Megacities, defined as agglomerations with a population above 10 million inhabitants (Gurjar and Lelieveld, 2005) often concentrate in a small area a substantial part of a country's population, economic activities, and thus air pollutant emissions. Emitted primary and subsequently formed secondary gas- or particulate-phase pollutants cause substantial health problems especially in megacities with rapidly growing industry and low pollution control (e.g., Gurjar et al., 2010). The impact of local emissions on air quality in many of these megacities was shown to be large (see, e.g., Molina and Molina, 2004, for a general overview; Hand et al., 2011, and Parrish et al., 2011, for Los Angeles; Molina et al., 2010, for Mexico City; Chan and Yao, 2008, for Chinese megacities). In addition to the local contribution, levels of gaseous species and of fine particulate matter (fine PM, with an aerodynamic diameter below 2.5 µm, PM2.5) are also affected by long-range transport from outside the cities. This important role of pollutant transport has been first shown for sulfate in New York City, a megacity located in a highly industrialized belt in the northeastern US (Rahn and Lowenthal, 1985; Dutkiewicz et al., 2004; Quin et al., 2006). Indeed, as a secondary aerosol component, sulfate needs some time to be formed from SO2 emissions by gaseous- or aqueous-phase oxidation pathways (Seinfeld and Pandis, 2006), important SO2 emissions of industrial origin are often located outside of a megacity. For nitrate, another important secondary inorganic aerosol component, and for organic aerosol (OA; both of primary and secondary origin), only few explicit estimations of the local versus advected contributions to their burden in megacities are available. While for Chicago (with about 8.75 million inhabitants is nearly a megacity) the advected part for both nitrate and OA is dominant (about 60 % (Hand et al., 2011)), it is only minor for New York City (about 30–40 %) (Lall and Thurston, 2006; Hand et al., 2011). Important contributions to nitrate from advection were qualitatively estimated for London (Harrison et al., 2012).

Inorganic secondary particulate species are directly related to their precursor gases SO2, NOx and NH3, with rather well known sources mostly related to fossil fuel burning (SO2, NOx) and agriculture (NH3). By contrast, sources of organic aerosol are widespread (road transport, residential heating including wood burning, open biomass burning, cooking activities, secondary formation from biogenic as well as anthropogenic volatile organic compound (VOC) precursors) and their relative contributions remain uncertain (Hallquist et al., 2009). Early source apportionment work on PM was based on the analysis of daily filter samples and analysis of correlations between organic aerosol and tracers for specific sources using different statistical methods (for a review of methods and results, see Viana et al., 2008). In these earlier studies the secondary OA was often determined as the part of OA which could not be attributed to primary sources.

More recently, the combination of aerosol mass spectrometer (AMS) measurements (DeCarlo et al., 2006; Drewnick et al., 2005) and factor analysis techniques (e.g., positive matrix factorization, PMF, Paatero and Tapper, 1994; Lanz et al., 2007; Ulbrich et al., 2009) has improved the high temporal resolution of source apportionment of primary and secondary OA (e.g., Jimenez et al., 2009; Zhang et al., 2011). Nevertheless, the biogenic versus anthropogenic origin of secondary aerosol in megacities and in megacity plumes is still an open question (Hallquist et al., 2009). The combination of radiocarbon (14C) measurements and AMS based source apportionment can provide valuable insights about the share of fossil fuel and contemporary organic aerosol sources (Lanz et al., 2007; Minguillón et al., 2011; El-Haddad et al., 2013; Zotter et al., 2014). Despite this progress, sources of organic aerosol in megacities with respect to their local versus advected and fossil fuel versus contemporary contribution are still highly uncertain. Comprehensive data sets are needed to better quantify these sources.

Here, we report measurements of air pollution of fine particles in the Paris agglomeration, a mid-latitude, post-industrial megacity with 10.84 million inhabitants (United Nations, 2015). We conducted two intensive field campaigns in the Greater Paris area during July 2009 and from mid-January to mid-February 2010 as part of the MEGAPOLI (Megacities: Emissions, urban, regional and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation) project (Butler, 2008; Baklanov et al., 2010). In addition, for 1 year (September 2009–September 2010), daily PM2.5 chemical composition measurements were performed at one urban background and several rural sites during the PARTICULES (Source Apportionment Of Airborne Particles In The Ile-de-France Region) project (Ghersi et al., 2010; Bressi et al., 2013, 2014). Our major aim is to quantify the contribution of local versus advected sources of fine aerosol in a post-industrial megacity, with a special focus on organic aerosol sources. This will be addressed by combining observations from both the MEGAPOLI and PARTICULES campaigns, and also considering results from chemical transport modeling and satellite observations. The analysis of organic aerosol sources will be addressed by a synthesis of AMS, radiocarbon and tracer measurements. Finally, we will place results found for Paris in a larger context of particulate matter origin in other selected megacities.

