20 A recent multiyear inversion in Paris showed a decreasing trend of around 2%/year in CO 2 emissions from 2016 to 2021. 21 A comparative analysis of Los Angeles and Washington, DC/Baltimore Metropolitan areas conducted inversions in these two cities for 2018–2020 to quantify COVID-19-related emissions reductions. 22 A study of the Salt Lake Valley used a multiyear CO 2 inversion to examine the relationship between emissions and urban population density. 23 Each of these studies took the approach of using a relatively small (range: 2–13) number of high-accuracy (∼0.1 ppm) monitoring sites to constrain CO 2 fluxes in an urban area.
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