Abstract. Strongly enhanced tropospheric ozone (O3) mixing ratios have been reported in the Arabian Basin, a region with intense solar radiation and high concentrations of O3 precursors such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). To analyze photochemical O3 production in the marine boundary layer (MBL) around the Arabian Peninsula, we use shipborne observations of NO, NO2, O3, OH, HO2, HCHO, the actinic flux, water vapor, pressure and temperature obtained during the summer 2017 Air Quality and Climate in the Arabian Basin (AQABA) campaign, and we compare them to simulation results from the ECHAM-MESSy Atmospheric Chemistry (EMAC) general circulation model. Net O3 production rates (NOPRs) were greatest over both the Gulf of Oman and the northern Red Sea (16 ppbv d−1) and over the Arabian Gulf (32 ppbv d−1). The NOPR over the Mediterranean, the southern Red Sea and the Arabian Sea did not significantly deviate from zero; however, the results for the Arabian Sea indicated weak net O3 production of 5 ppbv d−1 as well as net O3 destruction over the Mediterranean and the southern Red Sea with values of −1 and −4 ppbv d−1, respectively. Constrained by HCHO∕NO2 ratios, our photochemistry calculations show that net O3 production in the MBL around the Arabian Peninsula mostly occurs in NOx-limited regimes with a significant share of O3 production occurring in the transition regime between NOx limitation and VOC limitation over the Mediterranean and more significantly over the northern Red Sea and Oman Gulf.

Abstract. The Arabian Peninsula is characterized by high and increasing levels of photochemical air pollution. Strong solar irradiation, high temperatures and large anthropogenic emissions of reactive trace gases result in intense photochemical activity, especially during the summer months. However, air chemistry measurements in the region are scarce. In order to assess regional pollution sources and oxidation rates, the first ship-based direct measurements of total OH reactivity were performed in summer 2017 from a vessel traveling around the peninsula during the AQABA (Air Quality and Climate Change in the Arabian Basin) campaign. Total OH reactivity is the total loss frequency of OH radicals due to all reactive compounds present in air and defines the local lifetime of OH, the most important oxidant in the troposphere. During the AQABA campaign, the total OH reactivity ranged from below the detection limit (5.4 s−1) over the northwestern Indian Ocean (Arabian Sea) to a maximum of 32.8±9.6 s−1 over the Arabian Gulf (also known as Persian Gulf) when air originated from large petroleum extraction/processing facilities in Iraq and Kuwait. In the polluted marine regions, OH reactivity was broadly comparable to highly populated urban centers in intensity and composition. The permanent influence of heavy maritime traffic over the seaways of the Red Sea, Gulf of Aden and Gulf of Oman resulted in median OH sinks of 7.9–8.5 s−1. Due to the rapid oxidation of direct volatile organic compound (VOC) emissions, oxygenated volatile organic compounds (OVOCs) were observed to be the main contributor to OH reactivity around the Arabian Peninsula (9 %–35 % by region). Over the Arabian Gulf, alkanes and alkenes from the petroleum extraction and processing industry were an important OH sink with ∼9 % of total OH reactivity each, whereas NOx and aromatic hydrocarbons (∼10 % each) played a larger role in the Suez Canal, which is influenced more by ship traffic and urban emissions. We investigated the number and identity of chemical species necessary to explain the total OH sink. Taking into account ∼100 individually measured chemical species, the observed total OH reactivity can typically be accounted for within the measurement uncertainty (50 %), with 10 dominant trace gases accounting for 20 %–39 % of regional total OH reactivity. The chemical regimes causing the intense ozone pollution around the Arabian Peninsula were investigated using total OH reactivity measurements. Ozone vs. OH reactivity relationships were found to be a useful tool for differentiating between ozone titration in fresh emissions and photochemically aged air masses. Our results show that the ratio of NOx- and VOC-attributed OH reactivity was favorable for ozone formation almost all around the Arabian Peninsula, which is due to NOx and VOCs from ship exhausts and, often, oil/gas production. Therewith, total OH reactivity measurements help to elucidate the chemical processes underlying the extreme tropospheric ozone concentrations observed in summer over the Arabian Basin.

