Abstract. During the DOMINO (Diel Oxidant MechanismIn relation to Nitrogen Oxides) campaign in southwest Spain we measured simultaneously all quantities necessary to calculate a photostationary state for HONO in the gas phase. These quantities comprise the concentrations of OH, NO, and HONO and the photolysis frequency of NO 2 , j (NO 2 ) as a proxy for j (HONO). This allowed us to calculate values of the unknown HONO daytime source. This unknown HONO source, normalized by NO 2 mixing ratios and expressed as a conversion frequency (% h −1 ), showed a clear dependence on j (NO 2 ) with values up to 43 % h −1 at noon. We compared our unknown HONO source with values calculated from the measured field data for two recently proposed processes, the light-induced NO 2 conversion on soot surfaces and the reaction of electronically excited NO 2 * with water vapour, with the result that these two reactions normally contributed less than 10 % (<1 % NO 2 + soot + hν; and <10 % NO 2 * + H 2 O) to our unknown HONO daytime source. OH production from HONO photolysis was found to be larger (by 20 %) than the "classical" OH formation from ozone photolysis (O( 1 D)) integrated over the day.Correspondence to: M. Sörgel

We present version 3.0 of the atmospheric chemistry box model CAABA/MECCA. In addition to a complete update of the rate coefficients to the most recent recommendations, a number of new features have been added: chemistry in multiple aerosol size bins; automatic multiple simulations reaching steady-state conditions; Monte-Carlo simulations with randomly varied rate coefficients within their experimental uncertainties; calculations along Lagrangian trajectories; mercury chemistry; more detailed isoprene chemistry; tagging of isotopically labeled species. Further changes have been implemented to make the code more user-friendly and to facilitate the analysis of the model results. Like earlier versions, CAABA/MECCA-3.0 is a community model published under the GNU General Public License

Abstract. Atmospheric measurements of hydroxyl radicals (OH) are challenging due to a high reactivity and consequently low concentration. The importance of OH as an atmospheric oxidant has motivated a sustained effort leading to the development of a number of highly sensitive analytical techniques. Recent work has indicated that the laser-induced fluorescence of the OH molecules method based on the fluorescence assay by gas expansion technique (LIF-FAGE) for the measurement of atmospheric OH in some environments may be influenced by artificial OH generated within the instrument, and a chemical method to remove this interference was implemented in a LIF-FAGE system by . While it is not clear whether other LIF-FAGE instruments suffer from the same interference, we have applied this method to our LIF-FAGE HORUS (Hydroxyl Radical Measurement Unit based on fluorescence Spectroscopy) system, and developed and deployed an inlet pre-injector (IPI) to determine the chemical zero level in the instrument via scavenging the ambient OH radical.We describe and characterise this technique in addition to its application at field sites in forested locations in Finland, Spain and Germany. Ambient measurements show that OH generated within the HORUS instrument is a non-negligible fraction of the total OH signal, which can comprise 30 to 80 % during daytime and 60 to 100 % during the night. The contribution of the background OH varied greatly between measurement sites and was likely related to the type and concentration of volatile organic compounds (VOCs) present at each particular location. Two inter-comparisons in contrasting environments between the HORUS instrument and two different chemical ionisation mass spectrometers (CIMS) are described to demonstrate the efficacy of IPI and the necessity of the chemical zeroing method for our LIF-FAGE instrument in such environments.

Abstract. Measurements of OH and HO2 radicals were conducted in a pine-dominated forest in southern Finland during the HUMPPA-COPEC-2010 (Hyytiälä United Measurements of Photochemistry and Particles in Air – Comprehensive Organic Precursor Emission and Concentration study) field campaign in summer 2010. Simultaneous side-by-side measurements of hydroxyl radicals were conducted with two instruments using chemical ionization mass spectrometry (CIMS) and laser-induced fluorescence (LIF), indicating small systematic disagreement, OHLIF / OHCIMS = (1.31 ± 0.14). Subsequently, the LIF instrument was moved to the top of a 20 m tower, just above the canopy, to investigate the radical chemistry at the ecosystem–atmosphere interface. Comprehensive measurements including observations of many volatile organic compounds (VOCs) and the total OH reactivity were conducted and analysed using steady-state calculations as well as an observationally constrained box model. Production rates of OH calculated from measured OH precursors are consistent with those derived from the steady-state assumption and measured total OH loss under conditions of moderate OH reactivity. The primary photolytic sources of OH contribute up to one-third to the total OH production. OH recycling, which occurs mainly by HO2 reacting with NO and O3, dominates the total hydroxyl radical production in this boreal forest. Box model simulations agree with measurements for hydroxyl radicals (OHmod. / OHobs. = 1.00 ± 0.16), while HO2 mixing ratios are significantly under-predicted (HO2mod. / HO2obs. = 0.3 ± 0.2), and simulated OH reactivity does not match the observed OH reactivity. The simultaneous under-prediction of HO2 and OH reactivity in periods in which OH concentrations were simulated realistically suggests that the missing OH reactivity is an unaccounted-for source of HO2. Detailed analysis of the HOx production, loss, and recycling pathways suggests that in periods of high total OH reactivity there are additional recycling processes forming OH directly, not via reaction of HO2 with NO or O3, or unaccounted-for primary HOx sources. Under conditions of moderate observed OH reactivity and high actinic flux, an additional RO2 source of approximately 1 × 106 molec cm−3 s−1 would be required to close the radical budget. Nevertheless, a major fraction of the OH recycling occurs via the reaction of HO2 with NO and O3 in this terpene-dominated environment.

Abstract. In this study we report measurements of hydrogen peroxide (H 2 O 2 ), methyl hydroperoxide* (MHP* as a proxy of MHP based on an unspecific measurement of total organic peroxides) and formaldehyde (HCHO) from the HO x OVer EuRope (HOOVER) project (HO x = OH+HO 2 ). HOOVER included two airborne field campaigns, in October 2006 and July 2007. Measurement flights were conducted from the base of operation Hohn (Germany, 54 • N, 9 • E) towards the Mediterranean and to the subpolar regions over Norway. We find negative concentration gradients with increasing latitude throughout the troposphere for H 2 O 2 and CH 3 OOH * . In contrast, observed HCHO is almost homogeneously distributed over central and northern Europe and is elevated over the Mediterranean. In general, the measured gradients tend to be steepest entering the Mediterranean region, where we also find the highest abundances of the 3 species. Mixing ratios of these tracers generally decrease with altitude. H 2 O 2 and CH 3 OOH * show maxima above the boundary layer at 2-5 km, being more distinct over southern than over northern Europe.We also present a comparison of our data with simulations by two global 3-D-models, MATCH-MPIC and EMAC, and with the box model CAABA. The models realistically represent altitude and latitude gradients for both HCHO and hydroperoxides (ROOH). In contrast, the models have problems reproducing the absolute mixing ratios, in particular of H 2 O 2 . Large uncertainties about retention coefficients and Correspondence to: H. Fischer (horst.fischer@mpic.de) cloud microphysical parameters suggest that cloud scavenging might be a large source of error for the simulation of H 2 O 2 . A sensitivity study with EMAC shows a strong influence of cloud and precipitation scavenging on the budget of H 2 O 2 as simulations improve significantly with this effect switched off.