Atmospheric Aerosols Studies over Arctic: Initiatives for Long Term Data and Preliminary Results

By: Jai Prakash Chaubey,* S Suresh Babu,V K Krishna Moorthy,◆ Mukunda M Gogoi,F Manoj M R,■ Anoop Tiwari• and K Satheesan•
After the launch of Indian Arctic Expedition and establishment of the Indian station Himadri (78o9'N, 11o9'E; Ny Ålesund, Svalbard) in Year 2007, during the International Polar Year (IPY), there were scattered measurements of atmospheric aerosols by few Indian groups which generated data (only summer months) but planned measurements and long term data were lacking. Keeping all the information and knowledge gaps in the background and taking cue from a month-long campaign-based measurements during the 4th Indian Arctic Expedition, 2010, from Gruvebadet (a place slightly away from Himadri and also away from the town area of Ny Ålesund), a long term measurement programme was formulated and executed during the 5th Indian Arctic Expedition. This involves continuous measurements of distinct aerosol properties round the year from Gruvebadet, with intense measurements during the summer months. Measurements of black carbon (BC) mass concentration, scattering coefficient (σsc) of aerosols are being made continuously while the total mass concentration (MT) of composite aerosols, columnar aerosol optical depth (AOD) and number size distributions (NSD) were made during the summer months. These measurements generated year round data from the Arctic, the first effort of India for any polar region (Arctic and Antarctic). These data have added to the already existing database of the Arctic and at the same time, preliminary analysis provides many new insights. This includes the effect of local activities on the aerosol within boundary layer, giving a weak diurnal variation with daytime high concentrations of BC, AOD, MT, BC and σsc showed a slight decreasing trend from June to August, indicating the pristine nature of Arctic atmosphere during summer.
Policy

Introduction

Polar regions are expected to provide an early signal of global warming and change in the Earth’s climate system, due to feedback effects associated with the high albedo of snow and ice (Alterskjaer et al., 2010; Shindell, 2008; Wang and Key, 2005; Chaubey et al., 2010). Although snow and ice albedo feedbacks are the key concern in the polar region, distribution of aerosols in the atmosphere is also an important factor in controlling the Earth’s radiation budget and affecting snow and ice albedo (Nagel et al., 1998; Flanner et al., 2007). A small quantity of absorbing aerosols (anthropogenic or natural) over the highly reflecting snow might enhance the warming of the atmosphere and the deposition of these particles over the surface of the snow or ice may reduce the albedo (Hansen and Nazarenko, 2004). As such, there is an increased interest and need to investigate the properties of polar aerosols, and their spatial temporal and microphysical properties to understand their climate forcing potential (Herber et al., 1993; Schwartz and Andreae, 1996; Sharma et al., 2004; IPCC, 2007; Quinns et al., 2007; Babu et al., 2011; Chaubey et al., 2011). Out of the two polar regions, aerosol properties in Antarctic are still less influenced by anthropogenic activities than those over Arctic. Antarctica is a unique continent at the extreme south, separated from the other populated continental masses, making it one of the most pristine places on the Earth (Wall, 2005). Due to the pristine characteristics (compared to other snow covered regions like the Arctic or the high altitude mountains in the Northern Hemisphere), it provides an excellent environment to examine the natural and background aerosols in the atmosphere over snow and ice. In contrast to Antarctic, in the Arctic aerosol properties are different. The large difference in the anthropogenic influence between the two regions is primarily responsible for this distinct behaviour, which occurs due to the position of the Arctic Polar Front and the presence of human population in Europe, North America and North Asia located north of the Polar Front. The seasonal position of the Arctic Polar Front is around ~50°N during winter, allowing the long range transport of pollutants from mid-latitudes to the Arctic, resulting in high concentrations of aerosols and reactive gases, the so called Arctic haze (Shaw,1995). The aerosol concentration remains high during the spring and with the availability of snow and sunlight during this season, radiative effects of aerosols are more pronounced. On the other hand, the Polar Front, shifting further north during summer (~70°N), better shields the Arctic from the polluted air masses. In addition, during summer, low-level cloud-cover leads to drizzles, which in-turn results in scavenging of the aerosol from the lower layers of the atmosphere (Curry et al., 1996; Lawson et al., 2001; Koch and Hansen, 2005; Tjernstrom, 2005). These conditions result in a summer minimum in aerosol loading and impact the Arctic region making this season suitable for studying the near-background aerosols. On the other hand, increase in precipitation and abundance of solar radiation (in comparison to winter) resulting in the vertical as well as horizontal dispersion of aerosols, makes the summer atmosphere less stable as compared to winter (Alterskjaer et al., 2005). This would also lead to a vertical heterogeneity in aerosol characteristics and possible elevated aerosol layers resulting from the dynamics of the local ABL. While the aerosols properties over Arctic winter/spring atmospheric conditions (Arctic haze) have been explored to a great extent, the current understanding of the summertime aerosol properties over the Arctic is still limited (Iziomon et al., 2006). Summer time aerosol characterisations are important as they relate to the role of local surface pollution (emissions from increased shipping transport, tourism as well as scientific activities) to the Arctic aerosol system.

