The vertical profile of new particle formation (NPF) events was studied by comparing the aerosol size number distributions measured aloft and at surface level in a suburban environment in Madrid, Spain, using airborne instruments. The horizontal distribution and regional impact of the NPF events was investigated with data from three urban, urban background, and suburban stations in the Madrid metropolitan area. Intensive regional NPF episodes followed by particle growth were simultaneously recorded at three stations in and around Madrid during a field campaign in July 2016. The urban stations presented larger formation rates compared to the suburban station. Condensation and coagulation sinks followed a similar evolution at all stations, with higher values at urban stations. However, the total number concentration of particles larger than 2.5 nm was lower at the urban station and peaked around noon, when black carbon (BC) levels are at a minimum. The vertical soundings demonstrated that ultrafine particles (UFPs) are formed exclusively inside the mixed layer. As convection becomes more effective and the mixed layer grows, UFPs are detected at higher levels. The morning soundings revealed the presence of a residual layer in the upper levels in which aged particles (nucleated and grown on previous days) prevail. The particles in this layer also grow in size, with growth rates significantly smaller than those inside the mixed layer. Under conditions with strong enough convection, the soundings revealed homogeneous number size distributions and growth rates at all altitudes, which follow the same evolution at the other stations considered in this study. This indicates that UFPs are detected quasi-homogenously in an area spanning at least 17 km horizontally. The NPF events extend over the full vertical extension of the mixed layer, which can reach as high as 3000 m in the area, according to previous studies. On some days a marked decline in particle size (shrinkage) was observed in the afternoon, associated with a change in air masses. Additionally, a few nocturnal nucleation-mode bursts were observed at the urban stations, for which further research is needed to elucidate their origin.
In urban areas, traffic emissions are a major source of ultrafine particles (UFPs; Kumar et al., 2014; Ma and Birmili, 2015; Pey et al., 2008, 2009; Dall'Osto et al., 2012; Salma et al., 2014; Paasonen et al., 2016). These emissions include primary UFP exhaust emissions (Shi and Harrison, 1999; Shi et al., 2000; Charron and Harrison, 2003; Uhrner et al., 2007), the cooling of engine exhaust emissions, and the condensation of a semi-volatile-phase vapor species that creates new UFPs during dilution (Charron and Harrison, 2003; Kittelson et al., 2006; Robinson et al., 2007; Rönkkö et al., 2017). These are also considered primary particles, since they are formed near the source. Other relevant UFP sources include industrial emissions (Keuken et al., 2015; El Haddad et al., 2013), city waste incineration (Buonanno and Morawska, 2015), shipping (Kecorius et al., 2016; Johnson et al., 2014), airports (Cheung et al., 2011; Hudda et al., 2014; Keuken et al., 2015), and construction (Kumar and Morawska, 2014).
New particle formation (NPF) from gaseous precursors has been shown to cause high UFP episodes in relatively clean atmospheres due to low condensation sinks (CSs) originating from low pre-existing particle concentrations (e.g., Kulmala et al., 2000, 2004; Boy and Kulmala, 2002; Wiedensohler et al., 2002; Wehner et al., 2007; O'Dowd et al., 2010; Sellegri et al., 2010; Vakkari et al., 2011; Cusack et al., 2013a, b; Tröstl et al., 2016; Kontkanen et al., 2017). However, at mountain sites, precursors' availability seems to be the most influential parameter in NPF events, with higher values of CSs during NPF events than during non-NPF events (Boy et al., 2008; Boulon et al., 2010; García et al., 2014; Nie et al., 2014; among others). Tröstl et al. (2016) reported experimental results on nucleation driven by the oxidation of volatile organic compounds (VOCs), and Kirkby et al. (2016) reported pure biogenic nucleation.
