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Hydroxyl radicals play a central role in the troposphere as they control the lifetime of many trace gases. Measurement of OH reactivity (OH loss rate) is important to better constrain the OH budget and also to evaluate the completeness of measured VOC budget. Total atmospheric OH reactivity was measured for the first time in an European Megacity: Paris and its surrounding areas with 12 million inhabitants, during the MEGAPOLI winter campaign 2010. The method deployed was the Comparative Reactivity Method (CRM). The measured dataset contains both measured and calculated OH reactivity from CO, NO_x and VOCs measured via PTR-MS, GC-FID and GC-MS instruments. The reactivities observed in Paris covered a range from 10 s⁻¹ to 130 s⁻¹, indicating a large loading of chemical reactants. The present study showed that, when clean marine air masses influenced Paris, the purely local OH reactivity (20 s⁻¹) is well explained by the measured species. Nevertheless, when there is a continental import of air masses, high levels of OH reactivity were obtained (120–130 s⁻¹) and the missing OH reactivity measured in this case jumped to 75%. Using covariations of the missing OH reactivity to secondary inorganic species in fine aerosols, we suggest that the missing OH reactants were most likely highly oxidized compounds issued from photochemically processed air masses of anthropogenic origin

Abstract. Previous studies have provided some insight into the Saharan dust deposition at a few specific locations from observations over long time periods or intensive field campaigns. However, no assessment of the dust deposition temporal variability in connection with its regional spatial distribution has been achieved so far from network observations over more than 1 year. To investigate dust deposition dynamics at the regional scale, five automatic deposition collectors named CARAGA (Collecteur Automatique de Retombées Atmosphériques insolubles à Grande Autonomie in French) have been deployed in the western Mediterranean region during 1 to 3 years depending on the station. The sites include, from south to north, Lampedusa, Majorca, Corsica, by satellites, more than 80 % of the major dust deposition events are recorded at only one station, suggesting that the dust provenance, transport and deposition processes (i.e. wet vs. dry) of dust are different and specific for the different deposition sites in the Mediterranean studied area. The results tend to indicate that wet deposition is the main form of deposition for mineral dust in the western Mediterranean basin, but the contribution of dry deposition (in the sense that no precipitation was detected at the surface) is far from being negligible, and contributes 10 to 46 % to the major dust deposition events, depending on the sampling site.

Environmental context Volatile organic compounds are key compounds in atmospheric chemistry as precursors of ozone and secondary organic aerosols. To determine their impact at a megacity scale, a first important step is to characterise their sources. We present an estimate of volatile organic compound sources in Paris based on a combination of measurements and model results. The data suggest that the current emission inventory strongly overestimates the volatile organic compounds emitted from solvent industries, and thus needs to be corrected. Abstract A positive matrix factorisation model has been used for the determination of volatile organic compound (VOC) source contributions in Paris during an intensive campaign (May–June 2007). The major sources were traffic-related emissions (vehicle exhaust, 22% of the total mixing ratio of the measured VOCs, and fuel evaporation, 17%), with the remaining emissions from remote industrial sources (35%), natural gas and background (13%), local sources (7%), biogenic and fuel evaporation (5%) and wood-burning (2%). It was noted that the remote industrial contribution was highly dependent on the air-mass origin. During the period of oceanic influences (when only local and regional pollution was observed), this source made a relatively low contribution (<15%), whereas the source contribution linked to traffic was high (54%). During the period of continental influences (when additional continental pollution was observed), remote industrial sources played a dominant role, contributing up to 50% of measured VOCs. Finally, the positive matrix factorisation results obtained during the oceanic air mass-influenced period were compared with the local emission inventory. This comparison suggests that the VOC emission from solvent industries might be overestimated in the inventory, consistent with findings in other European cities.

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Environmental context Non-methane hydrocarbons play an important role in the formation of photochemical oxidants such as ozone. We investigate factors controlling the distribution of non-methane hydrocarbons in an urban area of the Middle East. The study highlights the importance of local emissions and atmospheric dynamics, and the limited effect of photochemistry at the measurement site. Abstract Measurements of over 70 C2-C16 non-methane hydrocarbons (NMHCs) were conducted in suburban Beirut (1.3 million inhabitants) in summer 2011 and winter 2012 during the Emission and Chemistry of Organic Carbon in the East Mediterranean (ECOCEM) field campaign. The levels of NMHCs observed exceeded by a factor of two in total volume the levels found in northern mid-latitude megacities (Paris and Los Angeles), especially for the unburned fossil fuel fraction. Regardless of the season, the major compounds, explaining 50% of the concentrations, were toluene, isopentane, butane, m,p-xylenes, propane and ethylene, emitted by mobile traffic and gasoline evaporation sources. Most NMHCs show a distinct seasonal cycle, with a summer maximum and a winter minimum, unlike seasonal cycles usually observed in the northern mid-latitude urban areas. We show that NMHC distribution is mainly driven by strong local emissions and local atmospheric dynamics, with no clear evidence of photochemical removal in summer or influence from long-range transport.

