CH3Br Emissions Research Articles

Unexplained high and persistent methyl bromide emissions in China

Methyl bromide (CH3Br) is an important ozone-depleting substance whose use is regulated under the Montreal Protocol. Quantifying emissions on the national scale is required to assess compliance with the Montreal Protocol and thereby ensure the timely recovery of the ozone layer. However, the spatial-temporal patterns of China’s national CH3Br emissions remain unclear. Here we estimate the national emissions of CH3Br in China during 2011−2020 using atmospheric observations at 10 sites across China combined with an inversion technique (top-down) and compare those with an updated inventory of identified emission sources (bottom-up). Measured CH3Br mole fractions are enhanced well above the background mole fractions, especially at sites in eastern China. Top-down emission estimates exceed bottom-up estimates by 5.5 ± 1.4 gigagrams per year, with the largest fraction (60%) of observationally derived CH3Br emissions arising from underestimated or unidentified emissions sources. This study shows the potential impacts of the unaccounted emissions on stratospheric ozone depletion, with implications for the Montreal Protocol.

Global methyl halide emissions from biomass burning during 2003-2021.

Methyl halides (CH3Cl, CH3Br, and CH3I) are ozone-depleting substances. Biomass burning (BB) is an important source of methyl halides. The temporal variations and global spatial distribution of BB methyl halide emissions are unclear. Thus, global methyl halide emissions from BB during 2003-2021 were estimated based on satellite data. A significant decreasing trend (p<0.01) in global methyl halide emissions from BB was found between 2003 and 2021, with CH3Cl emissions decreasing from 302 to 220Ggyr-1, CH3Br emissions decreasing from 16.5 to 11.7Ggyr-1, and CH3I emissions decreasing from 8.9 to 6.1Ggyr-1. From a latitudinal perspective, the northern high-latitude region (60-90° N) was the only latitude zone with significant increases in BB methyl halide emissions (p<0.01). Based on an analysis of the drivers of BB methyl halide emissions, emissions from cropland, grassland, and shrubland fires were more correlated with the burned area, while BB emissions from forest fires were more correlated with the emissions per unit burned area. The non-BB emissions of CH3Cl increased from 4749Ggyr-1 in 2003 to 4882Ggyr-1 in 2020, while those of CH3Br decreased from 136Ggyr-1 in 2003 to 118Ggyr-1 in 2020 (global total CH3I emissions are not available). The finding indicates that global CH3Cl and CH3Br emissions from sources besides BB increased and decreased during 2003-2020. Based on our findings, not only searching for unknown sources is important, but also re-evaluating known sources is necessary for addressing methyl halide emissions.

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

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 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 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 mole fractions of CH3Br obtained at the Gosan station on Jeju Island, South Korea, from 2008 to 2019. The background mole fractions of CH3Br in the atmosphere at Gosan declined from 8.5±0.8 ppt (parts per trillion) 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 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 CH3Br emissions from eastern China. Using an interspecies correlation (ISC) method with the reference trace species CFC-11 (CCl3F), we estimate anthropogenic CH3Br emissions from eastern China at an average of 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 most likely due to unreported or incorrectly reported QPS and non-QPS fumigation uses. These 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, and irradiation, and a possible lack of industry awareness of the need for regulation of CH3Br production and use.

Causes of Enhanced Bromine Levels in Alpine Ice Cores During the 20th Century: Implications for Bromine in the Free European Troposphere

AbstractTotal bromine (Br) was investigated in seasonally resolved alpine ice cores covering the 20th century. Results revealed increased Br concentrations in summer (from 0.7 ng g−1 in the late 1940s to 1.6 ng g−1 in the mid 1970s), followed by a slight decrease to 1.25 ng g−1 during the last decade of the 20th century. In winter, a more moderate increase was observed from before 1950 (0.35 ng g−1) to the 1970–2000 period (∼0.50 ng g−1). Measurements of lead suggest that ∼75% of the bromine increase in summer alpine ice between 1930–1950 and 1965–1985 was from bromine‐containing aerosol (PbBrCl) emitted during the combustion of leaded gasoline. This contribution decreased to ∼27% in 2000 following the large decrease of lead additive content in gasoline used in western Europe. Summer ice bromine concentrations, not related to PbBrCl aerosol (denoted Bry*), increased by 40% from the late 1940s to the 1990–2000 decade. In winter, the contribution of leaded‐gasoline aerosol to total bromine levels was weaker than in summer, with the winter bromine trend mainly caused by enhanced Bry* levels (∼40%) after 1950. These Bry* variations in glacial ice are discussed in terms of past atmospheric reactive bromine (Bry) changes in the European troposphere, including anthropogenic CH3Br emissions and transport from the stratosphere.

