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Study of the vertical variability of aerosol properties based on cable cars in-situ measurements
Atmospheric Pollution Research xxx (2017) 1e11
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Study of the vertical variability of aerosol properties based on cable cars in-situ measurements O. Zawadzka a, *, M. Posyniak b, K. Nelken c, P. Markuszewski d, e, M.T. Chilinski a, D. Czyzewska a, J. Lisok a, K.M. Markowicz a a
Institute of Geophysics, Faculty of Physics, University of Warsaw, Pasteura 7, 02-093, Warsaw, Poland Institute of Geophysics, Polish Academy of Sciences, Ksiecia Janusza 64, 01-453, Warsaw, Poland Institute of Physical Geography, Faculty of Geography and Regional Studies, University of Warsaw, Krakowskie Przedmiescie 30, 00-927, Warsaw, Poland d Institute of Oceanology, Polish Academy of Sciences, Powstancow Warszawy 55, 81-712, Sopot, Poland e Centre for Polar Studies, National Leading Research Centre, 60 Bedzinska Street, 41-200 Sosnowiec, Poland b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 November 2016 Received in revised form 28 March 2017 Accepted 28 March 2017 Available online xxx
This work presents the methodology for obtaining the vertical profiles of aerosol optical and microphysical properties based on cable-car and ground-based measurements in a mountain region. The presented data were collected during the winter workshop between 7th and 13th March 2016 in Krynica-Zdroj (southern Poland). During this campaign photoacoustic instruments were used to observe the single-scattering optical properties at two sites with a vertical separation distance of about 360 m. The micro-aethalometer AE-51 and the optical particle counter OPC-N2 were mounted on the cable car and used to measure profiles of black carbon concentration and aerosol size distribution. The mean extinction coefficients at the upper (37 Mm1) and lower (43 Mm1) sites were about three times lower than the long-term average for this season due to weather conditions, which did not favour the haze conditions. However, a significant correlation between temperature gradient and difference of extinction coefficient between the valley and mountain was found. During nights and stable thermodynamic conditions the values in the valley were higher than close to the top of the mountain. Profiles obtained from cable car measurements shown significant reduction of black carbon and aerosol concentration with altitude also during the day time. In addition, the effective radius, and the fine and coarse mode aerosol concentration was slightly changed with altitude when the relative humidity was below 100%. During condensation and cloud formation the significant variability in particles effective radius were found as a result of aerosol activation close to the top of the mountains. © 2017 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Black carbon Vertical profile Aerosol properties Cable car
1. Introduction Recent years have brought an increase in the understanding of the influence of air pollution on human health (IPCC, 2013; WHO, 2016). Furthermore, many studies show that high concentration of aerosols in close vicinity to the Earth's surface has a negative influence on the health conditions and life expectancy of the human population (Mauderly and Chow, 2008; WHO, 2016). Because
* Corresponding author. E-mail address: [email protected] (O. Zawadzka). Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control.
of the influence of aerosols on climate and health, it is important to characterise its concentration, size distribution and optical parameters at many different locations around the world at various times of the year and under a range of meteorological conditions, especially within the planetary boundary layer (PBL). Observation of PBL composition is also essential for understanding radiative balance in the atmosphere. As a result of absorption of solar radiation by highly absorbing aerosols particles (e.g. soot), the vertical temperature and humidity profiles change. This effect has an impact on the PBL properties (Stull, 1988; Garratt, 1992). Although being a very important mechanism, feedback between aerosols and the PBL is still not very well understood. Previous analyses were performed using only numerical models (Yu et al., 2002; Wendisch et al., 2008). These
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Please cite this article in press as: Zawadzka, O., et al., Study of the vertical variability of aerosol properties based on cable cars in-situ measurements, Atmospheric Pollution Research (2017), http://dx.doi.org/10.1016/j.apr.2017.03.009
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papers show that the absorption of radiation by aerosols should result in a decrease of the PBL's height, which is an important parameter used in air pollution studies. Recent studies described by Sinha et al. (2013), Dumka et al. (2015a, 2015b) showed that the influence of the changes in PBL is associated with the aerosol scattering and absorption properties as well as CCN concentrations. In general it is assumed that absorption of radiation by aerosols particles at the top of the PBL causes a slowdown of the vertical transport and increase of the atmosphere stability (Ramanathan et al., 2005; Wendisch et al., 2008). With the assumption of a constant source of aerosols, the stabilisation of the PBL causes a constant increase of pollution concentration which leads to further warming of the air, and, as a result, causes further strengthening of the PBL stability and limits vertical mixing in the atmosphere, as well as a slowdown of the PBL development. This stabilisation has severe effects on the ventilation of the polluted PBL and may also suppress or delay convective cloud formation on many days with moist conditions close to the top of the PBL (Wendisch et al., 2008). The newest studies are aimed mainly at miniaturisation and automatisation of equipment used for aerosol and meteorological observations. Since full-scale measurements from aeroplanes generate high costs, new methods of observation using small autonomous unmanned aerial vehicles (UAV) were developed in order to measure vertical profiles of BC (Ramana et al., 2007; Roberts et al., 2008; Corrigan et al., 2008; Chilinski et al., 2016). In addition, tethered balloons are used to profile the lowest troposphere and to measure aerosol optical and microphysical properties (Ferrero et al., 2011; Ferrero et al., 2016; Markowicz et al., 2017). According to the European Environment Agency report for 2015 (EFA Report, 2015) several cities located in southern Poland are included among most polluted in Europe. One of the reasons for that particular situation is that the structure of the Polish energy economy is heavily based on coal (Zawadzka et al., 2013). In general, high air pollution episodes occur most often in large cities, but in small towns and villages in mountain regions too. During nights with low horizontal pressure gradient a mountain breeze in the valley leads to accumulation of air pollution in the surface layer (Wanner and Hertig, 1984; Baumbach and Vogt, 1999; Lang et al., 2015; Chilinski et al., 2016). Such a phenomenon has an impact on the propagation of both longwave and, what is more important, shortwave radiation. Interaction of the pollutants with shortwave radiation is most frequent during mornings (until the inversion disappears) or afternoons (when the inversion starts to develop) (Leukauf et al., 2015). Accumulation of absorbing anthropogenic aerosols in basins during nocturnal temperature inversion creates favourable conditions for observations of mutual interactions between aerosols and the PBL (Cordova et al., 2016). Additionally, in some locations it is possible to install measurement devices on a cable car to perform vertical profiles of aerosol properties using in-situ methods (Seidel et al., 2016). So far the measurements of aerosols properties in mountainous areas have been carried out, among others, in the Alps on Jungfraujoch (Collaud Coen et al., 2011), in the laboratory in Zugspitze (Krüger et al., 2014), and in the region of Chamonix (Greenwald et al., 2006), but there was no profiling of aerosol properties during those measurements. In Poland, a very small number of regular observations of aerosol optical properties in mountainous areas is carried out. Insitu measurements of the concentration of particulate matter with particle diameter below 2.5 mm (PM2.5) or below 10 mm (PM10) are conducted by the air quality network of the Inspectorate of Environmental Protection. Nevertheless, those measurements are carried out mostly at the bottoms of valleys. What is more, the vertical structure of the atmospheric pollution is not studied. One
of the longest series of aerosol optical properties observations are based on direct solar radiation measurements (Linke Turbidity Factor) and were carried out in the Polish Tatra Mountains, in Zakopane and on Kasprowy Wierch (Markowicz and UsckaKowalkowska, 2015). However, those measurements do not allow the determination of detailed parameters characterising optical and microphysical properties (single scattering properties, size distribution) of atmospheric aerosols as in the case of data obtained from sunphotometers (Dubovik and King, 2000), and they do not allow the determination of the vertical profiles of those properties. Recent technical development has also brought new applications of mobile in-situ vertical profile measurements (Chilinski et al., 2016). Observations with the use of a mobile set for determining black carbon concentration were performed in Strzyzow in the region of Podkarpacie (Markowicz et al., 2014). In that case the profiles were performed carrying the measuring equipment during an uphill/ downhill walk or using unmanned aerial vehicles (UAV). Due to the location of those measurements, the height of the profiles was small (approx. 100 m), and the measurement method is slow and limits the number of profiles taken during the day. Use of cable cars can be convenient for PBL observation in mountainous areas. Such a situation allows continuous measurement the of air's vertical composition, such as exemplary liquid water content in relation to height (Wieprecht et al., 1970). Seidel et al. (2016) presented an example of comprehensive atmospheric observations that cover in-situ observation with ground-based mobile measurements (on a bus, on a vessel on a lake) and with airborne platforms (on a cable car, on a UAV and on a tethered balloon probe). It should be emphasised that all of the presented techniques have different characteristics related to spatial coverage, specific carrier system, climate elements measured and data analysis and evaluation. Cable car measurements can be deemed particularly interesting because of the existence of thousands of similar scheduled carrier systems in nearly all mountain areas of the world (Seidel et al., 2016). However, as in the case of all described systems, measurements can be affected simply by the moving platform. Moreover, the temporal and spatial bias of a moving sensor system should be taken into account. This paper is the result of a workshop organised by University of Warsaw in cooperation with the Institute of Oceanology of the Polish Academy of Sciences, the Institute of Geophysics of the Polish Academy of Sciences and Poznan University of Life Sciences, all belonging to the Poland-AOD consortium (www.polandaod.pl). The main aim of the workshop was to improve understanding of the physical processes involving atmospheric aerosol phenomena occurring in the lower troposphere in mountainous regions. The workshop concentrated on expansion of knowledge on modern research techniques, including in-situ and remote sensing methods and numerical tools used to conduct computer simulations of physical processes in the atmosphere. In addition to the lectures, the field measurements were conducted in the Beskid Mountains. The classes covered field observation of the optical and microphysical structure of smog forming in the region of Krynica-Zdroj and its impact on the local thermodynamic conditions. The results of these measurements are presented in the following paper. Section 2 contains a description of the strategy of observations, measurement sites, as well as used equipment. The next section is dedicated to analysis of the meteorological conditions during field measurements. Afterwards, in Section 4, temporal variability of single-aerosol properties obtained from optical devices is discussed. Section 5 is focused on vertical variability of the aerosol number and BC concentration. The last part of the paper contains a summary and discussion of the presented results as well as possible extension of the described measurements.
