Abstract. The Master Chemical Mechanism has been updated from MCMv3 to MCMv3.1 in order to take into account recent improvements in the understanding of aromatic photo-oxidation. Newly available kinetic and product data from the literature has been incorporated into the mechanism. In particular, the degradation mechanisms for hydroxyarenes have been revised following the observation of high yields of ring-retained products, and product studies of aromatic oxidation under relatively low NOx conditions have provided new information on the branching ratios to first generation products. Experiments have been carried out at the European Photoreactor (EUPHORE) to investigate key subsets of the toluene system. These results have been used to test our understanding of toluene oxidation, and where possible, refine the degradation mechanisms. The evaluation of MCMv3 and MCMv3.1 using data on benzene, toluene, p-xylene and 1,3,5-trimethylbenzene photosmog systems is described in a companion paper, and significant model shortcomings are identified. Ideas for additional modifications to the mechanisms, and for future experiments to further our knowledge of the details of aromatic photo-oxidation are discussed.

This paper presents the summary of the key objectives, instrumentation and logistic details, goals, and initial scientific findings of the Marie Curie Action FP7-EU SAPUSS project carried out in the Western Mediterranean Basin (WMB) from 20 September–20 October 2010. The experiment involved concurrent measurements of aerosols with multiple techniques occurring simultaneously. The key objective is to deduce point aerosol source characteristics and to understand the atmospheric processes responsible for their generations and transformations. The unique approach is the large variety of instrumentation deployed simultaneously in six monitoring sites in Barcelona (NE Spain) and around the city, including: a main road traffic site, two urban background sites, a regional background site and two tower sites (150 m and 545 m a.s.l., 150 m and 80 m above ground, respectively). The SAPUSS experiment allows us to interpret the variability of aerosols levels and composition in an Urban Mediterranean, an environment not well characterized so far. During SAPUSS different air mass scenarios were encountered, including warm Saharan, cold Atlantic, wet European and stagnant Regional ones and presenting different local meteorology and boundary layer conditions. Analysis of part of the data collected allows us to compare the monitoring sites as well as to draw scientific conclusions about relevant air quality parameters. High levels of traffic-related gaseous pollutants were measured at the urban ground level monitoring sites, whereas layers of tropospheric ozone were recorded at tower levels. Particularly, tower level night time average ozone concentrations (80 ± 25 &mu;g m^&minus;3) were up to double than ground level ones. Particle number concentrations (<i>N</i>_>5: 9980 ± 6500 cm^&minus;1, average of all measurements) were generally traffic dependent, although a contribution from two different types of nucleation events was also found. Analysis of the particulate matter (PM) mass concentrations shows an enhancement of coarse particles (PM_2.5-10) at the urban ground level (+64%, average 11.7 &mu;g m^&minus;3) but of fine ones (PM₁) at urban tower level (+28%, average 14.4 &mu;g m^&minus;3). Preliminary modeling findings reveal an underestimation of the fine accumulation aerosols. In summary, this paper lays the foundation of SAPUSS, an integrated study of relevance to many other similar urban Mediterranean coastal environment sites

Abstract. Secondary organic aerosol (SOA) affects atmospheric composition, air quality and radiative transfer. However major difficulties are encountered in the development of reliable models for SOA formation. Constraints on processes involved in SOA formation can be obtained by interpreting the speciation and evolution of organics in the gaseous and condensed phase simultaneously. In this study we investigate SOA formation from dark α-pinene ozonolysis with particular emphasis upon the mass distribution of gaseous and particulate organic species. A detailed model for SOA formation is compared with the results from experiments performed in the EUropean PHOtoREactor (EUPHORE) simulation chamber, including on-line gas-phase composition obtained from Chemical-Ionization-Reaction Time-Of-Flight Mass-Spectrometry measurements, and off-line analysis of SOA samples performed by Electrospray Ionisation Ion Trap Mass Spectrometry. The temporal profile of SOA mass concentration is relatively well reproduced by the model. Sensitivity analysis highlights the importance of the choice of vapour pressure estimation method. Comparisons of the simulated gaseous- and condensed-phase mass distributions with those observed show a generally good agreement. The simulated speciation has been used to (i) propose a chemical structure for the principal gaseous semi-volatile organic compounds and condensed monomer organic species and (ii) explore the possible contribution of a range of accretion reactions occurring in the condensed phase. We find that oligomer formation through esterification reactions gives the best agreement between the observed and simulated mass spectra.

In the UK on 5 November, Guy Fawkes Night is celebrated with bonfires and fireworks. An undesirable consequence of these activities is a statistically significant reduction (~25%) in atmospheric visibility nationwide. This reduction is caused by increased loading of atmospheric particulate matter generated by bonfires and fireworks. The effect of this increased loading on visibility is investigated in greater detail for the city of Nottingham, where larger visibility decreases compared to the national average are observed. Visibility reduction is more significant when the background particulate matter loading and/or the atmospheric relative humidity are high.

The Regents Park and Tower Environmental Experiment (REPARTEE) comprised two campaigns in London in October 2006 and October/November 2007. The experiment design involved measurements at a heavily trafficked roadside site, two urban background sites and an elevated site at 160–190 m above ground on the BT Tower, supplemented in the second campaign by Doppler lidar measurements of atmospheric vertical structure. A wide range of measurements of airborne particle physical metrics and chemical composition were made as well as measurements of a considerable range of gas phase species and the fluxes of both particulate and gas phase substances. Significant findings include (a) demonstration of the evaporation of traffic-generated nanoparticles during both horizontal and vertical atmospheric transport; (b) generation of a large base of information on the fluxes of nanoparticles, accumulation mode particles and specific chemical components of the aerosol and a range of gas phase species, as well as the elucidation of key processes and comparison with emissions inventories; (c) quantification of vertical gradients in selected aerosol and trace gas species which has demonstrated the important role of regional transport in influencing concentrations of sulphate, nitrate and secondary organic compounds within the atmosphere of London; (d) generation of new data on the atmospheric structure and turbulence above London, including the estimation of mixed layer depths; (e) provision of new data on trace gas dispersion in the urban atmosphere through the release of purposeful tracers; (f) the determination of spatial differences in aerosol particle size distributions and their interpretation in terms of sources and physico-chemical transformations; (g) studies of the nocturnal oxidation of nitrogen oxides and of the diurnal behaviour of nitrate aerosol in the urban atmosphere, and (h) new information on the chemical composition and source apportionment of particulate matter size fractions in the atmosphere of London derived both from bulk chemical analysis and aerosol mass spectrometry with two instrument types