Bromine biogeodynamics in the NE Atlantic: A perspective from natural wetlands of western Portugal

Bromine (Br) cycling in natural wetlands is highly complex, including abiotic/biotic processes and multiphase inorganic/organic Br-species. Wetland ecosystems receive Br primarily from the ocean, functioning as either sinks or sources of Br, with the overall imbalance largely decided by the prevailing climate. Aiming to trace the present-day transport of oceanogenic Br (i.e., derived from salt-water spray-droplets) and its uptake and storage in brackish and freshwater wetlands, we surveyed waters, autochthonous plants, and soils/sediments from coastal marshes and mountain peatlands in the westernmost fringe of northern Portugal. The calculated enrichment factors of bromide (Br−) relative to chloride in rainfall (EFsea = 16.8–75.3), rivers (EFsea = 1.3–13.9) and wetland waters, superficial (EFsea = 5.8–13.1) and interstitial (EFsea = 2.1–8.9), increased towards the inland highlands. We hypothesized that these values derived mostly from a known Br autocatalytic (heterogeneous) chemical cycle, starting at the seawater-aqueous interface and progressing in altitude. Br-bearing air masses are carried far from the Atlantic Ocean by moist westerlies, with Br− rainout from the atmosphere supplying the neighbouring mountain peatlands. Average [Br] in sampled wetland soils/sediments (111–253 mg/kg) agreed with values from other coastal regions, and they were directly correlated with the abundance of organic matter, varying irrespective the [Br−] of interstitial waters (129 μg/L–79 mg/L). According to the computed bioconcentration factors, the aqueous component was the major source of Br for all plant species investigated (BFplant/water = 2.1–508.0), as described elsewhere. However, Br contents in plants (14–173 mg/kg) evidenced interspecific differences, also suggesting a divergence from the acknowledged halophytic-glycophytic “model”. As plants are recognized producers of Br volatile molecules (e.g., methyl bromide, CH3Br), we interpreted translocation factors less than one in vascular species as explanatory of phytovolatilization rather than restriction of Br− upward movement in plants. Further investigation is needed, since considerable intrinsic plant variations in CH3Br emissions are mentioned in the literature.

Methyl chloride and methyl bromide emissions from baking: an unrecognized anthropogenic source

Methyl chloride and methyl bromide (CH3Cl and CH3Br) are the largest natural sources of chlorine and bromine, respectively, to the stratosphere, where they contribute to ozone depletion. We report the anthropogenic production of CH3Cl and CH3Br during breadbaking, and suggest this production is an abiotic process involving the methyl ester functional groups in pectin and lignin structural polymers of plant cells. Wide variations in baking styles allow only rough estimates of this flux of methyl halides on a global basis. A simple model suggests that CH3Br emissions from breadbaking likely peaked circa 1990 at approximately 200tonnes per year (about 0.3% of industrial production), prior to restrictions on the dough conditioner potassium bromate. In contrast, CH3Cl emissions from breadbaking may be of similar magnitude as acknowledged present-day CH3Cl industrial emissions. Because the mechanisms involve functional groups and compounds widely found in plant materials, this type of methyl halide production may occur in other cooking techniques as well.

Coastal emissions of methyl bromide and methyl chloride along the eastern Gulf of Mexico and the east coast of the United States

Measurements of methyl bromide (CH3Br) and methyl chloride (CH3Cl) were made during the Gulf of Mexico and the East Coast Carbon (GOMECC) cruise in the summer of 2007. This was the first large‐scale study of CH3Br and CH3Cl in the coastal ocean. The mean atmospheric mixing ratios were 10.1 ppt for CH3Br and 517 ppt for CH3Cl. Surface seawater concentrations ranged from 0.8 pM to 5.0 pM for CH3Br and from 61.5 pM to 179 pM for CH3Cl. Biological processes contribute to the elevated seawater concentrations of CH3Br, while a combination of biological processes and terrestrial sources may contribute to elevated seawater concentrations of CH3Cl. Vertical distributions show highest concentrations of CH3Br and CH3Cl in the subsurface seawater below the mixed layer due to high degradation rates near the surface. Good correlation of the production rates between CH3Br and CH3Cl suggests they have some common sources in the coastal ocean. After excluding the open ocean points, annual coastal emissions in the studied region were 0.01–0.06 Gg yr−1 for CH3Br and 0.3–1.6 Gg yr−1 for CH3Cl. By simple extrapolation, the global coastal emissions were estimated at 0.5–3.6 Gg yr−1 for CH3Br and 19–98 Gg yr−1 for CH3Cl. Including the coastal ocean emissions of CH3Br and CH3Cl will increase the estimate of global oceanic emissions by 1%–9% and 1%–8%, respectively.

