EGUs (Electricity Generation Units) are one of the point sectors (ptegu). For sources in the ptegu sector that could be matched to 2019 CEMS (Continuous Emissions Monitoring System) data, hourly CEMS NOx and SO2 emissions for 2019 from EPA’s Acid Rain Program were used rather than annual inventory emissions. In the SMOKE flat file egucems (egucems_2019NEI_SmokeFlatFile_POINT_20220325_07apr2022_v0), there are 100 facilities that can be matched to the units in ptegu in California and Nevada. I found the corresponding units to these facilities through their location information and compared the monthly SO2 and NOx emissions of units in ptegu that have been processed by SMOKE and the raw CEMS data. EPA used the NHONO NOx speciation profile in the ptegu sector to divide NOx into NO and NO2; the molar split factors for NO and NO2 are 0.9 and 0.1, respectively. As shown in the following two figures, the SO2 emissions of the units in ptegu and the corresponding facilities in CEMS are very similar; however, the sum of NO and NO2 emissions of units in ptegu are smaller than that from CEMS data. The monthly NOx emissions from CEMS are about 1.458 times as great as those of units in ptegu, which have been processed by SMOKE.
Figure 1. SO2 emissions of the 100 pairs of the matched facilities in CEMS and the corresponding units in ptegu from March to September 2019 in CA and NV.
Figure 2. NOx (NO+NO2) emissions of the 100 pairs of the matched facilities in CEMS and the corresponding units in ptegu from March to September 2019 in CA and NV.
The monthly CEMS NOx and SO2 emissions (unit: short tons) are downloaded from Clean Air Markets Program Data (CAMPD). The monthly NO, NO2 and SO2 emissions of units in ptegu are calculated from the inln_mole_ptegu_2019xxxx_ files, downloaded from AWS S3 Explorer. The unit of emissions is moles/s in these files. Therefore, I calculated the monthly emissions and converted the unit to short tons following these steps:
(1). There is one inln_mole_ptegu file for each day, and there are 25 hours in each file from 00:00 to next day 00:00. Therefore, I add up the emissions from 00:00 to 23:00 to get a daily emission. And then I add up the daily emissions of all days in a month to get a monthly emission.
(2). Now I have monthly emissions in unit of moles/s, and then I convert the unit to short tons using the equation:
Short tons/month = (moles/s summed across all hours) / 907185 (g/short tons) * 3600 (s/hr) * MW (g/mol)
MW(SO2) = 64; MW(NO2) = 46; MW(NO)=30.
My question is why the sum of NO and NO2 emissions of units in ptegu are always smaller than the NOx emissions from CEMS data (with a ratio of about 1.458) while the SO2 emissions are similar. Is it caused by any error in my calculation or SMOKE did some extra processing to NOx?