and sub-ppb v levels. In particular, the use of tunable infrared lasers has found widespread application during the past four decades. While instruments in the 1980s and 1990s deployed lead-chalcogenide tunable diode lasers [1][2][3][4], since about the year 2000 tunable quantum cascade lasers have mostly been used due to their superior behavior with respect to laser power, single mode operation and stability [5][6][7]. In particular, the deployment of Tunable Diode Laser Absorption Spectroscopy (TDLAS) or Quantum cascade Laser Absorption Spectroscopy (QLAS) instruments on research aircraft require compact, low weight and rigid designs to face the challenging demands due to vibrations, as well as cabin pressure and temperature fluctuations on the platform [5,6,[8][9][10][11][12]. Since 1997, we have deployed the multi-laser TRacer In-Situ Tdlas for Atmospheric Research (TRISTAR) on a number of airborne platforms during a total of 15 measurement campaigns, on 170 research flights with more than 800 flight hours for tropospheric and stratospheric measurements of carbon monoxide (CO), carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O) and formaldehyde (HCHO) [13,14]. In the present configuration TRISTAR deploys three liquid nitrogen cooled continuous wave quantum cascade lasers for CO, CH 4 and HCHO. As mentioned above, the measurement precision during airborne applications suffers from a non-ideal environment. In particular, changes of the cabin temperature and the cabin pressure can affect the optical alignment and the operation conditions of lasers and infra-red detectors. For species with mixing ratios in the ppb v to ppm v range like CO (typical tropospheric mixing ratios of 80 ppb v ) and CH 4 (~2 ppm v ) the absorptions correspond to optical densities in the 10 −2 range and the reported in-flight precisions, based on the reproducibility of in-flight calibrations, are in the sub per cent range [14], similar to results obtained with other airborne QCL spectrometers [15,16]. For HCHO, whose tropospheric mixing Abstract Airborne carbon monoxide (CO) measurements based on Quantum cascade Laser infrared Absorption Spectroscopy (QLAS) were performed on the German High-Altitude Long-range Observatory (HALO) aircraft during test flights in January 2015. Here we investigate the in-flight stability of TRISTAR (TRacer In-Situ Tdlas for Atmospheric Research), a multilaser QLAS instrument for the detection of tropospheric CO, methane and formaldehyde (HCHO). During one test flight the instrument was probed with tank air to measure a constant mixing ratio of CO and zero air for HCHO. Here we investigate the instrument stability for the CO channel of TRISTAR and identify potential noise sources as well as environmental processes that limit the stability of the instrument. The 1σ reproducibility of the constant CO measurement yields a value of 1.2% (2.9 ppb v ) corresponding to an optical density limit of 0.001 for a 5-s average. The CO precision is ultimately limited by an etalon fringe originating from the doub...

Non-methane hydrocarbons (NMHCs) such as ethane and propane are significant atmospheric pollutants and precursors of tropospheric ozone, while the Middle East is a global emission hotspot due to extensive oil and gas production. Here we compare in situ hydrocarbon measurements, performed around the Arabian Peninsula, with global model simulations that include current emission inventories (EDGAR) and state-of-the-art atmospheric circulation and chemistry mechanisms (EMAC model). While measurements of high mixing ratios over the Arabian Gulf are adequately simulated, strong underprediction by the model was found over the northern Red Sea. By examining the individual sources in the model and by utilizing air mass back-trajectory investigations and Positive Matrix Factorization (PMF) analysis, we deduce that Red Sea Deep Water (RSDW) is an unexpected, potent source of atmospheric NMHCs. This overlooked underwater source is comparable with total anthropogenic emissions from entire Middle Eastern countries, and significantly impacts the regional atmospheric chemistry.

Abstract. A total of 252 emission plumes of ships operating in the Mediterranean Sea and around the Arabian Peninsula were investigated using a comprehensive dataset of gas- and submicron-particle-phase properties measured during the 2-month shipborne AQABA (Air Quality and Climate Change in the Arabian Basin) field campaign in summer 2017. The post-measurement identification of the corresponding ship emission events in the measured data included the determination of the plume sources (up to 38 km away) as well as the plume ages (up to 115 min) and was based on commercially available historical records of the Automatic Identification System. The dispersion lifetime of chemically inert CO2 in the ship emission plumes was determined as 70±15 min, resulting in levels indistinguishable from the marine background after 260±60 min. Emission factors (EFs) as quantities that are independent of plume dilution were calculated and used for the investigation of influences on ship emission plumes caused by ship characteristics and the combustion process as well as by atmospheric processes during the early stage of exhaust release and during plume ageing. Combustion efficiency and therefore emission factors of black carbon and NOx were identified to depend mostly on the vessel speed and gross tonnage. Moreover, larger ships, associated with higher engine power, were found to use fuel with higher sulfur content and have higher gas-phase SO2, particulate sulfate, particulate organics, and particulate matter EFs. Despite the independence of EFs of dilution, a significant influence of the ambient wind speed on the particle number and mass EFs was observed that can be traced back to enhanced particle coagulation in the case of slower dilution and suppressed vapour condensation on particles in the case of faster dilution of the emission plume. Atmospheric reactions and processes in ship emission plumes were investigated that include NOx and O3 chemistry, gas-to-particle conversion of NOx and SO2, and the neutralisation of acids in the particle phase through the uptake of ambient gas-phase ammonia, the latter two of which cause the inorganic particulate content to increase and the organic fraction to decrease with increasing plume age. The results allow for us to describe the influences on (or processes in) ship emission plumes quantitatively by parameterisations, which could be used for further refinement of atmospheric models, and to identify which of these processes are the most important ones.