Please cite this article in press as: Zawadzka, O., et al., Study of the vertical variability of aerosol properties based on cable cars in-situ measurements, Atmospheric Pollution Research (2017), http://dx.doi.org/10.1016/j.apr.2017.03.009
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2. Equipment and field experiment 2.1. Strategy of measurements The field campaign was carried out between 7th and 13th March of 2016 in Krynica-Zdroj, which is a town in Nowy Sacz County, Lesser Poland Voivodeship (Southern Poland). It occupies an area of around 40 km2 and is inhabited by around 11,000 people (population density: 280/km2) and it is the biggest health-resort in Poland (http://www.krynica-zdroj.pl/en/). Krynica-Zdroj is a popular tourist and winter sports destination situated in the Beskidy Mountain range. A gondola lift built in 1997 on Jaworzyna Krynicka Mountain overlooking Krynica, and subsequent investment in modern skiing facilities, made Krynica one of the most important ski resorts in Poland. Krynica-Zdroj is a community of rural and urban character. Due to its settlement type, location, and tourist functions the main sources of air pollution are household combustion (mainly coal-based) and road transport. The measurements were performed in two sites (upper and lower station) and along the cable car route (Fig. 1). The lower station was located in Czarny Potok valley at 680 m a.s.l. (49.423 N, 20.922 E) while the upper station (1040 m a.s.l.) was below the top of Jaworzyna Krynicka Mountain at 1114 m a.s.l., 49.421 N, 20.893 E. Czarny Potok is the western part of Krynica-Zdroj, although it is separated from the main part of the town by a hill and is characterised by sparse settlement. The distance between both sites is 2050 m in horizontal and about 360 m in vertical direction. The equipment at the lower site was mounted in the measurement car with the aerosol inlet about 3 m above ground level. At the upper station the aerosol device was mounted in the building (mountain shelter) where the inlet was mounted about 8 m above ground level. On 9th March 2016 vertical profile of aerosol properties was measured based on equipment mounted on the one of the cable cars. The route profile of the cable car is presented in Fig. 2. The ride time between the lower and upper station was approximately 7 min.
Fig. 2. Profile of approximated route of Jaworzyna Krynicka cable car.
particle. The construction of the scattering chamber is based on the nephelometer technique. The detector measures scattering light between 6 and 174 . Two instruments that work on two different wavelengths (532 and 870 nm) were installed in the lower site (Czarny Potok) and another one (532 nm) operated in the upper station (Jaworzyna Krynicka) with 1 min resolution (Table 1). The detection limit for 60 s averaging for absorption and scattering coefficient is less than 0.25 Mm1 (870 nm) (Kok et al., 2010). The uncertainty in the measured scattering and absorption coefficient is about ±10% and ±20% respectively (Retama et al., 2015). PAX calibrations were provided two months before the campaign in the factory. Sampling inlets were mounted around 2.5 and 8 m above ground, in the lower and upper station, respectively. We used standard nephelometer inlets, not longer than 2 m. The micro-aethalometer AE-51 measures light transmission through a Teflon-coated fibre-glass filter at 880 nm (Table 1). The shortest integration time is 1 s with a maximum flow speed of 0.2 l/ min. Every part of the experiment was held with the same integration time and the flow mentioned above. During the measurements, results were automatically stored on a built-in logger.
2.2. Equipment The one-wavelength Photoacoustic Extinctiometer (PAX) is an instrument from the Droplet Measurement Technologies company, which measures aerosol single-scattering properties such as scattering and absorption coefficient, single scattering albedo (SSA), and black carbon (BC) mass concentration (Kok et al., 2010). It uses a photoacoustic method to retrieve absorption by detection of pressure waves initiated by emission of energy from the absorbing
Table 1 Instruments measuring aerosol properties during field campaign. Name
Acronym
l [nm]
Property
Dt
Station
Photoacoustics Extinctiometer Aethalometer Optical Counter
PAX
532, 870 532 880 658
sEXT, sABS, sSCAT
1 min
SSA, BC BC, sABS PM1, 2.5, 10
1s 1s
lower upper Profile Profile
AE-51 OPC-N2
Fig. 1. Illustrative map of the study area. Distance between both measurement sites (lower and upper) is 2050 m in horizontal and about 360 m in vertical direction. Travel time of the cable car was approx. 7 min.
Please cite this article in press as: Zawadzka, O., et al., Study of the vertical variability of aerosol properties based on cable cars in-situ measurements, Atmospheric Pollution Research (2017), http://dx.doi.org/10.1016/j.apr.2017.03.009
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According to the manufacture's information, the resolution of measurements is 0.001 mg/m3, with precision of ±0.1 mg/m3 for 1min averaging. The device's operational weight is approx. 280 g. Micro-aethalometer AE-51 reports the BC concentration in mg/m3 and attenuation (ATN). We calculated the BC from the time variation of ATN. In order to limit noise in a signal, the ATN value before signal derivative was filtered by running a mean filter, with an averaging window of 20 s. Afterwards, the filtered ATN was used to calculate BC concentration and absorption coefficient with the following formulas:
BC ¼
sAB $C$RðATNÞ sATN
sAB ¼
dATN A dt Q $C$RðATNÞ
where A is a sample spot area on the filter (7.1 106 m2), Q is the volumetric flow rate (100 ml/min), C is the multiple scattering optical enhancement factor (2.05 ± 0.03 Ferrero et al., 2011), R(ATN) is the aerosol loading factor, and sATN is the apparent mass attenuation cross section (12.5 m2/g at 880 nm). The R(ATN) term in the formula compensates for the nonlinear loading effect caused by the increase in aerosol absorption over time, which in turn results in reduction in the optical path. Schmid et al. (2006) found that it is required only when the ATN becomes higher than 20. Therefore, we controlled the value of the ATN and changed the filter when the ATN was near (16e18) the threshold. Reported BC concentrations are normalised to standard conditions (pressure 1013.25 hPa, temperature 273.15 K). An optical particle counter, OPC-N2, measures the light scattered at 658 nm by individual particles carried in a sample of air stream through a laser beam (Table 1). These measurements are used to determine the particle size, which is related to the intensity of light scattered via a calibration based on the Lorenz-Mie theory. Particle mass concentration PM1, PM2.5, and PM10 are then calculated from the particle size spectra and concentration data, assuming a particle density (1.65 g/ml) and refractive index (1.5 þ i0). Assuming both parameters as well as the relative humidity, which is not measured inside the device leads to a significant error in the PM estimation. For example, in the case of a Scanning Mobility Particle Sizer (SMPS) and an Aerodynamic Particle Sizer (APS) the uncertainty is close to 30% (Buonanno et al., 2009). The OPC-N2 employs a narrow inlet to physically constrain the airborne particles to pass through a uniform central part of the illuminating laser beam and ensure accurate sizing. The instrument classifies each particle size, at rates up to ~10,000 particles per second, recording the particle size to one of 16 bins covering the size range from 0.38 to 17 mm. The flow rate is about 220 ml/min. The data is transmitted via an SPI to a Raspberry Pi microcomputer every 1 s. This device was used during vertical profile measurements.
