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 (CH3CCl3) and process-based chemistry transport models often produce conflicting results. Here we use a previously tested transport model to simulate atmospheric CH3CCl3 for the period 1985–2018. Based on mismatches between model output and observations, we derive consistent anomalies in the inverse lifetime of CH3CCl3 (KG) 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 + CH3CCl3 reaction rate in the atmosphere. Small increases in KG due to “physical” influences are mostly driven by increases in the temperature-dependent reaction between OH and CH3CCl3 and resulted in a smoothly varying increase of 0.80% decade−1. Chemical effects on KG, 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 KG can be derived after 2001, but it persists only through 2015 and only if we assume that CH3CCl3 emissions decayed more slowly over time than our best estimate suggests. If global CH3CCl3 emissions dropped below 3 Gg year−1 after 2015, recent CH3CCl3 measurements indicate that the 2015–2018 loss rate of CH3CCl3 due to reaction with OH is comparable to its value 2 decades ago.
Bibliographical noteFunding Information:
CHCCl measurements continue to clarify more of the long‐lasting uncertainties in OH variability and trends, thanks to the dedicated efforts by our colleagues contributing to the AGAGE and NOAA measurement programs. The Mace Head, Trinidad Head, Ragged Point, Cape Matatula, and Cape Grim AGAGE stations are supported by the National Aeronautics and Space Administration (NASA) (grants NNX16AC98G to MIT, and NNX16AC97G and NNX16AC96G to SIO). Support also comes from the UK Department for Business, Energy & Industrial Strategy (BEIS) for Mace Head, the National Oceanic and Atmospheric Administration (NOAA) for Barbados, and the Commonwealth Scientific and Industrial Research Organization (CSIRO) and the Bureau of Meteorology (Australia) for Cape Grim. The balloon‐borne measurements at Hyderabad were conducted by a collaborative project between the Max‐Plank Institute of Aeronomy, Lindau and Indian Space Research Organisation (ISRO), Bangalore. Data archive: http://agage.mit.edu/data . M. Krol received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program under grant agreement No 742798. S. Naus is supported by NWO (grant 824.15.002). This study is partly supported by the Arctic Challenge for Sustainability II grant (JPMXD1420318865) of the Ministry of Education, Science, Culture and Sports (MEXT), Japan; the Environment Research and Technology Development Fund (JPMEERF20182002) of the Environmental Restoration and Conservation Agency of Japan. 3 3
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- hydroxyl radical (OH) interannual variability
- inverse lifetime of CHCCl
- methyl chloroform (CHCCl)