During spring 2020, the COVID-19 pandemic caused massive reductions in emissions from industry, ground and airborne transportation. To explore the resulting atmospheric composition changes, we conducted the BLUESKY campaign with two research aircraft and measured trace gases, aerosols, and cloud properties from the boundary layer to the lower stratosphere. From 16 May to 9 June 2020, we performed 20 flights in the early COVID-19 lockdown phase over Europe and the Atlantic Ocean. We found up to 50% reductions in boundary layer nitrogen dioxide concentrations in urban areas from GOME-2B satellite data, along with carbon monoxide reductions in the pollution hot spots. We measured 20 to 70% reductions in total reactive nitrogen, carbon monoxide and fine mode aerosol concentration in profiles over German cities compared to a 10-year data set from passenger aircraft. The total aerosol mass was significantly reduced below 5 km altitude, and the organic aerosol fraction also aloft, indicative of decreased organic precursor gas emissions. The reduced aerosol optical thickness caused a perceptible shift in sky color towards the blue part of the spectrum (hence BLUESKY) and increased shortwave radiation at the surface. We find that the 80% decline in air traffic led to substantial reductions in nitrogen oxides at cruise altitudes, in contrail cover, and in resulting radiative forcing. The light extinction and depolarization by cirrus were also reduced in regions with substantially decreased air traffic. General circulation-chemistry model simulations indicate good agreement with the measurements when applying a reduced emission scenario. The comprehensive BLUESKY dataset documents the major impact of anthropogenic emissions on the atmospheric composition.

Abstract. Shipborne measurements of nitryl chloride (ClNO2), hydrogen chloride (HCl) and sulfur dioxide (SO2) were made during the AQABA (Air Quality and climate change in the Arabian BAsin) ship campaign in summer 2017. The dataset includes measurements over the Mediterranean Sea, the Suez Canal, the Red Sea, the Gulf of Aden, the Arabian Sea, the Gulf of Oman, and the Arabian Gulf (also known as Persian Gulf) with observed ClNO2 mixing ratios ranging from the limit of detection to ≈600 pptv. We examined the regional variability in the generation of ClNO2 via the uptake of dinitrogen pentoxide (N2O5) to Cl-containing aerosol and its importance for Cl atom generation in a marine boundary layer under the (variable) influence of emissions from shipping and the oil industry. The yield of ClNO2 formation per NO3 radical generated was generally low (median of ≈1 %–5 % depending on the region), mainly as a result of gas-phase loss of NO3 dominating over heterogeneous loss of N2O5, the latter being disfavoured by the high temperatures found throughout the campaign. The contributions of ClNO2 photolysis and OH-induced HCl oxidation to Cl-radical formation were derived and their relative contributions over the diel cycle compared. The results indicate that over the northern Red Sea, the Gulf of Suez, and the Gulf of Oman the formation of Cl atoms will enhance the oxidation rates of some volatile organic compounds (VOCs), especially in the early morning.

Abstract. We present a newly constructed, two-channel thermal dissociation cavity ring-down spectrometer (TD-CRDS) for the measurement of NOx (NO+NO2), NOy (NOx+HNO3+RO2NO2+2N2O5 etc.), NOz (NOy−NOx) and particulate nitrate (pNit). NOy-containing trace gases are detected as NO2 by the CRDS at 405 nm following sampling through inlets at ambient temperature (NOx) or at 850 ∘C (NOy). In both cases, O3 was added to the air sample directly upstream of the cavities to convert NO (either ambient or formed in the 850 ∘C oven) to NO2. An activated carbon denuder was used to remove gas-phase components of NOy when sampling pNit. Detection limits, defined as the 2σ precision for 1 min averaging, are 40 pptv for both NOx and NOy. The total measurement uncertainties (at 50 % relative humidity, RH) in the NOx and NOy channels are 11 %+10 pptv and 16 %+14 pptv for NOz respectively. Thermograms of various trace gases of the NOz family confirm stoichiometric conversion to NO2 (and/or NO) at the oven temperature and rule out significant interferences from NH3 detection (<2 %) or radical recombination reactions under ambient conditions. While fulfilling the requirement of high particle transmission (>80 % between 30 and 400 nm) and essentially complete removal of reactive nitrogen under dry conditions (>99 %), the denuder suffered from NOx breakthrough and memory effects (i.e. release of stored NOy) under humid conditions, which may potentially bias measurements of particle nitrate. Summertime NOx measurements obtained from a ship sailing through the Red Sea, Indian Ocean and Arabian Gulf (NOx levels from <20 pptv to 25 ppbv) were in excellent agreement with those taken by a chemiluminescence detector of NO and NO2. A data set obtained locally under vastly different conditions (urban location in winter) revealed large diel variations in the NOz to NOy ratio which could be attributed to the impact of local emissions by road traffic.