NPF events also contribute significantly to ambient UFP concentrations in
urban environments (Costabile et al., 2009; Wegner et al., 2012; von
Bismarck-Osten et al., 2013, 2014; Ma and Birmili, 2015; Hofman et al., 2016;
Kontkanen et al., 2017). Common features enhancing urban NPF are high
insolation, low relative humidity, the availability of
Reche et al. (2011) evaluated the prevalence of primary versus newly formed UFPs in several European cities and found a different daily pattern for the southern European cities, in which the newly formed particles contributed substantially to the annual average concentrations, probably because of high insolation and possible site-specific chemical precursors. Brines et al. (2015) determined that NPF events lasting for 2 h or more occurred on 55 % of the days, and those extending to 4 h occurred on 28 % of the days, with NPF being the main contributor 14 %–19 % of the time in Mediterranean and subtropical climates (Barcelona, Madrid, Rome, Los Angeles, and Brisbane). The latter percentages reached 2 % and 24 %–28 % in Helsinki and Budapest, respectively (Wegner et al., 2012; Salma et al., 2016). Furthermore, Brines et al. (2015) calculated that 22 % of the annual average UFP number concentration recorded at an urban background site in Barcelona originated from NPF. Ma and Birmili (2015) reported that the annual contribution of traffic to the UFP number concentration was 7 %, 14 %, and 30 % at roadside, urban background, and rural sites, respectively, in and around Leipzig, Germany. On the other hand, traffic emissions contributed to 44 %–69 % of UFP concentrations in Barcelona (Pey et al., 2009; Dall'Osto et al., 2012; Brines et al., 2015), 65 % in London (Harrison et al., 2011; Beddows et al., 2015), and 69 % in Helsinki (Wegner et al., 2012).
Minguillón et al. (2015) and Querol et al. (2017) demonstrated that intensive NPF episodes take place inside the planetary boundary layer (PBL) in Barcelona, occurring around midday at surface level, when insolation and dilution of pollution are at their maxima. Earlier in the morning, NPF can only take place at upper atmospheric levels, at an altitude where pollutants are diluted, since at surface level, a high CS prevents particle formation.
While many studies have investigated NPF around the world, only a few have
focused on the vertical distribution of these events (Stratmann et al.,
2003; Wehner et al., 2010). In view of this, we devised a campaign with the
aim to study photochemical episodes, including high
The Madrid metropolitan area (MMA) lies in the center of the Iberian Peninsula at an elevation of 667 m a.s.l. (meters above sea level). It is surrounded by mountain ranges and river basins that channel the winds in a NE–SW direction. Having an inland Mediterranean climate, winters are cool and summers are hot, and precipitation occurs mainly in autumn and spring. Road traffic and residential heating in winter are the main sources of air pollutants, with small contributions made by industrial and aircraft emissions (Salvador et al., 2015).
In summer, the area is characterized by strong convection, which results in PBL heights as high as 3000 m a.g.l. (above ground level) and mesoscale recirculation caused by anabatic and katabatic winds in the surrounding mountain ranges (Plaza et al., 1997; Crespí et al., 1995), which can lead to the accumulation of pollutants if the recirculation persists for several days.
The cold and warm advection of air masses associated with the passage of upper-level troughs and ridges over the area gives rise to a sequence of accumulation and venting periods, respectively. During accumulation periods, pollutants accumulate in the area, and concentrations increase for 2–6 days, until a trough aloft brings a cold advection and a venting period starts. For a detailed description of the meteorological context during the campaign, see Querol et al. (2018).
A few studies have focused on NPF events in the area. For instance,
Gómez-Moreno et al. (2011) reported NPF episodes in Madrid to be “not a
frequent phenomenon”, since only 63 events per year were detected, with
17 % of the total days occurring mostly in spring and summer. However,
Brines et al. (2015) reported both intensive summer and winter NPF episodes
at the same station, which accounted for 58 % of the time as an annual
average, considering the prevalence of nucleation bursts for 2 h or more.
Alonso-Blanco et al. (2017) described the phenomenology of particle-shrinking
events, i.e., a decline in particle size caused by particle-to-gas
conversion, at an urban background station in Madrid (CIEMAT), stating that
they occur mainly between May and August in the afternoon, due to either a
change in wind direction or the reduction of photochemical processes.
Particle shrinkage following their growth is not a common phenomenon but has
been observed in a few areas around the world. Yao et al. (2010), Cusack et
al. (2013a, b), Young et al. (2013), Skrabalova et al. (2015), and
Alonso-Blanco et al. (2017) and references therein, reported shrinkage rates
ranging from
The data used in this study were collected during a summer campaign in and around Madrid in July 2016. Three air quality supersites were used, namely, an urban station, an urban background station, and a suburban station, in addition to a setting in a suburban environment with two tethered balloons that allowed for the study of the vertical distribution of aerosols and air pollutants. All stations are located within a range of 17 km. A map displaying all locations is shown in Fig. 1.
The CSIC (Consejo Superior de Investigaciones Científicas, the Spanish
national research council) urban station, operative from 9 to 20 July, was
located in the Institute of Agricultural Sciences (40
Location of the stations and sounding setting used in the campaign. The location of the airport is also shown. A white solid line marks the city limits of Madrid.
The CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y
Tecnológicas, Research center for energy, environment, and technology)
urban background station, operative from 4 to 20 July, was located on the
outskirts of Madrid, 4 km from the CSIC station (40
The ISCIII (Instituto de Salud Carlos III, Carlos III Institute of Health)
suburban station was located at the Carlos III Institute of Health in
Majadahonda, 15 km from the CSIC station (40
UFP instrument calibration was performed by the manufacturers: TSI in the
case of SMPS and CPCs and Airmodus for PSM. Particle sizing and counting
instrumentation at all stations were collocated next to windows or walls
where holes were available for inlets and equipped with individual
Regarding the vertical measurements, two tethered balloons carrying
miniaturized instrumentation were based at the Majadahonda (MJDH) rugby field
(40
Identification of NPF events was made via the method proposed by Dal Maso et
al. (2005). After the examination of the daily particle size distribution, if
the day was classified as an event day, we proceeded to calculate growth
rates (GRs), shrinking rates (SRs), condensation and coagulation sinks (CSs and
CoagSs), and formation rates (
The algorithm proposed by Hussein et al. (2005) was used to fit log-normal
modes to the particle size distribution, from which GRs were calculated by
following Eq. (1):
The CS, a measure of the removal rate of condensable vapor molecules due to
their condensation onto pre-existing particles (Kulmala et al., 2012), is
calculated using Eq. (2):
The formation rates of particles were calculated as 30 min averages,
following Eq. (4):
A rough estimation of the mixed layer height was determined using Hy-CPC measurements. The top of the mixed layer was considered to be at an altitude at which particle concentration decreases an order of magnitude quasi-instantaneously and remains constant above this altitude. All UFP profiles are included in Querol et al. (2018).
Additionally, bivariate polar plots of concentration have been used to relate wind speed and direction with total particle concentration using PSM data by means of the R package “openair” (Carslaw and Ropkins, 2012).
Figure S2 shows the evolution of the temperature, relative humidity, wind speed, and wind direction measured at CIEMAT from 5 to 20 July 2016. The evolution of temperatures during this period evidences a succession of accumulation and venting episodes. Rain gauges collected significant precipitation only on 6 July at midnight (not shown).
Summary of new particle formation events recorded during the campaign, showing the starting time, considered as the moment of first detection of the nucleation mode, the final time, considered as the time when the mode reaches 25 nm, the growth rate calculated in that period using SMPS and PSM data, and formation rates at the starting time. An asterisk marks the events that are detected simultaneously at all stations and were chosen for further analysis in this work.
Particle size distribution at CSIC, CIEMAT, and ISCIII (top to bottom), from 4 to 20 July 2016. Total particle concentration of particles with diameter greater than 2.5 nm is also shown.
The balloon field campaign, held from 11 to 14 July, coincided with the start of a venting period, the passage of an upper-level trough, and the transition to an accumulation period when the trough moved to the east of the Iberian Peninsula and a ridge passed over the area of study (see Fig. S3). Maxima and minima temperatures dropped, while strong westerly winds predominated until they veered to the NE on 12 July 18:00 UTC. High nocturnal wind speed peaks were recorded in this period and were often accompanied by a change in wind direction. For detailed information on the meteorological parameters during this campaign, see Querol et al. (2018).
In the following discussions, we group CSIC (urban) and CIEMAT (urban background) as urban stations and compare them to ISCIII (suburban). This grouping is done because of the availability of data during the period of interest. However, it has to be noted that CSIC is more influenced by traffic than CIEMAT, therefore it is more representative of urban environments, and, for this reason, CSIC data are chosen when possible. Eighteen NPF episodes have been identified on a total of 7 days throughout the campaign. A summary of these events is presented in Table 1. Out of these, a total of 14 events on 6 days had simultaneous data available for at least one of the urban stations (CSIC, CIEMAT) and the suburban station (ISCIII). These episodes, marked with an asterisk in Table 1, are selected for further analysis in this section. Figure 2 represents the aerosol number particle size distributions for the selected episodes (12–18 July 2016).
