Estimation of NOx and SO2 emissions from Sarnia, Ontario, using a mobile MAX-DOAS (Multi-AXis Differential Optical Absorption Spectroscopy) and a NOx analyzer

Abstract

Sarnia, Ontario, 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 the mobile MAX-DOAS (Multi-AXis Differential Optical Absorption Spectroscopy). Measurements were made using this technique from 21 March 2017 to 23 March 2017 along various driving routes to retrieve vertical column densities (VCDs) of NO₂ and SO₂ and to estimate emissions of NO_x and SO₂ 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∕NO₂ ratios across the urban plumes, allowing improved accuracy of NO_x emission estimates. Confidence in the use of near-surface-measured NO_x∕NO₂ 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 NO₂ 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–NO₂–O₃ photostationary state through VOC (volatile organic compound) oxidation. The average lower-limit emission estimate of NO_x from Sarnia was 1.60±0.34 t h⁻¹ using local 10 m elevation wind-speed measurements. Our estimates were larger than the downscaled annual 2017 NPRI-reported (National Pollution Release Inventory) industrial emissions of 0.9 t NO_x h⁻¹. Our lower-limit estimate of SO₂ emissions from Sarnia was 1.81±0.83 t SO₂ h⁻¹, equal within uncertainty to the 2017 NPRI downscaled value of 1.85 t SO₂ h⁻¹. Satellite-derived NO₂ 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 SO₂ emissions in relatively small, highly industrialized regions, especially when supplemented with mobile NO_x measurements.