Summertime photochemistry during CAREBeijing-2007: RO_x budgets and O₃ formation

Abstract. We analyze summertime photochemistry near the surface in 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 RO_x (OH + HO₂ + RO₂) radicals and O₃ formation. While the modeling analysis focuses on near-surface photochemical budgets, the implications for the budget of O₃ in the planetary boundary layer are also discussed. In terms of daytime average, the total RO_x primary production rate near the surface in Beijing is 6.6 ppbv per hour (ppbv h⁻¹, among the highest found in urban atmospheres. The largest primary RO_x source in Beijing is photolysis of oxygenated volatile organic compounds (OVOCs), which produces HO₂ and RO₂ at 2.5 ppbv h⁻¹ and 1.7 ppbv h⁻¹, respectively. Photolysis of excess HONO from an unknown heterogeneous source is the predominant primary OH source at 2.2 ppbv h⁻¹, much larger than that of O¹D+H₂O (0.4 ppbv h⁻¹). The largest RO_x 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 uptake coefficient value used. The daytime average O₃ production and loss rates near the surface 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 considerable O₃ loss and reduction of its lifetime. Our observation-constrained modeling analysis suggests that oxidation of VOCs (especially aromatics) and heterogeneous reactions (e.g. HONO formation and aerosol uptake HO₂) play potentially critical roles in the primary radical budget and O₃ formation in Beijing. One important ramification is that O₃ production is neither NO_x nor VOC limited, but in a transition regime where reduction of either NO_x 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 NO_x or VOC limited regime. The co-benefit of concurrent reduction of both NO_x and VOCs in reducing column O₃ production integrated in the planetary boundary layer is significant. Further research on the spatial extent of the transition regime over the polluted eastern China is critically important for controlling regional O₃ pollution.