Methyl Chloroform Continues to Constrain the Hydroxyl (OH) Variability in the Troposphere. Issue 4 (16th February 2021)

• Record Type: Journal Article
• Authors: Patra, P. K.; Krol, M. C.; Prinn, R. G.; Takigawa, M.; Mühle, J.; Montzka, S. A.; Lal, S.; Yamashita, Y.; Naus, S.; Chandra, N.; Weiss, R. F.; Krummel, P. B.; Fraser, P. J.; O'Doherty, S.; Elkins, J. W.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 4(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-02-16
• Subjects: hydroxyl radical (OH) interannual variability -- inverse lifetime of CH3CCl3 -- methyl chloroform (CH3CCl3); Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2020JD033862
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Trends and variability in tropospheric hydroxyl (OH) radicals influence budgets of many greenhouse gases, air pollutant species, and ozone depleting substances. Estimations of tropospheric OH trends and variability based on budget analysis of methyl chloroform (CH3 CCl3 ) and process‐based chemistry transport models often produce conflicting results. Here we use a previously tested transport model to simulate atmospheric CH3 CCl3 for the period 1985–2018. Based on mismatches between model output and observations, we derive consistent anomalies in the inverse lifetime of CH3 CCl3 ( K G ) using measurements from two independent observational networks (National Oceanic and Atmospheric Administration and Advanced Global Atmospheric Gases Experiment). Our method allows a separation between "physical" (transport, temperature) and "chemical" (i.e., abundance) influences on OH + CH3 CCl3 reaction rate in the atmosphere. Small increases in K G due to "physical" influences are mostly driven by increases in the temperature‐dependent reaction between OH and CH3 CCl3 and resulted in a smoothly varying increase of 0.80% decade −1 . Chemical effects on K G, linked to global changes in OH sources and sinks, show larger year‐to‐year variations (∼2%–3%), and have a negative correlation with the El Niño Southern Oscillation. A significant positive trend in K G can be derived after 2001, but it persists only through 2015 and only if we assume that CH3 CCl3 emissions decayed more slowly over time than our best estimate suggests. If global CH3 CCl3 emissions dropped below 3 Gg year −1 after 2015, recent CH3 CCl3 measurements indicate that the 2015–2018 loss rate of CH3 CCl3 due to reaction with OH is comparable to its value 2 decades ago. Key Points: High‐quality measurements of CH3 CCl3 from two independent measurement networks provide key information about tropospheric OH variability We separate effects of the "physical" climate on inverse lifetime of CH3 CCl3 ( K G ) from those arising from "chemical" changes in the atmosphere Robust variability in K G, correlating with the El Niño Southern Oscillation, has large implications, e.g., for the global methane budget