Oxidative capacity of the Mexico City atmosphere – Part 2: A RO x radical cycling perspective

A box model using measurements from the Mexico City Metropolitan Area study in the spring of 2003 (MCMA-2003) is presented to study oxidative ca- pacity (our ability to predict OH radicals) and ROx (ROx=OH+HO2+RO2+RO) radical cycling in a polluted (i.e., very high NOx=NO+NO2) atmosphere. Model simu- lations were performed using the Master Chemical Mech- anism (MCMv3.1) constrained with 10 min averaged mea- surements of major radical sources (i.e., HCHO, HONO, O3, CHOCHO, etc.), radical sink precursors (i.e., NO, NO2, SO2, CO, and 102 volatile organic compounds (VOC)), meteoro- logical parameters (temperature, pressure, water vapor con- centration, dilution), and photolysis frequencies. Modeled HOx (=OH+HO2) concentrations compare favor- ably with measured concentrations for most of the day; how- ever, the model under-predicts the concentrations of radicals in the early morning. This "missing reactivity" is highest during peak photochemical activity, and is least visible in a direct comparison of HOx radical concentrations. We con- clude that the most likely scenario to reconcile model predic- tions with observations is the existence of a currently uniden- tified additional source for RO 2 radicals, in combination with an additional sink for HO2 radicals that does not form OH. The true uncertainty due to "missing reactivity" is apparent in parameters like chain length. We present a first attempt to calculate chain length rigorously i.e., we define two pa- rameters that account for atmospheric complexity, and are based on (1) radical initiation, n(OH), and (2) radical termi- nation, !. We find very high values of n(OH) in the early morning are incompatible with our current understanding of ROx termination routes. We also observe missing reactivity in the rate of ozone production (P (O3)). For example, the integral amount of ozone produced could be under-predicted by a factor of two. We argue that this uncertainty is partly accounted for in lumped chemical codes that are optimized to predict ozone concentrations; however, these codes do not reflect the true uncertainty in oxidative capacity that is rele- vant to other aspects of air quality management, such as the formation of secondary organic aerosol (SOA). Our analysis highlights that apart from uncertainties in emissions, and me- teorology, there is an additional major uncertainty in chemi- cal mechanisms that affects our ability to predict ozone and SOA formation with confidence.