GAW Mt. Cimone global station (Italy, 2165 m a.s.l.)

Atmospheric Environment 141 (2016) 139e152

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Summer atmospheric composition over the Mediterranean basin: Investigation on transport processes and pollutant export to the free troposphere by observations at the WMO/GAW Mt. Cimone global station (Italy, 2165 m a.s.l.) P. Cristofanelli*, T.C. Landi, F. Calzolari, R. Duchi, A. Marinoni, M. Rinaldi, P. Bonasoni Institute of Atmospheric Sciences and Climate, National Research Council of Italy, Via Gobetti 101, 40129, Bologna, Italy

h i g h l i g h t s
Mt. Cimone (Italy) is strategic to study Mediterranean summer atmospheric composition.
We investigate processes affecting summer trace gas and aerosols at Mt. Cimone.
Impact of different atmospheric regimes was investigated.
77% of observations can be tagged to aged emissions or background conditions.
Tracer probability density functions are able to detect specific event.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 December 2015 Received in revised form 30 May 2016 Accepted 18 June 2016 Available online 22 June 2016

In this work, we analysed reactive gases (O3, CO, NOx) and aerosol properties (eqBC, ss and particle number concentration) collected at the WMO/GAW Mt. Cimone station (2165 m a.s.l., Italy) during the summer of 2012 in the framework of PEGASOS project. The major aim of this experiment is providing a characterization of the variability of summer atmospheric composition over the central Mediterranean basin, which is considered as a global “hot-spot” for atmospheric pollution and climate change. The atmospheric tracers have been analysed as a function of (i) meteorological parameters, (ii) synoptic-scale circulation and (iii) anthropogenic emission source proximity as estimated by O3/NOx ratio variability. In particular, we identified three O3/NOx regimes which tagged the distance of anthropogenic sources: near outflow (23% of hourly data), far-outflow (38% of data) and background (39% of data). The highest levels of anthropogenic pollutants (e.g. O3, CO, eqBC, accumulation particles) were concomitant with fresh emissions from northern Italy under near-outflow conditions: once injected to the free troposphere, these air-masses, rich in pollutants and climate-forcers (i.e. O3, eqBC) and soil dust, can be spread over a large region, thus significantly affecting regional climate. Moreover, based on the anthropogenic source proximity, atmospheric tracer variability and synoptic-scale atmospheric circulation, we categorized and characterised four types of atmospheric regimes associated with (1) air-mass transport from the free troposphere, (2) transport of fresh emitted pollutants from the PBL, (3) transport at regional/continental scale of aged anthropogenic (4) transport of air-mass rich in mineral dust from northern Africa (i.e. coming from more than 1000 km). Lastly, by analysing the probability density functions (PDFs) of trace gases and aerosol properties, “fingerprints” of the mentioned atmospheric regimes were pointed out. Such information is useful for the implementation of early-warning services, for the timely detection of event occurrence as well as for the definition of observation-based diagnostic for model verifications. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Mediterranean basin Po basin Short-lived climate forcers/pollutants Air-mass age Air-mass transport

1. Introduction * Corresponding author. E-mail address: [email protected] (P. Cristofanelli). http://dx.doi.org/10.1016/j.atmosenv.2016.06.048 1352-2310/© 2016 Elsevier Ltd. All rights reserved.

The Mediterranean Basin is considered a hot-spot region in terms of air-quality (Monks et al., 2009) and climate change (Giorgi

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and Lionello, 2008), due to the impact of anthropogenic and natural processes (Lelieveld et al., 2002; Kanakidou et al., 2011). During the warm season, events of ozone (O3) photochemical production frequently occur in this region favored by high pressure conditions (e.g. Vautard et al., 2005) and anthropogenic pollutants can be vented up to mid-troposphere by vertical thermal processes (i.e. Henne et al., 2004; Monks et al., 2009). Kalabokas et al. (2007), pointed out that during summertime enhanced O3 frequently exceeded the 8-h EU air quality standard (60 ppb) in the lower troposphere over the Eastern Mediterranean basin, while Safieddinne et al. (2014) showed that the central Mediterranean basin (i.e. from 5 E to 20 E) is the region were surface O3 is maximized in summer. As reported by Richards et al. (2013), local anthropogenic NOx emissions result in the overall largest sensitivity to near-surface and lower troposphere (below 700 hPa) summertime O3 distribution over the Mediterranean basin. Over the eastern Mediterranean basin, the presence of an O3 pool in the middle troposphere has been noticed (Zanis et al., 2014) which has been mainly related to stratosphere-troposphere exchange and subsidence from the upper troposphere. In addition, Saharan dust outbreaks from northern Africa (Querol et al., 2009) as well as widespread open biomass burning (Turquety et al., 2014) further exacerbate air-quality and the impact of anthropogenic emissions on the regional climate (Mallet et al., 2013). Because of the impact of these processes on air-quality and regional climate, ultimately on population health and ecosystem integrity, in the recent years experimental forecast and near-real time event detection services were launched for a suite of atmospheric compounds and disruptive events (e.g. Basart et al., 2012; Wagner et al., 2015 and references therein). In this study, we provide an almost comprehensive characterization of the summer atmospheric composition over the central Mediterranean basin, by using a wide set of continuous observations at the high mountain station of Mt. Cimone (CMN, 44120 N, 10 420 E; 2165 m a.s.l.), which is located in the Italian northern Apennines. Due to its elevation, this research site is typically exposed to air-masses from the planetary boundary layer (PBL) of the northern Italy (encompassing also the Po basin) during summer day-time, while it is representative of free troposphere during night-time (see e.g. Fischer et al., 2003; Rinaldi et al., 2015). Therefore, CMN represents a strategic site to investigate the impact of PBL air-mass export on the atmospheric composition of the Mediterranean free troposphere as well as to characterize its background conditions. In this paper, we present and discuss variability of reactive gases (ozone, carbon monoxide, nitrogen oxides) and aerosol properties (number concentration and size distribution, aerosol scattering, equivalent black carbon concentration) observed during an intensive field campaign (2015, 10th June e 10th July) carried out in the framework of the EU project PEGASOS (Pan-European GasAeroSOls Climate Interaction Study, see Rinaldi et al., 2015). Besides providing an overall characterization of the PEGASOS measurement period at CMN, we used qualitative information about the proximity of emission sources, combined with analysis of synoptic-scale atmospheric circulation regimes and atmospheric tracer variability for discriminating the occurrence of different classes of air-masses transport (fresh pollution, aged pollution, dust transport, upper tropospheric air-mass transport). Nitrogen oxides (NOx ¼ NO þ NO2) influence tropospheric radical chemistry, O3 formation and secondary aerosol by oxidation to aerosol nitrate. Carbon monoxide (CO) is emitted from combustion processes, but it is also formed by the oxidation of methane (CH4) and volatile organic compounds (VOCs). CO is also a major O3 precursor. O3 is a short-lived climate forcers/pollutants (SLCF/P) as being a greenhouse gas and a tropospheric oxidizing agent, thus affecting