In Sect. 2.1, we will present the sites and measurements used for this study during the MEGAPOLI and PARTICULES campaigns. The processing of the measurements will be described in Sect. 2.2. The analysis for attribution of local versus advected contributions is outlined in Sect. 2.3. Details of the combined use of 14C and AMS data are given in the Supplement in Sect. S1. Additional data sets used in this study are described in the Supplement: satellite data sets (aerosol optical depth (AOD) from the Advanced Along Track Scanning Radiometer (AATSR), NO2 columns from the SCIAMACHY instrument, both on the ENVISAT platform) in Sect. S2, black carbon (BC) and elemental carbon (EC) observations from other megacities than Paris in Sect. S3, emission inventories in Sect. S4 and modeling simulations with the PMCAMx and the CHIMERE models in Sect. S5.

The MEGAPOLI campaign results also provide important insights into specific sources of carbonaceous PM (the sum of OA and BC), which represents on average almost 70 % (summer) and 43 % (winter) of PM1 at the urban background site. Radiocarbon (14C) analysis clearly shows a dominant non-fossil fuel fraction of carbonaceous PM1: ∼62%±8% (uncertainty) during summer and ∼78%±14% during winter (Fig. 7). These values are much higher than those for other megacities reported in Hodzic et al. (2010), where contributions from non-fossil fuel carbon are about 30 and 40 %, respectively, during summer and winter for Tokyo, about 30 % during summer for Los Angeles, about 30 % during spring in Mexico City for days without significant contribution of wild-fires, and about 50 and 30 % in summer and winter in Beijing, respectively. Thus sources of non-fossil fuel carbon are much more important for Paris than for many other megacities.

Cooking-related OA (e.g., modern carbon) constitutes up to 30 % of carbonaceous PM1 in summer and nearly 15 % in winter. The high values at the urban site could be partly due to a large number of restaurants nearby, although the cooking source has also been identified at the SW suburban site (Crippa et al., 2013b) and in the Paris pollution plume (von der Weiden-Reinmüller et al., 2014a, b). This finding for Paris is consistent with the importance of cooking contributions found in several other cities (London (Allan et al., 2010), Barcelona (Mohr et al., 2012), Beijing (Huang et al., 2010), New York City (Sun et al., 2011), and Toronto (Slowik et al., 2010)).

During summer, the PMF OOA factor, a proxy for secondary organic aerosol (SOA), accounted for nearly 40 % of OA. Combined 14C/AMS-PMF analysis suggests that more than 90 % of this OOA is of non-fossil fuel origin with an uncertainty range between 60 and 100 % (Fig. 7). Secondary organic aerosol formation from non-fossil fuel precursors is thus clearly the dominant source for SOA advected to the Paris megacity during summer. Modeling results using CHIMERE including the volatility basis set (description in Sect. S5) confirm the origin of SOA: more than 60 % of SOA is of non-fossil fuel and mostly biogenic origin, which is within, albeit at the lower end of, the experimentally derived range of values (see Fig. 8). The simulations indicate two transport patterns during high SOA periods in Paris: either transport from the northeast with SOA of mixed anthropogenic and biogenic origin, or transport from the south with predominantly biogenic SOA, from large biogenic VOC emissions over southwestern France and northeastern Spain (Zhang et al., 2013). Apparently, rapid SOA formation from anthropogenic VOC precursors, as for example observed in Mexico City (e.g., Volkamer et al., 2006), is not strong and fast enough within the Paris agglomeration to compete with the imported biogenic SOA fraction. However, SOA within the Paris pollution plume at about 100 km downwind of the city is enriched with anthropogenic SOA after several hours of processing time, based on the analysis of aircraft data obtained during the summer campaign (Freney et al., 2014). It should also be noted that alternative PMF calculations of summer campaign AMS data slightly alter the results. For instance, the five-factor PMF analysis for the urban background site shows an additional factor related to biogenic marine emissions with high sulfur content and contributing a little more than 10 % to summer PM1 OA (Crippa et al., 2013c).

