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.

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) are major carriers of atmospheric bromine and chlorine, respectively, which can catalyze stratospheric ozone depletion. However, in our current understanding, there are missing sources associated with these two species. Here we investigate the effect of copper(II) on CH3Br and CH3Cl production from soil, seawater and model organic compounds: catechol (benzene-1,2-diol) and guaiacol (2-methoxyphenol). We show that copper sulfate (CuSO4) enhances CH3Br and CH3Cl production from soil and seawater, and it may be further amplified in conjunction with hydrogen peroxide (H2O2) or solar radiation. This represents an abiotic production pathway of CH3Br and CH3Cl perturbed by anthropogenic application of copper(II)-based chemicals. Hence, we suggest that the widespread application of copper(II) pesticides in agriculture and the discharge of anthropogenic copper(II) to the oceans may account for part of the missing sources of CH3Br and CH3Cl, and thereby contribute to stratospheric halogen load.

Methyl chloride (CH 3 Cl) and methyl bromide (CH 3 Br) are the predominant carriers of natural chlorine and bromine from the troposphere to the stratosphere, which can catalyze the destruction of stratospheric ozone. Here, penguin colony soils (PCS) and the adjacent tundra soils (i.e., penguin-lacking colony soils, PLS), seal colony soils (SCS), tundra marsh soils (TMS), and normal upland tundra soils (UTS) in coastal Antarctica were collected and incubated for the first time to confirm that these soils were CH 3 Cl and CH 3 Br sources or sinks. Overall, tundra soil acted as a net sink for CH 3 Cl and CH 3 Br with potential flux ranges from −18.1 to −2.8 pmol g −1 d −1 and −1.32 to −0.24 pmol g −1 d −1 , respectively. The deposition of penguin guano or seal excrement into tundra soils facilitated the simultaneous production of CH 3 Cl and CH 3 Br and resulted in a smaller sink in PCS, SCS, and PLS. Laboratory-based thermal treatments and anaerobic incubation experiments suggested that the consumption of CH 3 Cl and CH 3 Br was predominantly mediated by microbes while the production was abiotic and O 2 independent. Temperature gradient incubations revealed that increasing soil temperature promoted the consumption of CH 3 Cl and CH 3 Br in UTS, suggesting that the regional sink may increase with Antarctic warming, depending on changes in soil moisture and abiotic production rates.

No abstract

No abstract