ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-15-10219-2015In situ formation and spatial variability of particle number
concentration in a European megacityPikridasM.SciareJ.FreutelF.CrumeyrolleS.von der Weiden-ReinmüllerS.-L.BorbonA.SchwarzenboeckA.MerkelM.CrippaM.KostenidouE.PsichoudakiM.HildebrandtL.https://orcid.org/0000-0001-8378-1882EngelhartG. J.PetäjäT.https://orcid.org/0000-0002-1881-9044PrévôtA. S. H.DrewnickF.BaltenspergerU.WiedensohlerA.KulmalaM.https://orcid.org/0000-0003-3464-7825BeekmannM.PandisS. N.spyros@chemeng.upatras.grDepartment of Chemical Engineering, University of Patras, Patras, GreeceInstitute of Chemical
Engineering Sciences (ICEHT), FORTH, Patras, GreeceThe Cyprus
Institute, Environment Energy and Water Research Center, Nicosia, CyprusLaboratoire des Sciences du Climat et de l'Environnement (LSCE),
Gif-sur-Yvette, FranceMax Planck Institute for Chemistry,
Particle Chemistry Department, Mainz, GermanyLaboratoire
Meteorologie Physique (LaMP), 24 avenue des Landais, 63177 Aubière,
FranceLaboratoire Interuniversitaire des Systemes Atmospheriques,
CNRS, Universites Paris-Est & Paris Diderot, 61 av. Du Gal
de Gaulle, 94010 Cretéil, FranceLeibniz Institute for
Tropospheric Research, Leipzig, GermanyPaul Scherrer Institute,
Laboratory of Atmospheric Chemistry, Villigen, SwitzerlandDepartment of Chemical Engineering, Carnegie Mellon University,
Pittsburgh, USADepartment of Physics, University of Helsinki,
Helsinki, Finlandnow at: LOA, UMR8518, CNRS – Universiteì
Lille1, Villeneuve d'Ascq, FranceS. N. Pandis (spyros@chemeng.upatras.gr)15September20151517102191023713January201526February201531July201512August2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/15/10219/2015/acp-15-10219-2015.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/15/10219/2015/acp-15-10219-2015.pdf
Ambient particle number size distributions were measured in Paris, France,
during summer (1–31 July 2009) and winter (15 January to 15 February 2010)
at three fixed ground sites and using two mobile laboratories and one
airplane. The campaigns were part of the Megacities: Emissions, urban,
regional and Global Atmospheric POLlution and climate effects, and Integrated
tools for assessment and mitigation (MEGAPOLI) project. New particle formation (NPF) was observed only
during summer on approximately 50 % of the campaign days, assisted by the
low condensation sink (about
10.7 ± 5.9 × 10-3 s-1). NPF events inside the Paris
plume were also observed at 600 m altitude onboard an aircraft
simultaneously with regional events identified on the ground. Increased
particle number concentrations were measured aloft also outside of the Paris
plume at the same altitude, and were attributed to NPF. The Paris plume was
identified, based on increased particle number and black carbon
concentration, up to 200 km away from the Paris center during summer. The
number concentration of particles with diameters exceeding 2.5 nm measured
on the surface at the Paris center was on average
6.9 ± 8.7 × 104 and
12.1 ± 8.6 × 104 cm-3 during summer and winter,
respectively, and was found to decrease exponentially with distance from
Paris. However, further than 30 km from the city center, the particle number
concentration at the surface was similar during both campaigns. During
summer, one suburban site in the NE was not significantly affected by Paris
emissions due to higher background number concentrations, while the particle
number concentration at the second suburban site in the SW increased by a
factor of 3 when it was downwind of Paris.
Introduction
Urban areas in the developed and developing world have been growing annually
by 0.7 % in population since 2005 and comprised approximately 54 % of
the total population of the planet in 2014 (United Nations, 2014). In this
work, following the definition of the Organization for Economic Co-operation
and Development (OECD), urban areas are defined as corresponding to a
population density greater than 1500 inhabitants per km2 (OECD, 2013).
Several of these urban areas have increased in size to mega-centers,
attracting more than 10 million inhabitants. This has led to an increasing
demand for transportation, energy and industrial activity, which resulted in
concentrated emission of gases and particulate matter (PM) impacting local
air quality (Molina and Molina, 2004; Molina et al., 2004; Lawrence et al.,
2007; Gurjar et al., 2008). Several epidemiological studies suggest that the
risk of cancer, particularly lung cancer, is increased for people residing in
areas affected by urban air pollution (Barbone et al., 1995; Beeson et al.,
1998; Laden et al., 2006; Nyberg et al., 2000; Pope et al., 2002; Nafstad et
al., 2003). Pope et al. (2009) and Wang et al. (2008) showed that fine
particles with diameters smaller than 2.5 µm (PM2.5) are
related to increased mortality.
Aerosol particles can change climate patterns and the hydrological cycle on
regional and global scales (Chung et al., 2005; Lohmann and Feichter, 2005;
IPCC, 2007). Submicrometer particles, down to 100 nm, are the most effective
ones in scattering solar radiation. The uncertainties in the primary emission
rates of these pollutants and in their formation from gaseous precursors are
still large. On a global scale, new particle formation (NPF), that is
nucleation of low volatility vapors and subsequent condensational growth to
larger sizes, is the major reason for high particle number concentrations
(Kulmala et al., 2004). The mechanism behind this major particle formation
process is still not completely understood (Riccobono et al., 2014). This
uncertainty has a direct impact on our understanding of the role of nucleated
particles in climate change (Pierce and Adams, 2009). NPF is often a regional
phenomenon covering areas of several hundred square kilometers (Vana et al.,
2004; Stanier et al., 2004a; Komppula et al., 2006; Crumeyrolle et al.,
2010), but it can be space restricted when the source of one of the
nucleating vapors is space limited, as has been observed in coastal sites
(Wen et al., 2006).
During the past decade a number of studies reported ambient particle number
concentrations in urban areas. The measurement period spanned from a few
months (Hering et al., 2007; Wang et al., 2010; Dunn et al., 2004;
Baltensperger et al., 2002; McMurry et al., 2005), to 1 or more years (Woo et
al., 2001; Alam et al., 2003; Shi, 2003; Wehner and Wiedensohler, 2003;
Stanier et al., 2004b; Wehner et al., 2004; Wu et al., 2007; Rodríguez et
al., 2005;
Watson et al., 2006; Wåhlin, 2009). The majority of studies are based on
observations from one or at most two stationary sites, assuming that these
stations are representative of the area under investigation. Most of these
studies have found higher concentrations during winter due to both increased
emissions caused by higher energy demand, and lower boundary layer height.
Also, typically a diurnal pattern has been found that shows peaks due to
morning rush hour traffic during weekdays but not on weekends.
NPF has often been observed in urban areas (Woo et al., 2001; Baltensperger
et al., 2002; Laakso et al., 2003; Tuch et al., 2003; Stanier et al., 2004a;
Watson et al., 2006; Wu et al., 2007), but growth and nucleation rates are
rarely reported in these studies (Birmili and Wiedensohler, 2000; McMurry,
2000; Shi et al., 2007; Wehner et al., 2007; Manninen et al., 2010).
During the Megacities: Emissions, urban, regional and Global Atmospheric
POLlution and climate effects, and Integrated tools for assessment and
mitigation (MEGAPOLI) project (Baklanov et al., 2010), measurements were
conducted in and around the megacity of Paris. Gas- and particulate-phase
measurements from three fixed ground sites, two mobile laboratories, and one
airplane were collected for both summer 2009 and winter 2010. The residence
time of the air mass over land was found to influence PM levels, with longer
residence times leading to higher mass concentrations (Freutel et al., 2013).
