Abstract
Abstract. Secondary pollutants such as ozone, secondary inorganic aerosol, and secondary organic aerosol formed in the plumes of megacities can affect regional air quality. In the framework of the FP7/EU MEGAPOLI (Megacities: Emissions, urban, regional and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation) project, an intensive campaign was launched in the greater Paris region in July 2009. The major objective was to quantify different sources of organic aerosol (OA) within a megacity and in its plume. In this study, we use airborne measurements aboard the French ATR-42 aircraft to evaluate the regional chemistry-transport model CHIMERE within and downwind of the Paris region. Two mechanisms of secondary OA (SOA) formation are used, both including SOA formation from oxidation and chemical aging of primary semivolatile and intermediate volatility organic compounds (SI-SOA) in the volatility basis set (VBS) framework. As for SOA formed from traditional VOC (volatile organic compound) precursors (traditional SOA), one applies chemical aging in the VBS framework adopting different SOA yields for high- and low-NOx environments, while another applies a single-step oxidation scheme without chemical aging. Two emission inventories are used for discussion of emission uncertainties. The slopes of the airborne OA levels versus Ox (i.e., O3 + NO2) show SOA formation normalized with respect to photochemical activity and are used for specific evaluation of the OA scheme in the model. The simulated slopes were overestimated slightly by factors of 1.1, 1.7 and 1.3 with respect to those observed for the three airborne measurements, when the most realistic "high-NOx" yields for traditional SOA formation in the VBS scheme are used in the model. In addition, these slopes are relatively stable from one day to another, which suggests that they are characteristic for the given megacity plume environment. The configuration with increased primary organic aerosol (POA) emissions and with a single-step oxidation scheme of traditional SOA also agrees with the OA / Ox slopes (about ± 50 % with respect to the observed ones); however, it underestimates the background. Both configurations are coherent with observed OA plume buildup, but they show very different SI-SOA and traditional anthropogenic SOA (ASOA) contributions. It is hence concluded that available theoretical knowledge and available data in this study are not sufficient to discern the relative contributions of different types of anthropogenic SOA in the Paris pollution plume, while its sum is correctly simulated. Based on these simulations, for specific plumes, the anthropogenic OA buildup can reach between 8 and 10μg m−3. For the average of the month of July 2009, maximum OA increases due to emissions from the Paris agglomeration are noticed close to the agglomeration at various length scales: several tens (for primary OA) to hundreds (for SI-SOA and ASOA) of kilometers from the Paris agglomeration. In addition, BSOA (SOA formed from biogenic VOC precursors) is an important contributor to regional OA levels (inside and outside the Paris plume).
Highlights
The number of large agglomerations (“megacities”) is increasing due to population clustering in urban regions (UN, 2014)
The relative contribution of secondary organic aerosol (SOA) from traditional anthropogenic volatile organic compound (VOC) precursors (ASOA) and from semivolatile (SVOC) or intermediate volatility (IVOC) organic compounds (SI-SOA) is still under debate and difficult to constrain from field data
This overestimation can be related to the SOA formation scheme: it is likely that the high-SOA yields under low-nitrogen oxides (NOx) conditions are incorrect under plume conditions
Summary
The number of large agglomerations (“megacities”) is increasing due to population clustering in urban regions (UN, 2014). The advantage of the airborne measurements over the ground-based ones is that they allow following the evolution of the city plume over time and space up to 200 km downwind of the emissions Data from these flights will be used to extend the model evaluation performed in Zhang et al (2013) for urban and suburban sites in the Paris agglomeration to plume conditions. Among the various formulations that have been derived in the framework of the VBS scheme (for example, Lane et al, 2008a; Murphy and Pandis, 2009; Dzepina et al, 2011; Shrivastava et al, 2013; Zhao et al, 2015, etc.), two are chosen for this paper (as already for Zhang et al, 2013) because they either favor large ASOA or large SI-SOA buildup in the plume In this way, we intend to address uncertainty linked to the coexistence of different VBS schemes in the formation of different SOA types within the plume. From comparison of different setups of the VBS scheme, uncertainties in the formation of different SOA types in the Paris plume are discussed
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