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Probing the Tropospheric Oxidation Capacity in Pristine to Polluted Envrionments


This study presents ground observation of hydroxyl radicals (OH) during GoAmazon2014/5, ground and airborne observations of nitryl chloride (ClNO2) during KORUS-AQ2016, and ship-borne observation of molecular iodine (I2) during ARAON2018. Measurements were made with a Chemical Ionization Mass Spectrometer (CIMS). Observation data sets of the radicals and radical precursors are compared with observation constrained box model simulations in order to understand the oxidation capacity of the troposphere from polluted to pristine environments.

During the GoAmazon2014/5 campaign, OH was measured in February-March of 2016 during the wet season at the T3 site, ~ 60 km west of Manaus, Brazil. Measurements are compared to box model simulations embedded with the near-explicit Master Chemical Mechanism (MCM v3.3.1) and other mechanisms used in global models. The model was highly constrained with measured meteorology and other trace gas observations. The results show that the simulations agree well with the OH observations. This confirms the HOx-NOx-VOC chemistry in this high biogenic volatile organic compound (BVOC) and low NOX environment is well understood contrary to the recent studies of higher than expected OH in isoprene dominant forested regions.

ClNO2 measurements were carried out in May to early June in the Seoul Metropolitan Area (SMA) during the KORUS-AQ 2016 campaign. Ground observations were conducted at the Olympic Park site and Taehwa Research Forest, each representing a polluted site near the city center and a forested region downwind. Airborne observations were made on the NASA DC-8, which made measurements over the Korean peninsula and the yellow sea during the campaign period. Significant levels of daytime ClNO2 were measured at both ground sites with a positive correlation to Cl2, which was suppressed at low O3. Box model simulations show that this is likely due to the heterogeneous reaction of HOCl and ClONO2 on aerosols and the autocatalytic production of Cl2. In the early morning, a second ClNO2 peak was observed when there were predominant westerlies. Based on airborne observations, box model simulations, and backtrajectories, this morning peak is possibly due to a mix of vertical and horizontal transport from the west coast. Box model runs show that chlorine chemistry can enhance up to 25 % of net chemical production of O3 in the SMA.

Gas phase I2 and HOI were measured with a CIMS, on-board the Korean ice breaker R/V ARAON from late March to early May. Up to ~ 15 ppt of I2 was measured near the Antarctic Peninsula, which corresponds to previous studies that reported high IO levels in West Antarctica. Short O3 depletion events were observed simultaneous to enhanced levels of Br2 and I2. Sources of iodine precursors can be both from inorganic, like accelerated reactions within the ice matrix, and organic sources, like sea ice diatoms. Correlation between I2 and biological tracers like DMS and isoprene was observed.

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