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

Because the major atmospheric reaction products for aromatic hydrocarbons are 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 ring fragmentation products by model species MGLY (α-dicarbonyls) and AFG2 (other photoreactive products) with yields derived from aromatics–NOx experiments with blacklight or xenon arc light sources. In this study, parameter estimates for nine aromatic compounds were estimated using stochastic programming. The uncertainties estimated for these parameters range from about 29% (1σ relative to the mean) for the yield of AFG2 in the 135-trimethylbenzene mechanism to 71% for the yield of MGLY for p-xylene. Major causes are uncertainties in rate constants for the aromatics+OH and NO2+OH reactions, and the light intensity, parameters representing unknown chamber radical sources, and initial aromatic concentrations in the experiments. The chamber radical source parameters are estimated from CO–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 intensity. A companion paper examines how uncertainties in the chamber-derived aromatics parameters affect incremental reactivity estimates for volatile organic compounds.