Estimation of NO<sub><i>x</i></sub> and SO<sub>2</sub> Emissions from Sarnia, Ontario using Mobile-MAX-DOAS and a NO<sub><i>x</i></sub>-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 NO2 and SO2 and to estimate emissions of NOx and SO2 from the Sarnia region. A novel aspect of the current study was the installation of a NOx analyzer in the vehicle to allow real time measurement and characterization of near-surface NOx/NO2 ratios across the urban plumes, allowing improved accuracy of NOx emission estimates. Confidence in the use of near-surface measured NOx/NO2 ratios for estimation of NOx 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 oxidation. The average lower limit emission estimate of NOx 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 NOx hr−1. Our lower limit estimate of SO2 emissions from Sarnia was 1.81 ± 0.83 tonnes SO2 hr−1, equal within uncertainty to the 2017 NPRI downscaled value of 1.85 tonnes SO2 hr−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 NOx and SO2 emissions in relatively small, highly industrialized regions especially when supplemented with mobile NOx measurements.


in the DSCD early and late in the day. The sum of SCD(FRS) and SCD(SZA) is collectively known as the 163 DSCD offset . The DSCD offset (t i ) function was estimated by fitting a second order polynomial to multiple pairs of 164 DSCDs of spectra (non-zenith and zenith from the same sequence), described in detail in (Wagner et al., 2010).

165
The DSCD offset polynomial is most accurate when successive spectra in each sequence observe similar mixing ratio 166 fields, and measurements obtained many data-points over most of the daylight hours. However, routes on Days 2 167 and 3 included driving in and out of both high and low NO x regions within short time-periods and thus met neither 168 of the requirements listed above for the DSCD offset method. On these days, a second method was used where NO 2 169 DSCDs were fit against an FRS spectrum obtained close in time (<25 minutes) along each respective route in a lowphoton such that the air-mass factor (AMF) depended only on the viewing elevation angle, ∝, AMF trop (∝) ≈ 1 sin (α) 175 (Brinksma et al., 2008)(Wagner et al., 2010. This "geometric approximation" is most valid under low to moderate 176 aerosol loading and has been shown to deviate from the typically more accurate radiative transfer modelling by up to 177 ±20% under moderate aerosol loading (Shaiganfar et al., 2011). Day 1 VCDs were calculated following Eq. (1): Days 2 and 3 NO x and Day 1 SO 2 VCDs were calculated following Eq. (2):

257
relatively long sampling time of the NO x analyzer with a relatively fast driving speed on this route may also have led periods as the transects. Values of j NO2 were estimated using SLEA Moore Line station solar irradiance data ( Fig. 1; 338 Table S1) and solar zenith angle following the method in Wiegand and Bofinger (2000).

352
For the calculation of SO 2 emissions, SO 2 was assumed to have a sufficiently long lifetime in the boundary layer so 353 as to be conserved between the emission and measurement location. Note that cloud processing of SO 2 was assumed 354 to be negligible since SO 2 measurements were completed on a mostly cloud-free day.

358
The VCDs measured are shown in Fig. 3-6 while the NO x emissions calculated using Eqs.
(3) and (4) are shown in 359 Table 4. The values of VCD influx required for the calculations were typically determined from measurements of VCD 360 in low pollution transect areas. However, the VCD influx on Day 2 was not determined in this way since these DSCDs 361 were close to zero within error (Figs. 2 & 4). The VCD influx is expected to be low on Day 2 because the north wind-362 direction indicates that the air-masses originated from over Lake Huron. These low values were probably due to low (Nunnermacker et al., 2000). Very high NO mixing ratios in a power plant plume (i.e., > 40ppb) could completely