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. Sarnia, Ontario, experiences pollutant emissionsdisproportionate to its relatively small size. The small size of the citylimits traditional top-down emission estimate techniques (e.g., satellite)but a low-cost solution for emission monitoring is the mobile MAX-DOAS(Multi-AXis Differential Optical Absorption Spectroscopy).Measurements were made using this technique from 21 March 2017 to 23 March 2017along various driving routes to retrieve vertical column densities (VCDs) of NO2 and SO2 and to estimate emissions of NOx and SO2 from the Sarnia region. A novel aspect of the current study was theinstallation of a NOx analyzer in the vehicle to allow real timemeasurement and characterization of near-surface NOx∕NO2 ratiosacross the urban plumes, allowing improved accuracy of NOx emissionestimates. Confidence in the use of near-surface-measured NOx∕NO2 ratios for estimation of NOx emissions was increased by relativelywell-mixed boundary layer conditions. These conditions were indicated bysimilar temporal trends in NO2 VCDs and mixing ratios when measurementswere sufficiently distant from the sources. Leighton ratios withintransported plumes indicated peroxy radicals were likely disturbing theNO– NO2 – O3 photostationary state through VOC (volatile organic compound) oxidation. The averagelower-limit emission estimate of NOx from Sarnia was 1.60±0.34 t h−1 using local 10  m elevation wind-speed measurements. Ourestimates were larger than the downscaled annual 2017 NPRI-reported (National Pollution Release Inventory)industrial emissions of 0.9  t NOx h−1 . Our lower-limitestimate of SO2 emissions from Sarnia was 1.81±0.83 t SO2 h−1 , equal within uncertainty to the 2017 NPRI downscaledvalue of 1.85  t SO2 h−1 . Satellite-derived NO2 VCDsover Sarnia from the ozone monitoring instrument (OMI) were lower thanmobile MAX-DOAS VCDs, likely due to the large pixel size relative to thecity's size. The results of this study support the utility of themobile MAX-DOAS method for estimating NOx and SO2 emissions inrelatively small, highly industrialized regions, especially when supplementedwith mobile NOx measurements.

atmospheric chemistry and physics

Estimation of NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; and SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; emissions from Sarnia, Ontario, using a mobile MAX-DOAS (Multi-AXis Differential Optical Absorption Spectroscopy) and a NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; analyzer