Methyl Chloroform Continues to Constrain the Hydroxyl (OH) Variability in the Troposphere

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 (CH 3 CCl 3 ) and process‐based chemistry transport models often produce conflicting results. Here we use a previously tested transport model to simulate atmospheric CH 3 CCl 3 for the period 1985–2018. Based on mismatches between model output and observations, we derive consistent anomalies in the inverse lifetime of CH 3 CCl 3 ( 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 + CH 3 CCl 3 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 CH 3 CCl 3 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 CH 3 CCl 3 emissions decayed more slowly over time than our best estimate suggests. If global CH 3 CCl 3 emissions dropped below 3 Gg year −1 after 2015, recent CH 3 CCl 3 measurements indicate that the 2015–2018 loss rate of CH 3 CCl 3 due to reaction with OH is comparable to its value 2 decades ago.