In the selected episodes, intensive daytime nucleation and subsequent condensational growth processes took place simultaneously at urban and suburban stations, located 17 km apart, and, accordingly, we classify these as regional NPF episodes. If the episodes were caused by primary emissions, then we would observe different size distributions at all stations, because each one of them is differently influenced by traffic. The urban station is largely influenced by traffic emissions, whereas the suburban station is much less affected by these emissions. Since we observe the same size distribution at both stations, we can say that traffic emissions are not the origin of the observed distributions. Additional arguments include the fact that number concentrations of sub-25 nm particles peak at noon, when BC levels are at their minimum, as well as the concentration of particles measured by PSM being higher at the suburban station compared to the urban station, implying that the particles did not originate from traffic sources.
Concentrations of BC,
At urban stations, particles of the order of 10 nm are detected throughout the day, even during the night. Conversely, at the suburban station, such small particles are only detected during daytime. Additionally, during some days, a very intense short nucleation burst is registered at around midnight local time at urban stations, but nothing of this nature is detected at the suburban station. This phenomenon is analyzed in Sect. 3.4.2.
Despite the detection of sub-10 nm particles as early as 04:00 UTC (06:00 LT – local time) at the urban stations, only after around 09:00 UTC is the growth of the particles observed, occurring roughly at the same time in both urban and suburban stations. Newly formed particles grow until they have reached sizes of up to 50 nm, usually around 13:00 UTC (15:00 LT). After this, shrinkage is observed on 10 days, corresponding to 71 % of the days with available data. Consequently, the evolution of the particle size distribution is arc-shaped in these cases.
It should be noted that nucleation episodes coincide in time with the early
increases in
For the observed daily regional NPF events, GRs for the nucleation mode,
The calculated GR for the surface stations are shown in Table 1. GRs ranged
from 2.9 to 7.6 nm h
Mean daily cycles of
The GRs calculated in this study are also consistent with those observed in
other urban and suburban areas. Kulmala et al. (2004) concluded that typical
GRs are 1–20 nm h
Figure 4 shows the average daily cycles of particle concentrations in the
size ranges 9–25 nm (
Growth rates (GR
The average values of the formation rates agree with those reported at
similar stations around the world. For instance, Woo et al. (2001) reported
Particle size distribution with fitted log-normal modes (black dots)
measured during the balloon soundings at Majadahonda on 12 July 2016. An
estimation of the mixing layer height is represented with orange dots. The
altitude of the instrumentation is represented with a white line. Surface
level is 630 m above sea level. Time is UTC. Local time is UTC
Querol et al. (2018) studied the vertical profiles of UFP and
The NPF events described in Sect. 3.2 that took place between 12 and 14 July
were not only detected at surface level, but also in the upper layers with
the balloons soundings in Majadahonda. However, the measurements were not
continuous, since the balloons could not be operated safely if the wind speed
was above 8 m s
Figure 5 shows the fitted modes to the particle size distribution measured in the soundings on 12 July. The fact that sub-40 nm particles are not detected at the higher levels of the first flights suggests that convection is not very effective yet, and the sounding goes through different atmospheric layers, most likely the mixed layer and the residual layer. In the residual layer, Aitken-mode particles formed on previous days prevail (Stull, 1988). The interphase between the mixed layer and the residual layer, i.e., the mixed layer height, has been derived using the UFP vertical profiles (see Querol et al., 2018). From 10:00 UTC onwards, once convection has fully developed, the mixed layer covers all the sounding, and we see a homogeneous distribution at all levels, which is also comparable to those recorded with the instrumentation measuring at the surface. This agrees with the fact that UFPs are homogeneously distributed in the mixed layer and are detected at higher altitudes as the mixed layer rises.
In the early morning the size distribution is dominated by a 60 nm mode at
all altitudes, which grows to 100 nm at 11:00 UTC. Even though it is
detected at all levels, the mode decreases slightly in size when the
sounding ascends above the mixed layer limit, which is more clearly visible
on the second flight at around 09:00 UTC. This result suggests that there are
lower vapor concentrations in the residual layer, which inhibits particle
growth, whereas the mixed layer is more polluted, spurring faster particle
growth. The GRs calculated for this mode were 1.8 nm h
Particle size distribution with fitted log-normal modes (black dots)
measured during the balloon soundings at Majadahonda on 13 July 2016. The
altitude of the instrumentation is represented with a white line. Surface
level is 630 m a.s.l. Time is UTC. Local time is UTC
Moreover, during the morning, we observed particles growing inside the
mixing layer, from 10 nm at 07:00 UTC to 30 nm at midday. This mode is
observed simultaneously at ISCIII; therefore, we consider it for
calculation. The GR obtained is 3.5 nm h
Particle size distribution with fitted log-normal modes (black dots)
measured during the balloon soundings at Majadahonda on 14 July 2016. The
altitude of the instrumentation is represented with a white line. An
estimation of the mixing layer height is represented with orange dots.