photochemical processes. Atmospheric aerosols (pollution particles, smoke, mineral dust, marine particles) exert a highly uncertain effect on radiative climate forcing and can have serious impacts on human health, especially in the Mediterranean basin (e.g. Mallet et al., 2013). In particular, black carbon (BC) is also recognized as an important SLCF/P due to its impact on solar and thermal radiation and on human health (UNEP and WMO, 2011). Several studies in Europe have been carried out to investigate the impact of air-mass transport to atmospheric composition variability in the free troposphere as deduced by observations at high mountain observatories. As an instance, Cuevas et al. (2013) investigated the impact of long-range transport processes to long-term surface
a (Canaries, 2373 m O3 at the subtropical high mountain station Izan a.s.l.). By using air-mass back-trajectories and by analysing the O3CO relationship, these authors suggested significant impact of anthropogenic emissions from North America to O3 variability. }o } v et al. (2008) estimated background concenSimilarly, Balzani Lo trations of trace gases at the Jungfraujoch Alpine station (Switzerland, 3580 m a.s.l.) by using backward trajectories and atmospheric compound variabilities, showing that “primary” compounds appeared to be greatly influenced by long range transport during winter, while “secondary” compound levels and photochemistry impact increase until summer. For the same site, CollaudCoen et al. (2011) investigated 14 years of meteorological parameters, aerosol variables (absorption and scattering coefficients, aerosol number concentration) and trace gases (CO, NOx,SO2) as a function of different synoptic weather types providing a quantification of air-mass export to the free troposphere. Moreover, Herrmann et al. (2015) investigated the influence of free troposphere and boundary layer on aerosol size distribution at Jungfraujoch, by analyzing the last contact of air-masses with PBL (by using the FLEXPART dispersion model) and variability of NOy/CO ratio used to estimate the “age” of air masses. They found that number concentration of particles larger than 90 nm were strongly affected by PBL which influences the measurement site especially during summer. During the Pic 2005 field campaign (13 June e 7 July 2005), Gheusi et al. (2011) investigated the question of the vertical layering of O3 in a mountain area by an experimental set-up combining in situ ground-based observations at the high mountain station Puy du Midi (2875 m a.s.l., French Pyrenees) with O3 lidar at two lower sites in close vicinity. They provided evidences that mixing with free-tropospheric air, photochemistry and surface deposition in the valleys should be considered for accounting quantitatively for the observed O3 variations. Through PEGASOS intensive field campaign, it was possible to shed light on the typical level and variability of trace gases and aerosol properties as a function of transport regimes and proximity with emission sources, thus offering new insights into the sources of atmospheric pollutants and climate forcers above the central Mediterranean Basin during summer season. In particular, for the first time, NOx measurements have been used to disentangling the proximity of emission sources related to anthropogenic pollution observed at CMN. We also provide a set of information useful for the implementation of early-warning services based on background in-situ observations as well as for the definition of observation-based diagnostics for model validation, by identifying a set of “optimal” parameters useful for detection of different airmass transport processes at CMN. 2. Material and methods The measurements analysed in this paper were carried out at the Italian Climate Observatory “O. Vittori”, a research infrastructure which is part of the Mt. Cimone GAW/WMO Global Station (GAW ID: CMN, see www.isac.cnr.it/cimone). For all the observed

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parameters, time is expressed as UTC þ 1. 2.1. Trace gas observations Trace gas intake extends 2 m above the roof and 7 m above the ground and consists of a glass tube through which the sampled air is passed at a high flow rate (40 lt/s). Surface O3 was continuously determined by a UV-analyzer (Dasibi 1108 W/GEN) regularly calibrated (about every 3 months) with the laboratory transfer standard (Thermo 49iPS) which, on turn, is compared every 2 years against the SRP#15 primary standard hosted at the WMO/GAW World Calibration Center at EMPA (Swiss Federal Laboratory for Materials Science and Technology). According to Klausen et al. (2003), the current total combined uncertainty of 1-min average values is usually less than 2 ppb in the range 1e100 ppb. A Thermo Electron 48C-TL, using gas filter correlation and NDIR technology, was used to determine CO ambient mole fraction. With the aim of minimizing possible influence of water vapor in the NDIR detection, the ambient air pass through a drying system (Nafion© Dryer) and it is then injected in the measurement cell. The system is also equipped with a purge air unit (constituted of a Parker drying system and steel tube filled with Sofnocat 423). In order to minimize the influence of temperature on the measurements zero calibrations are performed every 30 min. A span check (500 ppb) is performed once a day, starting at 00:30 and ending at 00:45. Full calibration is performed every three months by dilution of a certified CO standard at 500 ppb. The accuracy of the measurement, in terms of relative standard deviation over daily repeated analysis of the 500 ppb certified CO standard (15 min per day) is 4%, with a total expanded uncertainty of 8% (k ¼ 2). Nitrogen oxides (NOx ¼ NO þ NO2) are measured by chemioluminescence detection (Thermo 42). NOx is measured after conversion of NO2 to NO by a heated (300  C) Molybdenum converter. As indicated by Steinbacher et al. (2007), NO2 reading obtained in this way can be overestimated up to ~50% due to the interference of other oxidized nitrogen compounds such as peroxyacetyl nitrate (PAN) and nitric acid: this limitation will be considered for the data discussion (see Section 3). The instrument was fully calibrated at the manufacturer laboratories before the start of the experimental campaign, while during the campaign zero checks were performed every 7 days. The uncertainty of the NO (NO2) is 5% (6%), with a total expanded uncertainty of 10% (14%) with a coverage factor k ¼ 2. 2.2. Aerosol physical property observations A specifically designed TSP (Total Suspended Particle) air intake is used for aerosol particle sampling. A sample flow of 150 lt/min is maintained to guarantee laminar flow within air-intake. T and RH within the air-intake were continuously monitored and a heating system allows dry sampling and avoids ice rimming blocking the sampling flow. A Condensation Particle Counter (TSI 3772) detects airborne particles with diameter ranging from 10 nm (counting efficiency > 50%) to 3 mm at an aerosol flow rate of 1.0 l/min, over a concentration range from 0 to 104 particles/cm3. According to the manufacturer, the particle concentration accuracy is ±10%. An optical particle counter (OPC, Grimm, Particle Size Analyzer Mod. 1.108) provides particle counts in the size range 0.3 mm < Dp < 20 mm. The instrument is based on the quantification of the 90 scattering of light by aerosol particles. According to the specifications, the reproducibility of the OPC in particle counting is ±2% (Putaud et al., 2004). Such measurements allow the determination of the accumulation (0.3 mm < Dp < 1 mm, Nacc) and coarse (1 mm < Dp < 20 mm, Ncoarse) aerosol particle number concentrations with a 1 min time resolution. At CMN, continuous

