Impact of isoprene and HONO chemistry on ozone and OVOC formation in a semirural South Korean forest

Journal Article · Wed Apr 29 00:00:00 EDT 2015 · Atmospheric Chemistry and Physics (Online)

DOI:https://doi.org/10.5194/acp-15-4357-2015· OSTI ID:1206591

Kim, S. ^[1]; Kim, S. -Y. ^[2]; Lee, M. ^[3]; Shim, H. ^[3]; Wolfe, G. M. ^[4]; Guenther, A. B. ^[5]; He, A. ^[1]; Hong, Y. ^[2]; Han, J. ^[2]

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1. Univ. of California, Irvine, CA (United States)
2. National Inst. Environmental Research, Incheon (South Korea)
3. Korean Univ., Seoul (South Korea)
4. Univ. of Maryland, Baltimore, MD (United States). Joint Center for Earth Systems Technology
5. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Atmospheric Science and Global Change Div. (ASGC)

Rapid urbanization and economic development in East Asia in past decades has led to photochemical air pollution problems such as excess photochemical ozone and aerosol formation. Asian megacities such as Seoul, Tokyo, Shanghai, Guangzhou, and Beijing are surrounded by densely forested areas, and recent research has consistently demonstrated the importance of biogenic volatile organic compounds (VOCs) from vegetation in determining oxidation capacity in the suburban Asian megacity regions. Uncertainties in constraining tropospheric oxidation capacity, dominated by hydroxyl radical, undermine our ability to assess regional photochemical air pollution problems. We present an observational data set of CO, NO_x, SO₂, ozone, HONO, and VOCs (anthropogenic and biogenic) from Taehwa research forest (TRF) near the Seoul metropolitan area in early June 2012. The data show that TRF is influenced both by aged pollution and fresh biogenic volatile organic compound emissions. With the data set, we diagnose HO_x (OH, HO₂, and RO₂) distributions calculated using the University of Washington chemical box model (UWCM v2.1) with near-explicit VOC oxidation mechanisms from MCM v3.2 (Master Chemical Mechanism). Uncertainty from unconstrained HONO sources and radical recycling processes highlighted in recent studies is examined using multiple model simulations with different model constraints. The results suggest that (1) different model simulation scenarios cause systematic differences in HO_x distributions, especially OH levels (up to 2.5 times), and (2) radical destruction (HO₂ + HO₂ or HO₂ + RO₂) could be more efficient than radical recycling (RO₂ + NO), especially in the afternoon. Implications of the uncertainties in radical chemistry are discussed with respect to ozone–VOC–NO_x sensitivity and VOC oxidation product formation rates. Overall, the NO_x limited regime is assessed except for the morning hours (8 a.m. to 12 p.m. local standard time), but the degree of sensitivity can significantly vary depending on the model scenarios. The model results also suggest that RO₂ levels are positively correlated with oxygenated VOCs (OVOCs) production that is not routinely constrained by observations. These unconstrained OVOCs can cause higher-than-expected OH loss rates (missing OH reactivity) and secondary organic aerosol formation. The series of modeling experiments constrained by observations strongly urge observational constraint of the radical pool to enable precise understanding of regional photochemical pollution problems in the East Asian megacity region.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC05-76RL01830
• OSTI ID: 1206591
• Journal Information: Atmospheric Chemistry and Physics (Online), Vol. 15, Issue 8; ISSN 1680-7324
• Publisher: European Geosciences Union
• Country of Publication: United States
• Language: English

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54 ENVIRONMENTAL SCIENCES