3. Meteorological conditions On 7th March Central Europe was under the influence of a lowpressure system (Fig. 3a). The front line was aligned longitudinally, and this resulted in warm polar marine air masses coming from the south to the area of Southern Poland. The 72-hour back-trajectories from the NOAA-HYSPLIT (Draxler and Rolph, 2010), which ended in Krynica at 12:00 UTC on selected days (9th, 11th, and 13th March), are presented in Fig. 4(aec). On the following days the low-pressure area moved north-
eastward and dissipated, leaving the quasi-stationary front line from Finland in the north, through the Baltic states, the eastern border of Poland, through the Balkan states to Greece in the south (Fig. 3b). The air from the height of about 3000 m came from the area of France, crossing the Mediterranean Sea Basin (Fig. 4a). The lower air layer travelled a much smaller distance, in particular air mass from 1000 m stayed in the Poland territory and near to the surface for the whole 72-hour period. The air pressure in southern Poland rose around 15 hPa within five days, from ca. 1005 hPa on 7th to ca. 1020 on 11th March (Fig. 3aec) (see Table 1). Back-trajectories of 11th of March show that the source area of the incoming air masses changed significantly (Fig. 4b). The air began to flow from the east and south-east, bringing warmer polar marine air masses from the central Russia source area. During the next two days (12th and 13th March) a high-pressure area above Scandinavia and Denmark began to develop. Respectively, the air pressure over Poland was gradually increasing, and reached 1030 hPa over southern Poland on 13th March (Fig. 3d). Thanks to that high-pressure system, the air in Central and Eastern Europe started to come from the north. Back-trajectories from HYSPLIT for 13th March clearly show the steadily stratified atmosphere (Fig. 4c). The air from the height of about 1000 and 2000 m came from almost the same altitude, while the higher layer (3000 m) settled from the level of over 6000 m. Transformed arctic air masses, coming from Scandinavia and the Arctic Ocean, resulted in a temperature decrease during the following days. At Jaworzyna Krynicka (1114 m a.s.l.) the temperature varied between 0 C up to 45 C during the first three days, and decreased during the following days (Table 2). Most of the time it was colder than in Krynica at 580 m a.s.l, but with the possibility of the occurrence of nocturnal inversions. Relative humidity was above 90% for the whole period, and minor snowfall/sleet occurred on a few days. Significant precipitation was observed in Krynica WMO station on 7th March (4.2 mm) and 13th March (2.6 mm). On the rest of the days there was only trace precipitation (Table 2). 4. Temporal variability of single-aerosol properties from PAX devices Fig. 5 shows temporal variability of aerosol scattering (a), absorption coefficient (b) and SSA (c) at 532 nm obtained from PAX devices. The black dots correspond to the upper station and red ones to the lower station. The grey points show very clean conditions (scattering coefficient below 5 Mm1) to indicate that the SSA is very uncertain. The aerosol extensive optical properties during the field campaign were very low. For example the mean extinction coefficients at 532 nm were 43.4 Mm1 and 37.1 Mm1 for the lower and upper station, respectively. For comparison, the mean extinction coefficient for March observed by the Aurora 4000 nephelometer and AE-31 aethalometer at the Poland-AOD station in Strzyzow (444 m a.s.l. and 85 km from Krynica) amounted to about 120 Mm1 (Markowicz et al., 2016). During the field campaign the extinction coefficient only once exceeded 100 Mm1, at the lower station. At the upper station the extinction coefficient every day dropped below 20 Mm1. Especially between 10th March 00:00 UTC and 11th March 10:00 UTC the air mass was very clean. Fig. 5a shows also some temporal variability between the lower and upper stations. During the night the values were usually larger in the valley while during day the larger extinction was generally observed in the upper station. We found significant correlation (r2 ¼ 0.57) between the extinction difference at both sites and air temperature gradient measured from Poprad-Ganovce radiosonde at 00:00 and 12:00 UTC (Fig. 6). The largest difference between lower and upper extinction was observed during the night and when the temperature gradient was small (stable conditions) as
Please cite this article in press as: Zawadzka, O., et al., Study of the vertical variability of aerosol properties based on cable cars in-situ measurements, Atmospheric Pollution Research (2017), http://dx.doi.org/10.1016/j.apr.2017.03.009
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Fig. 3. Synoptic analysis charts for 7th (a), 9th (b), 11th (c), and 13th (d) March. Charts were prepared in IMGW-PIB, CBPM in Krakow (http://www.pogodynka.pl/polska/mapa_ synoptyczna).
well as when the vertical mixture was negligible. During the day and in unstable conditions the extinction coefficients at both stations were much closer and even the values at higher altitude were larger. Provisos research shows that wind speed, height of the PBL top as well as the temperature profiles have a significant impact on the vertical distribution of the aerosol optical and microphysical properties in the lower troposphere (Zhang et al., 2009). In general, vertical and horizontal distribution of aerosol properties in the mountain area is very complicated due to several physical processes that can play an important role: mountain-valley breeze, distribution of emission, PBL modification over the mountain, dilution and accumulation process, and horizontal and vertical mixing. In the case of the absorption coefficient (Fig. 5b) the mean value of the absorption coefficient measured at the upper station was 8.0 Mm1 while in the valley it was only 4.1 Mm1. A significant difference in mean values was seen for measurements on 8th and 11th March. During the remaining days the difference in the absorption coefficient is rather small. In addition, significant temporal fluctuation of the absorption coefficient close to the top of Jaworzyna Krynicka was probably closely related to local emissions from coal central heating systems. The SSA at the upper station was lower during the whole campaign (Fig. 5c). Only close to noon were the SSA and also scattering and absorption coefficients at both
stations were similar, with the exception of 10th March. For the lower site the SSA changed between 0.7 and 0.98 while for upper between about 0.4 and 0.92. However, the very low SSA measured at both sites during the afternoon of 10th March is unrealistic. This value corresponds to very clean conditions (scattering coefficient of 3e7 Mm1), so the SSA is very uncertain. 5. Vertical variability of the aerosol number and BC concentration The vertical profiles of optical and microphysical properties were performed on 9th March 2016 between 8:00 and 15:00 UTC. Fig. 7 shows temporal variability of aerosol concentration from OPC-N2 (Fig. 9a) and BC concentration from AE-51 (Fig. 7b) which was mounted on the cable car. In addition, Fig. 7c and d present the extinction and absorption coefficient from PAX 532 nm, respectively, for the lower (red line) and upper (black line) station. In the case of the OPC-N2 and AE-51 the temporal oscillation is shown as a result of the change in altitude of the sensors. In addition, some variation exists for all quantities. Between 8:00 and 12:00 UTC the aerosol and BC concentration slightly decreased as well as the extinction and absorption coefficient at the lower and upper stations. Later, at 12e14 UTC, a significant increase of all optical and microphysical parameters was observed. During the last hour of the
Please cite this article in press as: Zawadzka, O., et al., Study of the vertical variability of aerosol properties based on cable cars in-situ measurements, Atmospheric Pollution Research (2017), http://dx.doi.org/10.1016/j.apr.2017.03.009
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Fig. 4. The Hybrid single particle Lagrangian integrated Trajectory model (HYSPLIT) back-trajectories obtained for Jaworzyna Krynicka at 12:00 UTC for 9th (a), 11th (b) and 13th (c) March. The HYSPLIT model was run for 72 h with meteorological data from the Global Data Assimilation System (GDAS).
measurements some reduction of aerosol concentration and extensive properties were detected. Moreover, slightly larger extinction and absorption coefficients were observed at the upper station in comparison to the values in the valley. Some opposite relation was observed during the morning (until 9:00 UTC) and late afternoon (after 18:00 UTC). Due to variability of daily aerosol conditions (Fig. 7), cable-car measurements (9th March 2016) were divided into four periods. Vertical profile data were averaged in layers 50 m high from 650 m
a.s.l. up to 1100 m a.s.l. In Fig. 8 the vertical aerosol and BC concentration profiles from OPC-N2 and AE-51 devices are presented. Between 08 and 10 UTC (Fig. 8a) both aerosol and BC concentrations show very similar vertical variability. BC decreases from 920 ng/m3 at about 650 m a.s.l. to 660 ng/m3 at about 1000 m a.s.l., while aerosol concentration reduces from 27.5 1/cm3 to 21.0 1/cm3. Close to the top both parameters slightly increase with altitude. The next profiles (10-12UTC, Fig. 8b) are very similar to the previous one; however, close to the top of the mountain an increase of BC
Please cite this article in press as: Zawadzka, O., et al., Study of the vertical variability of aerosol properties based on cable cars in-situ measurements, Atmospheric Pollution Research (2017), http://dx.doi.org/10.1016/j.apr.2017.03.009
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Table 2 Summary of meteorological parameters during the campaign: Tmax [ C] e maximum temperature, Tmin [ C] e minimum temperature, RH [%] - relative humidity, Prec. [mm] e daily sum of precipitation. Date
7.03. 8.03. 9.03. 10.03. 11.03. 12.03.
Jaworzyna Krynicka (1114 m a.s.l.)
Krynica (WMO station at 580 m a.s.l.)
Tmin [ C]
Tmax [ C]
Tmin [ C]
Tmax [ C]
RH [%}
Prec. [mm]
0.4 0.2 0.4 0.2 0.9 0.3
3.2 3.6 5.1 1.1 3.6 1.4
0.6 2.0 1.5 1.2 2.0 1.4
6.6 7.2 8.0 4.4 6.3 6.0
93.3 89.3 86.0 98.3 95.0 95.0
4.2 0.1 0.0 0.2 0.5 0.2
Fig. 6. Aerosol extinction coefficient difference between lower and upper station as a function of the air temperature measured at 00:00 and 12:00 UTC radiosonde launched from Poprad-Ganovce. Optical data was averaged ±2 h before and after radiosonde profile. The black line shows the linear fit.