During winter, PMF analysis of AMS measurements directly attributes 12 % of carbonaceous PM1 to wood burning. A large fraction (44 %) of carbonaceous PM1 is attributed to OOA (Fig. 7), similar to other megacities during winter (New York City and Tokyo (Zhang et al., 2007)). From the combined 14C/AMS-PMF analysis, at least about 80 % of this fraction is estimated to be of non-fossil fuel origin (Fig. 7). This points to either strong contributions of aged organic aerosol from wood burning or of SOA formation from biogenic VOCs, even if in wintertime biogenic emissions are expected to be much lower than during summer. Estimated wood burning contributions to OA from the Aethalometer model (Sandradewi et al., 2008) and a tracer approach (using levoglucosan as a tracer for wood-burning emissions) for this period are also compatible with a part of the identified OOA fraction originating from wood-burning emissions, within their large error bars (Crippa et al., 2013a). For instance Grieshop et al. (2009) and Heringa et al. (2011) showed that domestic wood-burning emissions from wood stoves form significant amounts of secondary organic aerosol. A question arises of the local versus regional origin of wood-burning OA in the Paris agglomeration since the dominant OA fraction was assigned to advection from outside in Sect. 3.1. The regional origin of primary or secondary wood-burning emissions is made evident by the strong correlation and similar levels of levoglucosan between the urban background and rural (southern) Paris sites of the PARTICULES project (R2=0.83, Fig. 9). However, more work is required at a regional scale to better separate wood-burning emissions from urban, suburban and rural areas. A large wood-burning contribution to OA (up to 60 % during wintertime) was also found for the Grenoble agglomeration in the French Alps (∼650 000 inhabitants) (Favez et al., 2010). In general, in Alpine valleys, wood-burning contributions to OA around 50 % were found (Herich et al., 2014). Crippa et al. (2014) showed that wood- and biomass-burning-related organic aerosol concentrations were significant across Europe even in spring and autumn.

Fossil fuel EC and HOA constitute about 20 and 15 %, respectively, of the carbonaceous PM1 in summer, and about 20 and 10 %, respectively, in winter (Fig. 7). According to emission inventories like those developed by MACC (Monitoring Atmospheric Composition and Climate, Pouliot et al., 2012) and EMEP (European Monitoring and Evaluation Program, Vestreng et al., 2007), the major contributor to this fossil-fuel-related fraction is road transport. Unlike biomass burning emissions, these contributions are expected to be dominated by local emissions within the agglomeration. This is confirmed for example by simulations with the PMCAMx model which attribute about 60 % of EC to emissions within the agglomeration (during the MEGAPOLI summer campaign).

The secondary inorganic aerosol fraction, mostly associated with regional transport (>70 %, see Sect. 3.2), represents about 30 % of PM1 during summer (19 % sulfate, 6 % nitrate and 7 % ammonium) and more than 50 % during winter (16 % sulfate, 28 % nitrate and 12 % ammonium). Again, MACC and EMEP emission inventories (Pouliot et al., 2012; Vestreng et al., 2007) indicate that inorganic aerosol precursors SO2, NOx and NH3 are mainly of anthropogenic origin. SO2 is mostly emitted by industrial sources (including energy production) and shipping. NOx emissions are dominated by transportation including shipping, while NH3 is mainly emitted by agriculture. Thus, unlike for organic aerosol with major non-fossil sources, transport- and industry-related fossil fuel combustion strongly contributes to fine inorganic PM. The dominant advected fraction of inorganic aerosol is explained by two factors: first, major source areas of its gaseous precursors are located outside the agglomeration (especially for SO2 and NH3); second, the time necessary for oxidation of the precursors does not allow for efficient aerosol formation from emissions within the megacity (as for nitrate formation from NOx emissions).

The strong regional control of fine PM in Paris is here compared to the situation in other megacities. We use BC as a tracer for the local megacity contribution to fine PM. A compilation of urban background BC and EC measurements in several megacities (Fig. 10 and Table 1) shows that values for Paris (1.8 µgm-3) are at the lower end, within a cluster of megacities like Los Angeles, London, or New York City (1–2 µgm-3). Several American and Asian megacities have intermediate BC levels (Rio de Janeiro, Mexico City, Seoul, 3–5 µgm-3), while Asian megacities in fast developing countries (Beijing, Delhi, etc.) and Cairo display the largest values (>6 µgm-3). Despite uncertainties due to different measurement protocols (Bond et al., 2013; Petzold et al., 2013) and site representativeness (approximately a factor of 2, Sect. S2), the general tendency towards lower values in post-industrial megacities is clear. Nevertheless, since BC and associated compounds like polycyclic aromatic hydrocarbons are known to have strong adverse health effects (e.g., Janssen et al., 2012), BC still represents an important air quality problem even for megacities with lower concentrations like the densely populated Paris agglomeration.