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measurements of equivalent black carbon concentration (eqBC) are obtained by a multi-angle absorption photometer (MAAP 5012, Thermo Electron Corporation, see Petzold et al., 2002) with an accuracy of 3.2% on the 1-min measurement of aerosol absorption coefficient (Müller et al., 2011). A M9003 integrating nephelometer (ECOTECH) has been measuring the aerosol scattering coefficient at 525 nm since May 2007. The measured values are automatically adjusted to standard temperature and pressure conditions (0  C; 1000 hPa). An additional heater on the measurement cell eliminates the effects of humidity particle growth on scattering behavior. A calibration on site with low span gas (filtered air) and high span gas (filtered carbon dioxide) is performed every 3 months. The instrument is operated with an internal averaging time of 1 min. 2.3. Meteorological observations At CMN, meteorological parameters (air temperature, relative humidity, atmospheric pressure, wind speed and direction) were continuously observed with an automatic integrated weather station. In particular, air temperature, atmospheric pressure and relative humidity were used to derive specific humidity (SH), a well known tracer for PBL air-masses at high mountain sites (see Zellweger et al., 2009). Solar radiation (wavelength: 350e1100 nm) is measured by a commercial silicon cell pyranometer (Skye SKS110) installed on the highest point of the station roof. 2.4. Air-mass back-trajectories With the aim of providing information about the synoptic-scale circulation scenarios which affect the CMN region, we calculated three-dimensional 5-day back-trajectories ending at CMN. We used the HYSPLIT model (Draxler and Hess, 1997) for calculating every 6 h (at 00, 06, 12 and 18 UTC) an ensemble of backtrajectories composed by seven trajectories, one of these ending at the real CMN location and the other slightly shifted in altitude (250 m) and horizontal location (±0.5 ). The model calculations are based on the GDAS meteorological field produced by NCEP, with a horizontal resolution of 1  1. As discussed by Cui et al. (2009), this horizontal resolution is sufficient to provide information about the synoptic-scale atmospheric circulation at mountain sites. In order to aggregate back-trajectories of common path, and to better characterize the synoptic-scale circulation occurring at CMN, a cluster aggregation technique was applied (see Draxler and Hess, 1998). A detailed description of this methodology can be found by Putero et al. (2014). The analysis of the path and origin of the back-trajectories enclosed in each cluster lead the identification of six “major” transport patterns to CMN during the experimental campaign (see Fig. S1 in the supplementary material): (1) air-masses moving from northern Africa (AF); (2) air-masses recirculating over the Mediterranean basin or continental Europe (REG), likely associated with weak synoptic forcing under high-pressure conditions; (3) air-masses originating in the north-eastern region of Atlantic Ocean and crossing western Europe continent within anti-cyclonic circulation and slow westerly flow (NW); (4 and 5) air-masses originating over central Atlantic Ocean (WES) or North America (AM) and travelling over western Europe under fast westerly flow; (6) air-masses linked with northerly circulation and originating at latitudes > 60 N (ARC). In general, AF, REG, and NW clusters are populated by a significant fraction of back-trajectories travelling at low altitudes (i.e. < 2000 m a.s.l.), thus supporting a significant contribution of mixing with PBL air-masses. On the opposite, NAM and ARC were populated by back-trajectories mostly originating at altitudes representative of the free troposphere (i.e. > 2000 m a.s.l.) and

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experiencing downward motion to CMN altitudes. 3. Results and discussion 3.1. Overview of the summer 2012 PEGASOS campaign at CMN In order to identify the different weather regimes which affected the measurement site region during the field campaign, we analysed the time series of meteorological parameters and solar radiations at CMN (see Fig. S2 in the supplementary material). During the first campaign period (10th e 14th June), bad weather conditions affected CMN: low temperature (<5  C), high RH (>95%) and fresh breezes coming from North-West were observed. These conditions were related to the passage of a pressure trough, which steered air-masses from higher latitudes (NAM and NW circulation) to CMN (Fig. 1). Later, high pressure conditions persisted for almost one week (15th e 20th June). During this period, especially from 15th to 17th June, diurnal wind breeze was active at CMN, as deduced by the diurnal cycle of relative humidity (RH) and wind speed (WS). The presence of the anticyclonic region also favored the occurrence of REG synoptic circulation for CMN (Fig. 1). A shallow trough affected CMN on 20the21st June, leading to the occurrence of south-westerly flows under AF circulation and to an increase of RH and WS at the measurement site. On 22nd e 23rd June, northerly AR circulation affected CMN: dry air-masses (RH < 40%) were observed at the measurement site during a period of atmospheric pressure rising. On the following days (27th e 29th June), REG and WES air-masses affected CMN: they were related to E-SE and W winds, respectively. After that, AF and REG circulations re-established until 2nd July (Fig. 1), with gentle westerly winds, high temperature and dry air condition (RH < 60%) at the measurement site. From 3rd July, atmospheric pressure slightly decreased at CMN and remained stable until the end of the field campaign: on 4th e 5th July, WES circulation were diagnosed, while continental-scale circulation (REG and NW) mostly characterised the period 6th e 10th July. In summary, the studied period was characterised by a significant meteorological variability which provide a relevant opportunity to investigate the impact of different weather and atmospheric circulation regimes to the atmospheric composition variability over the Mediterranean basin. 3.2. Comparison with “historical” observations To evaluate the PEGASOS filed campaign results in the framework of historical observations at CMN, we compared the values of

reactive gases and aerosol properties with those recorded on previous summer seasons (i.e. June e August). To obtain the “climatological” reference, we decided to consider all the observations available for each parameters, i.e. 1996e2011 for O3, 2007e2011 for CO, 2007e2011 for eqBC, 2002e2011 for Nacc and Ncoarse, 2005e2011 for ss. For each parameter, for the PEGASOS period and the “climatological” time-frame, we calculated basic percentiles and average values of hourly mean values (Fig. 2). During the PEGASOS period, CO appeared to be well comparable with reference data-set both in terms of average values and variability (see the very similar upper and lower percentiles). O3 shows, for the PEGASOS campaign, a shift of the distribution towards lower values, with 90th and 95th percentiles much lower than the climatological reference. eqBC and Nacc showed very similar median values between the two data-sets, however the 90th percentiles for the PEGASOS campaign are lower than the 75th percentile of the reference data-set, suggesting the lack of extremely high aerosol events related to anthropogenic pollution. In respect to the reference data-set, ss showed a shift towards higher values for the lowest percentiles and the inner quantile (i.e. 25th e 75th percentiles). Lastly, Ncoarse reported similar lower percentiles and inner quantile between the two data-sets (please take into account the logarithmic scale of y-axis) but with an evident shift toward higher values for 90th and 95th percentiles for the PEGASOS campaign. For both ss and Ncoarse this would indicate an higher “baseline” value in respect to the climatological reference. To investigate the possibility that the bad weather conditions occurring during the first 5 days of the experimental campaign could have biased the PEGASOS values of the atmospheric tracers, we considered a shorter measurement period excluding 10th e 14th June 2012. As reported by Fig. 2, for this shorter measurement period (PEG-S), the deviations of mean average values with the reference climatology decreased for O3, eqBC and Nacc which also showed almost identical median values with the climatology. Still higher values were observed for ss and Ncoarse. As will be shown later the motivation for such a large Ncoarse (and possibly ss) can be related to occurrence of two “major” dust outbreak events observed during PEGASOS. Direct comparison of PEGASOS results with observations from an extensive field campaign (MINATROC, see Fischer et al., 2003) carried out at CMN in June 2000, revealed similar O3 average values (58.2 ppb and 58 ppb, respectively), lower CO (110.8 ppb vs. 121 ppb) but higher NOx (0.7 ppb vs. 0.28 ppb). It should be noted that the higher NOx values observed during PEGASOS campaign can be, at least in part, related to the use of a CLD instrument equipped with Mo converter for NO2 determination (as discussed in Section 2.1 and Section 3.4).

Fig. 1. Daily time series of air-mass circulation at CMN as deduced by HYSPLIT back-trajectories: northern Africa (AF); Regional (REG); north-west (NW); west (WES); northern America (NAM); Artic (ARC). For each back-trajectory ensemble, calculated every 3 h, the y-axis reports the number of members belonging to each trajectory cluster.

P. Cristofanelli et al. / Atmospheric Environment 141 (2016) 139e152 1.0

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Fig. 2. Box-and-whiskers plot of 24-h averaged values for O3, CO, eqBC (first row), aerosol scattering, accumulation and coarse particle number (second row) at CMN on summer seasons 1996e2011 (JJA), for the whole PEGASOS campaign (PEG) as well as for the PEGASOS-short period (PEG-S). The box and whiskers denote the 10th, 25th, 50th, 75th and 90th percentiles, the dots represent the 5th and 95th percentiles and the bold lines the average values.

