Estimating nitrous oxide (N2O) emissions for the Los Angeles Megacity using mountaintop remote sensing observations

Abstract

Nitrous oxide (N2O) is an important greenhouse gas contributing both to global radiative forcing and ozone depletion. Though N2O emissions are largely derived from agricultural activities, urban sources of N2O also contribute significantly to anthropogenic emissions, but are not well understood and difficult to quantify. This study employs a top-down approach to derive urban N2O emissions for the Los Angeles megacity using a unique dataset from a mountaintop remote sensing instrument, which has been observing greenhouse gas mixing ratios in LA since 2011. CLARS-FTS observations yield a weighted mean of 15.0 ± 0.1 ppb excess XN2O above background in the LA basin from 2013 to 2019. Time series of XN2Oxs show a seasonal cycle with a peak-to-peak amplitude of 5.6 ± 2.5 ppb, where greater XN2Oxs values are observed during the winter/spring and minima occur in late summer/early fall. A tracer-tracer ratio method is applied using XN2Oxs and XCO2,xs observations to estimate top-down N2O emissions for the LA basin during 2013–2018. Estimated monthly emissions range from 6 to 19 Gg N2O per month and exhibit a similar seasonal cycle to that observed in XN2Oxs. Estimated annual emissions fall within the range 124–144 Gg per year for the years 2014–2018. These top-down annual estimates are roughly 3 times the official statewide bottom-up inventory for the same time period, but consistent considering uncertainties with other top-down estimates for the LA basin. The discrepancy between top-down emission estimates and the statewide bottom-up inventory highlights the difficulty in constraining N2O emissions, especially for an urban environment.