ENSO‐Driven Fires Cause Large Interannual Variability in the Naturally Emitted, Ozone‐Depleting Trace Gas CH3Br

• Published in: Geophysical research letters 2022-02, Vol.49 (3), p.n/a
• Main Authors: Nicewonger, M. R., Saltzman, Eric S., Montzka, S. A.
• Format: Article
• Language: English
• Subjects: Air sampling; Anthropogenic factors; Biological production; Biomass burning; Bromides; Burning; Chlorofluorocarbons; Depletion; El Nino; El Nino phenomena; El Nino-Southern Oscillation event; ENSO; Interannual variability; Methyl bromide; Montreal Protocol; Ocean models; Ozone; Ozone depletion; Ozone layer; Ozonosphere; Sea surface; Sea surface temperature; Southern Oscillation; Surface temperature; trace gas; Trace gases; Variability; Winds
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL094756

Methyl bromide (CH3Br) is an ozone depleting trace gas that is now mainly emitted from natural sources. Roughly 80% of anthropogenic production of CH3Br was phased‐out in response to the Montreal Protocol on Substances that Deplete the Ozone Layer beginning in 1999 and atmospheric levels of CH3Br have declined considerably since. Here we use surface measurements of CH3Br from NOAA's global air sampling network, along with a six‐box atmosphere/ocean model to explore interannual variability in atmospheric CH3Br mole fractions. We find that CH3Br mole fractions are strongly correlated with the El Niño Southern Oscillation (ENSO) phenomenon, but variability in winds, sea surface temperature, and biological production during ENSO are unlikely to drive the observed changes in atmospheric CH3Br directly. Rather, the results indicate that ENSO‐driven changes to biomass burning are an important cause of the observed interannual CH3Br variability. Plain Language Summary Methyl bromide (CH3Br) is a trace gas known to destroy stratospheric ozone. Atmospheric mole fractions of CH3Br have declined substantially over time because of Montreal Protocol controls on production of this ozone‐depleting gas. What remained unexplained were the substantial temporary variations observed in the measurement record. Here we find that those variations are related to El Niño Southern Oscillation (ENSO) events, and the increased burning during the warm phase of ENSO is determined to be the most likely cause of these interannual changes. Key Points Interannual variability in the global atmospheric methyl bromide (CH3Br) mole fraction is strongly correlated with the El Niño Southern Oscillation (ENSO) phenomenon Changes in the ocean (sea surface temperature and winds) during ENSO are unlikely to cause the observed variability in atmospheric CH3Br mole fraction The majority of interannual variability in atmospheric CH3Br mole fraction is explained by ENSO‐driven fire emissions