During specific pollution episodes, high PM concentrations can also be encountered in the Paris agglomeration. For instance, during March 2014 (for 10 days from 7 to 18 March), exceptionally high PM10 concentrations exceeded pollution alert levels (daily average of 80 µgm-3 PM10) on several occasions. During this episode, anticyclonic conditions led to regional PM accumulation over northwestern Europe. ACSM measurements at the SIRTA site showed a prevailing contribution of ammonium nitrate (∼50 % of PM1) and ammonium sulfate (∼15 %) suggesting a strong advective impact from outside the agglomeration, in line with the results of our study (Sciare, 2014). In addition, low dispersive conditions during this period also favored additional local pollution build-up.

London is another European megacity with expected low average local contributions to PM levels. Indeed, the regional contribution for organic aerosol, nitrate and sulfate was found to dominate based on observations during the REPARTEE (REgents PARk and Tower Environmental Experiment) campaigns in autumns 2006 and 2007 (Harrison et al., 2012), although a quantitative assessment could not be made with the available data at only one central London site. These dominant regional and correspondingly smaller local emission contributions to urban background PM levels are reflected in recently revised emission inventories. For both the Paris and London agglomerations, bottom-up emission inventories using geographically specific information on activity and emission factors yield about a factor of 3 lower per capita emission values for BC and PM than for national averages (Timmermans et al., 2013), due to lower per capita fuel consumption for large cities with increased population density (Grimm et al., 2008), and possibly also due to lower emission factors.

In contrast, Mexico City is a good example of a megacity with larger local pollution impact. PMCAMx simulations for the MILAGRO campaign in March 2006 (Molina et al., 2010) indicated a much higher contribution of local emissions to fine PM for Mexico City (60 % for PM2.5, 60 % for OA, 80 % for nitrate, less than 10 % for sulfate (Karydis et al., 2011)) than for Paris (Figs. 5 and 6). Mexico City is surrounded by mountain ridges, leading to less favorable dispersion conditions with average summer wind speed of about 4 ms-1 during MILAGRO at about 300 m a.g.l. (Fast et al., 2007) as compared to about 8 ms-1 in Paris during both the MEGAPOLI summer and winter campaign (from SODAR measurements at the SW suburban site at 200 m a.g.l.). Also, increased radiation intensity (enhancing the build-up of secondary pollutants already within the agglomeration), and higher per capita emissions than for Paris (consistent with larger BC values, Fig. 10) contribute to this enhanced local pollution impact. These different conditions result in a local PM2.5 contribution of about 1.4 µgm-3 per million inhabitants for Mexico City with 21 million inhabitants (United Nations, 2015) simulated during MILAGRO, but only 0.3 µgm-3 per million inhabitants for Paris (with 10.84 million inhabitants), observed on an annual average basis during PARTICULES. Thus, in terms of local versus advected contributions to PM, these two megacities might be two extremes.

Analysis of the MEGAPOLI and PARTICULES campaign observations of satellite data and of modeling results have allowed for the quantification of the advected/regional versus local origin of particulate matter (PM) and especially carbonaceous aerosol in the Paris agglomeration. On average over 1 year, about 70 % of the fine PM mass is transported into the megacity from upwind regions of France and continental Europe. Note that this number refers to urban background. For traffic sites, the local contribution would be larger. For organic aerosol (OA) and nitrate, also more than 70 % of their urban background concentrations, respectively, are advected to Paris. For high PM periods, the advected fractions can be even larger. At a measurement site located at the NE edge of the agglomeration, the strong advective source caused higher PM levels for air masses originating in the NE sector (continental origin) than those originating in SW, and thus having crossed the agglomeration before arriving at the site. Satellite AOD measurements and chemistry-transport modeling simulations further confirm these results.

In contrast to other megacities, primary fossil fuel combustion emissions constitute little of carbonaceous fine PM: less than about 20 % in winter and less than 40 % in summer. Instead, cooking activities and, during winter, residential wood burning are the major primary organic PM sources and contribute to non-fossil fuel aerosol. From combined 14C and PMF analysis of AMS data, a mainly non-fossil fuel origin of secondary organic aerosol can be inferred both during summer and winter, either due to formation of secondary aerosol from biogenic VOC precursors or to processing of wood-burning emissions.

Low BC/EC levels (in comparison to those at other megacities worldwide, and not precluding large episodic BC and PM levels) are consistent with relatively low emissions in a post-industrial megacity such as Paris. Indeed, more efficient per capita energy use, and lower emission factors than on national average contribute to these low emissions. Other post-industrial, mid-latitude, flat terrain megacities like New York City, London, and Tokyo probably show a similar type of behavior, while local sources prevail for megacities such as Mexico City or Los Angeles, affected by one or several factors such as larger local emissions, dispersion limited by orography and larger radiation.