Integration of airborne and ground observations of nitryl chloride in the Seoul metropolitan area and the implications on regional oxidation capacity during KORUS-AQ 2016

Abstract

Abstract. Nitryl chloride (ClNO2) is a radical reservoir species that releases chlorine radicals upon photolysis. An integrated analysis of the impact of ClNO2 on regional photochemistry in the Seoul metropolitan area (SMA) during the Korea–United States Air Quality Study (KORUS-AQ) 2016 field campaign is presented. Comprehensive multiplatform observations were conducted aboard the NASA DC-8 and at two ground sites (Olympic Park, OP; Taehwa Research Forest, TRF), representing an urbanized area and a forested suburban region, respectively. Positive correlations between daytime Cl2 and ClNO2 were observed at both sites, the slope of which was dependent on O3 levels. The possible mechanisms are explored through box model simulations constrained with observations. The overall diurnal variations in ClNO2 at both sites appeared similar but the nighttime variations were systematically different. For about half of the observation days at the OP site the level of ClNO2 increased at sunset but rapidly decreased at around midnight. On the other hand, high levels were observed throughout the night at the TRF site. Significant levels of ClNO2 were observed at both sites for 4–5 h after sunrise. Airborne observations, box model calculations, and back-trajectory analysis consistently show that these high levels of ClNO2 in the morning are likely from vertical or horizontal transport of air masses from the west. Box model results show that chlorine-radical-initiated chemistry can impact the regional photochemistry by elevating net chemical production rates of ozone by ∼25 % in the morning.

Taehwa Research Forest: A receptor site for severe pollution events in Korea during 2016

“…Downwind plume chemistry is also further investigated and fingerprinted using the onboard DC-8 observations and an explicitly constrained 0-D photochemical box model simulation. Constituent profiles at TRF were also measured using ground-based instrumentation such as O3 lidar (Sullivan et al, 2016;, electrochemical cell (ECC) O3-sondes, and aerosol backscatter. Surface observations of O3, NO2, toluene, and isoprene at TRF are also presented.…”

“…Recent chemical simulations performed during the KORUS-AQ period by Miyazaki et al, 2018 also reveal that observed boundary 10 layer O3 can be as much as 30 ppbv different with dynamic and stagnant flow regimes. Supporting work at TRF has also shown that fast oxidation rates (Kim et al, 2018) and overall oxidation capacity (Jeong et al, 2018) at TRF can exacerbate severe pollution events.…”

Contributions to OH reactivity from unexplored volatile organic compounds measured by PTR-ToF-MS – a case study in a suburban forest of the Seoul metropolitan area during the Korea–United States Air Quality Study (KORUS-AQ) 2016

“…A high-resolution PTR-ToF-MS (Ionicon Analytik GmbH) (de Gouw and Warneke, 2007;Jordan et al, 2009) was deployed at the TRF site. The instrument was operated with a drift tube temperature of 60 • C, 560 V drift voltage, and 2.27 mbar drift tube to maintain the field density ratio (E / N) of 126 Td.…”

“…As the only information we have is the molecular composition, we identified multiple isomers with different functional groups and thus different reactivity. We have extensively reviewed previous publications (Williams et al, 2001;De Gouw et al, 2003;de Gouw and Warneke, 2007;Jordan et al, 2009;Ruuskanen et al, 2011;Müller et al, 2012;Koss et al, 2017) identifying ambient VOCs using PTR-MS with both quadrupole and time-offlight systems to identify possible compounds. For example, for the m/z of 75.043, there are four possible compounds including hydroxy acetone, propionic acid, methyl acetate, and ethyl formate.…”

“…Thus, this site is an ideal natural laboratory to study contributions towards total OH reactivity from primary trace gas emissions from both natural and anthropogenic processes and their oxidation products. This motivated us to deploy a highresolution proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) to quantify trace amounts of VOCs with unknown molecular structures by taking advantage of the universal sensitivity of hydronium ion chemistry towards reactive VOCs (Graus et al, 2010;Jordan et al, 2009). Therefore, we intend to observationally constrain the contributions of conventionally unidentified or unmeasured VOCs towards OH reactivity.…”