaceous aerosol (Soot) measured in the lower stratosphere during POLARIS and its role in stratospheric photochemistry

This paper describes recent measurements of carbonaceous aerosol made by wi re impactors during the Photochemistry Ozone Loss in the Arctic Region in S ummer (POLARIS) campaign and assesses their role in stratospheric photochem istry. Ninety-five percent of the carbonaceous aerosol collected during thi s campaign was in the form of b lack carbon aerosol (BCA), or soot. A new m ethod of analyzing impactor samples is described that accounts for particle bounce and models the BCA as fractal aggregates to modify the aerodynamic collection efficiency and determine particle surface area. Results are comp ared to previously used methods. The new method results in an increase in t he measured BCA number density of 4 times, surface area density of similar to 15 times, and an increase in mass loading of 6.15 times over one previou sly used approach, Average values of number, surface area, and mass densiti es are 0.06 no./cm(3), 0.03 mu m(2)/cm(3), and 0.64 ng/m(3), respectively. BCA number densities are similar to 1% of total aerosol number density, and BCA surface area density is similar to 10% of the measured sulfuric acid a erosol surface area. Including heterogeneous reactions on BCA in a photoche mical model can affect photochemistry leading to renoxification and increas ed ozone depletion. However, these predicted effects are not supported by t he POLARIS observations, in particular, the NOx/NOy ratios, The laboratory data is not conclusive enough to determine to what extent the heterogeneous reaction is catalytic or carbon consuming. Including catalytic reactions o n BCA does not statistically improve the agreement between model and measur ement in any of the several scenarios considered. Furthermore, if the react ions cause even partial carbon oxidation, the BCA would be consumed at a ra te inconsistent with POLARIS observations. These inconsistencies lead us to conclude that the presence of BCA in the stratosphere did not affect strat ospheric photochemistry during POLARIS.