Quantifying Trends in Methane, Nitrous Oxide, and Carbon Monoxide Emissions in the Los Angeles Basin Using Remote Sensing Spectroscopy at CLARS: Final Report

Quantifying Trends in Emissions in the Using This project aims to (1) extend the CLARS-FTS data record for 2.5 additional years; (2) process the CLARS-FTS spectra to obtain dry air slant column abundances and mixing ratios of the key greenhouse gases CH 4 and CO 2 , as well as CO, N 2 O (nitrous oxide), water vapor and O 2 (used for filtering the effects of aerosols and clouds); and (3) calculate spatially and temporally-resolved CH 4 emissions. This project will provide CH 4 emissions data from late 2011 through the end of 2020. Abstract Legislation in (GHG) emissions by 2030 and of 80% by 2050. To establish compliance with California legislation, comprehensive, long-term observing strategies are required. In this report, we present the long-term trends of methane (CH 4 ), Nitrous Oxide (N 2 O), and Carbon Monoxide (CO) emissions beginning in 2011 using the California Laboratory for Atmospheric Remote Sensing (CLARS) facility on Mt. Wilson, which is the home of the CLARS-FTS instrument, a scanning Fourier transform spectrometer (FTS) which maps the spatial distributions of GHGs in the Los Angeles (LA) basin. The CLARS-FTS instrument points to a series of locations in the South Coast Air Basin (SoCAB), providing maps every 90 minutes of dry air mole fractions of CH 4 (XCH 4 ), N 2 O (XN 2 O), CO (XCO) and a number of other chemical species with a spatial resolution of a few km. In the first part of this report, we determine the emissions of CH 4 in LA from 2011 to 2019, extending the work of He et al. (2019), using CLARS-FTS observations based on a tracer-tracer ratio method with CO 2 emissions. We show that the pattern of CH 4 emissions contains both seasonal and non-seasonal contributions. We find that the seasonal component peaks in the winter and is correlated (R 2 =0.50) with utility natural gas consumption from the residential and commercial sectors and not from the industrial and gas-fired power plant sectors. The non-seasonal component is (14.0 ± 1.3) Gg CH 4 /month. If the seasonal correlation is causal, about (1.8 ± 0.18) % of the commercial and residential natural gas consumption in LA is released into the atmosphere. This report updates these numbers, which are different from He et al. (2019), with two more years of CLARS observations and updated background calculations. Identification of the sector(s) responsible for the CH 4 emissions and their temporal and spatial variability is a key step in achieving the goals of emission reduction. In we fitting from strong during removing 5.7 to quantify the correlation between the CLARS synthetic and WRF-Chem simulations. Both maps show a high degree of agreement for all hours of day (r 2 =0.69). CLARS synthetic maps, created from sampling the 33 surface reflection points, can explain more than 69% of the spatial variability of the “true” atmosphere from WRF-Chem. The CLARS synthetic maps show higher values than the model simulations (with a regression slope of 1.09). This is because of the heterogeneous distribution of XCO in the northern LA basin such that the reflected light path of CLARS is always passing through a high concentration atmosphere on its way to the CLARS instrument in summer. This result demonstrates the representativeness of the CLARS observing strategy towards capturing realistic XCO enhancements in the LA megacity.