Comment on acp-2021-699

<strong class="journal-contentHeaderColor">Abstract.</strong> Methyl bromide (<span class="inline-formula">CH₃Br</span>) is a potent ozone-depleting substance (ODS) that has both natural and anthropogenic sources. <span class="inline-formula">CH₃Br</span> has been used mainly for preplant soil fumigation, post-harvest grain and timber fumigation, and structural fumigation. Most non-quarantine and pre-shipment (non-QPS) uses were phased out by 2005 for non-Article 5 (developed) countries and by 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 <span class="inline-formula">CH₃Br</span> 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 mole fractions of <span class="inline-formula">CH₃Br</span> obtained at the Gosan station on Jeju Island, South Korea, from 2008 to 2019. The background mole fractions of <span class="inline-formula">CH₃Br</span> in the atmosphere at Gosan declined from <span class="inline-formula">8.5±0.8</span> ppt (parts per trillion) in 2008 to <span class="inline-formula">7.4±0.6</span> ppt in 2019 at a rate of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">0.13</mn><mo>±</mo><mn mathvariant="normal">0.02</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="0542a0c6fa095ebed628b4b0a60e60c7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-5157-2022-ie00001.svg" width="64pt" height="10pt" src="acp-22-5157-2022-ie00001.png"/></svg:svg></span></span> ppt yr<span class="inline-formula">^−1</span>. At Gosan, we also observed periods of persistent mole fractions (pollution events) elevated above the decreasing background in continental air masses from China. Statistical back-trajectory analyses showed that these pollution events are predominantly traced back to <span class="inline-formula">CH₃Br</span> emissions from eastern China. Using an interspecies correlation (ISC) method with the reference trace species CFC-11 (<span class="inline-formula">CCl₃F</span>), we estimate anthropogenic <span class="inline-formula">CH₃Br</span> emissions from eastern China at an average of <span class="inline-formula">4.1±1.3</span> Gg yr<span class="inline-formula">^−1</span> in 2008–2019, approximately <span class="inline-formula">2.9±1.3</span> Gg yr<span class="inline-formula">^−1</span> higher than the bottom-up emission estimates reported to UNEP. Possible non-fumigation <span class="inline-formula">CH₃Br</span> sources – rapeseed production and biomass burning – were assessed, and it was found that the discrepancy is most likely due to unreported or incorrectly reported QPS and non-QPS fumigation uses. These unreported anthropogenic emissions of <span class="inline-formula">CH₃Br</span> are confined to eastern China and account for 30 %–40 % of anthropogenic global <span class="inline-formula">CH₃Br</span> emissions. They are likely due to delays in the introduction of <span class="inline-formula">CH₃Br</span> alternatives, such as sulfuryl fluoride (<span class="inline-formula">SO₂F₂</span>), heat, and irradiation, and a possible lack of industry awareness of the need for regulation of <span class="inline-formula">CH₃Br</span> production and use.

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