Fig. 5. Comparison of PAX532 measurements (scattering, absorption coefficient and SSA) obtained between 8th and 13th March at lower station (red dots) and upper station (black dots). The grey dots show scattering coefficient below 5 Mm1 and corresponding SSA.
and aerosol concentration was not observed. On the contrary, the BC decreased from 900 to 530 ng/m3, and aerosol concentration from 25.0 to 17.5 1/cm3. The profiles averaged between 12 and 14 UTC (Fig. 8c) show significantly higher values in the whole layer. The decrease of the BC with altitude is more constant (gradient) in comparison to aerosol concentration. BC at the bottom of the valley was about 1450 ng/m3, and close to top it was 1200 ng/m3. The aerosol concentration reduced from 42 to 35.5 1/cm3. The last profiles (14e15 UTC, Fig. 8d) were performed in cloudy conditions. The cloud base was above 900 m a.s.l., while the top was above the mountain. Just below 1000 m a.s.l. the aerosol concentration increased while the BC decreased but in the lower atmosphere both parameters increased towards the valley. In case of the vertical variability the radio sounding can help with data interpretation. Radio sounding from Poprad-Ganovce, even though made around 60 km farther away, gives general information on thermodynamic conditions. Fig. 9 shows data
Fig. 7. Temporal variability of (a) aerosol concentration from OPC-N2, (b) BC concentration from AE-51, (c) extinction and (d) absorption coefficient from PAX 532 nm for the lower (red line) and upper (black line) station between 8:00 and 15:15 UTC on 9th March 2016.
obtained from 00:00 UTC and Fig. 10 from 12:00 UTC observations on 9 March 2016. Night time thermodynamics profiles indicate a humid layer (relative humidity about 100%) in the lower troposphere. The mean lapse rate up to 1100 m a.l.s. was about 0.35, which indicates a stable condition event for condensation. Above there is a potential instability (air temperature in red parallel to saturated adiabatic). The lower atmosphere is not dryer at any time. From surface (706 m a.s.l.) up to 1140 m a.s.l. the potential temperature is almost constant indicating well mixed PBL. At the top of the PBL a two-degree inversion separates the air mass in the free troposphere. Above the inversion (about 1200 m a.s.l.) the potential temperature increases slightly with altitude up to 2500 m a.s.l. (the second condensation level). Thus the morning profiles of aerosol and BC concentration showing larger gradient between the valley and the top of the mountain than the afternoon data can be explained by the stable thermodynamic conditions. The smaller
Please cite this article in press as: Zawadzka, O., et al., Study of the vertical variability of aerosol properties based on cable cars in-situ measurements, Atmospheric Pollution Research (2017), http://dx.doi.org/10.1016/j.apr.2017.03.009
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Fig. 8. Averaged vertical profiles of two parameters on 9th March 2016. Observation results were segregated into 4 periods (08e10 UTC, 10e12 UTC, 12e14 UTC, and 14e15 UTC). Red triangles represent the aerosol concentration [1/cm3] for size range from 0.38 to 17 mm, and the black squares show the BC concentration [ng/m3].
Fig. 9. Radio sounding from Poprad-Ganovce on 9th March 2016 at 00:00 UTC. The thick black line shows air temperature, red line shows the due point temperature, while the blue line shows potential temperature. In addition, symbols on the right side show wind speed and direction.
reduction of physical parameters with altitude in the last two cases is due to well mixed PBL (Fig. 11). However, there is still a significant difference in aerosol and BC concentration between the valley and the top of the mountain. This can be related to very low wind speed.
Fig. 10. Radio sounding from Poprad-Ganovce on 9th March 2016 at 12:00 UTC. The thick black line shows air temperature, red line shows due point temperature, while the blue line shows potential temperature. In addition, symbols on the right side show wind speed and direction.
Both radio sounding showed that the wind speed in the PBL was up to 2e2.5 m/s. The local pollution is mostly emitted in the valley by the heating system, so the cable-car measurements may shows both horizontal and vertical gradient between lower and upper
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Fig. 11. Averaged aerosol volume size distribution measured on the route of the cable car on 9th March 2016. Observation results were segregated into four periods (08e10 UTC, 10e12 UTC, 12e14 UTC, and 14e15 UTC).
stations. Note that horizontal distance between both points is about 2.3 km. Fig. 11 shows spatial volume distribution of aerosol particle concentration from OPC-N2. Plot shows only four of nine averaging levels for clarity. At the altitude level of 900 m during the hours 12e14 UTC significant increase of coarse mode concentration was observed. During the last hour the amount of coarse particles started increasing at 800 m which is the altitude of the cloud base. In the case of the first period of observation (08e12 UTC) the aerosol size distribution was almost constant with altitude. Profiles of aerosol number concentration for fine (smaller than 1 mm) and coarse (larger than 1 mm) particles indicates cloud development between 14:00 and 15:00 UTC (Fig. 12). A significant change in aerosol number concentration of coarse particles was observed from 800 m a.s.l. to the mountain top. In addition, the fine particles were reduced in whole cloud layer (Fig. 12d). The coarse mode concentration increased at the expense of a decrease in the smaller particles. This is the result of cloud droplet coalescence and the evolution of cloud size distribution. During the morning (before aerosol activation) the aerosol number concentration for both fine and coarse mode slightly decreased with altitude, but the variability was negligible. 6. Summary and discussion The presented paper describes results obtained from a pilot experiment carried out in the mountainous region of Krynica-Zdroj located in southern Poland. The novel approach was the installation of the measurement equipment on a cable car, which could be the future of aerosol and PBL observation in mountain areas. The aerosol extensive optical properties measured by PAX at 532 nm during the field campaign were very low due to the weather conditions. Measurements were made during cloudy conditions when the development of a night surface inversion is unlikely. The mean
extinction coefficients at 532 nm were about three times lower that long-term mean for March measured at the rural station in Strzyzow. At the upper station the extinction dropped below 20 Mm1 every day. During the night the values were usually larger in the valley, while during the day the larger extinction was observed in the upper station. The largest difference between lower and upper extinction coefficient was observed during the night and when the temperature gradient was small (stable conditions). We found significant correlation between the temperature lapse rate in the lower PBL and the gradient of the extinction coefficient between the lower and upper stations. When the lapse rate was close to dryadiabatic (or higher) then the difference in extinction was low, but during stable (nighttime) conditions it was up to 40 Mm1. The vertical profile of aerosol microphysical properties measured on the cable cars on 9th March 2016 shows significant variability with altitude. For all measurements the aerosol and BC concentration decreased with altitude, but the gradient was steeper during the morning due to more stable thermodynamic conditions. During the afternoon and well-mixed conditions both parameters still decreased with altitude. This can be explained by the very low wind speed in the whole PBL (about 2 m/s), which causes weak horizontal mixing. As a result of the vertical mixing the profiles of the aerosol number concentration indicated cloud developments between 14:00 and 15:00 UTC. The fine particles were reduced in whole cloud layer while the coarse mode concentration increased as a result of cloud droplet coalescence and evolution of the cloud size distribution. The OPC-N2 showed a significant increase in the effective radius above 800 m a.s.l. with maximum 1.8 mm at 970 m a.s.l., as well as a significant change in aerosol number concentration of coarse particles from 800 m a.s.l. to the mountain top. Developing this research method by placing the measuring set on the cable car (Seidel et al., 2016) will enable the measurement of profiles of aerosols microphysical properties during the carriage ride up or down within several minutes. This method is much more
Please cite this article in press as: Zawadzka, O., et al., Study of the vertical variability of aerosol properties based on cable cars in-situ measurements, Atmospheric Pollution Research (2017), http://dx.doi.org/10.1016/j.apr.2017.03.009
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Fig. 12. Averaged vertical aerosol concentration [1/cm3] profiles of particle sizes up to 1 mm (blue squares) and over 1 mm (green triangles) on 9th March 2016. Observation results were segregated into 4 periods (08e10 UTC, 10e12 UTC, 12e14 UTC, and 14e15 UTC).
effective in comparison to other types of measurements (e.g. taken while walking on foot) because it shortens the time of creating the profile and increases the number of profiles done within 24 h. Moreover, this solution is much simpler and cheaper to realise in comparison to airborne measurements (airplane or UAV) or a tethered balloon because it does not require an additional technical base or full-time maintenance, and significantly lowers the overall cost of the research (Seidel et al., 2016). Acquired data could be evaluated against other observations, for example LIDAR measurements. The limitation of the cable-car method is the height of the performed profile due to the height of the upper cableway station, but the elevation (approx. 400 m) is sufficient to study the pollution deposited in the PBL. The results of the described pilot experiment show the great potential of this research method. The installation of the measurement equipment on the cable car may be used in the future for constant monitoring of the state of atmospheric pollution. The use of cutting-edge miniature Particulate Monitor sensors allows measurements (vertical profiles or horizontal transects) to be taken which had not been possible using the stationary monitoring stations of the Inspectorate of Environmental Protection. Additionally, the cost of field measurements is much lower in comparison to stationary or even mobile monitoring stations. This is due to the low cost of construction and maintenance and ease of transportation. Such research will allow the determination of emission of anthropogenic aerosols affecting the development of the smog that fills mountain valleys, which in consequence will enable implementation of atmosphere protection programmes because towns situated in the mountainous regions of the south of Poland take notorious first place in the rankings of cities with the most polluted atmosphere.