Air masses from the Atlantic, which were dominating during the summer
campaign, led to relatively clean conditions (Freutel et al., 2013; Freney et
al., 2014). Cooking was identified as a significant local organic aerosol
source within Paris during summer, with vehicular traffic being second
(Crippa et al., 2013b). During winter, residential wood burning was found to
be a major source of organic aerosol (Crippa et al., 2013a). During both
MEGAPOLI campaigns, the contribution of primary transportation emissions to
submicrometer organic aerosol (OA) was around 6 % (Crippa et al., 2013b).
In the year of the MEGAPOLI campaigns, 61 % of the light-duty vehicles in
France were powered by diesel engines and 72 % of the consumed fuel was
diesel (World Bank, 2012). The sulfur content of diesel in France at that
time was 10 ppm compared for example to 500 ppm in 1998. The sulfur content
of fuel affects not only the total particle emissions but also the shape of
the corresponding aerosol size distribution (Platt et al., 2013; Bermúdez et
al., 2015).
Beekmann et al. (2015) have presented a synthesis of the MEGAPOLI PM mass
source attribution efforts based on the corresponding field measurements. In
parallel, several modeling efforts have also been conducted examining the
contribution of regional sources to fine PM (Skyllakou et al., 2014) and
investigating the organic aerosol sources in Paris (Couvidat et al., 2013;
Zhang et al., 2013). All of these studies focused on PM mass concentration
and not on particle number. The different size distributions of the aerosol
emitted by different sources usually result in very different source
contributions to particle number and mass (Zhou et al., 2004). There have
been a number of studies that tried to quantify the particle number sources
using available size distribution measurements (Wåhlin et al., 2001;
Hussein et al., 2004; Zhou et al., 2004; Chan and Mozurkewich, 2007).
However, the changes in these distributions due to new particle formation and
growth or other dynamic changes seriously limit the applicability of
techniques like positive matrix factorization (PMF). Zhou et al. (2004)
excluded the corresponding new particle formation periods from their data set
to overcome this problem.
In this work we focus on the particle number concentrations in Paris and its
surroundings during both (summer and winter) campaigns. The effect of the
Paris megacity on the downwind areas is assessed together with the spatial
extent of its influence. The frequency and spatial characteristics of new
particle formation events are also investigated.
Sampling sites
Month-long campaigns were conducted in the Parisian region during summer (1
July to 31 July 2009) and winter (15 January to 15 February 2010). They
included monitoring of the aerosol size distribution along with composition,
coupled with gas-phase and meteorological monitoring.
The city of Paris is an urbanized area covering about 3000 km2 with 2.2
million inhabitants. The greater Paris area, called Île de France (IDF),
is one of the largest metropolitan areas in Europe, including more than 12
million inhabitants. The administrative boundaries of Paris and IDF are shown
in Fig. 1 along with the population density map of the area.
Population density and administrative map of Paris. Outlined in red
is Île de France and, in green, Paris. The three ground stations (SIRTA,
LHVP and GOLF) are depicted with black dots. The map is separated into
sectors depicted by blue lines, formed by concentric circles centered at
kilometer zero of Paris (48.8534∘ N, 2.3488∘ E). The radius
of the circles is 0.15, 0.25, 0.4, 0.6, 0.8 and 1∘, which corresponds
to 16.7, 27.8, 44.4, 66.7, 88.9 and 111.1 km.
Detailed aerosol particle measurements were conducted at an urban site and
two sub-urban sites (Fig. 1). The Site Instrumental de Recherche par
Télédétection Atmosphérique (SIRTA, 48∘43′5′′ N,
2∘12′26′′ E) is located on the campus of Ecole Polytechnique
(Palaiseau), 20 km southwest of the Paris center in a semi-urban environment
inside the campus of Ecole Polytechnique. This site is surrounded by highways
at 3–6 km distance in all wind directions. Measurements in the Laboratoire
d'Hygiène de la Ville de Paris (LHVP, 48∘49′11′′ N and
2∘21′35′′ E), inside of Paris, were performed on a terraced roof
14 m above ground level (a.g.l.) and on the ground inside a research
container. This site includes a station of the AIRPARIF air quality
monitoring network and is representative of the Paris urban background air
pollution (Sciare et al., 2010; Favez et al., 2007). Finally, the sub-urban
station at Golf de la Poudrerie (GOLF, 48∘56′2′′ N,
2∘32′49′′ E) was located 20 km northeast of the Paris center
near a golf course and a forested park.
Two mobile platforms, named MoLa (Mobile Laboratory) and MOSQUITA
(Measurements Of Spatial QUantitative Immissions of Trace gases and
Aerosols), were operated by the Max Planck Institute for Chemistry (Drewnick
et al., 2012; von der Weiden-Reinmüller et al., 2014a) and the Paul
Scherrer Institute (Bukowiecki et al., 2002; Weimer et al., 2009),
respectively. The measurement path of both mobile platforms was decided based
on forecasts of the CHIMERE chemical transport model (Rouil et al., 2009;
Menut and Bessagnet, 2010; Menut et al., 2013). Three measurement strategies
were employed during both campaigns: stationary, axial and cross-sectional
measurements (von der Weiden-Reinmüller et al., 2014a, b).
Cross-sectional (mobile) measurements were carried out by maintaining
approximately constant distance from the Paris center while varying the
cardinal directions, allowing distinction between background concentrations
and Paris emissions. Axial (mobile) measurements were conducted by
maintaining approximately the same cardinal direction while varying the
distance with respect to the Paris center, thus monitoring the evolution of
the plume. Stationary measurements were conducted when the direction of the
Paris emissions, based on the CHIMERE model, were not stable enough to allow
cross-sectional or axial measurements. Stationary measurements were conducted
only by MoLa either downwind of Paris or upwind to assess background aerosol
loadings.
The airborne measurements were performed by an ATR-42 and a Piper Aztec
aircraft during summer and winter, respectively, operated by the French
Service des Avions Français Instrumentés pour la Recherche en
Environnement (SAFIRE). Each flight included a circle around IDF followed by
crossing the expected Paris plume multiple times, at a constant altitude of
600 and 500 m above sea level for the summer and winter campaigns,
respectively. During 1 July the flight path was kept at a constant altitude
of approximately 800 m. Flights were performed on 11 out of the 31 days of
the summer campaign. Figure 2 shows the flight patterns and sampling days of
the ATR-42 during summer. Flight days were selected based on CHIMERE
predictions. Higher PM concentration days were favored; thus, the observed
aerosol properties are expected to be biased toward more polluted conditions.
During winter two flights per sampling day were conducted for 4 days (27 and
31 January, 14 and 15 February). The first flight included a survey of the
aerosol properties around IDF and the second monitored the Paris plume,
following a flight path similar to the summer one.
Flight paths of the ATR-42 aircraft during the summer campaign.
Different colors correspond to different flight routes. The cities of Fecamp
and Paluel are also depicted on the map.
Instrumentation
The MEGAPOLI project focused on the properties of ambient aerosol, including
both mass and number concentration measurements. This work examines the
particle number concentration N during both MEGAPOLI campaigns; the
instruments and measurements relevant for this purpose are summarized in
Table 1. A number of additional measurements of concentrations of gas-phase
pollutants, radicals, etc., were conducted during the campaigns (Michoud et
al., 2012), but are not used in the present work because they did not
provide any additional insights.
Summary of the main MEGAPOLI measurements used in this study.
