Abstract
Airborne measurements of methane (CH4) and carbon dioxide (CO2) were taken over the rice growing region of California's Sacramento Valley in the late spring of 2010 and 2011. From these and ancillary measurements, we show that CH4 mixing ratios were higher in the planetary boundary layer above the Sacramento Valley during the rice growing season than they were before it, which we attribute to emissions from rice paddies. We derive daytime emission fluxes of CH4 between 0.6 and 2.0% of the CO2 taken up by photosynthesis on a per carbon, or mole to mole, basis. We also use a mixing model to determine an average CH4/CO2 flux ratio of −0.6% for one day early in the growing season of 2010. We conclude the CH4/CO2 flux ratio estimates from a single rice field in a previous study are representative of rice fields in the Sacramento Valley. If generally true, the California Air Resources Board (CARB) greenhouse gas inventory emission rate of 2.7 × 1010 g CH4/yr is approximately three times lower than the range of probable CH4 emissions (7.8–9.3 × 1010 g CH4/yr) from rice cultivation derived in this study. We attribute this difference to decreased burning of the residual rice crop since 1991, which leads to an increase in CH4 emissions from rice paddies in succeeding years, but which is not accounted for in the CARB inventory.
Highlights
[2] Methane (CH4) is a potent greenhouse gas whose emissions in California are regulated under Assembly Bill 32, signed into law as the Global Warming Solutions Act of 2006, which requires statewide greenhouse gas emissions not to exceed 1990 levels by the year 2020
McMillan et al [2007] measured fluxes at one rice paddy (39.28N latitude, 122.18W longitude) located in the Sacramento Valley of California. This was a multiyear project that included three growing seasons from 2000 to 2002. They showed that during the growing season, CH4 and CO2 emissions were negatively correlated during the daytime (i.e., CH4 was emitted while CO2 was taken up by the rice crop during photosynthesis), and positively correlated at nighttime (i.e., CH4 was emitted while CO2 was respired)
We show that CH4 and CO2 data over rice paddies are consistent with the findings of McMillan et al [2007], and use a mathematical model to determine that the ratios of CH4 and CO2 fluxes measured by McMillan et al [2007] from one rice paddy in 2000–2002 are representative of an area-wide survey of the majority of California’s rice paddies for one day in 2010
Summary
[2] Methane (CH4) is a potent greenhouse gas whose emissions in California are regulated under Assembly Bill 32, signed into law as the Global Warming Solutions Act of 2006, which requires statewide greenhouse gas emissions not to exceed 1990 levels by the year 2020. [5] We use data from two flights of a chemically instrumented National Oceanic and Atmospheric Administration (NOAA) P-3 research aircraft over the rice-growing region of the Sacramento Valley, where the majority of rice paddies in California are located (e.g., see http://www.nass.usda.gov/ Charts_and_Maps/Crops_County/ar-ha.asp), to examine the possible sources of CH4 emissions during the flights. [6] we use data from a flight of an instrumented Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter on 15 June 2011, during the California Airborne Biogenic Volatile Organic Compound Emission Research in Natural Ecosystem Transects (CABERNET) field campaign, which support the CH4/CO2 flux ratio determined by measurements from the NOAA P-3 in 2010 and the work of McMillan et al [2007]
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