Acknowledgements This research was made within Polish Grant No. 2012/05/E/ ST10/01578 of the National Science Centre coordinated by IGF UW and also project KNOW, Leading National Research Centre received by the Centre for Polar Studies for the period 2014e2018 established by regulation No. 152 (2013, Nov 14) of the Rector of the University of Silesia. We also want to thank all workshop participants: Karolina Bartkowska, Bogdan Chojnicki, Olga Krzywicka, Szymon Malinowski, Jakub Nowak, Paulina Pakszysz, Bartłomiej Pietras, Michał Piwoda, Anna Rozwadowska, Mateusz Samson, Adam Skomorowski, Urszula Syperowicz, Patrycja UlandowskaMonarcha, Krzysztof Wiejak, and Anna Zimniak.
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Study of the vertical variability of aerosol properties based on cable cars in-situ measurements O. Zawadzka a, *, M. Posyniak b, K. Nelken c, P. Markuszewski d, e, M.T. Chilinski a, D. Czyzewska a, J. Lisok a, K.M. Markowicz a a
Institute of Geophysics, Faculty of Physics, University of Warsaw, Pasteura 7, 02-093, Warsaw, Poland Institute of Geophysics, Polish Academy of Sciences, Ksiecia Janusza 64, 01-453, Warsaw, Poland Institute of Physical Geography, Faculty of Geography and Regional Studies, University of Warsaw, Krakowskie Przedmiescie 30, 00-927, Warsaw, Poland d Institute of Oceanology, Polish Academy of Sciences, Powstancow Warszawy 55, 81-712, Sopot, Poland e Centre for Polar Studies, National Leading Research Centre, 60 Bedzinska Street, 41-200 Sosnowiec, Poland b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 November 2016 Received in revised form 28 March 2017 Accepted 28 March 2017 Available online xxx
This work presents the methodology for obtaining the vertical profiles of aerosol optical and microphysical properties based on cable-car and ground-based measurements in a mountain region. The presented data were collected during the winter workshop between 7th and 13th March 2016 in Krynica-Zdroj (southern Poland). During this campaign photoacoustic instruments were used to observe the single-scattering optical properties at two sites with a vertical separation distance of about 360 m. The micro-aethalometer AE-51 and the optical particle counter OPC-N2 were mounted on the cable car and used to measure profiles of black carbon concentration and aerosol size distribution. The mean extinction coefficients at the upper (37 Mm1) and lower (43 Mm1) sites were about three times lower than the long-term average for this season due to weather conditions, which did not favour the haze conditions. However, a significant correlation between temperature gradient and difference of extinction coefficient between the valley and mountain was found. During nights and stable thermodynamic conditions the values in the valley were higher than close to the top of the mountain. Profiles obtained from cable car measurements shown significant reduction of black carbon and aerosol concentration with altitude also during the day time. In addition, the effective radius, and the fine and coarse mode aerosol concentration was slightly changed with altitude when the relative humidity was below 100%. During condensation and cloud formation the significant variability in particles effective radius were found as a result of aerosol activation close to the top of the mountains. © 2017 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Black carbon Vertical profile Aerosol properties Cable car
1. Introduction Recent years have brought an increase in the understanding of the influence of air pollution on human health (IPCC, 2013; WHO, 2016). Furthermore, many studies show that high concentration of aerosols in close vicinity to the Earth's surface has a negative influence on the health conditions and life expectancy of the human population (Mauderly and Chow, 2008; WHO, 2016). Because
* Corresponding author. E-mail address: [email protected] (O. Zawadzka). Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control.
of the influence of aerosols on climate and health, it is important to characterise its concentration, size distribution and optical parameters at many different locations around the world at various times of the year and under a range of meteorological conditions, especially within the planetary boundary layer (PBL). Observation of PBL composition is also essential for understanding radiative balance in the atmosphere. As a result of absorption of solar radiation by highly absorbing aerosols particles (e.g. soot), the vertical temperature and humidity profiles change. This effect has an impact on the PBL properties (Stull, 1988; Garratt, 1992). Although being a very important mechanism, feedback between aerosols and the PBL is still not very well understood. Previous analyses were performed using only numerical models (Yu et al., 2002; Wendisch et al., 2008). These
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papers show that the absorption of radiation by aerosols should result in a decrease of the PBL's height, which is an important parameter used in air pollution studies. Recent studies described by Sinha et al. (2013), Dumka et al. (2015a, 2015b) showed that the influence of the changes in PBL is associated with the aerosol scattering and absorption properties as well as CCN concentrations. In general it is assumed that absorption of radiation by aerosols particles at the top of the PBL causes a slowdown of the vertical transport and increase of the atmosphere stability (Ramanathan et al., 2005; Wendisch et al., 2008). With the assumption of a constant source of aerosols, the stabilisation of the PBL causes a constant increase of pollution concentration which leads to further warming of the air, and, as a result, causes further strengthening of the PBL stability and limits vertical mixing in the atmosphere, as well as a slowdown of the PBL development. This stabilisation has severe effects on the ventilation of the polluted PBL and may also suppress or delay convective cloud formation on many days with moist conditions close to the top of the PBL (Wendisch et al., 2008). The newest studies are aimed mainly at miniaturisation and automatisation of equipment used for aerosol and meteorological observations. Since full-scale measurements from aeroplanes generate high costs, new methods of observation using small autonomous unmanned aerial vehicles (UAV) were developed in order to measure vertical profiles of BC (Ramana et al., 2007; Roberts et al., 2008; Corrigan et al., 2008; Chilinski et al., 2016). In addition, tethered balloons are used to profile the lowest troposphere and to measure aerosol optical and microphysical properties (Ferrero et al., 2011; Ferrero et al., 2016; Markowicz et al., 2017). According to the European Environment Agency report for 2015 (EFA Report, 2015) several cities located in southern Poland are included among most polluted in Europe. One of the reasons for that particular situation is that the structure of the Polish energy economy is heavily based on coal (Zawadzka et al., 2013). In general, high air pollution episodes occur most often in large cities, but in small towns and villages in mountain regions too. During nights with low horizontal pressure gradient a mountain breeze in the valley leads to accumulation of air pollution in the surface layer (Wanner and Hertig, 1984; Baumbach and Vogt, 1999; Lang et al., 2015; Chilinski et al., 2016). Such a phenomenon has an impact on the propagation of both longwave and, what is more important, shortwave radiation. Interaction of the pollutants with shortwave radiation is most frequent during mornings (until the inversion disappears) or afternoons (when the inversion starts to develop) (Leukauf et al., 2015). Accumulation of absorbing anthropogenic aerosols in basins during nocturnal temperature inversion creates favourable conditions for observations of mutual interactions between aerosols and the PBL (Cordova et al., 2016). Additionally, in some locations it is possible to install measurement devices on a cable car to perform vertical profiles of aerosol properties using in-situ methods (Seidel et al., 2016). So far the measurements of aerosols properties in mountainous areas have been carried out, among others, in the Alps on Jungfraujoch (Collaud Coen et al., 2011), in the laboratory in Zugspitze (Krüger et al., 2014), and in the region of Chamonix (Greenwald et al., 2006), but there was no profiling of aerosol properties during those measurements. In Poland, a very small number of regular observations of aerosol optical properties in mountainous areas is carried out. Insitu measurements of the concentration of particulate matter with particle diameter below 2.5 mm (PM2.5) or below 10 mm (PM10) are conducted by the air quality network of the Inspectorate of Environmental Protection. Nevertheless, those measurements are carried out mostly at the bottoms of valleys. What is more, the vertical structure of the atmospheric pollution is not studied. One
of the longest series of aerosol optical properties observations are based on direct solar radiation measurements (Linke Turbidity Factor) and were carried out in the Polish Tatra Mountains, in Zakopane and on Kasprowy Wierch (Markowicz and UsckaKowalkowska, 2015). However, those measurements do not allow the determination of detailed parameters characterising optical and microphysical properties (single scattering properties, size distribution) of atmospheric aerosols as in the case of data obtained from sunphotometers (Dubovik and King, 2000), and they do not allow the determination of the vertical profiles of those properties. Recent technical development has also brought new applications of mobile in-situ vertical profile measurements (Chilinski et al., 2016). Observations with the use of a mobile set for determining black carbon concentration were performed in Strzyzow in the region of Podkarpacie (Markowicz et al., 2014). In that case the profiles were performed carrying the measuring equipment during an uphill/ downhill walk or using unmanned aerial vehicles (UAV). Due to the location of those measurements, the height of the profiles was small (approx. 100 m), and the measurement method is slow and limits the number of profiles taken during the day. Use of cable cars can be convenient for PBL observation in mountainous areas. Such a situation allows continuous measurement the of air's vertical composition, such as exemplary liquid water content in relation to height (Wieprecht et al., 1970). Seidel et al. (2016) presented an example of comprehensive atmospheric observations that cover in-situ observation with ground-based mobile measurements (on a bus, on a vessel on a lake) and with airborne platforms (on a cable car, on a UAV and on a tethered balloon probe). It should be emphasised that all of the presented techniques have different characteristics related to spatial coverage, specific carrier system, climate elements measured and data analysis and evaluation. Cable car measurements can be deemed particularly interesting because of the existence of thousands of similar scheduled carrier systems in nearly all mountain areas of the world (Seidel et al., 2016). However, as in the case of all described systems, measurements can be affected simply by the moving platform. Moreover, the temporal and spatial bias of a moving sensor system should be taken into account. This paper is the result of a workshop organised by University of Warsaw in cooperation with the Institute of Oceanology of the Polish Academy of Sciences, the Institute of Geophysics of the Polish Academy of Sciences and Poznan University of Life Sciences, all belonging to the Poland-AOD consortium (www.polandaod.pl). The main aim of the workshop was to improve understanding of the physical processes involving atmospheric aerosol phenomena occurring in the lower troposphere in mountainous regions. The workshop concentrated on expansion of knowledge on modern research techniques, including in-situ and remote sensing methods and numerical tools used to conduct computer simulations of physical processes in the atmosphere. In addition to the lectures, the field measurements were conducted in the Beskid Mountains. The classes covered field observation of the optical and microphysical structure of smog forming in the region of Krynica-Zdroj and its impact on the local thermodynamic conditions. The results of these measurements are presented in the following paper. Section 2 contains a description of the strategy of observations, measurement sites, as well as used equipment. The next section is dedicated to analysis of the meteorological conditions during field measurements. Afterwards, in Section 4, temporal variability of single-aerosol properties obtained from optical devices is discussed. Section 5 is focused on vertical variability of the aerosol number and BC concentration. The last part of the paper contains a summary and discussion of the presented results as well as possible extension of the described measurements.