Measurements of gaseous and particulate organic carbon were performed during the MEGAPOLI experiments, in July 2009 and January-February 2010, at the SIRTA observatory in suburban Paris. Measurements comprise primary and secondary volatile organic compounds (VOCs), of both anthropogenic and biogenic origins, including C-12-C-16 n-alkanes of intermediate volatility (IVOCs), suspected to be efficient precursors of secondary organic aerosol (SOA). The time series of gaseous carbon are generally consistent with times series of particulate organic carbon at regional scale, and are clearly affected by meteorology and air mass origin. Concentration levels of anthropogenic VOCs in urban and suburban Paris were surprisingly low (2-963 ppt) compared to other megacities worldwide and to rural continental sites. Urban enhancement ratios of anthropogenic VOC pairs agree well between the urban and suburban Paris sites, showing the regional extent of anthropogenic sources of similar composition. Contrary to other primary anthropogenic VOCs (aromatics and alkanes), IVOCs showed lower concentrations in winter (< 5 ppt) compared to summer (13-27 ppt), which cannot be explained by the gas-particle partitioning theory. Higher concentrations of most oxygenated VOCs in winter (18-5984 ppt) suggest their dominant primary anthropogenic origin. The respective role of primary anthropogenic gaseous compounds in regional SOA formation was investigated by estimating the SOA mass concentration expected from the anthropogenic VOCs and IVOCs (I/VOCs) measured at SIRTA. From an integrated approach based on emission ratios and SOA yields, 38% of the SOA measured at SIRTA is explained by the measured concentrations of I/VOCs, with a 2% contribution by C-12-C-16 n-alkane IVOCs. From the results of an alternative time-resolved approach, the average IVOC contribution to SOA formation is estimated to be 7 %, which is half of the average contribution of the traditional aromatic compounds (15 %). Both approaches, which are based on in situ observations of particular I/VOCs, emphasize the importance of the intermediate volatility compounds in the SOA formation, and support previous results from chamber experiments and modeling studies. They also support the need to make systematic the IVOCs' speciated measurement during field campaigns

Abstract. The ChArMEx (Chemistry and Aerosols Mediterranean Experiments) SOP2 (special observation period 2) field campaign took place from 15 July to 5 August 2013 in the western Mediterranean Basin at Ersa, a remote site in Cape Corse. During the campaign more than 80 volatile organic compounds (VOCs), including oxygenated species, were measured by different online and offline techniques. At the same time, an exhaustive description of the chemical composition of fine aerosols was performed with an aerosol chemical speciation monitor (ACSM). Low levels of anthropogenic VOCs (typically tens to hundreds of parts per trillion for individual species) and black carbon (0.1-0.9 µg m −3 ) were observed, while significant levels of biogenic species (peaking at the ppb level) were measured. Furthermore, secondary oxygenated VOCs (OVOCs) largely dominated the VOC speciation during the campaign, while organic matter (OM) dominated the aerosol chemical composition, representing 55 % of the total mass of non-refractory PM 1 on average (average of 3.74 ± 1.80 µg m −3 ), followed by sulfate (27 %, 1.83 ± 1.06 µg m −3 ), ammonium (13 %, 0.90 ± 0.55 µg m −3 ) and nitrate (5 %, 0.31 ± 0.18 µg m −3 ).Positive matrix factorization (PMF) and concentration field (CF) analyses were performed on a database containing 42 VOCs (or grouped VOCs), including OVOCs, to idenPublished by Copernicus Publications on behalf of the European Geosciences Union. 8838V. Michoud et al.: Organic carbon at a remote site of the western Mediterranean Basin tify the covariation factors of compounds that are representative of primary emissions or chemical transformation processes. A six-factor solution was found for the PMF analysis, including a primary and secondary biogenic factor correlated with temperature and exhibiting a clear diurnal profile. In addition, three anthropogenic factors characterized by compounds with various lifetimes and/or sources have been identified (long-lived, medium-lived and short-lived anthropogenic factors). The anthropogenic nature of these factors was confirmed by the CF analysis, which identified potential source areas known for intense anthropogenic emissions (north of Italy and southeast of France). Finally, a factor characterized by OVOCs of both biogenic and anthropogenic origin was found. This factor was well correlated with submicron organic aerosol (OA) measured by an aerosol chemical speciation monitor (ACSM), highlighting the close link between OVOCs and organic aerosols; the latter is mainly associated (96 %) with the secondary OA fraction. The source apportionment of OA measured by ACSM led to a three-factor solution identified as hydrogenlike OA (HOA), semi-volatile oxygenated OA (SV-OOA) and low volatility OOA (LV-OOA) for averaged mass concentrations of 0.13, 1.59 and 1.92 µg m −3 , respectively.A combined analysis of gaseous PMF factors with inorganic and organic fractions of aerosols helped distinguish between anthropogenic continental and biogenic influences on the aerosol-and gas-phase compositions.