Atmospheric Environment 35 (2001) 4489–4498 Heterogeneous NO x chemistry in the polluted PBL V.R. Kotamarthi*, J.S. Gaffney, N.A. Marley, P.V. Doskey Environmental Research, Argonne National Laboratory, 9700 South Cass Ave., Argonne, IL, USA Received 30 September 2000; accepted 3 April 2001 Abstract The significance of heterogeneous mechanisms in controlling gas-phase NO x (NO, NO 2 ) 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 NO y 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 NO 2 conversion to HONO on the surface of soot particles for further analysis with a box photochemical model. These calculations show that the conversion of NO 2 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 HO 2 in the polluted planetary boundary layer (PBL). The sensitivity of these results to the different reaction rate probabilities (g) and particle surface areas was also examined. Results are then evaluated to find the combination of g and surface areas that would mostly likely occur in the PBL within the limitations of the model. r 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction The focus of much urban-scale atmospheric chemistry is on the formation and destruction of ozone. The near- source chemical production of ozone is largely deter- mined by the available hydrocarbon and NO x (NO, NO 2 ) precursors in the urban plume and by local meteorological conditions (i.e., the number of days with temperature inversions, high temperature, or a combi- nation of the two). Aerosols are primarily considered a sink for source gases and secondary gases, from either condensation onto preexisting particles or nucleation of gas-phase constituents. The recognition is recent that heterogeneous chemical mechanisms have a role through hydrolysis or other alterations of the nature and the complexity of the gas-phase chemistry itself in different regions of the atmosphere. As field methods for particle characterization and our ability to measure short-lived free radicals have improved over the last several years, important deficiencies in our current understanding of the NO x and HO x (OH, HO 2 , H) chemical cycles have become apparent (Wennberg et al., 1994; Singh et al., 1995). 1.1. Reactions on atmospheric aerosol surfaces: possible significance for PBL chemistry Particle surfaces ranging from sulfuric acid droplets to soot have been shown capable of catalyzing the conversion of active forms of nitrogen oxides to reservoirs (reduction reactions; Mozurkewich and Cal- vert, 1988; Hanson and Ravishankara, 1991), and the results hint at possible conversion of reservoir com- pounds such as PAN and HNO 3 to more active forms of odd nitrogen such as NO and NO 2 (oxidation reactions; Ammann et al., 1998; Lary et al., 1997). In addition, non-catalytic reactions involving N 2 O 5 and NO 3 on wet aerosol surfaces have been proposed (Dentener and Crutzen, 1993). However, the significance and feasibility of these reactions in the different regions of the *Corresponding author. Tel.: +1-630-252-7164; fax: +1- 630-252-5498. E-mail addresses: vrkotamarthi@anl.gov (V.R. Kotamarthi). 1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII:S1352-2310(01)00221-7