VariableInstrumentGroupTime resolutionSample conditionATR-42Absorption (summer)PSAPaLaMPj1 sDryTrace gas concentrationHS PTR-QMS 500bCNRSk1 sDryAerosol number concentrationTSI 3025 CPCcCNRMl1 sDryAerosol number concentrationTSI 3010 CPCcLaMPj1 sDryAbsorption (winter)PSAPaCNRMl1 sDryMoLaAerosol number concentrationTSI 3786 UWCPCdMPICm1 sAmbientMOSQUITAAerosol number concentrationTSI 3010 CPCcPSIn1 sAmbientAerosol number concentrationUHSASePSIn1 sAmbientSIRTAAerosol number size distribution (10–500 nm)SMPSfCMUo10 minDryAerosol number size distribution (6–800 nm)DMPSgUoHp9 minAmbientLHVPAerosol number size distribution (3–630 nm)DMPSgIfTq10 minDryPositive/negative ion size distribution (0.8–40 nm)AIShUoHp3 minAmbientGOLFAerosol number size distribution (5 nm to 1 µm)EASiMPICm1 minAmbient
a PSAP: Particle Soot Absorption Photometer;
b HS PTR-QMS: High Sensitivity Proton Transfer
Reaction-Quadrupole Mass Spectrometer; c CPC: Condensation
Particle Counter; d UWCPC: Ultrafine Water Condensation Particle
Counter; e UHSAS: Ultra High Sensitivity Aerosol Spectrometer;
f SMPS: Scanning Mobility Particle Sizer; g DMPS:
Differential Mobility Particle Sizer; h AIS: Air Ion
Spectrometer; i EAS: Electrical Aerosol Spectrometer;
j LaMP: Laboratoire Meteorologie Physique; k CNRS:
Centre National de la Recherche Scientifique; l CNRM: Centre
National de Recherches Météorologiques; m MPIC: Max
Planck Institute for Chemistry; n PSI: Paul Scherrer Institute;
o CMU: Carnegie Mellon University; p UoH: University
of Helsinki; q IfT: Leibniz Institute for Tropospheric Research.
At SIRTA, two instruments were used to monitor the ambient particle number
distribution. A Scanning Mobility Particle Sizer (SMPS; TSI model 3936)
sampled aerosol particles from 10 to 500 nm in diameter through an inlet
located approximately at 4 m a.g.l. The particles were actively dried using a
Nafion dryer. A Differential Mobility Particle Sizer (DMPS, Aalto et al.,
2001) also monitored ambient number size distributions ranging from 6 to
800 nm during summer. At LHVP, the sampling inlet was located 6 m a.g.l.
and the aerosol sample was dried using a diffusion dryer as described in Tuch
et al. (2009) before entering a mobility particle size spectrometer
TROPOS-type TDMPS (Twin Differential Mobility Particle Sizer; Birmili et al.,
1999), which monitored the aerosol size distribution from 3 to 630 nm. At
the same site, an Air Ion Spectrometer (AIS; Mirme et al., 2007) monitored
the size distribution of ambient (not dried) positive and negative air ions
of mobility diameters ranging from 0.8 to 40 nm. To minimize particle
losses, the sampling line length of the AIS was 30 cm. At GOLF, the particle
size distribution between 5 nm and 1 µm was monitored with an
Electrical Aerosol Spectrometer (EAS, Airel Ltd.) and sampling was conducted
8 m a.g.l. Because the three aerosol size distribution instruments (SMPS,
TDMPS, EAS) used for the stationary ground measurements during both campaigns
overlap between 10 and 500 nm (mobility diameter), our analysis will focus
on this size range, denoted as N10–500.
MoLa, which was based at GOLF, monitored the total particle number
concentration via an Ultrafine Water Condensation Particle Counter (UWCPC,
TSI model 3786) with 50 % detection efficiency at 2.5 nm, which will be
denoted as N2.5. The aerosol inlet during stationary measurements was
located at approximately the same height as the stationary measurements at
GOLF (8 m a.g.l.). During mobile measurements, sampling occurred at about
2.4 m a.g.l. MOSQUITA monitored the total particle number concentration via
a butanol-based Condensation Particle Counter (CPC; TSI model 3010, 50 %
detection efficiency at 10 nm) during summer, further denoted as N10,
and via an Ultra High Sensitivity Aerosol Spectrometer (UHSAS; DMT model A)
during winter. The UHSAS monitored the size distribution, with respect to the
optical diameter, ranging from 60 nm to 1 µm.
Onboard the METEO-FRANCE aircraft (ATR-42), aerosols were sampled, under dry
conditions, through the community aerosol inlet and delivered to a
comprehensive suite of aerosol instruments. This isokinetic and isoaxial
inlet is based on the University of Hawaii shrouded solid diffuser designed
by A. Clarke and had been modified by Meteo France (McNaughton et al., 2007).
Particle number concentration was monitored directly during summer and winter
flights using a CPC with 10 nm (TSI model 3010) and 2.5 nm (TSI model 3025)
lower cutoff, respectively. Because the CPCs used during the summer and
winter campaigns had different lower detection limits, the corresponding
number concentrations will be denoted as N10 and N2.5,
respectively.
In order to quantify potential differences between instruments, at least one
of the mobile laboratories visited each site for 5–15 h during each
campaign. During summer, the differences in number concentration between the
CPC on board the visiting mobile laboratory (MOSQUITA) and the aerosol sizing
instrument at each of the stationary sites did not exceed 10 % (Fig. S1
in the Supplement). The CPC onboard MOSQUITA had a detection size limit equal
to approximately 10 nm. During winter, the MoLa CPC, with a lower detection
size limit of 2.5 nm, was employed for the intercomparisons. In this case,
the differences were higher and equal to 30, 18 and 19 % at SIRTA, LHVP
and GOLF, respectively. Taking into account that particles below 10 nm were
typically present at SIRTA during winter the corresponding discrepancy can be
partially explained by the different detection limits of the two instruments
(10 nm for the SMPS at SIRTA and 2.5 nm for the MoLa CPC). During both
campaigns the number concentrations monitored onboard MoLa and MOSQUITA were
also compared for approximately 8 h. The two instruments were found to agree
when the concentrations of the nucleation mode particles were moderate or
low. This is expected due to their different size detection limits. The
results of this intercomparison have been presented by von der
Weiden-Reinmüller et al. (2014a).
MethodsParticle formation event categorization
Particle formation events have been categorized in the past based on the
concentration of 1.6–7.5 nm air ions (Hirsikko et al., 2007; Vana et al.,
2008) and on the concentration of total ambient particles below 25 nm
(Stanier et al., 2004a; Dal Maso et al., 2005). At LHVP both air ions and
ambient particles were measured and therefore we used two classification
schemes, one based solely on ambient particles following Dal Maso et
al. (2005) and one that includes air ions, following Hirsikko et al. (2007).
In both cases, the observation period was divided into particle formation
event days, non-event days and undefined days. In general, a day is
classified as an event day if a nucleation mode (particles with sizes smaller
than 10 nm) is present for several hours and grows continuously during the
course of the day. If no traces of a nucleation mode are seen, a day is
classified as a non-event day. Days that did not clearly belong to either of
the aforementioned categories were classified as undefined. Examples of
event, undefined and non-event days are shown in Figs. 3, 4 and 5,
respectively.
Size distribution measurements during a nucleation event day (12
July 2009) at all ground sites. (a) AIS measurements at LHVP,
(b) SMPS measurements at SIRTA, (c) DMPS measurements at
LHVP, and (d) EAS measurements at GOLF. Time of day corresponds to
local standard time (UTC+1). Dp is the particle diameter.
Size distribution measurements during an undefined event day (10
July 2009): (a) AIS measurements at LHVP, (b) SMPS
measurements at SIRTA, (c) DMPS measurements at LHVP, and
(d) EAS measurements at GOLF. Time of day corresponds to local
standard time (UTC+1). Dp is the particle diameter.