3.3. Impact of thermal transport to variability of summer atmospheric composition We analysed the diurnal behavior of atmospheric parameters by considering the percentiles and average mean values of hourly observations (Fig. 3). As reported by Rinaldi et al. (2015) for the PEGASOS campaign and by previous studies (e.g. Fischer et al., 2003), summer atmospheric composition at CMN is affected by convective transport from lower altitudes during day-time and by the occurrence of residual layers or “free tropospheric” air-masses during night-time. The diurnal-scale influence of air-mass transport from PBL is reflected by the diurnal cycle in SH. On average, SH started to increase at 9:00 with maxima values from 11:00 to 18:00, indicating the time period typically influenced by vertical convection and thermal transport at the measurement site. A decreasing behavior to typical night-time values was observed until 23:00. The period characterised by the highest SH is also affected by the lowest WS: according to Gheusi et al. (2011) this indicated (on average) a prevailing influence of upward thermal wind. Conversely, higher WS are observed during night-time, indicating a stronger influence of large-scale dynamical forcing more representative of the synoptic-scale circulation. The occurrence of vertical transport of air-masses from the PBL is pointed out by the increase of CO, NOx, eqBc and Nacc during day-

time (their highest average values are observed around 17:00). The diurnal variability of O3/NOx and CO/NOx show the lowest averaged values (less than 100) from late morning (11:00) to evening (21:00), while larger values (greater than 200) occurred during the nightearly morning. This suggests an impact of nearer/recent anthropogenic source emissions during the central part of the day and farther/older during night-time (Morgan et al., 2010). On average, during the central part of the day, when PBL air-masses started to affected the measurement site (see also the high SH values) lower O3 occurs in respect to evening and night-time (on average, 4 ppb). This “reversal” O3 diurnal variations can be related to the transport from the PBL of air-masses poorer in O3 (but richer in CO, eqBC and NOx in respect to the conditions more representative of the free troposphere occurring during night-time) as well as to dry O3 deposition processes occurring along mountain slope (see Gheusi et al., 2011). According to Schuepbach et al. (2001), this transport is likely to mask in-situ photochemical production occurring during the central part of the day. On the other side, the positive correlation between O3 and CO on afternoon-evening suggested, for this period, an efficient transport of air-masses richer in O3 produced within the PBL of the northern Italy or present in residual layers above the PBL. It is interesting to note that the diurnal increases of atmospheric pollutants (CO, eqBC, NOx, Nacc) during day-time is accompanied by

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Fig. 3. Analysis of average diurnal variations for atmospheric tracers (O3, CO, NOx, eqBC, ss, Nacc, Ncoarse, Ncoarse excluding dust events), specific humidity (SH), wind speed (WS), O3/ NOx and CO/NOx ratios during the field campaign. The data percentiles (5th, 25th, 50th, 75th, 95th) and the mean average values (tick lines) were reported.

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Ncoarse ðiÞ

p ¼ thermal; African

i¼1

Where i ¼ 1,n represents the number of hours for which thermal and African outbreak regimes occurred (150 and 96, respectively). The results showed Nthermalcoarse, ¼ 90.9 cm3 h and NAfricancoarse, ¼ 186.9 cm3 h, thus pointing out that the contribution of thermal transport processes to the variability of Ncoarse over the Po basin is not completely negligible (see also Bucci et al., in preparation).

3.4. Variability of summer atmospheric composition as a function of emission source proximity With the purpose of better characterising the evolution of aerosol and trace gases observed at CMN, we investigated the variability of O3/NOx together with the CO/NOx ratios that, according to Morgan et al. (2010) and Parrish et al. (2009), can be considered as proxies for qualitatively evaluate the proximity to major emission sources and photochemical processing. Fig. S3 reports the scatterplot of CO/NOx vs O3/NOx hourly values with data categorized as a function of CO and eqBC values (proxies for combustion emissions). A strict linear relationship was evident between CO/NOx and O3/NOx (R2: 0.99; a: 0.6159 ± 0.0005) with the lower values (i.e. roughly below 100) as being characterised by the highest CO and eqBC values (i.e. higher than 120 ppb and 300 ng/ m3). The lower values of CO/NOx and O3/NOx were 50 and 20, respectively: according to Parrish et al. (2009) this would indicate that direct pollution sources did not affected CMN. This is also supported by the analysis of concurrent organic aerosol (OA) measurements (Rinaldi et al., 2015) showing almost no direct influence of freshly emitted primary aerosols to the observed OA load during the PEGASOS period. We categorized the hourly CMN data-set as a function of O3/NOx ratios. In agreement with the definition proposed by Morgan et al. (2010), the data were split among three photochemical regimes which tagged the distance of anthropogenic sources: near outflow (N-OUT, O3/NOx  50, 23% of hourly data), far-outflow (F-OUT, 50 < O3/NOx  90, 38% of data) and background (BKG; O3/NOx > 90, 39% of data). To specifically select these regimes, we analysed CO and eqBC variability as function of O3/NOx (Fig. 4). Due to NOx oxidation, dilution and photochemistry processes, O3/NOx is usually higher than 10 within air-masses just downwind of pollution

150 140 130 CO (ppb)

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sources (Neuman et al., 2009). The value 50 was selected for O3/NOx as threshold between N-OUT and F-OUT regimes, because of the discontinuity of CO and eqBC (i.e. a marked decrease) around this value (Fig. 4): indeed, primary pollutants (i.e. CO, eqBC) tend to decrease during transport due to dilution downwind of the pollution sources. CO and eqBC steadily decreased moving towards higher O3/NOx and stabilized for O3/NOx greater than 90, indicating the shift from the F-OUT to the BKG regime. In addition, a sensitivity study has been performed for investigating the impact of the possible NO2 overestimation due to the use of Molybdenum converter for NO2 determination (see Section 2.1) to O3/NOx calculation and then to regime identification. Fig. 4 also reports hourly CO and rec eqBC variability as function of O3/NOrec x , where NOx was obtained by halving the original NO2 values. Apart a clearer difference between F-OUT and BKG regime for CO, possibly pointing out the interference of other oxidized nitrogen compounds to the NO2 reading, no significant differences were pointed out for the regime identification, thus indicating the robustness of our selection. The analysis of trace gases and aerosol variability as a function of photochemical processing pointed out that all the atmospheric compounds showed decreasing values shifting from N-OUT to BKG with statistical significant (at the 95% confidence level) differences among the three regimes (Table 1). Overall, the highest decrease was observed for NOx (74%) and Nacc (60%), while the lowest decrease were observed for O3 (4.7%). With the purpose of taking into account the diurnal variability observed at CMN for the atmospheric tracers (Section 3.2), we classified trace gases and aerosol properties also as a function of time periods more (07e21 UTC þ 1, hereinafter “day-time”) or less (23e06 UTC þ 1, “nighttime”) influenced by air-masses originated from PBL. The first point to note is that, as expected, “nigh-time” observations were marginally affected by N-OUT regime (only 5% of data) and significantly affected by BKG regime (69.6% of data), stressing that during this time window CMN is well representative of the background conditions of the free troposphere. The decreasing trend of

eqBC (ng m )

a simultaneous increase of the Ncoarse, once that the Saharan dust events (see Section 3.5) are neglected. This would suggest that the north Italy PBL represents a source of coarse particle for the free troposphere of the Mediterranean basin. By analysing the diurnal variation of the Ncoarse/Nacc ratio (which maximized in the early morning and then decreased moving to evening, here not shown), we ruled out the possibility that the Ncoarse increase observed during afternoon-evening could be mainly due to aerosol aging. Thus, it is conceivable to suppose that this coarse aerosol (with mean particle diameter Dp ¼ 1.96 mm) can be related to soil particles emitted by wind erosion, traffic, agriculture or other anthropogenic activities during dry period along the summer season. Even if the amount of coarse particle related with thermal transport is lower in respect to the one observed during dust outbreaks from northern Africa, it appeared to be less “erratic” (at last during the summer period under dry weather conditions). To provide a quantitative comparison of the contributions to Ncoarse by thermal transport versus Africa dust outbreaks, we calculated the respective integral Ncoarse (Nthermalcoarse, Nafricancoarse) during PEGASOS. We defined, Nthermalcoarse, and Nafricancoarse as:

145

500 400 300 200 100 10 20 30 40 50 60 70 80 90 100 200 300 400 500 O3/NOx

Fig. 4. Relationship between CO and eqBC with O3/NOx ratio. The red dashed lines represents the regime boundaries (N-OUT, F-OUT, BKG) discussed in the paper. Red dots represent O3/NOx values obtained from original data, while blue dots denote O3/ NOx values obtained by halving the NO2 original values. Vertical bars denote the 95% confidence level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Average mean value (Avg) and number of data (ND) for O3 (ppb), CO (ppb), NOx (ppb), eqBC (ng m3), integrated fine particle number (Np, cm3), accumulation particle number (Nacc, cm3), aerosol scattering (ss, Mm1) as a function of the different photochemical processing (N-OUT, F-OUT, BKG). Upper: all data, middle: “day-time” data; bottom: “nigh-time” data. N-OUT Avg All data 62.5 ± 1.7 O3 CO 131.7 ± 2.5 NOx 1.62 ± 0.08 eqBC 395.2 ± 18.5 Np 2594.2 ± 209.7 37.7 ± 3.5 Nacc ss 55.7 ± 3.4 07e21 UTC þ 1 63.2 ± 1.7 O3 CO 133.4 ± 2.3 NOx 1.64 ± 0.08 eqBC 406.1 ± 15.6 Np 2621.8 ± 213.7 Nacc 38.8 ± 3.6 ss 56.9 ± 3.5 22e06 UTC þ 1 O3 55.5 ± 7.5 CO 109.3 ± 8.1 NOx 1.46 ± 0.23 eqBC 279.9 ± 66.5 Np 1762.4 ± 197.5 Nacc 24.4 ± 9.2 ss 42.0 ± 11.4

F-OUT

BKG

ND

Avg

ND

Avg

ND

120 116 131 131 102 121 130

57.9 ± 1.2 111.9 ± 1.9 0.88 ± 0.03 292.2 ± 14.2 1702.1 ± 115.5 22.6 ± 1.4 44.7 ± 2.6

232 209 228 242 221 235 241

59.1 ± 1.1 99.6 ± 1.6 0.42 ± 0.02 214.3 ± 13.1 1099.6 ± 76.7 14.2 ± 1.2 32.3 ± 2.4

234 188 193 241 235 238 241

110 108 121 121 99 111 120

56.8 ± 1.2 114.3 ± 1.9 0.86 ± 0.04 307.8 ± 17.0 1766.8 ± 142.6 23.5 ± 1.6 46.3 ± 3.1

157 145 166 167 155 163 166

57.0 ± 1.4 102.2 ± 2.2 0.42 ± 0.02 209.6 ± 15.7 1202.3 ± 132.0 14.1 ± 1.5 32.1 ± 3.4

98 84 91 105 101 102 105

8 7 8 8 2 8 8

59.5 ± 2.6 103.9 ± 5.1 0.89 ± 0.06 244.6 ± 25.6 1488.7 ± 222.2 20.8 ± 3.4 41.1 ± 5.4

57 47 44 57 50 54 57

60.6 ± 1.8 96.9 ± 2.6 0.42 ± 0.03 220.8 ± 23.3 1024.5 ± 102.6 14.3 ± 2.0 32.6 ± 3.9

108 80 81 108 107 108 108

atmospheric tracer values from N-OUT to BKG was evident also for “nigh-time” and “day-time” selections. Only for O3, an increasing tendency was observed for the “nigh-time” selection, probably indicating effect of NO titration in air-masses affected by fresh anthropogenic emissions (N-OUT). A further point to be stressed is that the average differences between “night-time” and “day-time” selections were minimized for the BKG regime, for which the two data-sets did not show statistically significant differences (at the 95% confidence level): this argues for the robustness of the BKG regime identification. The same was observed for F-OUT regime, but with statistically significant higher value for CO (þ10.4 ppb) and eqBC (þ63.2 ng m3) for “day-time” selection. In this context, we analysed the variability of atmospheric tracers as a function of wind direction and O3/NOx regimes (Fig. 5), by calculating the wind rose of the proportional contribution to the mean for two representative primary pollutants (i.e. CO and eqBC) and for O3 (secondary pollutant). Concerning the N-OUT regimes, SE and SW sectors appear to control the overall mean of CO, eqBC and O3. Especially for eqBC and O3, the highest values were related to SE winds, indicating the role of the Po basin as near source of pollutants during day-time (please remember that 93% of N-OUT data were observed during this time period). Shifting towards FOUT and BKG regimes (i.e. higher O3/NOx), the wind sectors from SW completely dominated the variability of atmospheric tracers/ pollutants, indicating that processed emissions or background contributions mostly influenced atmospheric composition at CMN even in summer, when the direct contribution from the Po Basin due to thermal transport is maximized. 3.5. Variability of summer atmospheric composition as a function of atmospheric regimes Due to its strategic location in the heart of the Mediterranean basin and its altitude which allows, during the summer period, alternate observations of free tropospheric and PBL air-masses, we

