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  1. Abstract. Glyoxal (CHOCHO), the simplest dicarbonyl in thetroposphere, is a potential precursor for secondary organic aerosol (SOA)and brown carbon (BrC) affecting air quality and climate. The airbornemeasurement of CHOCHO concentrations during the KORUS-AQ (KORea–US AirQuality study) campaign in 2016 enables detailed quantification of lossmechanisms pertaining to SOA formation in the real atmosphere. Theproduction of this molecule was mainly from oxidation of aromatics (59 %)initiated by hydroxyl radical (OH). CHOCHO loss to aerosol was found to bethe most important removal path (69 %) and contributed to roughly∼ 20 % (3.7 µg sm−3 ppmv−1 h−1,normalized with excess CO) of SOA growth in the first 6 h in SeoulMetropolitan Area. A reactive uptake coefficient (γ) of∼ 0.008 best represents the loss of CHOCHO by surface uptakeduring the campaign. To our knowledge, we show the first field observationof aerosol surface-area-dependent (Asurf) CHOCHO uptake, which divergesfrom the simple surface uptake assumption as Asurf increases in ambientcondition. Specifically, under the low (high) aerosol loading, the CHOCHOeffective uptake rate coefficient, keff,uptake, linearly increases(levels off) with Asurf; thus, the irreversible surface uptake is areasonable (unreasonable) approximation for simulating CHOCHO loss toaerosol. Dependence on photochemical impact and changes in the chemical andphysical aerosol properties “free water”, as well as aerosol viscosity,are discussed as other possible factors influencing CHOCHO uptake rate. Ourinferred Henry's law coefficient of CHOCHO, 7.0×108 M atm−1, is ∼ 2 orders of magnitude higher than thoseestimated from salting-in effects constrained by inorganic salts onlyconsistent with laboratory findings that show similar high partitioning intowater-soluble organics, which urges more understanding on CHOCHO solubilityunder real atmospheric conditions. 
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  2. null (Ed.)
    Abstract. Anthropogenic secondary organic aerosol (ASOA), formed from anthropogenicemissions of organic compounds, constitutes a substantial fraction of themass of submicron aerosol in populated areas around the world andcontributes to poor air quality and premature mortality. However, theprecursor sources of ASOA are poorly understood, and there are largeuncertainties in the health benefits that might accrue from reducinganthropogenic organic emissions. We show that the production of ASOA in 11urban areas on three continents is strongly correlated with the reactivityof specific anthropogenic volatile organic compounds. The differences inASOA production across different cities can be explained by differences inthe emissions of aromatics and intermediate- and semi-volatile organiccompounds, indicating the importance of controlling these ASOA precursors.With an improved model representation of ASOA driven by the observations,we attribute 340 000 PM2.5-related premature deaths per year to ASOA, which isover an order of magnitude higher than prior studies. A sensitivity casewith a more recently proposed model for attributing mortality to PM2.5(the Global Exposure Mortality Model) results in up to 900 000 deaths. Alimitation of this study is the extrapolation from cities with detailedstudies and regions where detailed emission inventories are available toother regions where uncertainties in emissions are larger. In addition tofurther development of institutional air quality management infrastructure,comprehensive air quality campaigns in the countries in South and CentralAmerica, Africa, South Asia, and the Middle East are needed for furtherprogress in this area. 
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  3. Dimethyl sulfide (DMS), emitted from the oceans, is the most abundant biological source of sulfur to the marine atmosphere. Atmospheric DMS is oxidized to condensable products that form secondary aerosols that affect Earth’s radiative balance by scattering solar radiation and serving as cloud condensation nuclei. We report the atmospheric discovery of a previously unquantified DMS oxidation product, hydroperoxymethyl thioformate (HPMTF, HOOCH 2 SCHO), identified through global-scale airborne observations that demonstrate it to be a major reservoir of marine sulfur. Observationally constrained model results show that more than 30% of oceanic DMS emitted to the atmosphere forms HPMTF. Coincident particle measurements suggest a strong link between HPMTF concentration and new particle formation and growth. Analyses of these observations show that HPMTF chemistry must be included in atmospheric models to improve representation of key linkages between the biogeochemistry of the ocean, marine aerosol formation and growth, and their combined effects on climate. 
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  4. Abstract

