Tropospheric ozone (O₃) is an atmospheric trace gas species which is principally formed via the photo-oxidation of nitrogen dioxide (NO₂) by sunlight, where NO₂ is emitted by both man-made (transport, industry) and natural (soil) sources. The growth in economic activity has resulted in a growth in man-made emissions, which has increased the background level of O₃ substantially over recent decades. This has consequences in that it exerts an influence on public health (respiratory conditions), air quality (cleansing ability of the atmosphere) and the global warming potential of the atmosphere by perturbing the atmospheric lifetime of important greenhouse gases such as CH4.

Since the eighties a concerted effort by laboratory scientists and atmospheric modellers has concluded that out of a large number of chemical reactions, there are a few key processes which essentially control background O₃ levels. One of these reactions involves the efficient production of NO₂ via the oxidation of NO with the HO₂ radical. Recent experiments performed by laboratory chemists in Orleans, France have applied sophisticated measurement techniques and found that rather than a 100% direct conversion of NO into NO₂, a small fraction is converted into nitric acid (HNO₃). This species essentially ‘locks’ reactive nitrogen (NO₂) away, thus lowering the production efficiency of O₃.

Further studies have revealed that this effect can be enhanced in the presence of water vapour whose concentration is high in the lower atmosphere, although measurements have only been made over a limited set of conditions relevant to the atmosphere (~1 atm, 298 K). This occurs via the formation of the complex HO₂-H₂O. To investigate the potential effects on tropospheric O₃ researchers in KS-CK have introduced this enhanced pathway into large scale chemical-transport simulations of the atmosphere.

The figure below shows the resulting increase in HNO₃ and the associated decreases in both reactive nitrogen oxides (NO_x) and tropospheric O₃ of between ~5-20%, where maximal changes in O₃ occur in the ‘damp’ tropics. Although it is desirable to expand on the experimental studies performed for other atmospheric conditions, this mechanism has the potential to act as a negative feedback mechanism towards tropospheric O₃ levels in a warmer and wetter climate.

Results were recently presented at the final SCOUT-O₃ meeting at Schliersee, Germany, where the poster version of the oral presentation is linked below.