used CMN observations to characterize a subset of four atmospheric regimes which are expected to influence the variability of atmospheric composition in the Mediterranean basin: (i) Fresh pollution transport from north Italy PBL (FRESH), (ii) Aged pollution transport occurring at regional-scale (AGED), (iii) Transport from the upper troposphere (UT) and (iv) Mineral dust transport from Africa (DUST). In particular, the different atmospheric regimes were categorized by using specific criteria which are based on the occurrence of favourable synoptic-scale atmospheric circulation and objective analysis of atmospheric tracer variability: (i) FRESH: occurrence of clear diurnal cycle in atmospheric pollutants (i.e. O3, CO, eqBC) well representative of the thermal export from PBL air- with a prevalence of N-OUT regime and REG/NW atmospheric circulation (15e18 June; 3e5 July); (ii) AGED: measurement periods characterised by enhanced primary pollutants (i.e. CO and eqBC higher than the PEGASOS averaged mean values) but without the appearance of well-defined diurnal cycle with a prevalence of F-OUT regime and REG/NW air-masses (17e20 June; 23e26 June; 2e3 July, 6 July); (iii) UT: low values of primary pollutants (i.e. CO and eqBC lower than the PEGASOS averaged mean values) with a prevalence of BKG regime and ARC/NAM air-masses (15e16 June, 22e24 June, 2e4 July); (iv) DUST: presence of large amount (i.e. > 0.5 cm3) of coarse particle (see Duchi et al., 2016) used as proxy for mineral dust with AF or REG air-mass circulation (20e21 June and 30 June e 1 July). Fig. 6 shows the time series of trace gases (O3, CO, NOx) and aerosol properties (number concentrations, eqBC, scattering coefficient) together with representative for the different atmospheric regimes, while Fig. 7 reports the probability density functions (PDF) of the observed atmospheric tracers. Overall, significant differences characterised the four regimes as a function for the variability of atmospheric composition (Table 2, Fig. 7). The UT regime showed an average O3 value (68.0 ± 0.2 ppb) exceeding the 75th percentile of the entire data-set, while the remaining atmospheric tracers showed values lower than the field campaign average values. For eqBC, the PDF showed a distribution peak at the lowest concentration boundary with frequency distribution dramatically decreasing moving towards higher values, while Nacc a frequency peak at very low number concentration values (around 7 cm3) was observed. The DUST regime was characterised by the lowest O3 values (52.5 ± 0.1 ppb) and the highest Ncoarse (1.95 ± 0.03 cm3) and ss (61.7 Mm1). The narrow PDF distributions observed for O3 and Ncoarse under UT and DUST regimes allows to unambiguously trace the specific impacts of free troposphere conditions and mineral dust transport at the measurement site. This behavior is also reflected by the ss: the PDF for the DUST regime showed a peculiar shape with a secondary maximum around 90 Mm1. Moreover, this regime is characterised by a larger baseline value for Nacc as deduced by the PDF shape showing completely lacking data lower than 10 cm3. For DUST, CO and NOx, did not show different PDF shape compared to the UT regime. Together with the very low average CO value (99.7 ppb), this would trace a lack of important CO sources over northern Africa during the investigated period. However, for NOx it might be argued that mineral dust particles could also favour the depletion of HNO3 and NO3 along the airmass travel (Ndour et al., 2008; Fairlie et al., 2010), thus suppressing their recycling and removing a significant fraction of NOx. Nonetheless, relatively high value of eqBC (333.5 ng/m3) were

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147

Fig. 5. Analysis of CO (1st raw), eqBC (2nd raw) and O3 (3rd raw) as a function of wind direction and photochemical processing (1st column: N-OUT; 2nd: column F-OUT; 3rd column: BKG). Tracer values are split into the intervals shown by the scale in each panel and into the different wind sector to magnify the contribution by each wind direction to their overall mean.

observed for these air-masses, thus suggesting the potential mixing with polluted air-masses during transport towards the measurement site. This was particularly evident during the first event (21st  22nd June), when the peak on Ncoarse was related to an increase of eqBC (up to 733 ng/m3). This would imply that mineral dust was mixed with polluted air-masses. This is evident also from the PDF analysis which shows an eqBC peak value around 300 ng/m3 and not negligible contribution up to 500 ng/ m3, with enhanced values in respect to the “background” UT conditions. Previous observations at CMN (Cristofanelli et al., 2009) pointed out that during summer, Saharan dust transport could be accompanied by plume of biomass burning occurring over the northern Africa coastlines. However, this possibility appears to be unlikely in this case: no significant open fires were detected by MODIS satellite for the considered period. Besides, the presence of light wind breeze from NE, would support a possible mixing with pollution from the regional PBL. The two pollution regimes (FRESH and AGED) were clearly characterised by the highest values of anthropogenic pollutants,

with similar values for O3 and CO (Table 2). As shown by PDF analysis, hourly O3 higher than 75 ppb were mostly related to the FRESH regime, while the highest occurrence of high eqBC (>500 ng/ m3) and Nacc (>70 cm3) were observed for both FRESH and AGED regimes, clearly tracing the advection of air-masses from the PBL. The AGED regime was characterised by lower NOx and Np in respect to FRESH regime, tracing farther emission sources. For both CO and NOx, the PDFs for the AGED regime showed a mixed condition between DUST/UT and FRESH regimes, indicating the influence of air-mass mixing/dilution and photochemical processes in respect to fresh/near emissions. To provide information about the impact of air-mass aging on the aerosol number size distribution, we calculated the ratios Nacc/Np for the different regimes. This ratio increased from 1  103 to 2  103 from the FRESH to the AGED regime, showing that the air-mass aging processes favored a higher contribution of accumulation particles to the overall size distribution. This relative higher contribution of particles with larger diameters in the AGED air-masses, probably determined the larger ss observed under this regime (Collaud Coen et al., 2004).

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Fig. 6. Reactive gas and aerosol property time series (hourly mean values) at CMN during the PEGASOS field campaign. On the top, the representative transport events are indicated: fresh pollution from Po basin (red), aged regional pollution (purple), free troposphere (blue), mineral dust advection (orange). In the bottom, the photochemical processing regime as deduced by the O3/NOx ratio. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3.6. O3-CO correlation as a function of the atmospheric regimes To provide hints about the net O3 production during the transport of air-masses to the site, we analysed CO-O3 correlations as a function of the different atmospheric regimes (i.e. FRESH, AGED, UT and DUST). In general, a positive correlation can be expected, under the assumption that CO is a proxy for the amount of reactive carbon used during O3 production (Fischer et al., 2003). Fig. S4, showed the scatterplots of O3 and CO at CMN for the different atmospheric

regimes together with their fitting parameters (R2 and slopes). Significant correlation was observed for the FRESH regime, with a slope of 0.30 ± 0.04, which is typical for rural and remote sites in moderately polluted boundary layer air (Fischer et al., 2003 and reference therein) and indicated a moderate impact of combustion emissions to O3 production at CMN. In the case of “aged” pollution (AGED regime), statistically significant O3-CO correlation was also found (R2: 0.47; m: 0.23 ± 0.06). This is in agreement with Cristofanelli et al. (2013) who estimated that for the warm period

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149

Fig. 7. Probability Density Function (PDF) of atmospheric tracers at CMN for the whole PEGASOS data-set (black), as well as for different atmospheric regimes (FRESH: red, AGED: blue; UT: green, DUST: red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

“April e September”, enhanced values of photochemical O3 and CO are mostly related to emission ages up to 7 days. No significant correlation between O3 and CO were observed for UT regime (R2: 0.10), probably indicating not intense photochemistry or stratosphere-troposphere exchange (STE) influence (Voulgarakis et al., 2011). A significant positive O3-CO correlation (R2: 0.31) but

with low linear slope (m: 0.14 ± 0.04) was found for the DUST regime: in agreement with the results showed before, this further indicates a not completely negligible influence of photochemical O3 production probably due to mixing of air-masses rich in mineral dust with PBL pollution.

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Table 2 Basic statistical parameters (Average mean value: Average; standard deviation: Dev.st; number of data: ND; 95% confidence level: 95% CL; 25th and 75th percentiles: 25th and 75th) for O3 (ppb), CO (ppb), NOx (ppb), eqBC (ng m3), integrated fine particle number (Np, cm3), accumulation particle number (Nacc, cm3), coarse particle number (Ncoarse, cm3), aerosol scattering (ss, Mm1) for the different atmospheric regimes and for all data.

