Estimation of NO x and SO 2 emissions from Sarnia, Ontario, using a mobile MAX-DOAS (Multi-AXis Differential Optical Absorption Spectroscopy) and a NO x analyzer

Abstract

Abstract. Sarnia, Ontario, experiences pollutant emissions\ndisproportionate to its relatively small size. The small size of the city\nlimits traditional top-down emission estimate techniques (e.g., satellite)\nbut a low-cost solution for emission monitoring is the mobile MAX-DOAS\n(Multi-AXis Differential Optical Absorption Spectroscopy).\nMeasurements were made using this technique from 21\xa0March\xa02017 to 23\xa0March\xa02017\nalong various driving routes to retrieve vertical column densities (VCDs) of\n NO2 and SO2 and to estimate emissions of NOx and SO2 from the Sarnia region. A novel aspect of the current study was the\ninstallation of a NOx analyzer in the vehicle to allow real time\nmeasurement and characterization of near-surface NOx∕NO2 ratios\nacross the urban plumes, allowing improved accuracy of NOx emission\nestimates. Confidence in the use of near-surface-measured NOx∕NO2 \nratios for estimation of NOx emissions was increased by relatively\nwell-mixed boundary layer conditions. These conditions were indicated by\nsimilar temporal trends in NO2 VCDs and mixing ratios when measurements\nwere sufficiently distant from the sources. Leighton ratios within\ntransported plumes indicated peroxy radicals were likely disturbing the\nNO– NO2 – O3 photostationary state through VOC (volatile organic compound) oxidation. The average\nlower-limit emission estimate of NOx from Sarnia was 1.60±0.34 t\u2009h−1 using local 10\u2009 m elevation wind-speed measurements. Our\nestimates were larger than the downscaled annual 2017 NPRI-reported (National Pollution Release Inventory)\nindustrial emissions of 0.9\u2009 t\u2009NOx\u2009h−1 . Our lower-limit\nestimate of SO2 emissions from Sarnia was 1.81±0.83 t\u2009SO2\u2009h−1 , equal within uncertainty to the 2017 NPRI downscaled\nvalue of 1.85\u2009 t\u2009SO2\u2009h−1 . Satellite-derived NO2 VCDs\nover Sarnia from the ozone monitoring instrument (OMI) were lower than\nmobile MAX-DOAS VCDs, likely due to the large pixel size relative to the\ncity s size. The results of this study support the utility of the\nmobile MAX-DOAS method for estimating NOx and SO2 emissions in\nrelatively small, highly industrialized regions, especially when supplemented\nwith mobile NOx measurements.