Identification and quantification of methyl halide sources in a lowland tropical rainforest

In conjunction with the OP3 campaign in Danum Valley, Malaysian Borneo, flux measurements of methyl chloride (CH 3Cl) and methyl bromide (CH 3Br) were performed from both tropical plant branches and leaf litter in June and July 2008. Live plants were mainly from the Dipterocarpaceae family whilst leaf litter samples were representative mixtures of different plant species. Environmental parameters, including photosynthetically-active radiation, total solar radiation and air temperature, were also recorded. The dominant factor determining magnitude of methyl halide fluxes from living plants was plant species, with specimens of the genus Shorea showing persistent high emissions of both gases, e.g. Shorea pilosa: 65 ± 17 ng CH 3Cl h −1 g −1 (dry weight foliage) and 2.7 ± 0.6 ng CH 3Br h −1 g −1 (dry weight foliage). Mean CH 3Cl and CH 3Br emissions across 18 species of plant were 19 (range, <LOD −76) and 0.4 (<LOD −2.9) ng h −1 g −1 respectively; fluxes from leaf litter were 1–2 orders of magnitude smaller per dry mass. CH 3Cl and CH 3Br fluxes were weakly correlated. Overall, the findings suggest that tropical rainforests make an important contribution to global terrestrial emissions of CH 3Cl, but less so for CH 3Br.

Recent trends in atmospheric methyl bromide: analysis of post-Montreal Protocol variability

Abstract. The atmospheric methyl bromide (CH3Br) burden has declined in recent years, in response to the phaseout of agricultural and structural fumigation consumption under the amendments to the Montreal Protocol. The timing and magnitude of this decrease represents an opportunity to examine our current understanding of atmospheric CH3Br and its budget, response to the phaseout, and response to interannual variability in biomass burning and global OH. In this study, simulations obtained from a time-dependent global model of atmospheric CH3Br emissions and uptake are compared to observations from the NOAA flask network. The model includes a detailed gridded ocean model coupled to a time-dependant atmospheric 2-box model. The phaseout of CH3Br production for agricultural uses began in 1998, concurrent with the pulse in biomass burning associated with the 1998 El Niño. The combined effects of three factors (biomass burning, global OH, and anthropogenic phaseout) appear to explain most of the observed atmospheric methyl bromide variability over the 1997–2008 period. The global budget remains imbalanced, with a large missing source indicated. The missing source does not exhibit a systematic decline during the phaseout period, and therefore, is not the result of significantly underestimating non-QPS agricultural CH3Br emissions. The model results suggest that the oceans should be less undersaturated than before the phaseout began.

Methyl bromide emissions to the atmosphere from temperate woodland ecosystems

AbstractThe environmental importance of methyl bromide (CH3Br) arises from its contribution to stratospheric ozone loss processes and, as a consequence, its emissions from anthropogenic sources are subject to the Montreal Protocol. A better understanding of the natural budget of CH3Br is required for assessing the benefit of anthropogenic emission reductions and for understanding any potential effects of environmental change on global CH3Br concentrations. Measurements of CH3Br flux in temperate woodland ecosystems, in particular, are very sparse, yet these cover a large fraction of terrestrial land surface. Results presented here from 18 months of field measurements of CH3Br fluxes in four static flux chambers in a woodland in Scotland and from enclosures of rotting wood and deciduous and coniferous leaf litter suggest net emissions from temperate woodlands. Net CH3Br fluxes in the woodland varied between the chambers, fluctuating between net uptake and net emissions (−73 to 279 ng m−2 h−1 across 161 individual measurements), and with no strong seasonality, but with time‐averaged net emission overall [27±57 (1 SD)] ng m−2 h−1]. This work demonstrates that scale‐up needs to be based on sufficient individual measurements to provide a reasonably constrained estimate of the long‐term mean. Mean (±1 SD) net CH3Br emissions from deciduous and coniferous leaf litter were 43 (±33) ng kg−1 (dry weight) h−1 and 80 (±37) ng kg−1 (dry weight) h−1, respectively, and ∼1–2 ng kg−1 (fresh weight) h−1 from rotting woody litter. Despite the intrinsic variability, data obtained here consistently point to the conclusion that the temperate forest soil/litter ecosystem is a net source of CH3Br to the atmosphere.

