Global Methyl Halide Emissions From Rapeseed (Brassica napus) Using Life Cycle Measurements

Abstract

Global budgets of methyl halides are not balanced between currently identified sources and sinks. Among biological sources, rapeseed is regarded as the second largest terrestrial source of CH 3 Br, extrapolated from laboratory-based incubations and limited field measurements. This study analyzes the CH 3 Br budget from rapeseed (Brassica napus "Empire"), using field-based life cycle measurements, yielding a globally scaled emission rate of 2.8 ± 0.7 Gg year −1. Though this verifies that rapeseed is a significant global source, it is just half of the previous estimation, even after accounting for the doubling of global annual rapeseed production since then. The ozone-depleting potential of rapeseed is further sustained through CH 3 Cl and CH 3 I emissions, which were measured for the first time and scaled to 5.3 ± 1.3 and 4.0 ± 0.8 Gg year −1 globally. Plain Language Summary Stratospheric ozone absorbs incoming solar UV radiation, attenuating the harmful radiation exposure for life on Earth's surface. Halogen atoms transported via halocarbons, including methyl halides, can catalyze ozone destruction efficiently in the stratosphere. Anthropogenic sources of halocarbons have been decreasing consistently since the implementation of the 1987 Montreal Protocol and its amendments. However, some natural sources, especially those influenced by anthropogenic activities, may offset some of the achievement of reduced halocarbon emissions. This study quantifies methyl halide emissions from cultivated rapeseed (Brassica napus, cultivar: Empire), based on life cycle measurements and normalized to seed production. This yields a global crop contribution of 2.8 ± 0.7 Gg of methyl bromide (CH 3 Br) annually, which is smaller than previous estimates (5.1-6.6 Gg), supporting the conventional view that there must be other unidentified or underestimated sources for CH 3 Br. This study also quantifies for the first time that rapeseed emits 5.3 ± 1.3 Gg of methyl chloride (CH 3 Cl) and 4.0 ± 0.8 Gg of methyl iodide (CH 3 I) each year. Due to the increasing demand on rapeseed products such as canola oil, its global methyl halide emissions are expected to grow in the future.

Application of copper(II)-based chemicals induces CH3Br and CH3Cl emissions from soil and seawater

“…Sample storage time in the syringe was less than 5 min. The air samples were cryotrapped and then cryo-focused prior to injection into a gas chromatograph-mass spectrometer (Agilent 6890 N/5973, Agilent Technologies, California, USA) for CH 3 Br and CH 3 Cl analysis 8 , 53 .…”

“…The initial headspace was filled with ambient air, either outdoor air or laboratory air. Headspace air samples from the jars were drawn into the sampling loop at 30 min intervals, and then drawn through a cryotrap, followed by desorption, cryofocusing, and injection into a gas chromatograph-mass spectrometer (Agilent 6890N/5973, Agilent Technologies, California, USA) for CH 3 Br and CH 3 Cl analysis 8 , 53 .…”

“…The present atmospheric CH 3 Br and CH 3 Cl originate from predominantly natural sources. These natural sources tend to be amplified under anthropogenic-induced influences, such as biomass burning 3 , global warming 4 , 5 , sea level rise 6 , 7 , agricultural cultivation 8 , land use change 9 , etc., posing persistent and growing challenges for the recovery of stratospheric ozone 10 , 11 .…”

Top-down and bottom-up estimates of anthropogenic methyl bromide emissions from eastern China

“…China is the third-largest producer of rapeseed in the world after the European Union and Canada, accounting for 12% of the total rapeseed production in 2015-2016, and the arable land lies mainly along the Yangtze River, which is suitable for growing rapeseed (Khir et al, 2017). Previous studies have reported that the global emissions of CH3Br by the rapeseed industry range from 2.8 ± 0.7 Gg yr -1 (Jiao et al, 2020) to 5 Gg yr -1 (Gan et al, 1998;Mead et al, 2008). Considering the proportion of eastern China in global rapeseed industry, the emissions of CH3Br by rapeseed in eastern China could be about 0.3-0.6 Gg yr -1 .…”

“…In the life cycle of rapeseed, CH3Br is largely emitted during the flowering period in the 2 months after sowing (Jiao et al, 2020). Rapeseed in the northern hemisphere generally blooms in the warm weather from March to May.…”