Surface level is 630 m above sea level. Time is UTC. Local time is
UTC
Vertical particle size distribution measured on 14 July during selected soundings.
The size distribution and the corresponding fitted modes for the soundings
made on 13 July are presented in Fig. 6. Although the balloons could not fly
until 10:30 UTC for safety reasons, at least two modes are detected from early
morning at the sounding location. A mode starting roughly at 40 nm at 07:00 UTC grows to 100 nm at 15:00 UTC. With a GR of 8.5 nm h
Figure 7 shows the particle size distribution and fitted modes for the
soundings made on 14 July. Correspondingly, in Fig. 8, the vertical
distribution of particles for some of the soundings is presented. The
earliest soundings revealed the existence of a residual layer aloft. In
order to verify this result, two constant altitude flights were made during
the morning. The extension of the wire was not modified during these
flights. However, changing wind conditions varied the altitude of
the instruments slightly. The altitude was chosen so that the instruments initially remained
outside the mixing layer, i.e., inside the residual layer. As the
insolation increased, so did the altitude of the mixing layer, until it
reached the altitude at which the balloons were positioned. As the mixing
layer reached the balloons, total particle concentration increased sharply
from
According to the abrupt decline in particle concentration, the boundary
between the mixing and residual layers was located at 1000 m at 09:00 UTC,
1200 m at 10:00 UTC, 1350 m at 11:00 UTC, and beyond 1800 m after 12:00 UTC.
This can be taken as an indicator of the effectiveness of convection,
meaning that after 12:00 UTC, the whole measured particle population was well
mixed throughout the sounding range. Inside the residual layer, particles
had a slower GR (0.5 nm h
Nucleation mode particles were detected exclusively inside the mixing layer
from 08:00 to 12:00 UTC, whereas growth was only observed from 09:00 to
11:00 UTC and from 12:00 UTC onwards. The time spacing between both growth periods
coincides with a marked decrease in wind speed. During the first period, GRs
at the sounding station, ISCIII, and CSIC were 6.2, 5.4, and 1.4 nm h
Overall, the soundings revealed that there is simultaneous growth and shrinking of nucleation and Aitken modes and that both of them grow and shrink at different rates. This was also observed in the surface measurements when comparing urban and suburban stations (see Sect. 3.2.2).
A further interesting feature is the presence of the Aitken mode on most days. Usually in the size range between 50–100 nm, reaching 110 nm in some cases, this mode does not correspond to newly formed particles, but it follows a parallel evolution (condensational growth and potential shrinkage). When looking at the evolution of aerosol size distributions on consecutive days, it is possible to see a connection between this 50–100 nm mode and the distribution of the previous days. The nucleated and grown mode from one day is still present the following day, and it continues to grow until it eventually fades away or grows beyond the detection limits of the instruments. On some occasions, the Aitken mode can be tracked for 2 or more consecutive days, alternating between the stages of growth and shrinkage.
The start of the shrinking phase coincides with a marked increase in wind
speed (Fig. S5); therefore, it is associated with dilution, which favors the
evaporation of semi-volatile vapors, resulting in a decline in particle
diameter and concentrations, as observed in most cases. The calculated
shrinkage rates are shown in Table S1. SRs for particles with a starting
diameter below 40 nm range from
Bipolar plot of
Although outside of the major focus of this study (photochemical nucleation),
other interesting events were detected which took place at night. From 6 to
11 and 17 to 19 July, high concentrations of 1.2–4 nm particles are
registered shortly after sunset for several hours, simultaneously at urban and suburban
stations (see Fig. 2). BC, NO, and
In order to determine the origin of these sub-25 nm particles, bivariate
polar plots of concentration have been used to relate wind speed and
direction measured at CIEMAT with total particle concentration of 1.2–2.5 nm
particles, BC,
In the discussion paper, we pointed out the airport Adolfo Suárez Madrid–Barajas, located NE of the city, as a possible source of these high UFP concentrations. However, the UFP peaks lasted for about 1 h on all days, whereas strong NE winds prevailed for a few hours. Moreover, the airport has flights all night; therefore, a longer period with high UFPs should be observed. Although other studies have linked aircraft emissions with nucleation bursts without growth (Cheung et al., 2011; Masiol et al., 2017), in this study we cannot affirm that the airport is the origin of these bursts. As mentioned before, these episodes were unexpected and were not the main focus of this study. To elucidate the origin of these UFP bursts, further research will be required.