Abiotic Methyl Bromide Formation from Vegetation, and Its Strong Dependence on Temperature

Methyl bromide (CH3Br) is the most abundant brominated organic compound in the atmosphere. It is known to originate from natural and anthropogenic sources, although many uncertainties remain regarding strengths of both sources and sinks and the processes leading to its formation. In this study a potential new CH3Br source from vegetation has been examined, analogous to the recently discovered abiotic formation of methyl chloride from plant pectin. Several plant samples with known bromine content, including ash (Fraxinus excelsior), saltwort (Batis maritima), tomato reference material (NIST-1573a), hay reference material (IAEA V-10), and also bromine enriched pectin, were incubated in the temperature range of 25-50 degrees C and analyzed for CH3Br emission using gas chromatography/mass spectrometry. All plant samples inspected showed an exponential increase in CH3Br emission as a function of temperature increase, i.e., emissions were observed to approximately double with every 5 degrees C rise in temperature. Next to temperature, it was found that emissions of CH3Br were also dependent on the bromine content of the plants. The highest CH3Br release rates were found for the saltwort which contained the highest bromine concentration. Arrhenius plots confirmed that the observed emissions were from an abiotic origin. The contribution of abiotic CH3Br formation from vegetation to the global budget will vary geographically as a result of regional differences in both temperature and bromide content of terrestrial plants.

An estimation of the global emission of methyl bromide from rapeseed ( Brassica napus) from 1961 to 2003

Gan et al. [Gan, J., Yates, S.R., Ohr, H.D., Sims, J.J., 1998. Production of methyl bromide by terrestrial higher plants. Geophysical Research Letters 25 (19), 3595–3598] have reported that plants of the family Brassicaceae take up bromide from soils and subsequently release methyl bromide (CH 3Br) to the atmosphere deriving a significant emission from this source of about 7 Gg(10 9 g) yr −1. In this study, we determine a yearly global emission rate for CH 3Br from one such species, rapeseed, from 1961 to 2003 using data on crop harvest index and growth rate in conjunction with global production data. This study suggests that CH 3Br emissions from rapeseed have increased 10-fold from 1961 to 2003 and by a factor of 3–4 since 1980. It also suggests that the geographical distribution of emissions has also changed substantially in the last 40 yr. The annual emission of CH 3Br from mustard and cabbage is also estimated; whilst relative levels emitted from these species are less significant, as these crops are continually exploited for new applications CH 3Br emissions are set to increase.

Temporal and spatial variation in methyl bromide flux from a salt marsh

Methyl bromide (CH3Br) is a trace gas involved in stratospheric ozone depletion with both anthropogenic and natural sources. Estimates of natural source strengths are highly uncertain. In this study, &gt;320 highly temporally and spatially resolved measurements of CH3Br emissions from a salt marsh in Scotland (56°00′N, 2°35′W) were made during one year using eight static enclosures. Net emissions showed both strong seasonal and diurnal cycles. Day‐to‐day maxima in emissions were associated with sunny days. Emissions dropped to zero when vegetation was removed. Mean measured CH3Br emission was 350 ng m−2 h−1, but a few “hot spots” (measured maximum 4000 ng m−2 h−1) dominated integrated emissions. A crude scale‐up of the annual mean emission yields an estimate for global CH3Br emission of ∼1 (0.5–3) Gg y−1 (range uses annual mean from lowest and highest emitting enclosures), ∼10% the global salt marsh emission regularly quoted in the literature.

NMHCs and halocarbons in Asian continental outflow during the Transport and Chemical Evolution over the Pacific (TRACE‐P) Field Campaign: Comparison With PEM‐West B

We present an overview of the spatial distributions of nonmethane hydrocarbons (NMHCs) and halocarbons observed over the western north Pacific as part of the NASA GTE Transport and Chemical Evolution over the Pacific (TRACE‐P) airborne field campaign (February–April 2001). The TRACE‐P data are compared with earlier measurements from the Pacific Rim during the Pacific Exploratory Mission‐West B (PEM‐West B), which took place in February–March 1994, and with emission inventory data for 2000. Despite the limited spatial and temporal data coverage inherent to airborne sampling, mean levels of the longer‐lived NMHCs (including ethane, ethyne, and benzene) were remarkably similar to our results during the PEM‐West B campaign. By comparison, mixing ratios of the fire extinguisher Halon‐1211 (CF2ClBr) increased by about 50% in the period between 1994 and 2001. Southern China (south of 35°N), and particularly the Shanghai region, appears to have been a substantial source of Halon‐1211 during TRACE‐P. Our previous analysis of the PEM‐West B data employed methyl chloroform (CH3CCl3) as a useful industrial tracer. However, regulations have reduced its emissions to the extent that its mixing ratio during TRACE‐P was only one‐third of that measured in 1994. Methyl chloroform mixing ratio “hot spots,” indicating regions downwind of continuing emissions, included outflow from the vicinity of Shanghai, China, but particularly high emission ratios relative to CO were observed close to Japan and Korea. Tetrachloroethene (C2Cl4) levels have also decreased significantly, especially north of 25°N, but this gas still remains a useful indicator of northern industrial emissions. Methyl bromide (CH3Br) levels were systematically 1–2 pptv lower from 1994 to 2001, in accord with recent reports. However, air masses that had been advected over Japan and/or South Korean port cities typically exhibited elevated levels of CH3Br. As a consequence, emissions of CH3Br from Japan and Korea calculated employing CH3Br/CO ratios and scaled to CO emission inventory estimates, were almost as large as for all of south China (south of 35°N). Total east Asian emissions of CH3Br and CH3Cl were estimated to be roughly 4.7 Gg/yr and 167 Gg/yr, respectively, in 2001.