Size distribution measurements during a non-event day (29 July
2009): (a) AIS measurements at LHVP, (b) SMPS measurements
at SIRTA, (c) DMPS measurements at LHVP, and (d) EAS
measurements at GOLF. Time of day corresponds to local standard time
(UTC+1). Dp is the particle diameter.
During 12 July, a nucleation mode appeared at 14:00 LST (local standard
time) simultaneously at all ground sites (Fig. 3). During this cloudy day,
nucleation was observed approximately 1 h after the solar intensity
increased by a factor of 3 (from 300 to 1070 W m-2). This day was
consequently classified as an event day. During 10 July, an increase in the
number concentration of particles above 10 nm in diameter was measured
simultaneously at LHVP and SIRTA at 14:00 LST (Fig. 4). It was unclear
whether the mode also appeared at GOLF due to interferences by local sources.
Particle growth was not continuous and the mode disappeared abruptly after
approximately 3 h, even though the direction of the wind did not change at
this time. At LHVP air ion bursts in the size range between 1.6 and 7.5 nm
did not follow a distinct pattern, but were random. As a result it was
unclear whether NPF occurred and the day was classified as undefined for all
sites. During 29 July, no nucleation event was observed, and the day was
consequently classified as a non-event day. During this day, the condensation
sink (CS) was rather high (9.0 ± 1.7 × 10-3,
20.3 ± 9.7 × 10-3 and
14.4 ± 4.1 × 10-3 s-1 at SIRTA, LHVP and GOLF,
respectively) from 08:00 to 16:00 LST, when NPF was expected to occur. These
sink values were above the summer average for all sites (see Sect. 3.3) and
contributed to the lack of a nucleation mode at all sites (Fig. 5).
Duration of nucleation events
The duration of nucleation events at LHVP was calculated based on AIS
measurements following the procedure described by Hirsikko et al. (2005) and
Pikridas et al. (2012). In brief, a normal distribution was fitted to the
time series of concentration of air ions with diameters between 2 and 5 nm.
The beginning of the event was determined by the initial increase in the air
ion concentration (assuming a stable air ion concentration before the event)
and the end by the peak of the normal distribution. A decrease in the number
concentration implies that the rate of particle production is lower than the
combined rates of coagulation and particle growth to diameters greater than
5 nm, or that the air mass is getting diluted; it does not necessarily imply
that the rate of production is zero. Our calculated event end is thus a lower
bound estimate.
Condensation sink
The condensation sink (CS) is defined as the condensational loss rate
constant of vapors (Kulmala et al., 2001; Dal Maso et al., 2002). The CS
values were calculated using
CS=2πD∫0∞Dpβm(Dp)n(Dp)dDp=2πD∑iDpiβmiNi,
where D is the diffusion coefficient of the condensing vapor,
Dpi is the diameter and Ni the particle number
concentration in size class. The term βmi corresponds to the
transition regime correction factor for the size class i, which was
calculated based on Fuchs and Sutugin (1971). The properties of the
condensable vapors are assumed to be similar to those of sulfuric acid,
without accounting for hydration, leading to an upper limit estimate. If the
aerosol sample was dried prior to the measurement, the diameter reduction due
to water loss was estimated using the Extended Aerosol Inorganic Model II
(E-AIM, http://www.aim.env.uea.ac.uk/aim/aim.php; Carslaw et al., 1995;
Clegg et al., 1998; Massucci et al., 1999). The hourly averaged inorganic
concentrations for sulfate, ammonium and nitrate measured by the aerosol mass
spectrometer (AMS; Jayne et al., 2000; Jimenez et al., 2003), and ambient
relative humidity (RH) measured at each site,
were used as inputs to the model, neglecting any contribution of organics to
the aerosol water content. The volume growth factor was determined following
the method of Engelhart et al. (2011), which assumes that all submicrometer
particles grow similarly by neglecting the Kelvin effects. The diameter
growth factor was calculated as the cubic root of the volume growth factor
and was applied to the whole particle distribution.
Mobile measurements
Due to the high frequency of local contamination events, mobile data were
post-processed by examining video footage recorded at the driver's cabin of
the mobile laboratory, based on Drewnick et al. (2012). Measurement periods
were omitted from analysis if traffic was identified less than 150 m from
the platform; if human activities (e.g., cooking, heating) were spotted; when
driving at low speed caused a possible contamination by the vehicle's own
exhaust; and when traveling inside tunnels. In order to reduce the number of
contaminated data, major roads were avoided. More details concerning the
conditioning of mobile measurements presented in this study can be found in
von der Weiden-Reinmüller et al. (2014a). Further analysis of the mobile
data set was conducted based on results from the FLEXPART particle dispersion
model performed in forward mode (Stohl et al., 2005). Particles were released
from an area whose borders were determined by the population density map
presented in Fig. 1, and included Paris. Based on these modeling results and
the respective measurement tracks, mobile measurements were attributed as
influenced or not by Paris emissions.
Meteorology
During summer, the lowest ambient temperature was 12 ∘C, observed at
SIRTA and GOLF, and the highest of 33 ∘C was measured at LHVP.
Campaign average temperatures during summer were 19.7, 21.1 and
18.7 ∘C at GOLF, LHVP and SIRTA, respectively. In general, the
temperature was higher inside the city center by 1 ∘C at least,
compared to the suburban sites. Diurnal variations of RH (ranging from 35 to
90 %) and temperature were similar at all sites during summer. There were
several cloudy periods and cloud coverage was geographically dependent.
During summer at all ground sites, solar radiation reached a maximum of
900 W m-2, while the presence of clouds could reduce it by a factor of
3. Precipitation as monitored at SIRTA occurred on 8 of the 31 days of the
campaign (8, 16–18, 22, 23, 27 and 30 July). The maximum observed
precipitation rate during the summer campaign was 0.5 mm min-1;
however, it rarely exceeded 0.1 mm min-1.
During winter the campaign average ambient temperatures were 2.6, 3.3 and
1.2 ∘C at GOLF, LHVP and SIRTA, respectively. RH varied from 40 to
90 % and exceeded 95 % on several occasions at all sites. Hourly
average global solar irradiance did not exceed 400 W m-2 during the
winter campaign and did not exceed 100 W m-2 on 14 of the 32 days of
observations. Precipitation occurred during winter on two-thirds (21 of 32
days) of the campaign days and the average precipitation rate was
approximately 0.15 mm min-1.
Aerosol number concentrations during the summer and winter
campaigns and characteristics of NPF during summer. σ is the
standard deviation.
Average ± 1σ number Average increase ± 1σ inGrowth rate ± 1σconcentration (10–500 nm) number concentration(nm h-1)1000 cm-3due to NPF (%)SiteSummerWinterSummerSummerGOLF13.3 ± 6.825.3 ± 15.1127 ± 1106.1 ± 1.8LHVP11.4 ± 5.115.6 ± 7.1100 ± 504.6 ± 1.9SIRTA5.3 ± 3.110.1 ± 5.7129 ± 595.5 ± 4.1
Figure 6 shows the wind direction distribution at all sites for each
campaign. Wind direction, measured at 10 m a.g.l., during summer was
predominantly SW at LHVP and GOLF and W at SIRTA (Fig. 6), indicating that
air masses often crossed the city center before reaching GOLF and that SIRTA
was mostly upwind of the city. During winter, wind directions were more
variable, with the wind equally coming from both NE and W (Fig. 6). During
the winter campaign, SIRTA was more often than GOLF influenced by air masses
that crossed the urban area before reaching the site.
Wind direction rose plots during the summer and winter campaigns at
each of the ground sites. Each rose segment corresponds to an angle bin of
π/18 (i.e., 20∘) and concentric circles at each site correspond
to 5 % relative frequency. Wind speed, in m s-1, corresponding to
each size bin is color coded inside each rose. Wind speeds below
1 m s-1 have been omitted from the graph.
