Tropospheric aerosol as a reactive intermediate

In tropospheric chemistry, secondary organic aerosol (SOA) is deemed an end product. Here, on the basis of new evidence, we make the case that SOA is a key reactive intermediate. We present laboratory results on the catalysis by carboxylate anions of the disproportionation of NO_2 'on water': 2NO_2 + H_2O = HONO + NO_3^− + H^+ (R1), and supporting quantum chemical calculations, which we apply to reinterpret recent reports on (i) HONO daytime source strengths vis-à-vis SOA anion loadings and (ii) the weak seasonal and latitudinal dependences of NO_x decay kinetics over several megacities. HONO daytime generation via R1 should track sunlight because it is generally catalyzed by the anions produced during the photochemical oxidation of pervasive gaseous pollutants. Furthermore, by proceeding on the everpresent substrate of aquated airborne particulates, R1 can eventually overtake the photolysis of NO_2: NO_2 + hν = NO + O(^3P) (R2), at large zenith angles. Thus, since R1 leads directly to ˙OH-radical generation via HONO photolysis: HONO + hν = NO + ˙OH, whereas the path initiated by R2 is more circuitous and actually controlled by the slower photolysis of O_3: O_3 + hν (+H_2O) = O_2 + 2˙OH, the competition between R1 and R2 provides a mechanistic switch that buffers ˙OH concentrations and NO_2 decay (via R1 and/or NO_2^+ ˙OH = HNO_3) from actinic flux variations.