Methyl bromide is a stratospheric ozone‐depleting substance with both natural and anthropogenic sources. The global budget of methyl bromide has never been fully understood as evidenced by the ...significant budget gap between the bottom‐up source estimates and calculated atmospheric losses. Atmospheric methyl bromide levels have declined significantly since Phase‐out under the Montreal Protocol began in 1999, and the atmosphere appears to have reached a new steady state during the past five years. Here, we reassess the global methyl bromide budget utilizing the 25‐year record of atmospheric methyl bromide measurements from the National Oceanic and Atmospheric Administration Global Monitoring Laboratory global flask network and a zonal 6‐box coupled global ocean/atmosphere model. Model inversions were used to estimate the total emissions required to account for the observed atmospheric methyl bromide levels. From 1995 to 2019, global land‐based emissions (natural and anthropogenic) declined from about 120 to 85 Gg y−1 and net ocean emissions increased from −5 to +5 Gg y−1. There remains an imbalance between the bottom‐up estimates of terrestrial sources and the inversion result. Based on the timing, magnitude, and spatial distribution of the imbalance we partition it into (a) a persistent or time invariant source located primarily in the tropics, and (b) a smaller time‐varying component that scales with the anthropogenic source during phase‐out. We hypothesize that the persistent source is likely natural and the time variant component is an artifact resulting from a slight underestimation of anthropogenic emissions.
Plain Language Summary
Methyl bromide is an ozone‐depleting gas for which production is globally controlled under the fully amended and adjusted Montreal Protocol. As a result, atmospheric methyl bromide levels have declined dramatically since 1999 as measured by the National Oceanic and Atmospheric Administration Global Monitoring Laboratory flask air network. The change in atmospheric methyl bromide abundance provides an opportunity to re‐examine our understanding of the global budget. We use a coupled ocean/atmosphere model to estimate the change in emissions required to explain the observed atmospheric changes. The study suggests that anthropogenic emissions may have previously been slightly underestimated and that there remain significant sources of unknown origin, located mainly in the tropics.
Key Points
As a result of the Montreal Protocol phase‐out, atmospheric methyl bromide declined over the past two decades and has recently stabilized
The ocean saturation state for methyl bromide has increased and the oceans are a small net source to the atmosphere
An imbalance between the known sources and losses of atmospheric methyl bromide remains, requiring additional emissions in the tropics
•At higher pCO2 concentrations Karenia brevis growth rates are significantly increased.•Growth rate and toxin production are not linked in Karenia brevis.•Temperature and pCO2 concentrations do not ...influence toxin production.•Future climate change has the potential of producing blooms with higher cell concentrations.
Increasing atmospheric CO2 is promoting ocean acidification and higher global temperatures and has been suggested as a possible factor for shifts in marine phytoplankton composition to more harmful species. Karenia brevis is the major harmful algal species in the Gulf of Mexico producing potent neurotoxins known as brevetoxins. We examined how changes in ocean inorganic carbon chemistry associated with pre-industrial (250ppm), recent (350ppm), and predicted at 2100 (1000ppm) pCO2 levels and increased temperature affect growth rates and brevetoxin production in K. brevis. At the predicted pCO2 levels for 2100, growth rate of K. brevis Wilson clone increased substantially by 46% at 25°C (0.43±0.01d−1) compared to recent and pre-industrial levels (0.29±0.01d−1). Growth rates also increased for a low brevetoxin-producing clone, SP1, from 0.24±0.02d−1 at lower pCO2 levels to 0.33±0.003d−1 at a pCO2 of 1000ppm. When grown at a higher temperature (30°C), growth rates for the Wilson clone significantly decreased at all three pCO2 by approximately 30%. However, even at the higher temperature, K. brevis growth rate significantly increased by 30% (0.30±0.01d−1) at the 1000ppm CO2 level when compared to recent and pre-industrial CO2 levels (0.21±0.01d−1). Although K. brevis growth rate decreased at higher temperatures, the growth rate at pCO2 level of 1000 and 30°C was slightly higher than at current conditions (pCO2 level of 350 and 25°C). Modification of pCO2 levels and temperatures did not have an effect on total brevetoxin production or brevetoxin profiles in either clone examined. Due to the increased growth rate, total brevetoxin production was significantly higher at the pCO2 level of 1000. Finally, from these results we conclude there is no connection between growth rate and brevetoxin per cell. Although neither pCO2 nor temperature influenced brevetoxin production per cell, we suggest that under predicted future climate conditions K. brevis blooms have the potential to produce higher cell concentrations and increased brevetoxin concentrations, which will pose an increased risk for ecosystem and human health.
