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Volume 15, issue 8
Atmos. Chem. Phys., 15, 4357-4371, 2015
https://doi.org/10.5194/acp-15-4357-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: Atmospheric impacts of Eastern Asia megacities

Atmos. Chem. Phys., 15, 4357-4371, 2015
https://doi.org/10.5194/acp-15-4357-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 29 Apr 2015

Research article | 29 Apr 2015

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

S. Kim1, S.-Y. Kim2, M. Lee3, H. Shim3, G. M. Wolfe4,5, A. B. Guenther6, A. He1, Y. Hong2, and J. Han2,* S. Kim et al.
  • 1Department of Earth System Science, School of Physical Sciences, University of California, Irvine, CA, USA
  • 2National Institute Environmental Research, Incheon, South Korea
  • 3Department of Earth and Environmental Sciences, Korean University, Seoul, South Korea
  • 4Joint Center for Earth Systems Technology, University of Maryland, Baltimore, MD, USA
  • 5Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 6Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland WA, USA
  • *now at: Department of Environmental & Energy Engineering, Anyang University, Anyang, South Korea

Abstract. 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, NOx, 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 HOx (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 HOx 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–NOx sensitivity and VOC oxidation product formation rates. Overall, the NOx 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.

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