Estimation of NO x and SO 2 Emissions from Sarnia, Ontario using Mobile-MAX-DOAS and a NO x -Analyzer

Abstract. Sarnia, ON experiences pollutant emissions disproportionate to its relatively small size. The small size of the city limits traditional top-down emission estimate techniques (e.g., satellite) but a low-cost solution for emission monitoring is Mobile-MAX-DOAS. Measurements were made using this technique from 21/03/2017 to 23/03/2017 along various driving routes to retrieve vertical column densities (VCDs) of NO 2 and SO 2 and to estimate emissions of NO x and SO 2 from the Sarnia region. A novel aspect of the current study was the installation of a NO x analyzer in the vehicle to allow real time measurement and characterization of near-surface NO x /NO 2 ratios across the urban plumes, allowing improved accuracy of NO x emission estimates. Confidence in the use of near-surface measured NO x /NO 2 ratios for estimation of NO x emissions was increased by relatively well-mixed boundary layer conditions. These conditions were indicated by similar temporal trends in NO 2 VCDs and mixing ratios when measurements were sufficiently distant from the sources. Leighton ratios within transported plumes indicated peroxy radicals were likely disturbing the NO-NO 2 -O 3 photostationary state through VOC oxidation. The average lower limit emission estimate of NO x from Sarnia was 1.60 ± 0.34 tonnes hr −1 using local 10 m elevation wind-speed measurements. Our estimates were larger than the downscaled annual 2017 NPRI reported industrial emissions of 0.9 tonnes NO x  hr −1 . Our lower limit estimate of SO 2 emissions from Sarnia was 1.81 ± 0.83 tonnes SO 2  hr −1 , equal within uncertainty to the 2017 NPRI downscaled value of 1.85 tonnes SO 2  hr −1 . Satellite-derived NO 2 VCDs over Sarnia from the Ozone Monitoring Instrument (OMI) were lower than Mobile-MAX-DOAS VCDs, likely due to the large pixel size relative to the city’s size. The results of this study support the utility of the Mobile-MAX-DOAS method for estimating NO x and SO 2 emissions in relatively small, highly industrialized regions especially when supplemented with mobile NO x measurements.