This paper reports the first depth profile measurements of methyl, ethyl, isopropyl and n‐propyl nitrates in the tropical Pacific Ocean. Depth profile measurements were made at 22 stations during the ...Project Halocarbon Air Sea Exchange cruise, in warm pool, equatorial, subequatorial, and gyre waters. The highest concentrations, up to several hundred pM of methyl nitrate, were observed in the central Pacific within 8 degrees of the equator. In general, alkyl nitrate levels were highest in the surface mixed layer, and decreased with depth below the mixed layer. The spatial distribution of the alkyl nitrates suggests that there is a strong source associated with biologically productive ocean regions, that is characterized by high ratios of methyl:ethyl nitrate. However, the data do not allow discrimination between direct biological emissions and photochemistry as production mechanisms. Alkyl nitrates were consistently detectable at several hundred meters depth. On the basis of the estimated chemical loss rate of these compounds, we conclude that deep water alkyl nitrates must be produced in situ. Possible sources include free radical processes initiated by radioactive decay or cosmic rays, enzymatically mediated reactions involving bacteria, or unidentified chemical mechanisms involving dissolved organic matter.
We have calculated from a 2° × 2° grid of oceanic properties the contribution of oceanic loss to the overall lifetimes of a number of anthropogenic and naturally produced trace gases involved in ...global warming and stratospheric ozone depletion. The model, originally developed for atmospheric methyl bromide, can be used for any well‐mixed trace gas where the seawater degradation rate constants and solubilities are known. Of the gases tested, it is clear that known oceanic chemical degradation processes alone are not significant sinks for most HFCs and HCFCs. Chemical degradation in the oceans is a substantial sink for COS (28%) and COCl2 (8%) and a minor sink for CH3Cl (<2%) and CH3I (2.5%), and it should be considered when determining atmospheric lifetimes and sink strengths for these gases. Biological degradation processes are likely to increase the oceanic uptake rates of many gases.
Data collected from the North Pacific Ocean during September and October 1999 were combined with data from other cruises to assess seasonal differences in the relationships between sea surface ...temperature (SST) and methyl bromide (CH3Br) saturation. We now are able to reproduce observed saturation anomalies substantially better with the revised, seasonal CH3Br‐SST equations than with those that were independent of season. The effect is most noticeable in temperate waters where data combined on an annual basis proved insufficient. The estimated, net global air‐sea flux of CH3Br remains negative at −10 to −18 Gg yr−1, which is consistent with extrapolations from observations.
Air and water concentrations of methyl bromide (CH3Br) and methyl chloride (CH3Cl) were measured in the Southern Ocean (latitudes 45°–67°S, longitudes 144°–139°E) from late October through ...mid‐December 2001. CH3Br and CH3Cl were undersaturated with mean saturation anomalies of −39 ± 11% and −37 ± 11% between 45° and 65°S. The minimum degradation rate constants needed to maintain these saturation anomalies are consistent with the observed total degradation rate constants, suggesting that there is no significant production of these gases in this region. Near the Antarctic coast (south of 65°S) the saturation anomalies for both gases decreased to approximately −80%, although CFC‐11 measurements suggest these extreme anomalies are associated with enhanced vertical mixing rather than with degradation in the surface waters.
PDF
