Oxidation processes in the eastern Mediterranean atmosphere: evidence from the modelling of HO_<i>x</i> measurements over Cyprus

• Main Authors: C. Mallik, L. Tomsche, E. Bourtsoukidis, J. N. Crowley, B. Derstroff, H. Fischer, S. Hafermann, I. Hüser, U. Javed, S. Keßel, J. Lelieveld, M. Martinez, H. Meusel, A. Novelli, G. J. Phillips, A. Pozzer, A. Reiffs, R. Sander, D. Taraborrelli, C. Sauvage, J. Schuladen, H. Su, J. Williams, H. Harder
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/10825/2018/acp-18-10825-2018.pdf
• ISSN: 1680-7316; 1680-7324

<p>The Mediterranean is a climatically sensitive region located at the crossroads of air masses from three continents: Europe, Africa, and Asia. The chemical processing of air masses over this region has implications not only for the air quality but also for the long-range transport of air pollution. To obtain a comprehensive understanding of oxidation processes over the Mediterranean, atmospheric concentrations of the hydroxyl radical (OH) and the hydroperoxyl radical (HO₂) were measured during an intensive field campaign (CYprus PHotochemistry EXperiment, CYPHEX-2014) in the northwest of Cyprus in the summer of 2014. Very low local anthropogenic and biogenic emissions around the measurement location provided a vantage point to study the contrasts in atmospheric oxidation pathways under highly processed marine air masses and those influenced by relatively fresh emissions from mainland Europe.</p><p>The CYPHEX measurements were used to evaluate OH and HO₂ simulations using a photochemical box model (CAABA/MECCA) constrained with CYPHEX observations of O₃, CO, NO_<i>x</i>, hydrocarbons, peroxides, and other major HO_<i>x</i> (OH&thinsp;+&thinsp;HO₂) sources and sinks in a low-NO_<i>x</i> environment (&lt;&thinsp;100&thinsp;pptv of NO). The model simulations for OH agreed to within 10&thinsp;% with in situ OH observations. Model simulations for HO₂ agreed to within 17&thinsp;% of the in situ observations. However, the model strongly under-predicted HO₂ at high terpene concentrations, this under-prediction reaching up to 38&thinsp;% at the highest terpene levels. Different schemes to improve the agreement between observed and modelled HO₂, including changing the rate coefficients for the reactions of terpene-generated peroxy radicals (RO₂) with NO and HO₂ as well as the autoxidation of terpene-generated RO₂ species, are explored in this work. The main source of OH in Cyprus was its primary production from O₃ photolysis during the day and HONO photolysis during early morning. Recycling contributed about one-third of the total OH production, and the maximum recycling efficiency was about 0.7. CO, which was the largest OH sink, was also the largest HO₂ source. The lowest HO_<i>x</i> production and losses occurred when the air masses had higher residence time over the oceans.</p>