Particle number concentrations and size distributionsStationary measurements
Average number concentrations of particles with diameters between 10 and
500 nm (N10–500), for all ground sites during both campaigns, are
summarized in Table 2. On average, the N10–500 concentrations
during winter were higher than during summer by a factor of 2 at SIRTA and
GOLF, and by 35 % at LHVP. The highest hourly averaged concentrations
were observed at GOLF (54.1 × 103 cm-3 and
72.2 × 103 cm-3 during summer and winter, respectively),
followed by the LHVP urban center station
(34.4 × 103 cm-3 and 45.5 × 103 cm-3
during summer and winter, respectively). The average ratio of the aerosol
number concentration observed at LHVP to the one observed at GOLF was 0.86
and 0.62 during summer and winter, respectively. The average ratio of the
aerosol number concentration observed at LHVP to the one observed at SIRTA
was 2.1 and 1.5 during summer and winter, respectively.
The particle number concentration at all sites followed the same diurnal
pattern during both seasons (Fig. 7). Regardless of site and season, minimum
concentrations were observed between 03:00 and 04:00 LST, when anthropogenic
activities are expected to be minimal. The concentration exhibited two
maxima: during morning traffic hours, peaking between 07:00 and 10:00 LST,
and during nighttime, between 08:00 and 09:00 LST. These diurnal profiles
are typical of urban areas (Ruuskanen et al., 2001; Woo et al., 2001; Watson
et al., 2006) and can be explained based on the evolution of the mixing layer
(Bukowiecki et al., 2005). In the afternoon atmospheric mixing reaches its
maximum and primary pollutant concentrations decrease due to dilution. The
mixing height remains fairly constant till nighttime, when it decreases,
resulting in increasing primary pollutant levels. Boundary layer measurements
using a Cloud and Aerosol Micro Lidar (Cimel model CE-370) at 355 nm that
were performed at SIRTA support this explanation. The magnitude and time of
the peaks varied depending on site and season. By comparing these maxima,
which correspond to the peak of anthropogenic activity, against the minimum
of the diurnal cycle, a rough estimate of the N10–500 local
contribution can be made for each site. During summer the increase was 84, 79
and 21 % at GOLF, LHVP and SIRTA, respectively, and during winter was
153, 133 and 141 %.
Number concentration diurnal profiles of the summer (left) and
winter (right) campaigns for size ranges from 10 to 30, 30 to 100 and 100 to
500 nm, respectively. Different scales are used for each season.
During summer, particles with diameters ranging from 30 to 100 nm dominated
the N10–500 at SIRTA, accounting on average for 53 %, followed
by particles with diameters ranging from 10 to 30 nm, which accounted for
30 % (Fig. 7). Similar behavior was observed at LHVP during summer, where
particles with diameters ranging from 30 to 100 nm accounted for 47 %
and particles with diameters ranging from 10 to 30 nm for 40 % of the
N10–500. However, N10–500 measured at GOLF was
dominated by particles with diameters ranging from 10 to 30 nm, which
accounted for 50 % of the N10–500, followed by particles with
diameters ranging from 30 to 100 nm that accounted for 42 %.
Campaign average particle number distributions for winter
(top) and summer (bottom) for the three ground sites based on measurements
of EAS at GOLF, DMPS at LHVP and SMPS at SIRTA. Each average size
distribution (solid red line) is deconvoluted to lognormal modes (dashed
blue lines). Note the different scaling of the y axes between sites.
Average size distributions for each site are shown in Fig. 8, along with the
corresponding lognormal modes. During summer, an Aitken mode centered
approximately at 35 nm dominated the number distributions at LHVP and SIRTA.
Nucleated particles grew to approximately this size during summer (see
Figs. 3 and 4) and could be identified for several hours after each event.
The average number size distribution in LHVP and SIRTA usually had two more
modes centered at 15 and 115 nm, respectively. The summertime number
distribution at GOLF was characterized by two modes centered at approximately
15 and 80 nm. Unlike SIRTA and LHVP, the 15 nm mode dominated the aerosol
number distribution at GOLF.
During winter the contribution of particles with diameters from 10 to 30 nm
to N10–500 was almost equal to that from particles with diameters
from 30 to 100 nm at SIRTA (42 and 39 %, respectively) and LHVP (44 and
40 %, respectively). At GOLF the contribution of particles with diameters
from 10 to 30 nm increased even further (compared to summer), reaching
56 %, and the contribution of particles with diameters from 30 to 100 nm
decreased to 34 %. The average size distribution, shown in Fig. 8,
indicates a dominating mode centered below 20 nm at all sites and a smaller
second mode at 60, 80 and 50 nm at SIRTA, LHVP and GOLF, respectively.
Similar shifts of the aerosol distribution to lower sizes during winter have
been observed elsewhere (Bukowiecki et al., 2003), where an inverse
temperature dependence of the particle number concentration was reported.
Particles larger than 100 nm accounted for less than 20 % of
N10–500 during both campaigns at all sites.
Taking into account the location of each site, the contribution of small
particles (diameters 10–30 nm) to N10–500 increases when
moving from the SW (SIRTA) to the NE of Paris (GOLF). Consequently, the
contribution of particles with sizes 30–100 nm to the N10–500
exhibits a decreasing (opposite) trend from the SW to the NE of Paris.
Both trends were observed during both seasons and indicate a persistent
source of particles with diameters smaller than 30 nm NE of Paris, where
GOLF was located. This conclusion is further supported by mobile
measurements (Sect. 5.3) that showed that the background N2.5 was
relatively stable in the area further than GOLF during summer.
Impact of Paris on its surroundings
To investigate the impact of the emissions from the city center on number
concentrations at the two satellite sites (GOLF, LHVP), the measurements were
separated with respect to wind direction, excluding periods when the wind
speed was below 1 m s-1 (Fig. 9). Taking into account that the area is
relatively flat, it was assumed that the urban center influences each of the
satellite sites at certain wind directions (215 ± 30∘ and
65 ± 30∘ for GOLF and SIRTA, respectively), noted with red in
Fig. 9. This analysis is complicated by the variability in aerosol load due
to air mass origin difference. During most of the summer campaign, clean air
masses from the Atlantic reached Paris (Freutel et al., 2013). Air masses of
different origin, which accounted for only two subsequent days during the
summer campaign, were omitted to minimize any discrepancy. During winter, air
mass origin was more variable and a common background could not be ensured,
limiting this analysis only to the summer campaign.
Number concentrations measured at the two satellite sites during
summer with respect to wind direction/air mass transport direction measured
at the respective site. The angles that indicate that the air mass traveled
through the city center prior to reaching the site are depicted in red. The
horizontal dashed black line corresponds to the campaign average for each
site. Periods with wind speeds below 1 m s-1 were omitted from the
analysis.
During summer, the highest N10–500 measured at SIRTA was observed
when the air masses crossed the city center
9.8 ± 3.5 × 103 cm-3 and the lowest when the wind
originated from the opposite direction
(4.2 ± 2.3 × 103 cm-3), considered later on as the
background concentration. The urban emissions led thus to an increase in the
number concentration by a factor of 2 at SIRTA. On the contrary, at GOLF the
N10–500 was not clearly affected by the wind direction during July
2009. N10–500 measurements at GOLF were higher than at SIRTA,
located at the same distance from Paris but in the opposite direction, by a
factor of 3 when either site was not influenced by Paris. These results do
not imply that Paris did not affect its surroundings during summer, but
rather that the effect of the city was not large enough to be observed due to
higher background concentrations at the GOLF site in the NE of Paris with
respect to those at the SIRTA site in the SW. Mobile measurements that
covered mainly the N–NE area with respect to Paris support this result (see
Sect. 5.3). The possibility that these observations were due to temperature
changes (Bukowiecki et al., 2003) was also investigated. However, no clear
dependence between temperature and N10–500 was established. As an
example, at SIRTA the lowest temperatures (around 17 ∘C on average)
were observed both when air masses were influenced by Paris and when the wind
came from the opposite direction.
