Summertime photochemistry during CAREBeijing-2007: RO<sub>x</sub> budgets and O<sub>3</sub> formation
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<sub>x</sub> (OH + HO<sub>2</sub> + RO<sub>2</s...
| Main Authors: | , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2012-08-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/12/7737/2012/acp-12-7737-2012.pdf |
| Summary: | 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<sub>x</sub> (OH + HO<sub>2</sub> + RO<sub>2</sub>) radicals and O<sub>3</sub> formation. While the modeling analysis focuses on near-surface photochemical budgets, the implications for the budget of O<sub>3</sub> in the planetary boundary layer are also discussed. In terms of daytime average, the total RO<sub>x</sub> primary production rate near the surface in Beijing is 6.6 ppbv per hour (ppbv h<sup>−1</sup>, among the highest found in urban atmospheres. The largest primary RO<sub>x</sub> source in Beijing is photolysis of oxygenated volatile organic compounds (OVOCs), which produces HO<sub>2</sub> and RO<sub>2</sub> at 2.5 ppbv h<sup>−1</sup> and 1.7 ppbv h<sup>−1</sup>, respectively. Photolysis of excess HONO from an unknown heterogeneous source is the predominant primary OH source at 2.2 ppbv h<sup>−1</sup>, much larger than that of O<sup>1</sup>D+H<sub>2</sub>O (0.4 ppbv h<sup>−1</sup>). The largest RO<sub>x</sub> sink is via OH + NO<sub>2</sub> reaction (1.6 ppbv h<sup>−1</sup>), followed by formation of RO<sub>2</sub>NO<sub>2</sub> (1.0 ppbv h<sup>−1</sup>) and RONO<sub>2</sub> (0.7 ppbv h<sup>−1</sup>). Due to the large aerosol surface area, aerosol uptake of HO<sub>2</sub> 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 O<sub>3</sub> production and loss rates near the surface are 32 ppbv h<sup>−1</sup> and 6.2 ppbv h<sup>−1</sup>, respectively. Assuming NO<sub>2</sub> to be the source of excess HONO, the NO<sub>2</sub> to HONO transformation leads to considerable O<sub>3</sub> 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 HO<sub>2</sub>) play potentially critical roles in the primary radical budget and O<sub>3</sub> formation in Beijing. One important ramification is that O<sub>3</sub> production is neither NO<sub>x</sub> nor VOC limited, but in a transition regime where reduction of either NO<sub>x</sub> or VOCs could result in reduction of O<sub>3</sub> production. The transition regime implies more flexibility in the O<sub>3</sub> control strategies than a binary system of either NO<sub>x</sub> or VOC limited regime. The co-benefit of concurrent reduction of both NO<sub>x</sub> and VOCs in reducing column O<sub>3</sub> 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 O<sub>3</sub> pollution. |
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| ISSN: | 1680-7316 1680-7324 |