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Nitryl chloride (ClNO 2) can play an important role in determining ozone and particulate matter (PM) chemistry as a nighttime reservoir of NO X and a source of chlorine radical (Cl•) in the daytime in polluted environments. We present a quantitative evaluation of the impact of tropospheric ClNO 2 chemistry on air quality in South Korea using extensive observations during the Korea-United States Air Quality (KORUS-AQ) field study and a 3-D chemical transport model (CTM). We first estimated anthropogenic emissions of reactive chlorine in South Korea to implement it along with the latest Chinese emissions in the model. A comparison between the simulated and observed ClNO 2 concentrations during the KORUS-AQ campaign shows that the model captures the observed spatial and temporal variations, including local and transboundary transport of ClNO 2 and its precursors. The model simulation highlights that ClNO 2 plays a significant role in nighttime NO X chemistry by efficiently converting NO to NO 2. In addition, it causes a noticeable acceleration of the NO X -O 3 cycle. When averaged over the campaign, adding ClNO 2 chemistry into the model leads to an increase of O 3 (1.1%), NO X (3.1%), •OH (2.0%), HO 2 • (0.8%), and Cl• (507.8%) and a decrease of TNO 3 (HNO 3 + aerosol nitrate, 1.7%). • Natural sources dominate ClNO 2 in Korea with nonnegligible anthropogenic influences. • GEOS-Chem reproduces ClNO 2 enhancement from natural/anthropogenic sources. • The meteorological patterns significantly affect the distributions of ClNO 2 in the model. • ClNO 2 chemistry interrupts the nighttime NO X sink process and increases O 3. • ClNO 2 chemistry enhances the daytime productions of OH, HO 2 , and Cl.•. [ABSTRACT FROM AUTHOR]

Abstract High concentrations of PM 2.5 have become a serious environmental issue in South Korea, which ranked 1st or 2nd among OECD countries in terms of population exposure to PM 2.5. Quantitative understanding of PM 2.5 source attribution is thus crucial for developing efficient air quality mitigation strategies. Here we use a suite of extensive observations of PM 2.5 and its precursors concentrations during the international KORea-US cooperative Air Quality field study in Korea (KORUS-AQ) in May–June 2016 to investigate source contributions to PM 2.5 in South Korea under various meteorological conditions. For the quantitative analysis, we updated a 3-D chemical transport model, GEOS-Chem, and its adjoint with the latest regional emission inventory and other recent findings. The updated model is evaluated by comparing against observed daily PM 2.5 and its component concentrations from six ground sites (Bangnyung, Bulkwang, Olympic park, Gwangju, Ulsan, and Jeju). Overall, simulated concentrations of daily PM 2.5 and its components are in a good agreement with observations over the peninsula. We conduct an adjoint sensitivity analysis for simulated surface level PM 2.5 concentrations at five ground sites (except for Bangnyung because of its small population) under four different meteorological conditions: dynamic weather, stagnant, extreme pollution, and blocking periods. Source contributions by regions vary greatly depending on synoptic meteorological conditions. Chinese contribution accounts for almost 68% of PM 2.5 in surface air in South Korea during the extreme pollution period of the campaign, whereas an enhanced contribution from domestic sources (57%) occurs for the blocking period. Results from our sensitivity analysis suggest that the reduction of domestic anthropogenic NH 3 emissions could be most effective in reducing population exposure to PM 2.5 in South Korea (effectiveness = 14%) followed by anthropogenic SO 2 emissions from Shandong region (effectiveness = 11%), domestic anthropogenic NO x emissions (effectiveness = 10%), anthropogenic NH 3 emissions from Shandong region (effectiveness = 8%), anthropogenic NO x emissions from Shandong region (effectiveness = 7%), domestic anthropogenic OC emissions (effectiveness = 7%), and domestic anthropogenic BC emissions (effectiveness = 5%). Highlights • Source contributions to PM 2.5 in South Korea are investigated during the KORUS-AQ. • Source contributions by regions vary greatly depending on synoptic meteorological conditions. • Chinese contributions are up to ~68% of PM 2.5 in surface air in South Korea. • An enhanced contribution from domestic sources (57%) occurs for the blocking period. [ABSTRACT FROM AUTHOR]

Despite its profound influence on air quality, the aerosol acidity, quantifiable by aerosol pH is poorly characterized in South Korea where high concentrations of fine particles have become a serious environmental issue. In this study, we estimated the pH values of PM 2.5 using observations of inorganic species and precursor gases concentrations, ambient temperature and relative humidity (RH), measured at the Olympic Park in Seoul during the KORUS-AQ campaign (10 May to June 19, 2016). For the aerosol pH calculation, the ISORROPIA and E-AIM thermodynamic equilibrium models in forward mode were used, in which we found the phase state assumption, either stable (solid + liquid) or metastable (liquid only) had no significant impact on pH predictions. As reported previously, the unrealistic pH prediction of ∼7.6 by ISORROPIA, which largely misrepresents ammonia-rich aerosol contents in the Seoul atmosphere was mainly due to coding errors in the standard ISORROPIA model (i.e., forward mode with stable state assumption). Using the revised ISORROPIA, pH predictions obtained with stable assumption approached those obtained with the metastable state assumption. Following such amendment, fine particle samples were predicted to be acidic with pH values ranging from 1.5 to 4.0 as determined by both ISORROPIA and E-AIM calculations. These pH values were higher than those reported in the United States and Europe and lower than those reported in northern China. A comparison of ISORROPIA and E-AIM models revealed that the pH values obtained using ISORROPIA were consistently but only ∼0.4 pH unit higher than those obtained using E-AIM, demonstrating the precision and consistent utility of thermodynamic equilibrium model in the quantification of PM 2.5 pH in Seoul. • PM 2.5 pH in Seoul during KORUS-AQ by ISORROPIA and E-AIM ranged between 1.4 and 4.0 • Particle pH by ISORROPIA was systematically higher than E-AIM but only 0.4 pH unit. • Assumed particle phase did not significantly affect pH calculations of both models. • Ammonia and aerosol water content played a critical role in determining particle pH. [ABSTRACT FROM AUTHOR]