Abstract. Formaldehyde (HCHO) and hydrogen peroxide (H2O2) play a key role in atmospheric oxidation processes. They act as sources and sinks for HOx radicals (OH + HO2), with OH as the primary oxidant that governs the atmospheric self-cleaning capacity. Measurements of these species allow evaluation of global chemistry-transport models which need to account for multifarious source distributions, transport and mixing, complex photochemical reaction pathways and deposition processes. HCHO is an intermediate produced during the oxidation of VOCs and is an indicator of photochemical activity and combustion related emissions. Due to its many production pathways and its rather short lifetime of only several hours at noon, accurate modelling of this species is challenging. In this study, we use in situ observations in the marine boundary layer (MBL) to evaluate results of the general circulation model EMAC (ECHAM5/MESSy2 Atmospheric Chemistry). The dataset was obtained during the AQABA ship campaign around the Arabian Peninsula in summer 2017. This region is characterized by high mixing ratios of photochemical air pollution, high humidity and strong solar irradiation, especially in the area around the Suez Canal and the Arabian Gulf. We find that EMAC fails to predict absolute mixing ratios of HCHO, especially during high pollution events, but it reproduces most of the HCHO variability seen in the different regions, while it systematically overestimates H2O2. This is mainly attributed to missing primary VOC emissions and the overestimation of HOx radicals, and also related to the models coarse spatial resolution.
Abstract. Formaldehyde (HCHO), hydrogen peroxide (H2O2) and organic hydroperoxides (ROOH) play a key role in atmospheric oxidation processes. They act as sources and sinks for HOx radicals (OH + HO2), with OH as the primary oxidant that governs the atmospheric self-cleaning capacity. Measurements of these species allow for evaluation of chemistry-transport models which need to account for multifarious source distributions, transport, complex photochemical reaction pathways and deposition processes of these species. HCHO is an intermediate during the oxidation of volatile organic compounds (VOCs) and is an indicator of photochemical activity and combustion-related emissions. In this study, we use in situ observations of HCHO, H2O2 and ROOH in the marine boundary layer (MBL) to evaluate results of the general circulation model EMAC (ECHAM5/MESSy2 Atmospheric Chemistry; European Center HAMburg, Modular Earth Submodel System). The dataset was obtained during the Air Quality and Climate Change in the Arabian Basin (AQABA) ship campaign around the Arabian Peninsula in summer 2017. This region is characterized by high levels of photochemical air pollution, humidity and solar irradiation, especially in the areas around the Suez Canal and the Arabian Gulf. High levels of air pollution with up to 12 ppbv HCHO, 2.3 ppbv ROOH and relatively low levels of H2O2 (≤0.5 ppbv) were detected over the Arabian Gulf. We find that EMAC failed to predict absolute mixing ratios of HCHO and ROOH during high-pollution events over the Arabian Gulf, while it reproduced HCHO on average within a factor of 2. Dry deposition velocities were determined for HCHO and H2O2 at 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. The photochemical budget of H2O2 revealed elevated HOx radical concentrations in EMAC, which resulted in an overestimation of H2O2 by more than a factor of 5 for the AQABA dataset. The underestimated air pollution over the Arabian Gulf was related to EMAC's coarse spatial resolution and missing anthropogenic emissions in the model.
Abstract. Measurements of hydroperoxides help improve the understanding of atmospheric oxidation processes. We introduce an instrumental setup designed for airborne hydroperoxide measurements. The instrument has been deployed on the German High-Altitude and Long-range Observatory (HALO) aircraft. The HYdrogen Peroxide and Higher Organic Peroxides (HYPHOP) monitor based on dual-enzyme fluorescence spectroscopy enables measurements up to ambient pressure of approximately 150 hPa pressure altitude (13.5–14 km). The main goal of this work is the characterization of the measurement method and data acquisition with special emphasis on potential interferences impacting instrumental uncertainty. Physically driven interferences were examined based on a dedicated test flight to investigate potential measurement inconsistencies arising from the dynamic movement patterns of the aircraft. During the test flight, the hydroperoxide monitor was operated in the background air sampling mode with purified air by scrubbing atmospheric trace gases, to investigate the instrumental stability and potential parameters that might affect the measurements. We show that technical and physical challenges during flight maneuvers do not critically impact the instrumental performance and the absolute measurements of hydroperoxide levels. Dynamic processes such as convective transport in the South Atlantic Convergence Zone (SACZ) are well-resolved as shown in the overview of a recent measurement campaign, Chemistry of the Atmosphere Field Experiment in Brazil in December 2022–January 2023 (CAFE-Brazil). The instrument precision based on the measurement results during CAFE-Brazil for hydrogen peroxide and the sum of organic hydroperoxides is estimated to be 6.4 % (at 5.7 ppbv) and 3.6 % (at 5.8 ppbv), respectively, and the corresponding detection limits 20 pptv and 19 pptv for a data acquisition frequency of 1 Hz, subsequently integrated over 120 second time intervals.
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