Summertime photochemistry during CAREBeijing-2007: RO x budgets and O3 formation
1School of Earth and Atmospheric Science, Georgia Institute of Technology, Atlanta, USA
2College of Environmental Sciences and Engineering, Peking University, Beijing, China
3Institute for Atmospheric Pollution, National Research Council (CNR-IIA), Rome, Italy
4Research Center for Environmental Changes (RCEC), Academic Sinica, Taipei, China
*now at: the Pacific Northwest National Laboratory, Richland, Washington, USA
Abstract. We analyze summertime photochemistry near the surface over Beijing, China, using a 1-D photochemical model (Regional chEmical and trAnsport Model, REAM-1D) constrained by in situ observations, focusing on the budgets of ROx (OH + HO2 + RO2) radicals and O3 formation. The daytime average of total ROx primary production rate in Beijing is ~6.6 ppbv h−1, among the highest found in urban atmospheres. The largest primary ROx source in Beijing is photolysis of oxygenated volatile organic compounds (OVOCs), which produces HO2 and RO2 at average daytime rates of 2.5 ppbv h−1 and 1.7 ppbv h−1, respectively. Photolysis of excess HONO from the unknown heterogeneous source is a predominant primary OH source at 2.2 ppbv h−1, much larger than that of O1D + H2O (0.4 ppbv h−1). The largest ROx sink is via OH + NO2 reaction (1.6 ppbv h−1), followed by formation of RO2NO2 (1.0 ppbv h−1) and RONO2 (0.7 ppbv h−1). Due to the large aerosol surface area, aerosol uptake of HO2 appears to be another important radical sink, although the estimate of its magnitude is highly variable depending on the reactive uptake coefficient value used. The daytime average O3 production and loss rates are 32 ppbv h−1 and 6.2 ppbv h−1, respectively. Assuming NO2 to be the source of excess HONO, the NO2 to HONO transformation leads to significant O3 loss and reduction of its lifetime.
Our observation-based modeling analyses suggest that VOCs and heterogeneous reactions (e.g. HONO formation and aerosol uptake HO2) play major roles in the primary radical budget and O3 formation in Beijing. Among the VOC precursors for OVOCs, which strongly affect ROx budgets and O3 formation, aromatics are the largest contributor. One important ramification is that O3 production is neither NOx nor VOC limited, but in a transition regime, where reduction of either NOx or VOCs could result in reduction of O3 production. The transition regime implies more flexibility in the O3 control strategies than a binary system of either NOx or VOC limited regime. Further research on the spatial extent of the transition regime over the polluted eastern China is critically important for controlling regional O3 pollution.