On 21 July, MoLa performed stationary measurements 38 km north of Paris,
which is almost twice the distance of each of the stationary sites (20 km)
from the city center. Initially, air masses reaching MoLa were influenced by
Paris emissions, based on FLEXPART simulations, and N2.5 was equal to
14.1 × 103 cm-3. However, the wind direction shifted
while sampling and the N2.5 decreased by 40 %, reaching
approximately 8.5 × 103 cm-3.
Spatial evolution of particle numbers in Paris and its
surroundings
The majority of mobile measurements were conducted downwind of Paris in order
to characterize its effect on its surroundings (von der Weiden-Reinmüller
et al., 2014a, b). These measurements were conducted at different distances
from the center of Paris, under various meteorological conditions and
different air mass origins (marine, continental), and were affected by the
diurnal pattern (Fig. 7) of Paris emissions. The mobile measurements were
further affected by wind direction shifts that resulted in monitoring of
background concentrations instead of Paris emissions.
Paris emission measurements were identified during data analysis using
FLEXPART in forward mode (Sect. 3.4). During summer, marine air masses
predominantly resulted in a relatively stable and low PM background. During
winter, air mass origin was not as stable as during summer, yet Paris
emissions were also higher, thus facilitating the analysis. Variations in the
number concentration due to meteorology effects or Paris emissions
fluctuations can be dealt with by examining short case-study periods when
these variables are relatively stable. However, because such periods span a
few hours only, the measurement sample is small. If measurements throughout
each campaign are considered, the sample size is satisfactory, yet it
reflects the different conditions mentioned above. In this work both
approaches were considered and are presented to quantify the behavior of the
Paris plume downwind of the city.
Mobile measurements were separated, based on location, into concentric rings
with borders at 0.15, 0.25, 0.4, 0.6, 0.8 and 1∘ (16.7, 27.8, 44.4,
66.7, 88.9 and 111.1 km) radius centered at kilometer zero of Paris (the
official Paris center) as shown in Fig. 1. The first ring includes Paris and
highly populated areas surrounding it, while the second one includes the
greater Paris area where the two stationary sites (GOLF, SIRTA) are located.
During summer, when SW winds were predominant, the majority of the mobile
measurements took place N–NE of Paris. The N2.5 decreased exponentially
with distance, reaching 1.3± 1.6 × 104 cm-3
approximately 100 km away from the Paris center (Fig. 10), which is not
statistically different at the 95 % confidence interval from the average
background (not influenced by Paris emissions) concentration
(1.4 ± 1.6 × 104 cm-3) measured during summer
upwind at distances greater than 30 km from the city center by MoLa.
However, at distances shorter than 30 km, where GOLF is located, the
background N2.5 was almost twice as large
(2.5 ± 1.1 × 104 cm-3), indicating a significant
regional number source affecting this area. During 13 July 2009, axial
measurements with respect to Paris were performed under relatively stable
meteorological conditions and the results, shown as black dots in Fig. 10,
are in good agreement with the campaign average values, following the same
exponential decrease. Similar behavior in that area was observed for other
pollutants during the same period (von der Weiden-Reinmüller et al.,
2014b).
Average number concentration (N2.5) with respect to distance
from the city center measured by the mobile platforms during summer (red) and
winter (blue). During both campaigns an exponential decrease in the number
concentration with respect to distance was observed. The number concentration
measured in an axial measurement on a case study day is also depicted in the
graph for summer (black dots) and winter (green triangles).
During winter, N2.5 exhibited an exponential decrease with increasing
distance from the Paris center similar to summer. However, at the center,
N2.5 was 75 % higher than during summer. This difference was
diminished in the Paris suburbs (second bar in Fig. 10), reaching 20 %.
At distances greater than 30 km from the Paris center, no statistical
difference at the 95 % confidence interval between N2.5 measured
during summer and winter was observed. Measured N2.5 further than 70 km
away from Paris remained stable (≈ 1.4 ± 1.9 × 104) and was not statistically different
from the background N2.5 concentrations measured during winter
(1.1 ± 1.4 × 104 cm-3) or summer
(1.4 ± 1.6 × 104 cm-3). During 19 January 2010,
axial measurements were performed and the results (shown as green triangles
in Fig. 10) are also in good agreement with the winter campaign averages.
New particle formation at ground level
A summary of the particle formation categorization for both campaigns can be
found in Fig. 11. During the summer campaign, air ion bursts (of both
polarities) for particles of sizes between 2 and 5 nm were picked up by the
AIS at LHVP on a daily basis (Fig. 11), with the exception of 29 July.
Concentrations of negatively charged particles between 2 and 10 nm were
higher by 1 order of magnitude compared to positively charged particles. In
Fig. 11 we present the NPF categorization based on the negative ions that
provided a more sensitive way of identifying nucleation events.
Nucleation analysis results during summer and winter for all
ground sites. Events, non-events, undefined and lack of data are depicted in
blue, grey, light blue and white, respectively.
During the summer campaign we observed 14 events at SIRTA, 14 at LHVP and 7
at GOLF based on SMPS, DMPS and EAS measurements, respectively. When NPF was
identified at SIRTA it also took place at the city center (Fig. 11) with one
exception (7 July). Due to technical issues of the DMPS, data for 5 days are
not available at the LHVP site. Nucleation events, if identified at two or
more of the ground sites, always occurred practically simultaneously
(< 10 min difference). N10–500 typically doubled at GOLF
due to NPF. At LHVP, an increase of N10–500 ranging between 50 and
150 % was observed due to NPF. The greatest increase in
N10–500, often exceeding 100 %, due to NPF was observed at
SIRTA.
The highest particle growth rate (17.6 nm h-1), based on SMPS
measurements, was observed at SIRTA on 4 July during a regional event
observed at all ground sites, while the lowest growth rate
(1.6 nm h-1) was observed on 15 July at LHVP, where typically lower
daily growth rates compared to the two satellite sites were observed. The
average growth rates were 6.1 ± 1.8, 4.6 ± 1.9 and
5.5 ± 4.1 nm h-1, at GOLF, LHVP and SIRTA, respectively, during
the summer campaign (Table 2). Growth rates for events that occurred on all
sites on the same day were 5.9 ± 2.4, 4.5 ± 2.0 and
8.3 ± 5.6 nm h-1, at GOLF, LHVP and SIRTA, respectively.
During 28 July, nocturnal particle formation was observed at LHVP, which was
identified by an increase in the ion number concentration within the
1.2–1.7 nm size range. An apparent growth of cluster ions to larger
diameters than the upper limit of the preexisting ion pool was evident, but
air ions did not grow above 2 nm. Nocturnal cluster growth has been observed
in remote areas (Junninen et al., 2008; Kalivitis et al., 2012; Hirsikko et
al., 2012) and has been linked to the presence of monoterpenes (Ortega et
al., 2012). Sulfuric acid generation due to nighttime oxidation processes has
also been observed before (Mauldin et al., 2003).
The CS during the summer campaign for all sites is shown in Fig. S2 of the
supplementary information, where event and undefined days are marked with
blue and light blue labels, respectively. During summer the CS was half the
value of that in winter at GOLF
(11.7 ± 11.6 × 10-3 s-1 in summer compared to
21.5 ± 14.4 × 10-3 s-1 in winter) and SIRTA
(5.7 ± 3.5 × 10-3 compared to
12.3 ± 6.8 × 10-3 s-1) and 30 % lower at LHVP
(12.8 ± 7.5 × 10-3 s-1 compared to
17.0 ± 8.6 × 10-3 s-1). During summer at SIRTA and
LHVP, NPF events occurred when the CS was lower than the seasonal average by
45 and 25 %, respectively. Undefined events at both sites were associated
with CS similar to the seasonal average value and non-event days with
25–30 % higher CS compared to the seasonal average. In winter, the high
CS values in conjunction with the low solar intensity (see Sect. 4) most
likely prevented nanoparticle growth and resulted in only five events without
significant growth, identified only by the AIS at LHVP.