    We use observations from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign to constrain the proposed mechanism of Cl2production from ClNO2reaction in acidic particles. To reproduce Cl2concentrations observed during WINTER with a chemical box model that includes ClNO2reactive uptake to form Cl2, the model required the ClNO2reaction probability, γ (ClNO2), to range from 6 × 10−6to 7 × 10−5, with a mean value of 2.3 × 10−5(±1.8 × 10−5). These field‐determined γ (ClNO2) are more than an order of magnitude lower than those determined in previous laboratory experiments on acidic surfaces, even when calculated particle pH is ≤2. We hypothesize this is because thick salt films in the laboratory enhanced the reactive uptake ClNO2compared to that which would occur in submicron aerosol particles. Using the reacto‐diffusive length‐scale framework, we show that the field and laboratory observations can be reconciled if the net aqueous‐phase reaction rate constant for ClNO2(aq) + Cl(aq) in acidic particles is on the order of 104s−1. We show that wet particle diameter and particulate chloride mass together explain 90% of the observed variance in the box model‐derived γ (ClNO2), implying that the availability of chloride and particle volume limit the efficiency of the reaction. Despite a much lower conversion of ClNO2into Cl2, this mechanism can still be responsible for the nocturnal formation of 10–20 pptv of Cl2in polluted regions, yielding an atmospherically relevant concentration of Cl atoms the following morning.

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  5. Abstract

    Secondary organic aerosol (SOA) from pollution sources is thought to be a minor component of organic aerosol (OA) and fine particulate matter beyond the urban scale. Here we present airborne observations of OA in the northeastern United States, showing that 58% of OA over the region during winter is secondary and originates from pollution sources. We observed a doubling of OA mass from SOA formation in aged emissions, with unexpected similarity to OA growth observed in polluted areas in the summer. A regional model with a simple SOA parameterization based on summer measurements reproduces these winter observations and shows that pollution SOA is widespread, accounting for 14% of submicron particulate matter in near‐surface air. This source of particulate matter is largely unaccounted for in air quality management in the northeastern United States and other polluted areas.

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  6. Abstract

    We present a comparison of instruments measuring nitrogen oxide species from an aircraft during the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign over the northeast United States. Instrument techniques compared here include chemiluminescence (CL), thermal dissociation laser‐induced fluorescence (TD‐LIF), cavity ring‐down spectroscopy (CRDS), high‐resolution time of flight, iodide‐adduct chemical ionization mass spectrometry (ICIMS), and aerosol mass spectrometry. Species investigated include NO2, NO, total nitrogen oxides (NOy), N2O5, ClNO2, and HNO3. Particulate‐phase nitrate is also included for comparisons of HNO3and NOy. Instruments generally agreed within reported uncertainties, with individual flights sometimes showing much better agreement than the data set taken as a whole, due to flight‐to‐flight slope changes. NO measured by CRDS and CL showed an average relative slope of 1.16 ± 0.01 across all flights, which is outside of combined uncertainties. The source of the error was not identified. For NO2measured by CRDS and TD‐LIF the average was 1.02 ± 0.00; for NOymeasured by CRDS and CL the average was 1.01 ± 0.00; and for N2O5measured by CRDS and ICIMS the average was 0.89 ± 0.01. NOybudget closure to within 20% is demonstrated. We observe nonlinearity in NO2and NOycorrelations at concentrations above ~30 ppbv that may be related to the NO discrepancy noted above. For ClNO2there were significant differences between ICIMS and TD‐LIF, potentially due in part to the temperature used for thermal dissociation. Although the fraction of particulate nitrate measured by the TD‐LIF is not well characterized, it improves comparisons to include particulate measurements.

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