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  1. Abstract

    Formaldehyde (HCHO) is generated from direct urban emission sources and secondary production from the photochemical reactions of urban smog. HCHO is linked to tropospheric ozone formation, and contributes to the photochemical reactions of other components of urban smog. In this study, pollution plume intercepts during the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign were used to investigate and characterize the formation of HCHO in relation to several anthropogenic tracers. Analysis of aircraft intercepts combined with detailed chemical box modeling downwind of several cities suggests that the most important contribution to observed HCHO was primary emission. A box model analysis of a single plume suggested that secondary sources contribute to 21 ± 10% of the observed HCHO. Ratios of HCHO/CO observed in the northeast US, from Ohio to New York, ranging from 0.2% to 0.6%, are consistent with direct emissions combined with at most modest photochemical production. Analysis of the nocturnal boundary layer and residual layer from repeated vertical profiling over urban influenced areas indicate a direct HCHO emission flux of 1.3 × 1014molecules cm−2h−1. In a case study in Atlanta, GA, nighttime HCHO exhibited a ratio to CO (0.6%–1.8%) and was anti‐correlated with O3. Observations were consistent with mixing between direct HCHO emissions in urban air masses with those influenced by more rapid HCHO photochemical production. The HCHO/CO emissions ratios determined from the measured data are 2.3–15 times greater than the NEI 2017 emissions database. The largest observed HCHO/CO was 1.7%–1.8%, located near co‐generating power stations.

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  2. Abstract

    Sulfur dioxide (SO2) is emitted in large quantities from coal‐burning power plants and leads to various harmful health and environmental effects. In this study, we use plume intercepts from the Wintertime INvestigation of Transport, Emission and Reactivity (WINTER) campaign to estimate the oxidation rates of SO2under wintertime conditions and the factors that determine SO2removal. Observations suggest that OH governs the rate SO2oxidation in the eastern United States during winter. The range of mean oxidation rates during the day from power plants were 0.22–0.71%/hr, producing SO2lifetimes of 13–43 days, if SO2consumption is assumed to occur during 10.5 hr of daylight in cloudless conditions. Though most nighttime rate measurements were zero within uncertainty, there is some evidence of nighttime removal, which suggests alternate oxidation mechanisms. The fastest nighttime observed SO2oxidation rate was 0.25±0.07%/hr, producing a combined day/night SO2lifetime of 8.5–21 days. The upper limit of the oxidation rate (the mean+1σof the fastest day and night observations) is 16.5%/day, corresponding to a lifetime of 6.1 days. The analysis also quantifies the primary emission of sulfate from power plants. The median mole percentage of SO4‐2from observed plumes was 1.7% and the mean percentage sulfate was 2.8% for intercepts within 1 hr of transit to power plants. The largest value observed from close intercepts was over 7% sulfate, and the largest extrapolated value was 18%, based on intercepts further from their source and fastest observed oxidation rate.

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