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2. Equipment and field experiment 2.1. Strategy of measurements The field campaign was carried out between 7th and 13th March of 2016 in Krynica-Zdroj, which is a town in Nowy Sacz County, Lesser Poland Voivodeship (Southern Poland). It occupies an area of around 40 km2 and is inhabited by around 11,000 people (population density: 280/km2) and it is the biggest health-resort in Poland (http://www.krynica-zdroj.pl/en/). Krynica-Zdroj is a popular tourist and winter sports destination situated in the Beskidy Mountain range. A gondola lift built in 1997 on Jaworzyna Krynicka Mountain overlooking Krynica, and subsequent investment in modern skiing facilities, made Krynica one of the most important ski resorts in Poland. Krynica-Zdroj is a community of rural and urban character. Due to its settlement type, location, and tourist functions the main sources of air pollution are household combustion (mainly coal-based) and road transport. The measurements were performed in two sites (upper and lower station) and along the cable car route (Fig. 1). The lower station was located in Czarny Potok valley at 680 m a.s.l. (49.423 N, 20.922 E) while the upper station (1040 m a.s.l.) was below the top of Jaworzyna Krynicka Mountain at 1114 m a.s.l., 49.421 N, 20.893 E. Czarny Potok is the western part of Krynica-Zdroj, although it is separated from the main part of the town by a hill and is characterised by sparse settlement. The distance between both sites is 2050 m in horizontal and about 360 m in vertical direction. The equipment at the lower site was mounted in the measurement car with the aerosol inlet about 3 m above ground level. At the upper station the aerosol device was mounted in the building (mountain shelter) where the inlet was mounted about 8 m above ground level. On 9th March 2016 vertical profile of aerosol properties was measured based on equipment mounted on the one of the cable cars. The route profile of the cable car is presented in Fig. 2. The ride time between the lower and upper station was approximately 7 min.
Fig. 2. Profile of approximated route of Jaworzyna Krynicka cable car.
particle. The construction of the scattering chamber is based on the nephelometer technique. The detector measures scattering light between 6 and 174 . Two instruments that work on two different wavelengths (532 and 870 nm) were installed in the lower site (Czarny Potok) and another one (532 nm) operated in the upper station (Jaworzyna Krynicka) with 1 min resolution (Table 1). The detection limit for 60 s averaging for absorption and scattering coefficient is less than 0.25 Mm1 (870 nm) (Kok et al., 2010). The uncertainty in the measured scattering and absorption coefficient is about ±10% and ±20% respectively (Retama et al., 2015). PAX calibrations were provided two months before the campaign in the factory. Sampling inlets were mounted around 2.5 and 8 m above ground, in the lower and upper station, respectively. We used standard nephelometer inlets, not longer than 2 m. The micro-aethalometer AE-51 measures light transmission through a Teflon-coated fibre-glass filter at 880 nm (Table 1). The shortest integration time is 1 s with a maximum flow speed of 0.2 l/ min. Every part of the experiment was held with the same integration time and the flow mentioned above. During the measurements, results were automatically stored on a built-in logger.
2.2. Equipment The one-wavelength Photoacoustic Extinctiometer (PAX) is an instrument from the Droplet Measurement Technologies company, which measures aerosol single-scattering properties such as scattering and absorption coefficient, single scattering albedo (SSA), and black carbon (BC) mass concentration (Kok et al., 2010). It uses a photoacoustic method to retrieve absorption by detection of pressure waves initiated by emission of energy from the absorbing
Table 1 Instruments measuring aerosol properties during field campaign. Name
Acronym
l [nm]
Property
Dt
Station
Photoacoustics Extinctiometer Aethalometer Optical Counter
PAX
532, 870 532 880 658
sEXT, sABS, sSCAT
1 min
SSA, BC BC, sABS PM1, 2.5, 10
1s 1s
lower upper Profile Profile
AE-51 OPC-N2
Fig. 1. Illustrative map of the study area. Distance between both measurement sites (lower and upper) is 2050 m in horizontal and about 360 m in vertical direction. Travel time of the cable car was approx. 7 min.
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According to the manufacture's information, the resolution of measurements is 0.001 mg/m3, with precision of ±0.1 mg/m3 for 1min averaging. The device's operational weight is approx. 280 g. Micro-aethalometer AE-51 reports the BC concentration in mg/m3 and attenuation (ATN). We calculated the BC from the time variation of ATN. In order to limit noise in a signal, the ATN value before signal derivative was filtered by running a mean filter, with an averaging window of 20 s. Afterwards, the filtered ATN was used to calculate BC concentration and absorption coefficient with the following formulas:
BC ¼
sAB $C$RðATNÞ sATN
sAB ¼
dATN A dt Q $C$RðATNÞ
where A is a sample spot area on the filter (7.1 106 m2), Q is the volumetric flow rate (100 ml/min), C is the multiple scattering optical enhancement factor (2.05 ± 0.03 Ferrero et al., 2011), R(ATN) is the aerosol loading factor, and sATN is the apparent mass attenuation cross section (12.5 m2/g at 880 nm). The R(ATN) term in the formula compensates for the nonlinear loading effect caused by the increase in aerosol absorption over time, which in turn results in reduction in the optical path. Schmid et al. (2006) found that it is required only when the ATN becomes higher than 20. Therefore, we controlled the value of the ATN and changed the filter when the ATN was near (16e18) the threshold. Reported BC concentrations are normalised to standard conditions (pressure 1013.25 hPa, temperature 273.15 K). An optical particle counter, OPC-N2, measures the light scattered at 658 nm by individual particles carried in a sample of air stream through a laser beam (Table 1). These measurements are used to determine the particle size, which is related to the intensity of light scattered via a calibration based on the Lorenz-Mie theory. Particle mass concentration PM1, PM2.5, and PM10 are then calculated from the particle size spectra and concentration data, assuming a particle density (1.65 g/ml) and refractive index (1.5 þ i0). Assuming both parameters as well as the relative humidity, which is not measured inside the device leads to a significant error in the PM estimation. For example, in the case of a Scanning Mobility Particle Sizer (SMPS) and an Aerodynamic Particle Sizer (APS) the uncertainty is close to 30% (Buonanno et al., 2009). The OPC-N2 employs a narrow inlet to physically constrain the airborne particles to pass through a uniform central part of the illuminating laser beam and ensure accurate sizing. The instrument classifies each particle size, at rates up to ~10,000 particles per second, recording the particle size to one of 16 bins covering the size range from 0.38 to 17 mm. The flow rate is about 220 ml/min. The data is transmitted via an SPI to a Raspberry Pi microcomputer every 1 s. This device was used during vertical profile measurements.