The solar intensity influence on NPF event occurrence was evident at SIRTA
and LHVP. During NPF events at these two sites solar intensity was on
average 30 and 20 % higher, respectively, compared to non-event days.
At GOLF, solar intensity during non-event days was found to be higher by
8 % compared to actual event periods.
At GOLF, seven NPF events were identified, corresponding to a monthly
frequency of 23 %. The event frequency difference between GOLF and the
other two ground stations was partially due to a higher frequency (23 %)
of undefined days (Fig. 11) caused by interferences of nearby traffic. When
no event was identified at all sites, the CS at GOLF was double
(14.7 ± 4.5 × 10-3 s-1) compared to event days
(7.3 ± 0.8 × 10-3 s-1), indicating that, similarly
to the other sites, the CS was contributing to the inhibition of NPF
occurrence. On several occasions (2, 6, 8, 23 and 28 July), NPF events were
identified at LHVP and SIRTA (on 8 July it was not clear whether NPF at SIRTA
occurred) but not at GOLF (Supplement Fig. S3). During these days CS values
at GOLF were similar to event days and lower by 30 % compared to the
campaign average, indicating that at least the CS was not suppressing NPF. On
two occasions (6 and 8 July), the observations show a continuous mode below
30 nm, either due to electrometer noise or local interferences, which
prevented identification of NPF. Both days were listed as non-event days, but
NPF may have occurred. During 2 July, a nucleation mode was observed at LHVP
for more than an hour, but nucleated particles did not grow above 20 nm
(Class II events based on Dal Maso et al., 2005). During the same time, an
air ion burst of between 2 and 5 nm particle diameter was picked up by the
AIS at the same site, but at GOLF the nucleation mode was not observed. The
size distribution at SIRTA was not monitored. It is uncertain whether
nucleation occurred and ions did not grow to a detectable size; thus, this
day was listed as a non-event. On 23 July NPF was identified at SIRTA, but at
LHVP only the size distribution below 40 nm was monitored by AIS, due to
technical issues. Air masses crossed SIRTA before reaching GOLF and a fresh
Aitken mode appeared at GOLF 3 h later. Wind direction was constant during
that period and the lag was consistent with the time needed for an air mass
to travel between the two sites at the observed wind speeds (3 m s-1).
Similarly to 23 July, on 28 July an NPF event was identified at SIRTA and
LHVP, while at GOLF a new Aitken mode appeared after approximately 3 h. From
all this, it can be concluded that the event frequency difference between
GOLF and the other two sites can be explained to a large extent by local
interferences and uncertainty in identifying nucleation events.
Inhomogeneities with respect to the extent of NPF between locations a few
tens of kilometers away, similar to this work, have been reported before
(Wehner et al., 2007) and were attributed to cloud cover in combination with
a boundary layer evolution scheme that allowed such behavior. However, in
the cases investigated in this work, cloud cover did not appear to dictate
non-event days at GOLF. Additionally, the beginning of events at all sites
always coincided, unlike the cases reported by Wehner et al. (2007). Despite
these differences, that work also noted the importance of CS in urban areas.
Airborne measurements
Airborne measurements of N10 during summer and winter showed increased
number concentrations downwind of Paris accompanied by increases in light
absorption measured by the PSAP (Fig. 12). These results were attributed to
PM emissions of Paris and are referred to henceforth as the “Paris plume”.
This plume identification method assumes that the only black carbon source in
the area under investigation is the greater Paris region. However, local
sources of black carbon, such as wildfires during summer or domestic heating
during winter, could interfere. To investigate the validity of our
assumption, fire maps derived from satellite information, utilizing a
detection algorithm that includes small fires (Randerson et al., 2012), were
examined for the two periods (summer and winter) under investigation. During
both periods no biomass burning activity was identified, ruling out
interferences due to this source. During winter, areas where simultaneous
increases in absorption and number concentration were identified and
attributed to local sources and not the Paris plume. The particle number
concentrations in these areas were relatively low though. The potential
interference of these sources has a modest to small effect on our estimates
regarding the evolution of the Paris aerosol number plume. A similar method
of plume identification that involves aerosol absorption was also implemented
by Freney et al. (2014) for the same campaign. Increased concentrations of
toluene and benzene, both of which are anthropogenic, were also encountered
in these plumes.
Flight trajectories for 9 (a, b) and 1 (c, d)
July 2009, color coded for black carbon and number concentrations
(N10), respectively. Black carbon concentrations are used as tracers of
the Paris plume (a, c); its direction relative to the city center indicates
wind direction. Red, green and black dots within the figure correspond to
the locations of SIRTA, LHVP and GOLF, respectively. Increased number
concentrations were observed outside of the plume. During 9 July (b) the
area where the number concentration increased was located upwind of the city
center and NPF was identified at all ground sites. During 1 July (d) the
particle number increase was observed along the plume. The number and black
carbon concentration corresponding to (c) and (d) are also shown with respect to
time in Fig. S3.
Due to air traffic restrictions, the ATR-42 was not allowed to get closer
than 30 km to the Paris center, but the Paris plume could be identified as
far as 200 km away from the city. As stated earlier, airborne measurements
were conducted on days when pollution levels were above average and the
flight paths were determined a priori based on forecasted values of the
CHIMERE numerical model; thus, the sample is positively biased. Mobile
laboratories on the ground sampled closer to Paris during the whole campaign,
but separating the plume from the background was cumbersome (von der
Weiden-Reinmüller et al., 2014a).
During summer the averaged aircraft measured N10 within the Paris plume
was 10.1 ± 5.6 × 103 cm-3, which was 14 % higher
than the concentrations observed outside of the Paris plume (8.8 ± 6.5 × 103 cm-3),
defining the background concentrations.
The high background number concentrations in this N to E quadrant where all
of the summer flights but one took place (grey, blue and green lines in Fig. 2) are consistent with the ground (stationary and mobile) observations.
During all summer flights, with the exception of 25 July, “hot spots”
outside of the Paris plume where particle number concentrations similar to or
higher than those of the Paris plume were identified without increase in
black carbon or anthropogenic volatile organic compounds (VOCs; benzene,
toluene). The hot spots where the particle number increase occurred were
separated into three groups based on their location with respect to the Paris
plume as “upwind”, “alongside” and “local”.
The upwind events were identified upwind of Paris four times, always near IDF
(Fig. 12b) and simultaneously with regional nucleation events observed at
least at two of the ground sites. The number concentration increases were
thus attributed to NPF. Assessment of the spatial extension of these events
was complicated by the presence of the plume and limited by the designated
flight paths (Fig. 2). In general, the N10 measured upwind was 40 %
higher than that measured in the plume during these upwind NPF events.
The alongside events occurred at an average distance of 40 km from the plume
edge and were attributed to NPF (Fig. 12d). The average number concentration
increased by 47 % in comparison to the concentration within the Paris
plume. The area in between the Paris plume and the hot spot area always
exhibited at least 20 % lower concentrations than the latter two
(Fig. 12d shows the number concentration with respect to cardinal coordinates
and Supplement Fig. 4 as a time series). The alongside events occurred during
four flights (1, 15, 21, and 28 July), two of which were non-event days for
all ground sites and two when NPF was identified at SIRTA and LHVP, but not
at GOLF. The high N10 areas covered approximately 3000 km2 along
the plume.
