Estimating Nitrous Oxide (N2O) Emissions for the Los Angeles Megacity using mountaintop remote sensing observations

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. Maps of N2O mixing ratios above background levels (XN2Oxs) highlight the coastal region of LA as having consistently elevated XN2Oxs compared to the interior basin and indicate a diurnal cycle in which XN2Oxs increases throughout the day. Time series of XN2Oxs show a seasonal cycle with a peak-to-peak amplitude of 10±3 ppb, where greater XN2Oxs values are observed during the winter/spring and a minimum occurs in late summer/ early fall. A tracer-tracer correlation method applied using XN2Oxs and XCO2,xs observations to estimate top-down N2O emissions for the LA basin during 2013-2018 yields a mean annual emissions value of 171 ±17 Gg N2O per year. This top-down estimate is 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 using bottom-up methods to constrain N2O emissions, especially for an urban environment.