3. Meteorological conditions On 7th March Central Europe was under the influence of a lowpressure system (Fig. 3a). The front line was aligned longitudinally, and this resulted in warm polar marine air masses coming from the south to the area of Southern Poland. The 72-hour back-trajectories from the NOAA-HYSPLIT (Draxler and Rolph, 2010), which ended in Krynica at 12:00 UTC on selected days (9th, 11th, and 13th March), are presented in Fig. 4(aec). On the following days the low-pressure area moved north-
eastward and dissipated, leaving the quasi-stationary front line from Finland in the north, through the Baltic states, the eastern border of Poland, through the Balkan states to Greece in the south (Fig. 3b). The air from the height of about 3000 m came from the area of France, crossing the Mediterranean Sea Basin (Fig. 4a). The lower air layer travelled a much smaller distance, in particular air mass from 1000 m stayed in the Poland territory and near to the surface for the whole 72-hour period. The air pressure in southern Poland rose around 15 hPa within five days, from ca. 1005 hPa on 7th to ca. 1020 on 11th March (Fig. 3aec) (see Table 1). Back-trajectories of 11th of March show that the source area of the incoming air masses changed significantly (Fig. 4b). The air began to flow from the east and south-east, bringing warmer polar marine air masses from the central Russia source area. During the next two days (12th and 13th March) a high-pressure area above Scandinavia and Denmark began to develop. Respectively, the air pressure over Poland was gradually increasing, and reached 1030 hPa over southern Poland on 13th March (Fig. 3d). Thanks to that high-pressure system, the air in Central and Eastern Europe started to come from the north. Back-trajectories from HYSPLIT for 13th March clearly show the steadily stratified atmosphere (Fig. 4c). The air from the height of about 1000 and 2000 m came from almost the same altitude, while the higher layer (3000 m) settled from the level of over 6000 m. Transformed arctic air masses, coming from Scandinavia and the Arctic Ocean, resulted in a temperature decrease during the following days. At Jaworzyna Krynicka (1114 m a.s.l.) the temperature varied between 0 C up to 45 C during the first three days, and decreased during the following days (Table 2). Most of the time it was colder than in Krynica at 580 m a.s.l, but with the possibility of the occurrence of nocturnal inversions. Relative humidity was above 90% for the whole period, and minor snowfall/sleet occurred on a few days. Significant precipitation was observed in Krynica WMO station on 7th March (4.2 mm) and 13th March (2.6 mm). On the rest of the days there was only trace precipitation (Table 2). 4. Temporal variability of single-aerosol properties from PAX devices Fig. 5 shows temporal variability of aerosol scattering (a), absorption coefficient (b) and SSA (c) at 532 nm obtained from PAX devices. The black dots correspond to the upper station and red ones to the lower station. The grey points show very clean conditions (scattering coefficient below 5 Mm1) to indicate that the SSA is very uncertain. The aerosol extensive optical properties during the field campaign were very low. For example the mean extinction coefficients at 532 nm were 43.4 Mm1 and 37.1 Mm1 for the lower and upper station, respectively. For comparison, the mean extinction coefficient for March observed by the Aurora 4000 nephelometer and AE-31 aethalometer at the Poland-AOD station in Strzyzow (444 m a.s.l. and 85 km from Krynica) amounted to about 120 Mm1 (Markowicz et al., 2016). During the field campaign the extinction coefficient only once exceeded 100 Mm1, at the lower station. At the upper station the extinction coefficient every day dropped below 20 Mm1. Especially between 10th March 00:00 UTC and 11th March 10:00 UTC the air mass was very clean. Fig. 5a shows also some temporal variability between the lower and upper stations. During the night the values were usually larger in the valley while during day the larger extinction was generally observed in the upper station. We found significant correlation (r2 ¼ 0.57) between the extinction difference at both sites and air temperature gradient measured from Poprad-Ganovce radiosonde at 00:00 and 12:00 UTC (Fig. 6). The largest difference between lower and upper extinction was observed during the night and when the temperature gradient was small (stable conditions) as
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Fig. 3. Synoptic analysis charts for 7th (a), 9th (b), 11th (c), and 13th (d) March. Charts were prepared in IMGW-PIB, CBPM in Krakow (http://www.pogodynka.pl/polska/mapa_ synoptyczna).
well as when the vertical mixture was negligible. During the day and in unstable conditions the extinction coefficients at both stations were much closer and even the values at higher altitude were larger. Provisos research shows that wind speed, height of the PBL top as well as the temperature profiles have a significant impact on the vertical distribution of the aerosol optical and microphysical properties in the lower troposphere (Zhang et al., 2009). In general, vertical and horizontal distribution of aerosol properties in the mountain area is very complicated due to several physical processes that can play an important role: mountain-valley breeze, distribution of emission, PBL modification over the mountain, dilution and accumulation process, and horizontal and vertical mixing. In the case of the absorption coefficient (Fig. 5b) the mean value of the absorption coefficient measured at the upper station was 8.0 Mm1 while in the valley it was only 4.1 Mm1. A significant difference in mean values was seen for measurements on 8th and 11th March. During the remaining days the difference in the absorption coefficient is rather small. In addition, significant temporal fluctuation of the absorption coefficient close to the top of Jaworzyna Krynicka was probably closely related to local emissions from coal central heating systems. The SSA at the upper station was lower during the whole campaign (Fig. 5c). Only close to noon were the SSA and also scattering and absorption coefficients at both
stations were similar, with the exception of 10th March. For the lower site the SSA changed between 0.7 and 0.98 while for upper between about 0.4 and 0.92. However, the very low SSA measured at both sites during the afternoon of 10th March is unrealistic. This value corresponds to very clean conditions (scattering coefficient of 3e7 Mm1), so the SSA is very uncertain. 5. Vertical variability of the aerosol number and BC concentration The vertical profiles of optical and microphysical properties were performed on 9th March 2016 between 8:00 and 15:00 UTC. Fig. 7 shows temporal variability of aerosol concentration from OPC-N2 (Fig. 9a) and BC concentration from AE-51 (Fig. 7b) which was mounted on the cable car. In addition, Fig. 7c and d present the extinction and absorption coefficient from PAX 532 nm, respectively, for the lower (red line) and upper (black line) station. In the case of the OPC-N2 and AE-51 the temporal oscillation is shown as a result of the change in altitude of the sensors. In addition, some variation exists for all quantities. Between 8:00 and 12:00 UTC the aerosol and BC concentration slightly decreased as well as the extinction and absorption coefficient at the lower and upper stations. Later, at 12e14 UTC, a significant increase of all optical and microphysical parameters was observed. During the last hour of the
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Fig. 4. The Hybrid single particle Lagrangian integrated Trajectory model (HYSPLIT) back-trajectories obtained for Jaworzyna Krynicka at 12:00 UTC for 9th (a), 11th (b) and 13th (c) March. The HYSPLIT model was run for 72 h with meteorological data from the Global Data Assimilation System (GDAS).
measurements some reduction of aerosol concentration and extensive properties were detected. Moreover, slightly larger extinction and absorption coefficients were observed at the upper station in comparison to the values in the valley. Some opposite relation was observed during the morning (until 9:00 UTC) and late afternoon (after 18:00 UTC). Due to variability of daily aerosol conditions (Fig. 7), cable-car measurements (9th March 2016) were divided into four periods. Vertical profile data were averaged in layers 50 m high from 650 m
a.s.l. up to 1100 m a.s.l. In Fig. 8 the vertical aerosol and BC concentration profiles from OPC-N2 and AE-51 devices are presented. Between 08 and 10 UTC (Fig. 8a) both aerosol and BC concentrations show very similar vertical variability. BC decreases from 920 ng/m3 at about 650 m a.s.l. to 660 ng/m3 at about 1000 m a.s.l., while aerosol concentration reduces from 27.5 1/cm3 to 21.0 1/cm3. Close to the top both parameters slightly increase with altitude. The next profiles (10-12UTC, Fig. 8b) are very similar to the previous one; however, close to the top of the mountain an increase of BC
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Table 2 Summary of meteorological parameters during the campaign: Tmax [ C] e maximum temperature, Tmin [ C] e minimum temperature, RH [%] - relative humidity, Prec. [mm] e daily sum of precipitation. Date
7.03. 8.03. 9.03. 10.03. 11.03. 12.03.
Jaworzyna Krynicka (1114 m a.s.l.)
Krynica (WMO station at 580 m a.s.l.)
Tmin [ C]
Tmax [ C]
Tmin [ C]
Tmax [ C]
RH [%}
Prec. [mm]
0.4 0.2 0.4 0.2 0.9 0.3
3.2 3.6 5.1 1.1 3.6 1.4
0.6 2.0 1.5 1.2 2.0 1.4
6.6 7.2 8.0 4.4 6.3 6.0
93.3 89.3 86.0 98.3 95.0 95.0
4.2 0.1 0.0 0.2 0.5 0.2
Fig. 6. Aerosol extinction coefficient difference between lower and upper station as a function of the air temperature measured at 00:00 and 12:00 UTC radiosonde launched from Poprad-Ganovce. Optical data was averaged ±2 h before and after radiosonde profile. The black line shows the linear fit.
