Investigating the global OH radical distribution using steady-state approximations and satellite data

Abstract

Abstract. We present 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 is a timely study as OH observations are predominantly from spatially sparse field and infrequent aircraft campaigns, so there is a requirement 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. Due to the short lifetime of OH (∼1 s), SSAs of varying complexities can be used to model its concentration and offer a tool to examine the OH budget in different regions of the atmosphere. Here, we use the well-evaluated TOMCAT three-dimensional chemistry transport model 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 is applied to aircraft measurements from the Atmospheric Tomography Mission (ATom) and compares well with the observed OH concentrations within ∼26 %, yielding a correlation of 0.78. We apply 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 here could be further developed as a constraint on the tropospheric OH distribution as additional satellite data become available in the future.

Technical Note: Constraining the hydroxyl (OH) radical in the tropics with satellite observations of its drivers: First steps toward assessing the feasibility of a global observation strategy

“…The level of agreement between observed and predicted OH, particularly for ATom1 and 3, is comparable or better than that of other methods to infer OH from space. For example, Pimlott et al (2022) found an r of 0.78 (r 2 = 0.61) when estimating ATom OH using a steady state approach, with r values ranging from 0.51 to 0.85 (r 2 of 0.26 to 0.72) for the different deployments. The level of agreement we show here therefore demonstrates the validity of the machine learning method to capture the variability of OH.…”

“…Using machine learning, chemical transport model output, and retrievals of NO2 and HCHO, Zhu et al (2022a) developed a method to estimate surface OH in North American urban areas. Finally, Pimlott et al (2022) used a steady state approximation of OH, including primary production from H2O and O3 and loss from CO, CH4, and O3, to estimate OH between 600 and 700 hPa using observations from IASI (Infrared Atmospheric Sounding Interferometer). A logical next step, building on the results of these studies, is the development of a methodology to constrain OH that ingests multiple satellite retrievals, encompasses the breadth of OH chemical and dynamical drivers, and spans a significant enough portion of the globe to inform variability and trends in CH4 and CO loss.…”

Enhanced HONO Formation from Aqueous Nitrate Photochemistry in the Presence of Marine Relevant Organics: Impact of Marine-Dissolved Organic Matter (m-DOM) Concentration on HONO Yields and Potential Synergistic Effects of Compounds within m-DOM

“…The hydroxyl (•OH) radical is the most significant atmospheric oxidizer, as it reacts with trace tropospheric gases and it is critical in the formation of secondary organic aerosols including secondary marine aerosols. − Despite its significance, •OH spatial and temporal information is still challenging to project by atmospheric models, mainly due to uncertainties in its sources. , A major source of •OH in the troposphere is the photodissociation of nitrous acid, or HONO, which occurs at wavelengths between 300 to 400 nm. , Over the last decade, several formation pathways of atmospheric HONO have been reported. These include direct emissions from soils, NO 2 hydrolysis, , NO 2 reduction in the presence of photosensitizers, , surface mediated reactions of nitrated surfaces, − and aqueous and solid particulate nitrate photolysis. − For nitrate photolysis, there are two reaction pathways, one leading to gas-phase NO 2 and the other to HONO at low pH (and NO 2 – at pH values greater than ca.…”

“…1−6 Despite its significance, •OH spatial and temporal information is still challenging to project by atmospheric models, mainly due to uncertainties in its sources. 7,8 A major source of •OH in the troposphere is the photodissociation of nitrous acid, or HONO, which occurs at wavelengths between 300 to 400 nm. 9,10 Over the last decade, several formation pathways of atmospheric HONO have been reported.…”

Trends and Interannual Variability of the Hydroxyl Radical in the Remote Tropics During Boreal Autumn Inferred From Satellite Proxy Data

“…Zhu et al (2022b) then used this satellite-constrained OH to determine trends in urban OH and their relationship to ozone (O 3 ) production. Pimlott et al (2022) used multiple retrievals from IASI (Infrared Atmospheric Sounding Interferometer) to constrain OH globally between 600 and 700 hPa and investigated the role of various drivers on IAV on the hemispheric scale. Finally, Anderson et al (2023) combined observations from multiple satellites to produce a tropospheric column OH (TCOH) product over the tropical oceans.…”