In the backdrop of the above, aerosol properties were measured during the summer of 4th Indian Arctic Expedition, 2010, from Gruvebadet (~10 m msl), at Ny Ålesund (78.9°N, 11.83°E), Spitsbergen in the Svalbard Archipelago. Based on the analysis of the limited data collected during the 4th Indian Arctic Expedition (July-August 2010), an initiative was taken for the generation of long term aerosol data and it was implemented successfully during the 5th Indian Arctic Expedition (2011). Continuous measurements of black carbon mass concentration (MB) and scattering coefficients (σsc) have been made (and are continuing). In this paper, we present the preliminary results of the first ever long term measurements by India from Ny Ålesund, in the Norwegian Arctic region.

 

 

Site Description

The measurements are carried out from Ny Ålesund (78.9º N, 11.9º E) of Svalbard archipelago which falls in the Norwegian Arctic region and well inside the Arctic Circle. In Figure 1(a), we show the Ny Ålesund town area and the location of Indian station Himadri. The closer view of the Indian station Himadri is shown in Figure 1(b). It is clear that the aerosol sampling from Himadri will be influenced by the local population and pollution in the Ny Ålesund town area. Due to this reason, aerosol sampling at Ny Ålesund is carried out at Gruvebadet (~10 m msl), slightly away from Himadri and just below the Zeppelin observatory (~475 m msl). In Figure 1(c) and 1(d), we show respective locations of Gruvebadet and Zeppelin stations during two different seasons (Spring 2012 and Summer 2011). Gruvebadet was chosen for sampling due to its slight elevation as well separation from the local anthropogenic activities which are maximum during summer. Overall, the sampling location is representative of clean Arctic conditions. It is also clear that, the total snow cover of the region shows significant variations, with maximum during the winter/spring and minimum during the peak summer months.

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Instrumentation

Extensive aerosol properties were measured using different aerosol instruments. In Table 1, we list the instruments, their make, aerosol properties measured and the period of measurements. The long-term data set generated from these measurements, constructed the first time data base, from any of the polar region by India.

Near-surface real- time measurements of the black carbon (BC) mass concentration (MB) were made continuously since 10 June 2011 using a multi-channel aethalometer (model AE-31, Magee Scientific). The ambient air was aspirated at a rate of 5 lpm from a height of 5 m above ground level. The measurement time base was kept at 30 minutes. Aethalometer is a rugged field instrument extensively used by the aerosol research community for continuous measurements of ambient BC mass concentration over a variety of environments (for eg., Hansen et al., 1984; Novakov et al., 2003; Moorthy and Babu, 2006; Schmid et al., 2006; Eleftheriadis et al., 2009; Chaubey et al., 2010; Babu et al., 2011). Following the error budget described in several earlier papers, the maximum uncertainty in the measured BC was as high as 20 per cent, with the higher percentage of error being applicable to lower concentrations (Corrigan et al., 2006; Moorthy et al., 2007).

Table 1

Measurements of aerosol scattering coefficients were made using multi-channel integrating nephelometer. The integrating nephelometer directly measures the total scattering (7º to 170º) and hemispheric backscattering (90o to 170o) coefficients at three wavelengths 450, 550 and 700 nm. Nephelometer was sampling air from a height of 5 m and sampling duration was 5 minutes. Since the accuracy of nephelometer measurements is directly related to its calibration, the instrument was calibrated prior to installing it at Arctic, using CO2 as high span gas and air as low span gas. Uncertainties associated with the angular truncation error and non-ideality in the geometry of the instrument was corrected using the spectral information of scattering coefficient, following Anderson and Ogren (1998). In general, uncertainties in the nephelometer measurements were <10 per cent (Anderson et al., 1996; Nair et al., 2009; Babu et al., 2011).

