Mapping CH<sub>4</sub> : CO<sub>2</sub> ratios in Los Angeles with CLARS-FTS from Mount Wilson, California

Abstract

Abstract. The Los Angeles megacity, which is home to more than 40 % of the population in California, is the second largest megacity in the United States and an intense source of anthropogenic greenhouse gases (GHGs). Quantifying GHG emissions from the megacity and monitoring their spatiotemporal trends are essential to be able to understand the effectiveness of emission control policies. Here we measure carbon dioxide (CO 2 ) and methane (CH 4 ) across the Los Angeles megacity using a novel approach -ground-based remote sensing from a mountaintop site. A Fourier transform spectrometer (FTS) with agile pointing optics, located on Mount Wilson at 1.67 km above sea level, measures reflected nearinfrared sunlight from 29 different surface targets on Mount Wilson and in the Los Angeles megacity to retrieve the slant column abundances of CO 2 , CH 4 and other trace gases above and below Mount Wilson. This technique provides persistent space-and time-resolved observations of path-averaged dry-air GHG concentrations, XGHG, in the Los Angeles megacity and simulates observations from a geostationary satellite. In this study, we combined high-sensitivity measurements from the FTS and the panorama from Mount Wilson to characterize anthropogenic CH 4 emissions in the megacity using tracer-tracer correlations. During the period between September 2011 and October 2013, the observed XCH 4 : XCO 2 excess ratio, assigned to anthropogenic activities, varied from 5.4 to 7.3 ppb CH 4 (ppm CO 2 ) −1 , with an average of 6.4 ± 0.5 ppb CH 4 (ppm CO 2 ) −1 compared to the value of 4.6 ± 0.9 ppb CH 4 (ppm CO 2 ) −1 expected from the California Air Resources Board (CARB) bottom-up emission inventory. Persistent elevated XCH 4 : XCO 2 excess ratios were observed in Pasadena and in the eastern Los Angeles megacity. Using the FTS observations on Mount Wilson and the bottom-up CO 2 emission inventory, we derived a topdown CH 4 emission of 0.39 ± 0.06 Tg CH 4 year −1 in the Los Angeles megacity. This is 18-61 % larger than the state government's bottom-up CH 4 emission inventory and consistent with previous studies.

Temporal variations of atmospheric CO<sub>2</sub> and CO at Ahmedabad in western India

“…If the emissions of CO 2 are known for a study location, the emissions of CO can be estimated by multiplying the correlation slopes and molecular mass mixing ratios (Wunch et al, 2009;Wong et al, 2015). Final emissions of CO will depend on choosing the values of the correlation slopes.…”

“…The vehicular as well as industrial emissions contribute large fluxes of CO 2 and CO to the atmosphere in urban regions. Several simultaneous ground-based and aircraftbased studies of CO and CO 2 have been carried out in the past in different parts of the world (Turnbull et al, 2006;Wunch et al, 2009;Wang et al, 2010;Newman et al, 2013) but such a study has not been done in India except for recently reported results from weekly samples for three Indian sites by Lin et al (2015).…”

Simulated retrievals for the remote sensing of CO<sub>2</sub>, CH<sub>4</sub>, CO, and H<sub>2</sub>O from geostationary orbit

“…As a proposed mission at this point in time, it is designed to measure high-resolution spectra of reflected sunlight in near-infrared bands and to deliver simultaneous retrievals of column-averaged dry air mole fractions of CO 2 , CH 4 , CO, and H 2 O (denoted XCO 2 , XCH 4 , XCO, and XH 2 O, respectively) at different times of day over North America. A prototype of the instrument on top of Mount Wilson, in the San Gabriel Mountains just north of Los Angeles, California, has been collecting high-quality data over that megacity since August 2011 (Key et al, 2012;Fu et al, 2014;Wong et al, 2015). The main scientific objectives of GeoFTS are the following:…”

Southern California Megacity CO<sub>2</sub>, CH<sub>4</sub>, and CO flux estimates using remote sensing and a Lagrangian model

“…7). For the first test, we filter out data where the observations differ from the Tracer-tracer relations were used for the TCCON (Wunch et al, 2009(Wunch et al, , 2016, CLARS (Wong et al, 2015, and in situ observations (Hsu et al, 2010;Wennberg et al, 2012). model above a threshold.…”

“…CLARS has a viewing geometry that is more sensitive to the mixing layer than TCCON and nadir-viewing satellites, which leads to larger typical enhancements in CO 2 and CH 4 (Wong et al, 2015. For comparability we exclude enhancements from CLARS and in situ observations (e.g., in this section.…”