A comparison of chemical mechanisms based on TRAMP-2006 field data

A comparison of a model using five widely known mechanisms (RACM, CB05, LaRC, SAPRC-99, SAPRC-07, and MCMv3.1) has been conducted based on the TexAQS II Radical and Aerosol Measurement Project (TRAMP-2006) field data in 2006. The concentrations of hydroxyl (OH) and hydroperoxy (HO₂) radicals were calculated by a zero-dimensional box model with each mechanism and then compared with the OH and HO₂ measurements. The OH and HO₂ calculated by the model with different mechanisms show similarities and differences with each other and with the measurements. First, measured OH and HO₂ are generally greater than modeled for all mechanisms, with the median modeled-to-measured ratios ranging from about 0.8 (CB05) to about 0.6 (SAPRC-99). These differences indicate that either measurement errors, the effects of unmeasured species or chemistry errors in the model or the mechanisms, with some errors being independent of the mechanism used. Second, the modeled and measured ratios of HO₂/OH agree when NO is about 1 ppbv, but the modeled ratio is too high when NO was less and too low when NO is more, as seen in previous studies. Third, mechanism-mechanism HO_x differences are sensitive to the environmental conditions - in more polluted conditions, the mechanism-mechanism differences are less. This result suggests that, in polluted conditions, the mechanistic details are less important than in cleaner conditions, probably because of the dominance of reactive nitrogen chemistry under polluted conditions.