title: Tropospheric aerosol as a reactive intermediate.
abstract: 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 NO2 'on water': 2NO2 + H2O = HONO + NO(9-) + 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 NO2: NO2 + hv = NO + O(3P) (R2), at large zenith angles. Thus, since R1 leads directly to *OH-radical generation via HONO photolysis: HONO + hv = NO + *OH, whereas the path initiated by R2 is more circuitous and actually controlled by the slower photolysis of O3: O3 + hv (+H2O) = O2 + 2*OH, the competition between R1 and R2 provides a mechanistic switch that buffers *OH concentrations and NO2 decay (via R1 and/or NO2 + *OH = HNO3) from actinic flux variations.
authors.affiliation: Ronald and Maxine Linde Center for Global Environmental Science, California Institute of Technology, Pasadena, California 91125, USA. ajcoluss@caltech.edu
authors.forename: Agustín J
authors.initials: AJ
authors.lastname: Colussi
authors.affiliation: The Hakubi Center for Advanced Research, Kyoto University, Kyoto 606-8302, Japan.
authors.forename: Shinichi
authors.initials: S
authors.lastname: Enami
authors.affiliation: Department of Molecular Engineering, Kyoto University, Kyoto 615-8510, Japan.
authors.forename: Akihiro
authors.initials: A
authors.lastname: Yabushita
authors.affiliation: Ronald and Maxine Linde Center for Global Environmental Science, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Hoffmann
authors.affiliation: Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Wei-Guang
authors.initials: WG
authors.lastname: Liu
authors.affiliation: Ronald and Maxine Linde Center for Global Environmental Science, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Himanshu
authors.initials: H
authors.lastname: Mishra
authors.affiliation: Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: William A
authors.initials: WA
authors.lastname: Goddard