Abstract
Abstract. 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 C12–C16 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 C12–C16 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.
Highlights
Since 1950, the number of people living in urban areas has steadily increased
The value of the organic matter (OM) / combustion sources (CO) enhancement ratio is determined from the fit of the correlation of data with an O3 / CO ratio higher than 0.5, which corresponds to the upper limit of the scatterplot envelope, implying that a maximum OM enhancement is considered here
– The seasonal variation of anthropogenic volatile organic compounds (VOCs) is the one usually depicted in urban areas with higher wintertime concentrations as a result of a combination of (i) a shallower boundary layer in winter than in summer, (ii) photochemistry processes less active in winter and (iii) emissions sources of different nature and/or intensity
Summary
Since 1950, the number of people living in urban areas has steadily increased. According to the United Nations, the urban population has risen from 30 to 50 % of the world population within the last 50 years, and it will reach about 70 % by 2050 (United Nations, 2012). Once emitted into the atmosphere, primary VOCs can be photolyzed or/and oxidized by atmospheric oxidants like the hydroxyl radical (OH), the nitrate radical (NO3) or ozone (O3), and other VOCs are produced (Atkinson and Arey, 2003) These newly produced VOCs or secondary VOCs, generally oxygenated (OVOC), can themselves undergo further reactions and produce different oxidants as by-products, including ozone (Jenkin and Hayman, 1999; Tanaka et al, 2003), whose adverse effects on climate and people are well-known (Heiden et al, 1999; Wildt et al, 2003; Filleul et al, 2006)
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