UT

DUST

FRESH

AGED

ALL

Average Dev.st. ND CL 95% 25th 75th Average Dev.st. ND CL 95% 25th 75th Average Dev.st. ND CL 95% 25th 75th Average Dev.st. ND CL 95% 25th 75th Average Dev.st. ND CL 95% 25th 75th

O3

CO

NOx

eqBC

Npt

Nacc

Ncoarse

ss

68.0 4.1 2585 0.2 65.8 70.7 52.5 4.2 5630 0.1 49.7 55.2 67.2 10.2 2272 0.4 56.9 75.9 65.2 9.9 3925 0.3 56.9 72.8 58.2 9.6 40,795 0.1 50.9 64.9

96.5 9.3 948 0.6 90.5 102.0 99.7 17.0 2546 0.7 88.5 110.5 131.9 22.7 1055 1.4 124.9 145.8 120.2 14.7 1697 0.7 111.0 131.3 110.8 19.0 14,527 0.3 97.0 123.7

0.28 0.18 2587 0.01 0.15 0.39 0.39 0.27 5760 0.01 0.18 0.54 1.49 0.95 2422 0.04 1.12 2.03 0.91 0.35 4074 0.01 0.65 1.14 0.70 0.56 37,758 0.01 0.32 1.01

207.6 104.8 1897 4.7 122.0 283.0 333.5 130.4 5609 3.4 247.0 400.0 404.2 165.2 2233 6.9 295.0 518.0 361.5 134.2 4000 4.2 264.0 452.0 296.6 155.9 33,159 1.7 173.0 396.0

758.1 332.1 218 44.1 572.3 898.7 959.8 467.5 509 40.6 650.3 1171.0 2781.7 1471.2 185 212.0 1835.3 3869.5 1592.4 422.9 300 47.9 1474.7 1896.1 1528.8 986.6 3163 34.4 813.5 1863.5

10.1 4.6 2587 0.2 6.8 12.9 26.7 8.8 5760 0.2 20.1 31.5 29.1 15.6 1578 0.8 19.1 33.3 26.7 12.1 4011 0.4 18.9 31.7 20.5 16.0 40,134 0.2 8.2 28.7

0.18 0.20 2587 0.01 0.08 0.19 1.95 1.11 5760 0.03 1.09 2.37 0.26 0.10 1578 0.00 0.21 0.30 0.51 0.43 4011 0.01 0.25 0.52 0.47 0.76 40,134 0.01 0.12 0.38

26.49 11.05 2565 0.43 20.22 35.08 61.74 21.40 5710 0.56 42.03 76.60 41.24 17.53 2339 0.71 22.91 53.26 46.39 15.19 4038 0.47 35.91 55.38 39.53 22.59 41,136 0.22 22.25 54.71

4. Discussion and conclusions In the framework of the PEGASOS project (Pan-European GasAeroSOls-climate interaction Study) international field experiments were performed for one-month period during the summer of 2012 in the Po Basin region (Rinaldi et al., 2015). As being located in the northern part of the Mediterranean basin, this region is considered a global hot-spot in terms of air-quality and anthropogenic emission impact on it. A wide set of measurement programmes carried out at the WMO/GAW Mt. Cimone (2165 m a.s.l.) Global Station provided the opportunity to investigate the summer atmospheric composition variability more than 2000 m a.s.l. over the Po Basin, with a special emphasis on the impact of anthropogenic pollution transport to the free troposphere. In particular, we categorized the hourly CMN data-set within three regimes representative of the proximity of anthropogenic source emissions: nearoutflow (N-OUT,. O3/NOx  50, 23% of hourly data), far-outflow (FOUT, 50 < O3/NOx  90, 38% of data) and background (BKG; O3/ NOx > 90, 39% of data). To some extent this is an arbitrary classification, however the differences of atmospheric tracer values (especially for CO and eqBC) among the different regimes, suggest that this classification is rather robust and sounding. In this work, for the first time at CMN, a new approach using the combined investigation of O3/NOx regimes, atmospheric composition tracer variability (i.e. exceedance of threshold values and appearance of diurnal cycles), synoptic-scale circulation types, was used to identify and characterize four specific atmospheric regimes influenced by: (1) air-mass transport from the free troposphere; (2) transport of fresh emissions from the northern Italy (i.e. through convective PBL); (3); regional scale transport of aged anthropogenic emissions; (4) transport of air-mass rich in mineral dust from northern Africa. If the first 4 days of observations were neglected, the average and median values of O3, CO, eqBc and Nacc (i.e. anthropogenic

pollutants) during summer 2012 were comparable with historical observations at CMN: not “extreme” values was observed during PEGASOS. Higher average and median values were observed for ss and Ncoarse, which can be related to the occurrence of two “major” Saharan dust events, according to the definition proposed in Duchi et al. (2016). However, it should be stressed that the fraction of days affected by dust outbreaks (18%) is also comparable with the climatological values reported by Duchi et al. (2016) for June (23%) and July (17%). Despite O3 and CO, the aerosol tracers (i.e. eqBC, ss, Nacc, Ncoarse) showed higher values for the lower percentiles (up to 25th) in respect to the historical summer observations. This can be attributed to the relatively low amount of rain precipitation that affected the northern Italy during summer 2012 (ARPA EmiliaRomagna, 2013) and that could have limited the influence of wet scavenging to atmospheric aerosols. We have found out that the highest levels of anthropogenic pollutants (O3, CO, eqBC, accumulation particles) were observed in concomitance of fresh emissions (i.e. low O3/NOx ratios). This regime is typically associated with the occurrence of enhanced diurnal variability of trace gases and aerosol as well as significant photochemical O3 production, well traced by O3-CO correlation (R2: 0.60; m: 0.30 ± 0.04). Together with the high correlation with SH values (see Rinaldi et al., 2015) this clearly indicates the prominent role of diurnal thermal transport (i.e. slope and valley wind breezes, through the convective PBL dynamic and possible influence of residual layers) in favoring the transport of polluted air-masses and soil particles from PBL to CMN. The highest pollutant values were related to wind from SE (i.e. from the Po basin), which contributed for about 30% of the observed fresh pollution conditions. Since CMN is located at the interface between PBL and the free troposphere, during warm months, this suggests that these air-masses are effectively injected to the free troposphere were they can be spread over larger regions. This is a point of high concern since some of

