Summertime photochemistry during CAREBeijing-2007: RO<sub> x</sub> budgets and O<sub>3</sub> formation

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 RO_x (OH + HO₂ + RO₂) radicals and O₃ formation. The daytime average of total RO_x primary production rate in Beijing is ~6.6 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 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 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 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. <br></br> 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 RO_x budgets and O₃ formation, aromatics are the largest contributor. 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. Further research on the spatial extent of the transition regime over the polluted eastern China is critically important for controlling regional O₃ pollution

Organosulfates from Pinene and Isoprene over the Pearl River Delta, South China: Seasonal Variation and Implication in Formation Mechanisms

“…During the day, NO would compete against RO 2 and HO 2 to react with hydroperoxy radical 1; and NO 3 reaction could be ignored due to its high photolysis rate. Thus, the fractional yield of hydroxynitrate in the OH pathway is calculated by f OH = k 7 2 ] levels might be comparable in the ambient (e.g., 6.8 × 10 8 molecules cm −3 vs 4.5 × 10 8 molecules cm −3 in summer of Beijing), 64 here we assumed that [RO 2 ] had the same levels as [HO 2 ] . Thus, the f OH and f NO3 were calculated as 0.23 and 1.2 × 10 −4 , respectively with the rate constants in Scheme 1 at T = 298.15 K. Although both hydroperoxy radical 1 and nitroperoxy radical 1 are potential sources of hydroxynitrate, the hydroperoxy pathway initiated by OH in the day is far more efficient.…”

Surface and free tropospheric sources of methanesulfonic acid over the tropical Pacific Ocean

“…Previously, we have applied the 1-D REAM to investigate polar photochemistry at the South Pole [Wang et al, 2007], urban photochemistry in China [Liu et al, 2010[Liu et al, , 2012, and sulfur chemistry over the tropical Pacific [Gray et al, 2011]. In this work, we use the 1-D REAM, which incorporates modules for O 3 -NO x -hydrocarbon photochemistry, marine sulfur chemistry, cloud/aerosol scavenging, turbulent and convective transport, and dry/wet deposition, to analyze the vertical distribution and sources of MSA over the tropical Pacific.…”

Summertime distributions of peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) in Beijing: Understanding the sources and major sink of PAN

“…b Data inferred from (Shao et al, 2009). c OH concentration was assumed to be constant and estimated to be 5 Â 10 6 molecules cm À3 during summertime in Beijing (Liu et al, 2012). d Recommended by Atkinson et al (2006) at 298 K. e Recommended by Darnall et al (1979).…”