Emissions of CH3Br, organochlorines, and organoiodines from temperate macroalgae

The production rates of a range of low molecular weight halogenated organics have been determined in cultures of five temperate species of macroalgae collected from the north coast of Norfolk, England. Compounds studied included CH3Br, the chlorinated organics CH3Cl, CH2Cl2 and CHCl3, and the iodinated organics CH3I, C2H5I, and CH2ClI. Measurements of a wider range of halocarbon concentrations in an isolated rockpool and in air over the seaweed bed were also conducted to evaluate the local impact of the seaweeds on halocarbon concentrations in the natural environment. Estimates for the global emissions of some of the key halogenated compounds from macroalgae have been derived. In general macrophytes appear not to be globally significant producers of the particular halocarbons studied. In coastal regions, however, the impact on local atmospheric composition and chemistry could be greater.

A study of methyl bromide emissions from automobiles burning leaded gasoline using standardized vehicle testing procedures

This study measured CH3Br emissions directly from vehicular exhaust while operating under standardized dynamometer conditions, and generated response curves to see if the bromine content of the fuel could be correlated to and used as a predictor for tailpipe emissions. Four lead and bromine fuel concentrations were run in two non‐catalyst automobiles using the standard Federal Test Procedure (FTP) driving cycle and exhaust samples were analyzed for CH3Br using Gas Chromatography‐Electron Capture Detection (GC‐ECD). Total FTP exhaust emissions of CH3Br produced a range of average values from below detection to 26 mg (kg fuel burned)−1. Total FTP Emission Rates were plotted and response curves demonstrated that fuel bromine content was indeed predictive of CH3Br emission rates. Overall average CH3Br mass emission factor was 2.4% (2.1 – 2.7%) which was lower than previous work. Emission results of this study were found to be about 10‐fold lower than indirect methods.

Biomass burning emissions and vertical distribution of atmospheric methyl halides and other reduced carbon gases in the South Atlantic region

The NASA TRACE A experiment (September – October 1992) investigated effects of dry season biomass burning emissions from both South America and southern Africa on the tropical South Atlantic troposphere. Whole air canister samples were collected aboard the NASA DC‐8 aircraft and analyzed for a wide range of nonmethane hydrocarbons (NMHCs) and halocarbons. Fast response in situ quantification of CH4, CO, and CO2 were also performed on the DC‐8. Sampling took place over Brazilian agricultural areas and southern African savanna where there was active biomass burning. The vertical distribution of the measured gases revealed that the concentrations of most hydrocarbons, methyl halides, CH4, CO, and CO2, were enhanced in the boundary layer of these regions principally as a result of biomass fires. Brazilian and African biomass burning emission ratios were calculated for CH3Br, CH3Cl, CH3I, and NMHCs relative to CO and CO2. Although both fire regions were dominated by efficient (flaming) combustion (CO/CO2 ratios &lt;0.1), the Brazilian samples exhibited the lower (more flaming) CO/CO2 ratio of 0.037 compared with the African savanna value of 0.062. This difference in combustion efficiency was reflected in lower ratios versus CO2 for all correlated gases. However, the gases more closely associated with smoldering combustion (e.g., C3H8 and CH3Cl) exhibited significantly higher ratios relative to CO for the Brazilian fires, while the African samples exhibited higher values versus CO for compounds associated more closely with flaming combustion (e.g., C2H2). This variation in the trace gas ratios versus CO is most likely caused by different fuel characteristics. On the basis of the emission ratios obtained, the total biomass burning emission rates for savannas and worldwide were calculated for the hydrocarbons and methyl halides. From these it was estimated that roughly 25% and 20% of global CH3Cl and CH3Br emissions, respectively, derive from biomass burning but that the contribution of biomass burning to total CH3I emissions was not significant.