Abstract
Methyl bromide (CH3Br) is a potent ozone-depleting substance (ODS) that has both natural and anthropogenic sources. CH3Br has been used mainly for preplant soil fumigation, post-harvest grain and timber fumigation, and structural fumigation. Most non-quarantine/pre-shipment (non-QPS) uses have been phased-out in 2005 for non-Article 5 (developed) countries and in 2015 for Article 5 (developing) countries under the Montreal Protocol on Substances that Deplete the Ozone Layer; some uses have continued under critical use exemptions (CUEs). Under the Protocol, individual nations are required to report annual data on CH3Br production and consumption for quarantine/pre-shipment (QPS) uses, non-QPS uses and CUEs to the United Nations Environment Programme (UNEP). In this study, we analyzed high precision, in situ measurements of atmospheric concentrations of CH3Br obtained at the Gosan station on Jeju island, Korea, from 2008 to 2019. The background concentrations of CH3Br in the atmosphere at Gosan declined from 8.5 ± 0.8 ppt in 2008 to 7.4 ± 0.6 ppt in 2019 at a rate of −0.13 ± 0.02 ppt yr−1. At Gosan, we also observed periods of persistent concentrations (pollution events) elevated above the decreasing background in continental air masses from China. Statistical back trajectory analyses showed that these pollution events predominantly trace back to CH3Br emissions from eastern China. Using an inter-species correlation (ISC) method with the reference trace species CFC-11 (CCl3F), we estimate anthropogenic CH3Br emissions from eastern China at 4.1 ± 1.3 Gg yr−1 in 2008–2019, approximately 2.9 ± 1.3 Gg yr−1 higher than the bottom-up emission estimates reported to UNEP. Possible non-fumigation CH3Br sources – rapeseed production and biomass burning – were assessed and it was found that the discrepancy is more likely due to unreported or incorrectly reported QPS and non-QPS fumigation uses. These largely-unreported anthropogenic emissions of CH3Br are confined to eastern China and account for 30–40 % of anthropogenic global CH3Br emissions. They are likely due to delays in the introduction of CH3Br alternatives, such as sulfuryl fluoride (SO2F2), heat, irradiation and a possible lack of industry awareness of the need for regulation of CH3Br production and use.
Highlights
Methyl bromide (CH3Br) is a colorless, odorless, non-flammable chemical that is a powerful ozone-depleting substance (ODS)
A statistical backward trajectory analysis showed that emissions of CH3Br in the region were highest from eastern China compared to other surrounding countries
Top-down emissions estimates of CH3Br from eastern China were determined by using an ISC method with CFC-11 as the reference tracer defining anthropogenic CH3Br emissions
Summary
Methyl bromide (CH3Br) is a colorless, odorless, non-flammable chemical that is a powerful ozone-depleting substance (ODS). An inverse modeling study (Vollmer et al, 2009) using high-frequency ground 90 measurements suggested emissions from China had decreased to 0.24 Gg yr-1 in 2006–2008 Those results were based on a limited period of observations (e.g., few months to years) and could not analyze the long-term variations and trends in CH3Br emissions. The annual average CH3Br baseline concentrations decreased steadily from 8.5 ± 0.8 ppt in 2008 to 7.4 ± 0.6 ppt in 2019 125 (Table 1), declining at a rate of -0.13 ± 0.02 ppt yr-1 (-1.5% yr-1) This rate of decline for CH3Br is consistent with the global trend of atmospheric CH3Br determined from AGAGE in situ and NOAA (National Oceanic and Atmospheric Administration) flask data in 2011–2012 (Carpenter and Reimann et al, 2014), which has been attributed to the influence of the CH3Br restrictions on non-QPS use imposed by the Montreal Protocol. Note that there are data missing for several months in 2016, 2017, and 2018, mainly because of typhoon damage to the Gosan station in early summer/fall
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