In order to investigate why the alongside events occurred only on one side of
the Paris plume during these flights, each flight path was separated into
three areas: (1) the area with high N10 outside of the plume, (2) the
plume area and (3) the area on the other side of the plume, where no increase
in particle number was observed. The observed differences between both sides
of the Paris plume with respect to the CS, solar intensity and isoprene
concentration, which has been reported as a potential inhibitor of NPF in
forested areas (Kiendler-Scharr et al., 2009; Kanawade et al., 2011), were
12, 5 and 6 %, respectively (Supplement Fig. S5). These relatively small
differences probably cannot explain the observed phenomenon. Other pollutants
such as benzene, toluene, monoterpenes, methacrolein, methyl vinyl ketone,
O3 and CO, but also meteorological parameters such as temperature and
RH, were investigated in order to identify potential reasons for the
different particle number concentrations between the two sides of the plume.
Differences in all the investigated parameters were less than 10 %. These
events clearly require more investigation with instrumentation that can
sample particles smaller than 10 nm in combination with trace gas
measurements relevant to NPF (e.g., SO2). Unfortunately, there were no
ground measurements in the areas in which the alongside events were
identified.
The local events were the most frequent (6 out of the 11 study cases) and
occurred either at the northern coast of France downwind of the city of
Fecamp (four events) and were associated with high or medium tide height
(indicating influence of ship emissions?), or near the city of
Aulnoye-Aymeries (four events). On two occasions these events were identified
at both locations during the same flight. Because the local events were
always associated with specific areas, the particle number concentration
increase was attributed to local sources.
During the three winter flights, the Paris plume N2.5 was 45 %
higher than the background and no hot spots were identified, consistent with
ground measurements where no NPF was identified.
Summary and conclusions
Ambient aerosol number concentrations were monitored at the center of Paris
(LHVP) along with two satellite suburban stations (SIRTA, SW, and GOLF, NE).
Mobile measurements were performed by two mobile laboratories and the SAFIRE
aircrafts during July 2009 (summer, ATR-42) and January–February 2010
(winter, Piper Aztec).
During summer, N10–500 (number concentration for particles between
10 and 500 nm diameter) at the city center was lower by 14 % than at the
downwind (GOLF) site and 54 % higher than at the upwind (SIRTA) suburban
site, respectively. The contribution of particles with diameters between 10
and 30 nm to N10–500 increased from the mostly upwind suburban
site (30 % at SIRTA) over the city center (40 % at LHVP) to the
mostly downwind suburban site (50 % at GOLF). The contribution of
particles with diameters between 30 and 100 nm ranged between 40 and
50 % and followed the opposite trend (highest upwind, lowest downwind).
During summer at SIRTA, N10–500 increased to 9.9 ± 2.4 × 103 cm-3
when the site was downwind of Paris and decreased to
4.2 ± 2.5 × 103 cm-3 when the site was upwind. At
GOLF, located at approximately the same distance from the city center as
SIRTA but in the opposite direction (NE), the effect of Paris emissions was
not clear, suggesting a high background N10–500 at the measurement
location for all wind directions.
NPF events were observed at all sites during summer. At SIRTA and LHVP,
events were identified every second day and, at GOLF, once every 4 days on
average. The lower frequency of NPF events at GOLF was mainly due to
interferences from nearby traffic and instrumental limitations that did not
allow clear event identification. NPF occurred during periods when the CS was
lower by 45, 25 and 50 % at SIRTA, LHVP and GOLF, respectively, in
comparison to each site's average value, indicating that the CS may have been
a controlling factor for the frequency of events. Solar intensity was higher
by 30 and 20 % on event days compared to non-event days at SIRTA and
LHVP, respectively. At GOLF, solar intensity was higher by 8 % during
non-event days compared to event days. On average, NPF events caused
N10–500 to double at all stationary measurement sites.
Average particle growth rates were 5.5, 4.6 and 6.1 nm h-1 at SIRTA,
LHVP and GOLF, respectively. The differences between these average growth
rates were not statistically significant.
The particle number concentration within the Paris emission plume was found
to decrease exponentially on the ground with distance from the Paris center
during both campaigns. At distances from the city center greater than 70 km,
N2.5 was approximately 1.4 × 104 cm-3 regardless of
season or whether the measurements were affected by the Paris plume. However,
during summer background conditions (not affected by Paris), N2.5 close
to GOLF (second circle in Fig. 1) was approximately a factor of 2 higher, in
agreement with N10–500 measurements at GOLF that indicated a
higher background in the region NE of Paris.
The Paris plume was identified by aircraft measurements at an altitude of
600 m, using black carbon as a tracer, as far as 200 km away from the city
center. Averaged N10 outside and within the Paris plume was
8.8 ± 6.5 × 103 and
10.1 ± 5.6 × 103 cm-3, respectively, which
corresponds to a 33 % increase. During summer, hot spots of high particle
number concentrations were identified outside of the Paris plume at 600 m
altitude. On four occasions the particle number concentration increase was
located upwind of the ground stations simultaneously with regional NPF
observed on the ground at least at two of the sites. These increases
therefore were attributed to NPF. Increased particle number concentrations
were also identified along one side of the plume on four occasions. A number
of parameters were investigated, including CS, solar irradiance,
anthropogenic and biogenic VOC concentrations among others, as possible
explanations for this asymmetry. All differences observed between both sides
of the Paris plume were approximately 10 % or lower, so none of these
could explain the observations.
During winter the absolute N10–500 was higher by a factor of 2 at
both suburban sites and by 36 % at the city center compared to summer. At
LHVP particles from 10 to 30 nm accounted for 44 % of the
N10–500 on average and those from 30 to 100 nm for 40 %. At
GOLF, similar to summer, the N10–500 was dominated by particles
with diameters between 10 and 30 nm, which accounted for 56 %, followed
by particles from 30 to 100 nm (33 %), following the same trends as
during summer. At SIRTA the contribution of particles from 10 to 30 nm and
from 30 to 100 nm to the N10–500 was 42 and 39 %,
respectively. Regardless of site or season, a mode, centered at a diameter
below 20 nm, was always present and dominated during winter at all sites.
During winter the higher CS and lower solar intensity compared to summer
prevented particles from growing to sizes larger than 10 nm.
A complete year of air ion measurements (including the two intensive
campaigns discussed in the present paper) has been recently presented by Dos
Santos et al. (2015). These measurements took place in the MEGAPOLI site in
the center of Paris (LHVP station) from July 2009 to September 2010. During
this year, the highest NPF frequency in Paris was observed during July 2009
(the summer campaign examined in this work) and the lowest during the winter
(which includes the winter campaign in this work). Therefore, our analysis
above focused on two extreme NPF periods in Paris: during summer under clean
conditions and peak NPF frequency and during winter under polluted conditions
and minimal NPF frequency.
The Supplement related to this article is available online at doi:10.5194/acp-15-10219-2015-supplement.
Acknowledgements
Parts of the research leading to these results have received funding from the
European Union's Seventh Framework Programme FP7 within the MEGAPOLI project,
grant agreement no. 212520, and the ATMOPACS FP7 IDEAS project. The research
conducted by MPIC was supported by internal funds. Support from the French
ANR project MEGAPOLI – PARIS (ANR-09-BLAN-0356) and from the CNRS-INSU/FEFE
via l'ADEME (no. 0962c0018) is acknowledged. We are grateful for the
logistical support in the field by IPSL/SIRTA, by Laboratoire d'Hygiène
de la Ville de Paris (LHVP) and by the staff of the Golf Départemental de
la Poudrerie. The SAFIRE team is acknowledged and thanked for performing
ATR-42 flights and measurements. Edited by:
R. MacKenzie
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