Reactivity estimates for aromatic compounds. Part 1. Uncertainty in chamber-derived parameters

Because the major atmospheric reaction products for aromatic hydrocarbons a re unknown, they are represented in air quality models using parameterized mechanisms derived by modeling environmental chamber data. Uncertainties in rate constants, experimental conditions, and chamber artifacts affect the resulting parameter estimates. The SAPRC-97 mechanism represents aromatic r ing fragmentation products by model species MGLY (alpha-dicarbonyls) and AF G2 (other photoreactive products) with yields derived from aromatics-NOx ex periments with blacklight or xenon are light sources. Tn this study, parame ter estimates for nine aromatic compounds were estimated using stochastic p rogramming. The uncertainties estimated for these parameters range from abo ut 29% (la relative to the mean) for the yield of AFG2 in the 135-trimethyl benzene mechanism to 71% for the yield of MOLY for p-xylene. Major causes a re uncertainties in rate constants for the aromatics + OH and NO2 + OH reac tions, and the light intensity, parameters representing unknown chamber rad ical sources, and initial aromatic concentrations in the experiments. The c hamber radical source parameters are estimated from GO-NOx and n-butane-NOx experiments, and are sensitive to uncertainties in the rate constants for n-butane or CO + OH, NO2 + OH. HONO photolysis and the experimental light i ntensity. A companion paper examines how uncertainties in the chamber-deriv ed aromatics parameters affect incremental reactivity estimates for volatil e organic compounds. (C) 2000 Elsevier Science Ltd. All rights reserved.