California
methane (CH4) emissions are quantified for
three years from two tower networks and one aircraft campaign. We
used backward trajectory simulations and a mesoscale Bayesian inverse
model, initialized by three inventories, to achieve the emission quantification.
Results show total statewide CH4 emissions of 2.05 ±
0.26 (at 95% confidence) Tg/yr, which is 1.14 to 1.47 times greater
than the anthropogenic emission estimates by California Air Resource
Board (CARB). Some of differences could be biogenic emissions, superemitter
point sources, and other episodic emissions which may not be completely
included in the CARB inventory. San Joaquin Valley (SJV) has the largest
CH4 emissions (0.94 ± 0.18 Tg/yr), followed by the
South Coast Air Basin, the Sacramento Valley, and the San Francisco
Bay Area at 0.39 ± 0.18, 0.21 ± 0.04, and 0.16 ± 0.05
Tg/yr, respectively. The dairy and oil/gas production sources in the
SJV contribute 0.44 ± 0.36 and 0.22 ± 0.23 Tg CH4/yr, respectively. This study has important policy implications for
regulatory programs, as it provides a thorough multiyear evaluation
of the emissions inventory using independent atmospheric measurements
and investigates the utility of a complementary multiplatform approach
in understanding the spatial and temporal patterns of CH4 emissions in the state and identifies opportunities for the expansion
and applications of the monitoring network.
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