Tropospheric OH

The hydroxyl radical (OH) is one of the most important trace species in the atmosphere. It is a very reactive oxidant, responsible for removing many pollutant species and reacts in many chemical cycles. It has a very short chemical lifetime (~seconds).

Deriving OH from Satellite Data

Pimlott et al. (2022) presented a novel approach to derive indirect global information on the hydroxyl radical (OH), one of the most important atmospheric oxidants, using state-of-the-art satellite trace gas observations (key sinks and sources of OH) and a steady-state approximation (SSA). This was a timely study as OH observations are predominantly from spatially sparse field and infrequent aircraft campaigns, so there is a strong incentive for further approaches to infer spatial and temporal information on OH and its interactions with important climate (e.g. methane, CH₄) and air quality (e.g. nitrogen dioxide, NO₂) trace gases.

We used TOMCAT to identify atmospheric regions where different complexities of the SSAs are representative of OH. In the case of a simplified SSA (S-SSA), where we have observations of ozone (O₃), carbon monoxide (CO), CH₄ and water vapour (H₂O) from the Infrared Atmospheric Sounding Interferometer (IASI) on board ESA's MetOp-A satellite, it is most representative of OH between 600 and 700 hPa (though suitable between 400–800 hPa) within ∼20 %–30 % of TOMCAT modelled OH. The same S-SSA was applied to aircraft measurements from the Atmospheric Tomography Mission (ATom) and compared well with the observed OH concentrations. We applied the S-SSA to IASI data spanning 2008–2017 to explore the global long-term inter-annual variability of OH. Relative to the 10-year mean, we find that global annual mean OH anomalies ranged from −3.1 % to +4.7 %, with the largest spread in the tropics between −6.9 % and +7.7 %. Investigation of the individual terms in the S-SSA over this time period suggests that O₃ and CO were the key drivers of variability in the production and loss of OH. For example, large enhancement in the OH sink during the positive 2015/2016 El Niño–Southern Oscillation (ENSO) event was due to large-scale CO emissions from drought-induced wildfires in South East Asia.

The methodology described in the paper could be further developed as a constraint on the tropospheric OH distribution as additional satellite data become available in the future.

TOMCAT Publications

McNorton, J., C. Wilson, M. Gloor, R.J. Parker, H. Boesch, W. Feng, R. Hossaini, and M.P. Chipperfield, Attribution of recent increases in atmospheric methane through 3-D inverse modelling, Atmos. Chem. Phys., 18, 18149-18168, doi:10.5194/acp-18-18149-2018, 2018.

Pimlott, M.A., R.J. Pope, B.J. Kerridge, B.G. Latter, D.S. Knappett, D.E. Heard, L.J. Ventress, R. Siddans, W. Feng and M.P. Chipperfield, Investigating the Global OH Radical Distribution Using Steady-State Approximations and Satellite Data, Atmos. Chem. Phys., 22, 10467-10488, doi:10.5194/acp-22-10467-2022, 2022.