Abstract
Abstract. The self-cleaning or oxidation capacity of the atmosphere is principally controlled by hydroxyl (OH) radicals in the troposphere. Hydroxyl has primary (P) and secondary (S) sources, the former mainly through the photodissociation of ozone, the latter through OH recycling in radical reaction chains. We used the recent Mainz Organics Mechanism (MOM) to advance volatile organic carbon (VOC) chemistry in the general circulation model EMAC (ECHAM/MESSy Atmospheric Chemistry) and show that S is larger than previously assumed. By including emissions of a large number of primary VOC, and accounting for their complete breakdown and intermediate products, MOM is mass-conserving and calculates substantially higher OH reactivity from VOC oxidation compared to predecessor models. Whereas previously P and S were found to be of similar magnitude, the present work indicates that S may be twice as large, mostly due to OH recycling in the free troposphere. Further, we find that nighttime OH formation may be significant in the polluted subtropical boundary layer in summer. With a mean OH recycling probability of about 67 %, global OH is buffered and not sensitive to perturbations by natural or anthropogenic emission changes. Complementary primary and secondary OH formation mechanisms in pristine and polluted environments in the continental and marine troposphere, connected through long-range transport of O3, can maintain stable global OH levels.
Highlights
The removal of most natural and anthropogenic gases from the atmosphere – important for air quality, the ozone layer and climate – takes place through their oxidation by hydroxyl (OH) radicals in the troposphere
Mainz Organics Mechanism (MOM) computes the chemistry of saturated and unsaturated hydrocarbons, including terpenes and aromatics (Cabrera-Perez et al, 2016). We use it to estimate the role of radical production through reactions of oxidized volatile organic carbon (VOC), referred to as the OVOC recycling mechanism of OH, being contrasted with the NOx and Ox recycling mechanisms of OH. Based on this scheme, implemented in the ECHAM/MESSy Atmospheric Chemistry general circulation model (EMAC), we provide an update of global OH calculations, sources, sinks, tropospheric distributions, OH reactivity, and the lifetime of CH4 and carbon monoxide (CO), and we discuss implications for atmospheric chemistry
Since conversions between HO2 and OH play a key role in OH recycling, we address the budget of HOx (OH + HO2), which is dominated by HO2
Summary
The removal of most natural and anthropogenic gases from the atmosphere – important for air quality, the ozone layer and climate – takes place through their oxidation by hydroxyl (OH) radicals in the troposphere. MOM computes the chemistry of saturated and unsaturated hydrocarbons, including terpenes and aromatics (Cabrera-Perez et al, 2016) We use it to estimate the role of radical production through reactions of oxidized VOC, referred to as the OVOC recycling mechanism of OH, being contrasted with the NOx and Ox recycling mechanisms of OH. Based on this scheme, implemented in the ECHAM/MESSy Atmospheric Chemistry general circulation model (EMAC), we provide an update of global OH calculations, sources, sinks, tropospheric distributions, OH reactivity, and the lifetime of CH4 and CO, and we discuss implications for atmospheric chemistry. We show that complementary OH recycling mechanisms in terrestrial, marine, pristine and polluted environments, interconnected through atmospheric transport, sustain stable levels of hydroxyl in the global troposphere
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