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We present the first ambient measurements of a new marine emission methane sulfonamide (MSAM: CH5NO2S), along with dimethyl sulfide (DMS) and dimethyl sulfone (DMSO2) over the Arabian Sea. Two shipborne transects (W → E, E → W) were made during the AQABA (Air Quality and Climate Change in the Arabian Basin) measurement campaign. Molar mixing ratios in picomole of species per mole of air (throughout this paper abbreviated as ppt) of DMS were in the range of 300–500 ppt during the first traverse of the Arabian Sea (first leg) and 100–300 ppt on the second leg. On the first leg DMSO2 was always below 40 ppt and MSAM was close to the limit of detection. During the second leg DMSO2 was between 40 and 120 ppt and MSAM was mostly in the range of 20–50 ppt with maximum values of 60 ppt. An analysis of HYSPLIT back trajectories combined with calculations of the exposure of these trajectories to underlying chlorophyll in the surface water revealed that most MSAM originates from the Somalia upwelling region, known for its high biological activity. MSAM emissions can be as high as one-third of DMS emissions over the upwelling region. This new marine emission is of particular interest as it contains both sulfur and nitrogen, making it potentially relevant to marine nutrient cycling and marine atmospheric particle formation.

Hydroxyl (OH) radical reactivity (kOH) has been measured for 18 years with different measurement techniques. In order to compare the performances of instruments deployed in the field, two campaigns were conducted performing experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich in October 2015 and April 2016. Chemical conditions were chosen either to be representative of the atmosphere or to test potential limitations of instruments. All types of instruments that are currently used for atmospheric measurements were used in one of the two campaigns. The results of these campaigns demonstrate that OH reactivity can be accurately measured for a wide range of atmospherically relevant chemical conditions (e.g. water vapour, nitrogen oxides, various organic compounds) by all instruments. The precision of the measurements (limit of detection −1 at a time resolution of 30 s to a few minutes) is higher for instruments directly detecting hydroxyl radicals, whereas the indirect comparative reactivity method (CRM) has a higher limit of detection of 2 s−1 at a time resolution of 10 to 15 min. The performances of the instruments were systematically tested by stepwise increasing, for example, the concentrations of carbon monoxide (CO), water vapour or nitric oxide (NO). In further experiments, mixtures of organic reactants were injected into the chamber to simulate urban and forested environments. Overall, the results show that the instruments are capable of measuring OH reactivity in the presence of CO, alkanes, alkenes and aromatic compounds. The transmission efficiency in Teflon inlet lines could have introduced systematic errors in measurements for low-volatile organic compounds in some instruments. CRM instruments exhibited a larger scatter in the data compared to the other instruments. The largest differences to reference measurements or to calculated reactivity were observed by CRM instruments in the presence of terpenes and oxygenated organic compounds (mixing ratio of OH reactants were up to 10 ppbv). In some of these experiments, only a small fraction of the reactivity is detected. The accuracy of CRM measurements is most likely limited by the corrections that need to be applied to account for known effects of, for example, deviations from pseudo first-order conditions, nitrogen oxides or water vapour on the measurement. Methods used to derive these corrections vary among the different CRM instruments. Measurements taken with a flow-tube instrument combined with the direct detection of OH by chemical ionisation mass spectrometry (CIMS) show limitations in cases of high reactivity and high NO concentrations but were accurate for low reactivity (−1) and low NO (

Since the start of industrialization and increasingly since the 1950s, anthropogenic activities have altered the Earth’s atmospheric composition significantly with consequences for climate, weather, and the health of both humans and ecosystems. Reactive trace gases are, on the one hand, part of the anthropogenic emissions that fuel air pollution and impact climate, and on the other hand, are impacted by the manmade changes in environmental conditions in a feedback loop. A way to quantify the total atmospheric load of reactive trace gases is the measurement of total OH reactivity, i.e. the loss rate of the most important tropospheric oxidant, the hydroxyl (OH) radical. In this doctoral project, total OH reactivity measurements were used to investigate two points in the feedback loop of human activity and atmospheric reactants: Firstly, the indirect impact of anthropogenic climate change and deforestation, which will lead to an increasing frequency of drought and heat events in the Amazon rainforest, is thought to influence biogenic trace gas emissions. This was investigated using total OH reactivity observations during an extreme El Niño event. Secondly, direct human impact above the seaways around the Arabian Peninsula, detectable by anthropogenically emitted trace gases from ships and oil/gas production, was studied with total OH reactivity observations and a regional ozone formation assessment. The method-oriented part of this doctoral project was based on the need for robust, accurate long-term observations of total OH reactivity for understanding atmospheric photochemistry in the Anthropocene epoch. During the drought and heat conditions of the extreme 2015/16 El Niño event, the diel cycle of total OH reactivity in the Amazon rainforest exhibited a striking difference to "normal" diel behavior. After the usual early afternoon OH reactivity maximum, a second, higher peak was observed during the sunset hours. A possible explanation for the increased sunset reactivity was found in stronger turbulent transport inside and above the canopy related with the changed meteorological conditions, combined with a stress-related release of monoterpenes and other (unmeasured) BVOCs by vegetation. Total OH reactivity measured around the Arabian Peninsula was comparable to highly populated urban areas, due to a combination of shipping emissions and petrochemical pollution. The extreme regional ozone concentrations could be explained by a favorable mixture of NOx and VOCs coupled with intense solar irradiation, causing rapid photochemical reactions. A new Comparative Reactivity Method (CRM) instrument for long-term autonomous measurements of total OH reactivity was successfully characterized and tested in Helsinki. Interferences were quantified and compared to a model of the CRM reactor chemistry. The total OH reactivity observed in winter in Helsinki was with an overall median of 7.6 s^−1 at the lower end of worldwide urban observations. In the first comprehensive intercomparison of OH reactivity measurements, the CRM method was compared to all other available instrument types by simultaneous measurements at an atmospheric simulation chamber. Results showed that the CRM device is suited for a range of atmospheric mixtures. However, significant deviations were seen under terpene-dominated and high NO conditions. A sensitivity towards ozone, which also impacted the size of the NO2 interference, was newly discovered for CRM. Photolysis inside the reactor and the HO2 concurrently produced with OH were identified as major sources of interferences and uncertainties. With the aim of improving the method, laboratory studies were conducted in the aftermath of the intercomparison, focusing on reducing photolysis and increasing CRM sensitivity.