Abstract
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.
Highlights
NOx (NO + NO2) and volatile organic compounds (VOCs) are two important precursors that drive HOx radical cycles (Levy, 1971)
We present atmospheric observations of NOx, CO, VOCs, ozone, and HONO in the Taehwa research forest (TRF) in the Seoul metropolitan area (SMA), South Korea
TRF has a sampling tower located in the middle of a coniferous tree plantation (200 m by 200 m) with a canopy height of 18 m (Pinus koraiensis), surrounded by a deciduous forest mostly composed by oak
Summary
NOx (NO + NO2) and volatile organic compounds (VOCs) are two important precursors that drive HOx radical cycles (Levy, 1971). These new findings in the isoprene oxidation process are closely related to recent findings in unexpectedly high OH concentrations (Hofzumahaus et al, 2009; Lelieveld et al, 2008) and substantial missing OH sinks known as unexpectedly high OH reactivity in high isoprene environments (Di Carlo et al, 2004; Edwards et al, 2013; Kim et al, 2011; Lou et al, 2010) These new findings have significant implications for regional air quality especially regarding photochemical ozone and secondary organic aerosols (SOA) production. We discuss current uncertainty in OHisoprene photochemistry with perspectives of constraining photochemical ozone production and OVOCs precursors of secondary organic aerosols in addition to the roles of unconstrained HONO sources in radical distributions
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