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

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 CH3Br, extrapolated from laboratory-based incubations and limited field measurements. This study analyzes the CH3Br 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 CH3Cl and CH3I 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 (CH3Br) 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 CH3Br. This study also quantifies for the first time that rapeseed emits 5.3 +/- 1.3 Gg of methyl chloride (CH3Cl) and 4.0 +/- 0.8 Gg of methyl iodide (CH3I) 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.