Global concentrations of atmospheric CH 4 have substantially increased since 1750 due to the expansion of human agriculture, waste management, and fossil fuel usage (Dlugokencky et al., 2011). CH 4 is a 28 times stronger greenhouse gas than carbon dioxide (CO 2 ) on a century timescale and increased atmospheric CH 4 accounts for 25% of anthropogenic climate forcing to date (Etminan et al., 2016). CH 4 has also contributed to increases in tropospheric ozone, an air pollutant and short-lived greenhouse gas (Fiore et al., 2008), and to stratospheric water vapor, which acts as a sink for stratospheric ozone (Myhre et al., 2013). Additionally, CH 4 has an atmospheric lifetime of 9.8 years, so its atmospheric abundance quickly responds to changes in its emissions (Saunois et al., 2016). Furthermore, there are proven and cost-effective ways to reduce emissions (UNEP, 2011). These characteristics make reducing CH 4 emissions an attractive policy option for mitigating the effects of climate change in the near and intermediate term.The US state of California is committed to reducing its CH 4 emissions to 40% of 2013 levels by 2030 as part of its climate change policy (CARB, 2017). The state has funded specific policies to reduce its emissions, including from its large dairy sector, which makes up 19% of the United States's milk production (CDFA, 2020). CH 4 emissions from the dairy sector are from two main sources: enteric fermentation emissions produced by dairy cows' digestion process and manure management emissions created by the anaerobic decomposition of dairy waste. Emissions from the dairy sector are estimated by the state as 880 Gg/yr, accounting for about half of California's total CH 4 emissions (CARB, 2019). The dairy industry is centered in the San Joaquin Valley (SJV), which contains 87% of the state's 1.7 million large dairy herd (CDFA, 2020). The SJV has also been identified as a large North American CH 4 emissions hotspot in many studies, highlighting the global importance of the region's dairy
Black carbon (BC) is a product of incomplete combustion and is primarily emitted from vehicles, power plants, residential heating, and wildfires (Bond et al., 2013;Boucher et al., 2013). It is a potent absorber of solar radiation, converting incoming light to atmospheric heating. In the atmosphere, air masses containing BC-aerosol are warmed which further affects climate through processes like cloud formation and cloud albedo (Albrecht, 1989;Twomey, 1977). These processes have large and mostly off-setting radiative forcing, but they are not well constrained meaning that the net impact of BC in climate models is highly uncertain (
Source attribution of natural gas emissions from fossil fuels in New Mexico's San Juan Basin (SJB) is challenging due to source heterogeneity and emissions transience. We demonstrate that ethane (C2H6) to methane (CH4) mixing ratios can identify and separate sources over different scales using various measurement techniques. We report simultaneous CH4 and C2H6 observations near a coal mine vent and oil and gas (O&G) emission sources using ground‐based in situ measurements in 2020/2021. During these campaigns, we observed a stable coal vent C2H6:CH4 ratio of 1.28% ± 0.11%, discernibly different than nearby O&G source ratios ranging from 0.9% to 16.8%. We analyze airborne observations of the SJB taken in 2014/2015 that exhibit similar coal vent ratios and further show the region's heterogeneity. We identify episodic O&G sources, including a gas plant source detected in 2014/2015 that is absent in our 2020/2021 data. We examine total column observations of C2H6 and CH4 made in 2013 with a solar spectrometer and find a C2H6:CH4 ratio of 1.3% ± 0.4% for the coal vent. The stable and unique coal vent ratio relative to other O&G sources in the region is used to demonstrate that consistent attribution is possible using various measurement methods at multiple scales across many years. Finally, we demonstrate that using C2H6 as a proxy for fossil CH4 inversions can inform detailed basin‐scale inversions, provided we understand source specific changes in the C2H6:CH4 ratio like we report in the SJB.
Abstract. With global wildfires becoming more widespread and severe, tracking their emissions of greenhouse gases and air pollutants is becoming increasingly important. Wildfire emissions have primarily been characterized by in situ laboratory and field observations at fine scales. While this approach captures the mechanisms relating emissions to combustion phase and fuel properties, their evaluation on regional-scale plumes has been limited. In this study, we report remote observations of total column trace gases and aerosols during the 2020 wildfire season from smoke plumes in the Sierra Nevada of California with an EM27/SUN solar Fourier transform infrared (FTIR) spectrometer. We derive total column aerosol optical depth (AOD), emission factors (EFs) and modified combustion efficiency (MCE) for these fires and evaluate relationships between them, based on combustion phase at regional scales. We demonstrate that the EM27/SUN effectively detects changes in CO, CO2, and CH4 in the atmospheric column at ∼10 km horizontal scales that are attributed to wildfire emissions. These observations are used to derive total column EFCO of 120.5±12.2 and EFCH4 of 4.3±0.8 for a regional smoke plume event in mixed combustion phases. These values are consistent with in situ relationships measured in similar temperate coniferous forest wildfires. FTIR-derived AOD was compared to a nearby AERONET (AErosol RObotic NETwork) station and observed ratios of XCO to AOD were consistent with those previously observed from satellites. We also show that co-located XCO observations from the TROPOspheric Monitoring Instrument (TROPOMI) satellite-based instrument are 9.7±1.3 % higher than our EM27/SUN observations during the wildfire period. Finally, we put wildfire CH4 emissions in context of the California state CH4 budget and estimate that 213.7±49.8 Gg CH4 were emitted by large wildfires in California during 2020, about 13.7 % of the total state CH4 emissions in 2020. Our work demonstrates a novel application of the ground-based EM27/SUN solar spectrometers in wildfire monitoring by integrating regional-scale measurements of trace gases and aerosols from smoke plumes.
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