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these pollutants (i.e. O3 and eqBC) are known to be short-lived climate forcers (UNEP and WMO, 2011) and, once in the free troposphere, can significantly affect regional climate. Interestingly, a diurnal variation was observed also for the coarse particle number concentration. As also suggested by Bucci et al. (in preparation) by analysing simultaneous LIDAR observation in the Po basin, this would suggest that soil dust emitted during dry summer periods can be vented to CMN altitude, thus representing a not completely negligible source of mineral aerosol for the Mediterranean free troposphere. As deduced by the analysis of the O3/NOx, about 36% of CMN observations can be related to relatively “aged” pollution emissions, mostly related to south-westerly wind circulation. Even if characterised by high O3, eqBC and CO, In respect to the “fresh” pollution, this regime featured a more aged aerosol (i.e. higher Nacc/Np ratio) and lower NOx values, thus tracing more distant/older anthropogenic sources. A statistically significant O3-CO correlation was also found (R2: 0.47; m: 0.23 ± 0.06), thus indicating the occurrence of photochemical O3 production. “Clean” atmospheric conditions were observed when air-masses originated in the free troposphere. This regime was characterised by significant amount of O3 which, however, cannot be traced to photochemical production related with combustion emissions, as testified by the absence of correlation with CO (R2: 0.10). Dust outbreaks from north Africa favored large increases in the number concentration of coarse particles. Such phenomenon is associated with the highest value of ss observed at CMN (67 Mm1 on average), nearly two-fold in respect to the average summer value, indicating the possible climatic impact on regional scale associated with these events. As indicated by other studies (e.g. Hanisch and Crowley, 2003; Fairlie et al., 2010), the DUST regime was characterised by relatively low values of O3 and NOx, probably due to the impact of heterogeneous chemistry occurring at the dust particle surface and to the ability of dust particle in modify the actinic flux (as testified by the high associated scattering coefficient). However, it should be stressed that air-masses originated from northern Africa are expected to be characterised by relatively low NOx due to the low amount of anthropogenic sources. Interestingly, we found out significant amount of eqBC on air-masses from northern Africa during the dust outbreak on 21st June. In this case, the analysis of atmospheric tracer diurnal variations (CO, eqBC, SH, CO/NOx, see Fig. 6) and the positive O3-CO correlation, indicated a not negligible influence of fresh PBL pollution (as also deduced by the CO/NOx never exceeding 150) by thermal transport also during the DUST regime. This is in agreement with the finding by Bucci et al. (in preparation) who, for 20th e 21st June 2012, showed a wide layer characterised by the presence of mineral dust and PBL pollution extending from the Po Basin surface up to CMN altitude over the Po basin, thus indicating mixing between longrange transported mineral dust and local/regional combustion emissions. Finally, we analysed trace gases and aerosol Probability Density Functions (PDFs) related to different atmospheric regimes: some of them (UT, FRESH, and DUST) were tagged to specific tracer values and PDF shapes which can be used to unambiguously detect the event occurrence. Our findings demonstrated that this approach might be useful to implement a methodology for a real-time automatic identification of specific atmospheric regimes as well as for process diagnostic in the verification of model outputs in the Mediterranean basin. Acknowledgements CMN continuous observation programme during PEGASOS summer experiment 2012 were supported in part by the National

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Project of Interest NextData, funded by the Italian Ministry for University, Research and Education and by the ACTRIS Research Infrastructure Project (2011e2015) supported by the European Union Seventh Framework Programme (EU FP7/2007e26013) under grant agreement no. 262254. HYSPLIT model was provided by NOAA-ARL, GDAS meteorological files used for HYSPLIT calculation were provided by NCEP. Figures 5 and S3 are produced by using R “Openair” package (Carslaw and Ropkins, 2012). The authors would like to thank the Italian Air Force e CAMM Mt. Cimone and the “Magera Team” for the technical and logistic assistance at CMN. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2016.06.048. References ARPA Emilia e Romagna, 2013. Annuario dei dati Ambientali 2012. In: Arpa EmiliaRomagna, Via Po, pp. 5e40139 (Bologna). Basart, S., et al., 2012. Development and evaluation of the BSC-DREAM8b dust regional model over Northern Africa, the Mediterranean and the Middle East. Tellus B 64, 1e23. Bucci, et al., 2016. Transport regimes analysis over Po valley during summer 2012: impacts on planetary boundary layer variability and aerosol content. To be Submitt. Atmos. Chem. Phys. in preparation. Carslaw, D.C., Ropkins, K., 2012. Openair d an R package for air quality data analysis. Environ. Model. Softw. 27e28, 52e61. Collaud Coen, M., Weingartner, E., Schaub, D., Hueglin, C., Corrigan, C., et al., 2004. Saharan dust events at the Jungfraujoch: detection by wavelength dependence of the single scattering albedo and first climatology analysis. Atmos. Chem. Phys. 4, 2465e2480. vo ^ t, A.S.H., Collaud Coen, M., Weingartner, E., Furger, M., Nyeki, S., Pre Steinbacher, M., Baltensperger, U., 2011. Aerosol climatology and planetary boundary influence at the Jungfraujoch analyzed by synoptic weather types. Atmos. Chem. Phys. 11, 5931e5944. http://dx.doi.org/10.5194/acp-11-59312011. , U., Calzolari, F., Colombo, T., Cristofanelli, P., Marinoni, A., Arduini, J., Bonafe Decesari, S., Duchi, R., Facchini, M.C., Fierli, F., Finessi, E., Maione, M., Chiari, M., Calzolai, G., Messina, P., Orlandi, E., Roccato, F., Bonasoni, P., 2009. Significant variations of trace gas composition and aerosol properties at Mt. Cimone during air mass transport from North Africa e contributions from wildfire emissions and mineral dust. Atmos. Chem. Phys. 9, 4603e4619. http://dx.doi.org/10.5194/ acp-9-4603-2009. Cristofanelli, P., Fierli, F., Marinoni, A., Calzolari, F., Duchi, R., Burkhart, J., Stohl, A., Maione, M., Arduin, J., Bonasoni, P., 2013. Influence of biomass burning and anthropogenic emissions on ozone, carbon monoxide and black carbon at the Mt. Cimone GAW-WMO global station (Italy, 2165 m a.s.l.). Atmos. Chem. Phys. 13, 15e30. http://dx.doi.org/10.5194/acp-13-15-2013. Cui, J., Sprenger, M., Staehelin, J., Siegrist, A., Kunz, M., Henne, S., Steinbacher, M., 2009. Impact of stratospheric intrusions and intercontinental transport on ozone at Jungfraujoch in 2005: comparison and validation of two Lagrangian approaches. Atmos. Chem. Phys. 9, 3371e3383. http://dx.doi.org/10.5194/acp-93371-2009. lez, Y., Rodríguez, S., Guerra, J.C., Go mez-Pel Cuevas, E., Gonza aez, A.J., Alonsorez, S., Bustos, J., Milford, C., 2013. Assessment of atmospheric processes Pe driving ozone variations in the subtropical North Atlantic free troposphere. Atmos. Chem. Phys. 13, 1973e1998. http://dx.doi.org/10.5194/acp-13-19732013. Draxler, R.R., Hess, G.D., 1997. Description of the HYSPLIT_4 Modeling System. NOAA Tech. Memo. ERL ARL-224. NOAA Air Resources Laboratory, Silver Spring, MD, 24 pp. Draxler, R.R., Hess, G.D., 1998. An overview of the HYSPLIT_4 modeling system of trajectories, dispersion, and deposition. Aust. Meteorol. Mag. 47, 295e308. Duchi, R., Cristofanelli, P., Landi, T.C., Arduini, J., Bonafe’, U., et al., 2016. Long-term (2002e2012) investigation of Saharan dust transport events at Mt. Cimone GAW global station, Italy (2165 m a.s.l.), 4, 000085. http://dx.doi.org/10.12952/ journal.elementa.000085. Fairlie, T.D., Jacob, D.J., Dibb, J.E., Alexander, B., Avery, M.A., et al., 2010. Impact of mineral dust on nitrate, sulfate, and ozone in transpacific Asian pollution plumes. Atmos. Chem. Phys. 10, 3999e4012. €nigstedt, R., Parchatka, U., Mühle, J., Fischer, H., Kormann, R., Klüpfel, T., Gurk, Ch., Ko Rhee, T.S., Brenninkmeijer, C.A.M., Bonasoni, P., Stohl, A., 2003. Ozone production and trace gas correlations during the June 2000 MINATROC intensive measurement campaign at Mt. Cimone. Atmos. Chem. Phys. 3, 725e738. http:// dx.doi.org/10.5194/acp-3-725-2003. Gheusi, F., Ravetta, F., Delbarre, H., Tsamalis, C., Chevalier-Rosso, A., Leroy, C., Augustin, P., Delmas, R., Ancellet, G., Athier, G., Bouchou, P., Campistron, B., Cousin, J.-M., Fourmentin, M., Meyerfeld, Y., 2011. Pic 2005, a field campaign to

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