Fig. 5. Comparison of PAX532 measurements (scattering, absorption coefficient and SSA) obtained between 8th and 13th March at lower station (red dots) and upper station (black dots). The grey dots show scattering coefficient below 5 Mm1 and corresponding SSA.
and aerosol concentration was not observed. On the contrary, the BC decreased from 900 to 530 ng/m3, and aerosol concentration from 25.0 to 17.5 1/cm3. The profiles averaged between 12 and 14 UTC (Fig. 8c) show significantly higher values in the whole layer. The decrease of the BC with altitude is more constant (gradient) in comparison to aerosol concentration. BC at the bottom of the valley was about 1450 ng/m3, and close to top it was 1200 ng/m3. The aerosol concentration reduced from 42 to 35.5 1/cm3. The last profiles (14e15 UTC, Fig. 8d) were performed in cloudy conditions. The cloud base was above 900 m a.s.l., while the top was above the mountain. Just below 1000 m a.s.l. the aerosol concentration increased while the BC decreased but in the lower atmosphere both parameters increased towards the valley. In case of the vertical variability the radio sounding can help with data interpretation. Radio sounding from Poprad-Ganovce, even though made around 60 km farther away, gives general information on thermodynamic conditions. Fig. 9 shows data
Fig. 7. Temporal variability of (a) aerosol concentration from OPC-N2, (b) BC concentration from AE-51, (c) extinction and (d) absorption coefficient from PAX 532 nm for the lower (red line) and upper (black line) station between 8:00 and 15:15 UTC on 9th March 2016.
obtained from 00:00 UTC and Fig. 10 from 12:00 UTC observations on 9 March 2016. Night time thermodynamics profiles indicate a humid layer (relative humidity about 100%) in the lower troposphere. The mean lapse rate up to 1100 m a.l.s. was about 0.35, which indicates a stable condition event for condensation. Above there is a potential instability (air temperature in red parallel to saturated adiabatic). The lower atmosphere is not dryer at any time. From surface (706 m a.s.l.) up to 1140 m a.s.l. the potential temperature is almost constant indicating well mixed PBL. At the top of the PBL a two-degree inversion separates the air mass in the free troposphere. Above the inversion (about 1200 m a.s.l.) the potential temperature increases slightly with altitude up to 2500 m a.s.l. (the second condensation level). Thus the morning profiles of aerosol and BC concentration showing larger gradient between the valley and the top of the mountain than the afternoon data can be explained by the stable thermodynamic conditions. The smaller
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Fig. 8. Averaged vertical profiles of two parameters on 9th March 2016. Observation results were segregated into 4 periods (08e10 UTC, 10e12 UTC, 12e14 UTC, and 14e15 UTC). Red triangles represent the aerosol concentration [1/cm3] for size range from 0.38 to 17 mm, and the black squares show the BC concentration [ng/m3].
Fig. 9. Radio sounding from Poprad-Ganovce on 9th March 2016 at 00:00 UTC. The thick black line shows air temperature, red line shows the due point temperature, while the blue line shows potential temperature. In addition, symbols on the right side show wind speed and direction.
reduction of physical parameters with altitude in the last two cases is due to well mixed PBL (Fig. 11). However, there is still a significant difference in aerosol and BC concentration between the valley and the top of the mountain. This can be related to very low wind speed.
Fig. 10. Radio sounding from Poprad-Ganovce on 9th March 2016 at 12:00 UTC. The thick black line shows air temperature, red line shows due point temperature, while the blue line shows potential temperature. In addition, symbols on the right side show wind speed and direction.
Both radio sounding showed that the wind speed in the PBL was up to 2e2.5 m/s. The local pollution is mostly emitted in the valley by the heating system, so the cable-car measurements may shows both horizontal and vertical gradient between lower and upper
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Fig. 11. Averaged aerosol volume size distribution measured on the route of the cable car on 9th March 2016. Observation results were segregated into four periods (08e10 UTC, 10e12 UTC, 12e14 UTC, and 14e15 UTC).
stations. Note that horizontal distance between both points is about 2.3 km. Fig. 11 shows spatial volume distribution of aerosol particle concentration from OPC-N2. Plot shows only four of nine averaging levels for clarity. At the altitude level of 900 m during the hours 12e14 UTC significant increase of coarse mode concentration was observed. During the last hour the amount of coarse particles started increasing at 800 m which is the altitude of the cloud base. In the case of the first period of observation (08e12 UTC) the aerosol size distribution was almost constant with altitude. Profiles of aerosol number concentration for fine (smaller than 1 mm) and coarse (larger than 1 mm) particles indicates cloud development between 14:00 and 15:00 UTC (Fig. 12). A significant change in aerosol number concentration of coarse particles was observed from 800 m a.s.l. to the mountain top. In addition, the fine particles were reduced in whole cloud layer (Fig. 12d). The coarse mode concentration increased at the expense of a decrease in the smaller particles. This is the result of cloud droplet coalescence and the evolution of cloud size distribution. During the morning (before aerosol activation) the aerosol number concentration for both fine and coarse mode slightly decreased with altitude, but the variability was negligible. 6. Summary and discussion The presented paper describes results obtained from a pilot experiment carried out in the mountainous region of Krynica-Zdroj located in southern Poland. The novel approach was the installation of the measurement equipment on a cable car, which could be the future of aerosol and PBL observation in mountain areas. The aerosol extensive optical properties measured by PAX at 532 nm during the field campaign were very low due to the weather conditions. Measurements were made during cloudy conditions when the development of a night surface inversion is unlikely. The mean
extinction coefficients at 532 nm were about three times lower that long-term mean for March measured at the rural station in Strzyzow. At the upper station the extinction dropped below 20 Mm1 every day. During the night the values were usually larger in the valley, while during the day the larger extinction was observed in the upper station. The largest difference between lower and upper extinction coefficient was observed during the night and when the temperature gradient was small (stable conditions). We found significant correlation between the temperature lapse rate in the lower PBL and the gradient of the extinction coefficient between the lower and upper stations. When the lapse rate was close to dryadiabatic (or higher) then the difference in extinction was low, but during stable (nighttime) conditions it was up to 40 Mm1. The vertical profile of aerosol microphysical properties measured on the cable cars on 9th March 2016 shows significant variability with altitude. For all measurements the aerosol and BC concentration decreased with altitude, but the gradient was steeper during the morning due to more stable thermodynamic conditions. During the afternoon and well-mixed conditions both parameters still decreased with altitude. This can be explained by the very low wind speed in the whole PBL (about 2 m/s), which causes weak horizontal mixing. As a result of the vertical mixing the profiles of the aerosol number concentration indicated cloud developments between 14:00 and 15:00 UTC. The fine particles were reduced in whole cloud layer while the coarse mode concentration increased as a result of cloud droplet coalescence and evolution of the cloud size distribution. The OPC-N2 showed a significant increase in the effective radius above 800 m a.s.l. with maximum 1.8 mm at 970 m a.s.l., as well as a significant change in aerosol number concentration of coarse particles from 800 m a.s.l. to the mountain top. Developing this research method by placing the measuring set on the cable car (Seidel et al., 2016) will enable the measurement of profiles of aerosols microphysical properties during the carriage ride up or down within several minutes. This method is much more
Please cite this article in press as: Zawadzka, O., et al., Study of the vertical variability of aerosol properties based on cable cars in-situ measurements, Atmospheric Pollution Research (2017), http://dx.doi.org/10.1016/j.apr.2017.03.009
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O. Zawadzka et al. / Atmospheric Pollution Research xxx (2017) 1e11
Fig. 12. Averaged vertical aerosol concentration [1/cm3] profiles of particle sizes up to 1 mm (blue squares) and over 1 mm (green triangles) on 9th March 2016. Observation results were segregated into 4 periods (08e10 UTC, 10e12 UTC, 12e14 UTC, and 14e15 UTC).
effective in comparison to other types of measurements (e.g. taken while walking on foot) because it shortens the time of creating the profile and increases the number of profiles done within 24 h. Moreover, this solution is much simpler and cheaper to realise in comparison to airborne measurements (airplane or UAV) or a tethered balloon because it does not require an additional technical base or full-time maintenance, and significantly lowers the overall cost of the research (Seidel et al., 2016). Acquired data could be evaluated against other observations, for example LIDAR measurements. The limitation of the cable-car method is the height of the performed profile due to the height of the upper cableway station, but the elevation (approx. 400 m) is sufficient to study the pollution deposited in the PBL. The results of the described pilot experiment show the great potential of this research method. The installation of the measurement equipment on the cable car may be used in the future for constant monitoring of the state of atmospheric pollution. The use of cutting-edge miniature Particulate Monitor sensors allows measurements (vertical profiles or horizontal transects) to be taken which had not been possible using the stationary monitoring stations of the Inspectorate of Environmental Protection. Additionally, the cost of field measurements is much lower in comparison to stationary or even mobile monitoring stations. This is due to the low cost of construction and maintenance and ease of transportation. Such research will allow the determination of emission of anthropogenic aerosols affecting the development of the smog that fills mountain valleys, which in consequence will enable implementation of atmosphere protection programmes because towns situated in the mountainous regions of the south of Poland take notorious first place in the rankings of cities with the most polluted atmosphere.
Acknowledgements This research was made within Polish Grant No. 2012/05/E/ ST10/01578 of the National Science Centre coordinated by IGF UW and also project KNOW, Leading National Research Centre received by the Centre for Polar Studies for the period 2014e2018 established by regulation No. 152 (2013, Nov 14) of the Rector of the University of Silesia. We also want to thank all workshop participants: Karolina Bartkowska, Bogdan Chojnicki, Olga Krzywicka, Szymon Malinowski, Jakub Nowak, Paulina Pakszysz, Bartłomiej Pietras, Michał Piwoda, Anna Rozwadowska, Mateusz Samson, Adam Skomorowski, Urszula Syperowicz, Patrycja UlandowskaMonarcha, Krzysztof Wiejak, and Anna Zimniak.
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Please cite this article in press as: Zawadzka, O., et al., Study of the vertical variability of aerosol properties based on cable cars in-situ measurements, Atmospheric Pollution Research (2017), http://dx.doi.org/10.1016/j.apr.2017.03.009