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 XN₂O_(xs) show a seasonal cycle with a peak-to-peak amplitude of 5.6 ± 2.5 ppb, where greater XN₂O_(xs) values are observed during the winter/spring and minima occur in late summer/early fall. A tracer-tracer ratio method is applied using XN₂O_(xs) and XCO_(2,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 XN₂O_(xs). 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.