Abstract. Alkyl nitrates and secondary organic aerosol (SOA) produced during the oxidation of isoprene by nitrate radicals has been observed in the SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) chamber. A 16 h dark experiment was conducted with temperatures at 289-301 K, and maximum concentrations of 11 ppb isoprene, 62.4 ppb O 3 and 31.1 ppb NO x . We find the yield of nitrates is 70±8% from the isoprene + NO 3 reaction, and the yield for secondary dinitrates produced in the reaction of primary isoprene nitrates with NO 3 is 40±20%. We find an effective rate constant for reaction of NO 3 with the group of first generation oxidation products to be 7×10 −14 molecule −1 cm 3 s −1 . At the low total organic aerosol concentration in the chamber (max=0.52 µg m −3 ) we observed a mass yield ( SOA mass/ isoprene mass) of 2% for the entire 16 h experiment. However a comparison of the timing of the observed SOA production to a box model simulation of first and second generation oxidation products shows that the yield from the first generation products was <0.7% while the further oxidation of the initial products leads to a yield of 14% (defined as SOA/ isoprene 2x where isoprene 2x is the mass of isoprene which reacted twice with NO 3 ). The SOA yield of 14% is consistent with equilibrium partitioning of highly functionalized C 5 products of isoprene oxidation.

Gaseous nitrous acid (HONO) is an important precursor of tropospheric hydroxyl radicals (OH). OH is responsible for atmospheric self-cleansing and controls the concentrations of greenhouse gases like methane and ozone. Due to lack of measurements, vertical distributions of HONO and its sources in the troposphere remain unclear. Here, we present a set of observations of HONO and its budget made onboard a Zeppelin airship. In a sunlit layer separated from Earth's surface processes by temperature inversion, we found high HONO concentrations providing evidence for a strong gas-phase source of HONO consuming nitrogen oxides and potentially hydrogen oxide radicals. The observed properties of this production process suggest that the generally assumed impact of HONO on the abundance of OH in the troposphere is substantially overestimated.

Most pollutants in the Earth's atmosphere are removed by oxidation with highly reactive hydroxyl radicals. Field measurements have revealed much higher concentrations of hydroxyl radicals than expected in regions with high loads of the biogenic volatile organic compound isoprene 1-8 . Different isoprene degradation mechanisms have been proposed to explain the high levels of hydroxyl radicals observed 5,9-11 . Whether one or more of these mechanisms actually operates in the natural environment, and the potential impact on climate and air quality, has remained uncertain 12-14 . Here, we present a complete set of measurements of hydroxyl and peroxy radicals collected during isoprene-oxidation experiments carried out in an atmospheric simulation chamber, under controlled atmospheric conditions. We detected significantly higher concentrations of hydroxyl radicals than expected based on model calculations, providing direct evidence for a strong hydroxyl radical enhancement due to the additional recycling of radicals in the presence of isoprene. Specifically, our findings are consistent with the unimolecular reactions of isoprenederived peroxy radicals postulated by quantum chemical calculations 9-11 . Our experiments suggest that more than half of the hydroxyl radicals consumed in isoprene-rich regions, such as forests, are recycled by these unimolecular reactions with isoprene. Although such recycling is not sufficient to explain the high concentrations of hydroxyl radicals observed in the field, we conclude that it contributes significantly to the oxidizing capacity of the atmosphere in isoprene-rich regions.Biogenic emissions by terrestrial vegetation are responsible for 90% of the volatile organic compounds (VOCs) in the atmosphere 15,16 . Isoprene (C 5 H 8 ) being the most abundant biogenic VOC has the highest emission rates in tropical forests owing to the light-and temperature-dependent emission process. The high isoprene emission rates coincide with high production rates of the main atmospheric oxidant, the hydroxyl radical (OH), which is mainly formed by photodissociation of ozone (O 3 ) and subsequent reaction of electronically excited oxygen atoms with water vapour. Isoprene reacts readily with OH (ref. 5). This process constitutes the main sink of atmospheric isoprene, and in turn influences the OH concentration, which determines the atmospheric lifetime of greenhouse gases (for example, methane) and pollutants (for example, CO).The atmospheric oxidation efficiency of OH is greatly enhanced by nitrogen monoxide (NO)-dependent radical chain reactions, which regenerate OH. The reaction of OH with isoprene, for example, forms hydroxy-peroxy isoprene radicals (RO 2 ), which can be converted to hydroperoxy radicals (HO 2 ) through their reaction with NO followed by the reaction of HO 2 with NO reforming OH ROOH IEPOX+OH +OH 17% 25% +HO 2 Isoprene+OH+ O 2 1 ,5 -H s h if t 29% <1% MVK, MACR +HCHO+OH y(OH): 0.29 /<0.01 1 ,6 -H s h if t +NO RO 2 41% 62% 13% 13% LIM-J LIM-CS MVK, MACR +HCHO+HO 2 y(OH): 0.29/0.43 HPAL...

Abstract. Emission of biogenic volatile organic compounds (VOC) which on oxidation form secondary organic aerosols (SOA) can couple the vegetation with the atmosphere and climate. Particle formation from tree emissions was investigated in a new setup: a plant chamber coupled to a reaction chamber for oxidizing the plant emissions and for forming SOA. Emissions from the boreal tree species birch, pine, and spruce were studied. In addition, α-pinene was used as reference compound. Under the employed experimental conditions, OH radicals were essential for inducing new particle formation, although O 3 (≤80 ppb) was always present and a fraction of the monoterpenes and the sesquiterpenes reacted with ozone before OH was generated. Formation rates of 3 nm particles were linearly related to the VOC carbon mixing ratios, as were the maximum observed volume and the condensational growth rates. For all trees, the threshold of new particle formation was lower than for α-pinene. It was lowest for birch which emitted the largest fraction of oxygenated VOC (OVOC), suggesting that OVOC may play a role in the nucleation process. Incremental mass yields were ≈5% for pine, spruce and α-pinene, and ≈10% for birch. α-Pinene was a good model compound to describe the yield and the growth of SOA particles from coniferous emissions. The mass fractional yields agreed well with observations for boreal forests. Despite the somewhat enhanced VOC and OH concentrations our results may be up-scaled to eco-system level. Using the mass fractional yields observed for the tree Correspondence to: Th. F. Mentel (t.mentel@fz-juelich.de) emissions and weighting them with the abundance of the respective trees in boreal forests SOA mass concentration calculations agree within 6% with field observations. For a future VOC increase of 50% we predict a particle mass increase due to SOA of 19% assuming today's mass contribution of pre-existing aerosol and oxidant levels.

Abstract. Hydroxyl (OH) radical reactivity (k OH ) 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 s −1 at a time resolution of 30 s to a few minutes) is higher for instruments directly detecting hydroxyl radicals, whereas the indirect comparative reactivPublished by Copernicus Publications on behalf of the European Geosciences Union. H. Fuchs et al.: OH reactivity comparison in SAPHIRity 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 flowtube 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 (< 15 s −1 ) and low NO (< 5 ppbv) conditions.