We investigated the phenomenology of regional and secondary new particle formation (NPF) episodes in central Spain. To this end, we set up three supersites (an urban, an urban background, and a suburban site) 17 km apart in and around Madrid. We were able to characterize six NPF events, and, in all cases, the evolution of the particle size distribution (PSD) was very similar at all stations; around sunrise, nucleation-mode particles appear and start growing, and in the afternoon, a decline in particle sizes, i.e., shrinkage, is observed. The regional origin of the NPF is supported by the simultaneous variation in PSD in the nucleation mode, particle number concentrations, and growth and shrinkage rates. Furthermore, temporal evolutions of condensation and coagulation sinks were similar at all stations, having minimum values shortly before sunrise and increasing after dawn towards the maximum value after midday in the early afternoon. In spite of the 17 km scale and the simultaneous processes affecting particle number concentrations, the following relevant differences between urban and suburban stations were observed: (i) the urban stations presented larger formation rates as compared to the suburban stations, and (ii) in general, the sinks were higher at the urban stations.
Regarding the vertical soundings of the NPF events, we observed that, in the early morning, the vertical distribution of newly formed particles is differentiated into two layers. The lower layer (mixed layer – ML), in which convection is effective, is well-mixed and has a homogeneous PSD. This ML heightens throughout the day, as insolation is more pronounced, extending beyond the sounding limits around midday. NPF occurs throughout this ML, and GRs and concentrations are homogeneous. The upper layer is a stable residual layer (RL), in which particles formed or transported during the previous days prevail. In the RL, growth is inhibited or even completely restrained, compared with the growth of the same particles in the ML. Overall, the soundings demonstrate that particles are formed inside the ML, but they can prevail and be displaced and stored at upper levels and continue to evolve on following days.
In this campaign we could not measure during the earliest stages of NPF due to the safety requirements imposed on the balloon flights early in the morning. We think it is important for future work to carry out soundings during the nucleation phase of the episodes. However, miniaturized instruments able to measure smaller particles would be needed and are not available at the present time. Carrying out these soundings would allow us to determine whether secondary NPF takes place throughout the ML or occurs at the surface and is transported upwards afterwards by convection. If the former is true, then locations with high ML could produce more secondary particles than we have considered, and they could affect a larger population or influence climate to a greater extent.
Additionally, a few nocturnal bursts of nucleation-mode particles were observed in the urban stations, and further research is needed to elucidate their origin.
We cannot determine whether the NPF episodes were triggered by the pollution generated in the city that extended to the region or caused by a broader phenomenon. Either way, it can be concluded that, in summer, the particle number concentrations are dominated by NPF in a wide area. The impact of traffic emissions on concentrations of UFPs is much smaller than that of NPF, even near the city center, where the pollution load is at the highest. This result is in line with other studies performed in cities from high-insolation regions (e.g., Kulmala et al., 2016). Given the extent of the episodes, the health effects of NPF can affect a vast number of people, considering that the Madrid metropolitan area, with more than 6 million inhabitants, is the most populated area in Spain and one of the most populated in Europe (UN, 2008). For this reason, we believe that the study of health effects related to newly formed particle inhalation is crucial.
All data used in this study can be accessed here:
The supplement related to this article is available online at:
Data analysis was done by CC, NP and LD. CC, NP, LD, PP, VK, BT, NM, DB, RH, TP, MK, AA and XQ contributed to the discussion and interpretation of the results. CC and XQ wrote the manuscript. NP, CR, MEa, GT, HL, HE, YP, EM, MEs, FG, EA, EC, AS, BT, NM, DB, RH, KA, AA and XQ carried out the measurements. All the authors commented on the manuscript.
The authors declare that they have no conflict of interest.
This work was supported by the Spanish Ministry of Agriculture, Fishing,
Food and Environment; the Ministry of Economy, Industry and Competitiveness;
the Madrid City Council and Regional Government; FEDER funds under the
project HOUSE (CGL2016-78594-R); the CUD of Zaragoza (project CUD 2016-05);
the Government of Catalonia (AGAUR 2017 SGR44); and the Korean Ministry of
Environment through “The Eco-Innovation project”. The funding received by
ERA-PLANET (