Spectral measurements of columnar aerosol optical depth (AOD) at 340, 380, 500, 675 and 870 nm were made at every 30 minutes by using a freshly calibrated hand held microtops Sun Photometer (Solar Light Company, USA) whenever the solar disc and its neighbourhoods were free from visible clouds. This instrument is widely used for AOD measurements and its advantages and limitations, as well as the precautions to be taken care have been extensively given in several earlier papers (Morys et al., 2001; Ichoku et al., 2002; Porter et al., 2001; Moorthy et al., 2005). The AODs estimated from microtops have a typical uncertainty of ~0.01.

Mass concentrations of the composite aerosols were estimated using a single stage high volume sampler (HVS; model GHV 2000P1, Thermo Anderson). The sampler was operated at a flow rate of 0.6 m3 min−1 for different durations ranging from a few hours to a few days to obtain detectable loading on predesiccated, tare-weighed, numbered, and sealed quartz fibre filter substrates. After sampling, the filters were sealed in their respective self-sealing envelopes and taken to the laboratory, where they were desiccated and weighed using the same microbalance. Total mass concentration (MT) was estimated from the difference in the initial and final masses, the flow rate, and the sampling time.

 

 

Meteorological features

Meteorological parameters at a height ~10 m amsl, were obtained from the climate change tower at Ny Ålesund for 2011. In Figure 2(a), we show the day to day variations of temperature (T, in °C), pressure (P, in mb) and relative humidity (RH, in per cent) in the top, middle and bottom panels respectively, and Figure 2b represents the polar diagram of daily wind speed (in ms-1) and wind direction (in degrees).

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It is clear from Figure 2 (a) that, T and P decrease towards the end of the year. These general synoptic meteorological features over the study area correspond to the transition from the Arctic summer to winter. The surface pressures measured over this region are comparatively lower during the winter (October to February) than summer (June to September). Unlike P, RH has not shown any seasonal pattern but it showed significant day to day variations and also variations within a day. Similar to P, T also showed a clear seasonal change in temperature, it remained above 0°C in summer and below 0°C in winter, ranging from a maximum T during summer (~ 15°C) and minimum during winter (~-20°C). Winds were comparatively calmer at the location and it mostly remained below 8 m s-1, but occasional high wind speeds (~20 m s-1) are also observed at the location, more frequently in the winter months.

 

 

Preliminary Results

Aerosol optical depth

Temporal variation of columnar aerosol optical depth (AOD) at 500 nm wavelength is shown in Figure 3. The gap in the data resulted is partly due to the frequent cloudy conditions and partly due to the measurement gap.

AOD at 500 nm ranged from about 0.19 to 0.04 for all observations made during the study period, showing a five fold decrease in the columnar loading from early June to late August. The high AOD values during the June (day 160 to 180) are the residues of aerosols in the column indicative of transported pollution (Shaw, 1991) which is more during the winter and spring months and very uncommon in the summer, due to the shrinking of Polar Front and increased drizzle and settling of aerosol particles. AOD measured at the end of study period are indicative of clean summertime AOD and are characteristic of Arctic background conditions (Shaw, 1982; Dutton et al., 1984; Stone, 2001).

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Total mass concentrations 

The total mass concentrations (MT) were obtained for each sample collected by high volume sampler for the study period (June-July 2011) and are shown in the Figure 4, where the sample number 1 belongs to the 11 June and the sample number 26 belongs to the 9 July, thus showing the temporal variation of MT, in the summer atmosphere. MT showed significant day-to-day variations and it ranged from a minimum value of 1.5 μg m-3 to a maximum of 14 μg m-3, and comparatively lower values are found at the end of study period. Similar to the columnar AOD, minimum values at the end of the study period shows the pristine nature of Arctic summer atmosphere. The larger day to day variability indicates the influence of local activities in the absence of long range transport.

