Oxidation precursors around the Arabian Peninsula – Evaluation of EMAC model results with ship-based measurements of HCHO, H2O2, ROOH and HOx

Formaldehyde (HCHO), hydrogen peroxide (H₂O₂) and organic hydroperoxides (ROOH) play a key role in atmospheric oxidation processes. They act as sources and sinks for HO_x radicals (OH + HO₂), with OH as the primary oxidant that governs the atmospheric self-cleaning capacity. In this study, we use in situ observations in the marine boundary layer (MBL) to calculate trace gas budgets, determine dry deposition velocities and evaluate results of the general circulation model EMAC (ECHAM5/MESSy2 Atmospheric Chemistry).

The dataset was obtained during the AQABA (Air Quality and climate change in the Arabian BAsin) ship campaign around the Arabian Peninsula in summer 2017. This region is famous for high levels of anthropogenic air pollution related to the oil and gas industry, especially in the areas around the Suez Canal and the Arabian Gulf. High levels of air pollution with up to 12 ppb_V HCHO, 2.3 ppb_V ROOH but relatively low levels of H₂O₂ (≤ 0.5 ppb_V) were detected over the Arabian Gulf.

We find that EMAC predicted mixing ratios of HCHO and ROOH mostly within a factor of 2, while the model overestimated ROOH in cleaner conditions and it failed to resolve the encountered high pollution events over the Arabian Gulf. Dry deposition velocities (V_dep) were determined for HCHO and H₂O₂ during night with 0.77 ± 0.29 cm s^–1 for HCHO and 1.03 ± 0.52 cm s^–1 for H₂O₂ over the Arabian Sea, which were matched by EMAC. V_dep was underestimated over the Mediterranean Sea by more than a factor of 2, which was mostly related to the models resolution and its wind speed dependency. Determination of the photochemical budget of H₂O₂ revealed overestimated HO_x in EMAC, which resulted in an elevated net photochemical production over most regions. Results of the regional model WRF-Chem (Weather Research and Forecasting-Chem) increased the accuracy of H₂O₂ for most regions, while the model did not resolve the complex air pollution encountered over the Arabian Gulf, which may lead to missing anthropogenic emissions in the region.