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Black carbon mass concentrations

Temporal variation of monthly mean features of MB are shown by the box and whisker plot in Figure 5. Monthly mean MB showed a decrease from June (25 ± 22 ng m-3 ) to October (19 ng m-3 with a high standard deviation of 65 ng m-3) and started increasing thereafter reaching 49 ng m-3 (in December), showing the seasonality of BC in the Arctic atmosphere, with summer low and winter high. This variation is clearly seen in the median values (middle line of the box) and in the minimum values. It is also noteworthy that, mean MB values in every month are higher than the median value, indicating the influence of a fewer higher values occurring at the location. The high standard deviation during each months also indicates the influence of local population and tourism which increases during summer and this aspect needs further investigation.

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Diurnal variations and their seasonality are also important in understanding the effect of local meteorology and atmospheric boundary layer (ABL) dynamics. Diurnal variations were prominent in all the months. The monthly averaged diurnal variations of MB for three representative months (June, September and December) are shown in Figure 6. Diurnal variations reveal a conspicuous pattern with an afternoon high which may be due to the boundary layer, local emissions (which are maximum during the day time activity period) affecting the measurement location.

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Aerosol scattering coefficients

A temporal variation of the scattering coefficient (σsc) at 550 nm wavelength is shown in the Figure 7. The σsc ranged from a very low value of 0.03 Mm-1 to as high as 16 Mm-1, showing a mean value of 2.6 Mm-1 with a standard deviation of 2 Mm-1. The median value (~2 Mm-1) is comparable to the mean value indicating the minimal high values of σsc, occurring at the locations. Overall, there is a small decreasing trend from June to August and it showed slight increase towards the September-October months.

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Discussions

The current understanding of winter and spring time aerosols over the Arctic is high but at the same time, summertime aerosol characterisation over the Arctic is minimal. Our measurements from the Gruvebadet (concentrated more in summer) showed a decrease in the columnar as well as surface aerosol properties from initial summer days (June) to peak summer months (August). Although our measurements generally record low aerosol conditions (as median values are lower than the mean values) which is typical of background locations, but events of high MB and total mass concentrations (MT) are frequently observed at the location. This may be due to the local influences or due to the long range transport episodes. The measurements presented in this paper are carried out at a moderate elevation of ~10 msl in comparison to the Zeppelin station (~475 m msl) and will be a better location for characterising the effect of local activities, which are more in summer and not taken into account for representation in the models. Long range transport episodes will affect both the locations in a similar way but the local influences have more effects visible on the low altitude location, which will be more enhanced during the absence of strong convection (solar radiation) and occurrence of low level temperature inversions. Simultaneous measurements from Gruvebadet and Zeppelin will help delineation of local influences and episodic long range transports. Recently, Storm et al., 2009 reported the variation in particle number density in the Arctic boundary layer during summer and its strong modulation by mixing and dilution by performing measurements at two different heights. This also demands for the concurrent observations of aerosol properties at two different heights in this location. In this regard the continuous measurements being carried out at the Gruvebadet needs to be strengthened with more focus on the experiments during summer.

 

 

Conclusion

Extensive measurements of aerosol properties were carried out from Gruvebadet (~10 m msl), just below the Zeppelin station (~475 m msl) and slightly away from the Indian station Himadri at Ny Ålesund in the Svalbard Archipelago of the Norwegian Arctic region. The measurements reported are the first time long-term measurements of aerosols by India from the Arctic and contribution to the global data base of the climate-sensitive Arctic region. Our preliminary analysis showed a strong seasonality in the aerosol characteristics with a significant decrease in the aerosol properties from June to September and increase from October to December. The locations remained in the background condition during summer. For the study period, columnar AOD showed a mean of 0.12 ± 0.02. The surface aerosol properties showed large day to day variability with mean total mass concentration (MT) of 7.6 ± 3.7 μg m-3, mean MB of 27 ± 11 ng m-3 and mean scattering coefficient (σsc) of 2.5 ± 2 Mm-1. Concurrent measurements from Gruvebadet and Zeppelin are required to delineate the local influence on the aerosol properties over the Ny Ålesund, which are enhanced during the summer season and require detailed investigations.

 

 

Acknowledgement

This experiment was conducted as a part of the Aerosols Radiative Forcing over India (ARFI) project of ISRO- Geosphere Biosphere Programme. We thank Director, National Centre for Antarctic and Oceanic Research (NCAOR), and Programme Director for Arctic studies in NCAOR for providing the necessary support, for carrying out the experiments. We also acknowledge the support provided by Dr. Vito Vitale of IASC, Centre for National Research, Italy and members of Norwegian Polar Institute at Ny Ålesund.

 

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