Direct Gravimetric Determination of Aerosol Mass Concentration in

Dec 8, 2010 - the plateau of central Antarctica (Dome C, East Ant- ... Measurements were car- .... to fill this gap with reference to the area of Dome...
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Anal. Chem. 2011, 83, 143–151

Direct Gravimetric Determination of Aerosol Mass Concentration in Central Antarctica Anna Annibaldi,* Cristina Truzzi, Silvia Illuminati, and Giuseppe Scarponi Department of Marine Science, Polytechnic University of Marche - Ancona, Via Brecce Bianche, 60131, Ancona, Italy In Antarctica, experimental difficulties due to extreme conditions have meant that aerosol mass has rarely been measured directly by gravimetry, and only in coastal areas where concentrations were in the range of 1-7 µg m-3. The present work reports on a careful differential weighing methodology carried out for the first time on the plateau of central Antarctica (Dome C, East Antarctica). To solve problems of accurate aerosol mass measurements, a climatic room was used for conditioning and weighing filters. Measurements were carried out in long stages of several hours of readings with automatic recording of temperature/humidity and mass. This experimental scheme allowed us to sample from all the measurements (up to 2000) carried out before and after exposure, those which were recorded under the most stable humidity conditions and, even more importantly, as close to each other as possible. The automatic reading of the mass allowed us in any case to obtain hundreds of measurements from which to calculate average values with uncertainties sufficiently low to meet the requirements of the differential weighing procedure ((0.2 mg in filter weighing, between (7% and (16% both in aerosol mass and concentration measurements). The results show that the average summer aerosol mass concentration (aerodynamic size e10 µm) in central Antarctica is about 0.1 µg m-3, i.e., about 1/10 of that of coastal Antarctic areas. The concentration increases by about 4-5 times at a site very close to the station. The Antarctic atmospheric aerosol, particularly that of central Antarctica, is undoubtedly the most rarefied and cleanest on Earth. Nevertheless, knowledge of its content and chemical composition is essential, e.g., to discover the origin, transport pathways, and deposition processes of substances reaching the continent (the most isolated on the globe), to interpret ice core chemical data (e.g., to correlate observed past atmospheric variations to natural or anthropogenic changes), and to understand the climatic role of atmospheric particulate matter in areas of highly reflective surfaces (by measurements of optical depth and calculation of radiative forcing of aerosols).1-12 * Corresponding author. Phone: +390712204981. Fax: +390712204650. E-mail: [email protected]. (1) Shaw, G. E. Rev. Geophys. 1988, 26, 89–112. (2) Robinson, E.; Bodhaine, B. A.; Komhyr, W. D.; Oltmans, S. J.; Steele, L. P.; Tans, P.; Thompson, T. M. Rev. Geophys. 1988, 26, 63–80. 10.1021/ac102026w  2011 American Chemical Society Published on Web 12/08/2010

However, the extreme conditions of temperature, wind, and day/night alternation, which make Antarctica a unique natural laboratory and stimulate exciting research interests, also constitute a challenge for men and equipment operating in such an inhospitable environment. Thus, experimental difficulties have meant that aerosol mass has rarely been measured in Antarctica by direct gravimetry (i.e., differential weighing of exposed/ unexposed collection filters).10,13-18 Indeed, although the literature concerning Antarctic aerosol is extensive (see, for example, the entire volume edited by Wolff, Legrand, and Wagenbach6), generally the overall elemental or ionic compositions are determined,19-34 or a few reference elements are measured and used to compute the masses of major aerosol components (mainly (3) Bales, R. C.; Wolff, E. W. Eos, Trans., AGU 1995, 76, 477–483. (4) Bodhaine, B. A. In Chemical Exchange Between the Atmosphere and Polar Snow; Wolff, E. W.; Bales, R. C., Eds.; Springer-Verlag: Berlin, 1996; pp 145-172. (5) Wolff, E. W.; Bales, R. C.; Eds. Chemical Exchange Between the Atmosphere and Polar Snow; NATO ASI Series, Ser. I; Springer-Verlag: New York, 1996. (6) Wolff, E. W.; Legrand, M. R.; Wagenbach, D. J. Geophys. Res. 1998, 103, 10927–11070. (7) Legrand, M.; Wolff, E.; Wagenbach, D. Ann. Glaciol. 1999, 29, 66–72. (8) Harder, S.; Warren, S. G.; Charlson, R. J. J. Geophys. Res. 2000, 105, 22825– 22832. (9) Delmas, R. J.; De Angelis, M.; Fujii, Y.; Goto-Azuma, K.; Kamiyama, K.; Petit, J. R.; Watanabe, O. Mem. Natl. Inst. Polar Res. 2003, 57, 105–120. (10) Gadhavi, H.; Jayaraman, A. Curr. Sci. 2004, 86, 296–304. (11) Six, D.; Fily, M.; Blarel, L.; Goloub, P. Atmos. Environ. 2005, 39, 5041– 5050. (12) Tomasi, C.; Vitale, V.; Lupi, A.; Di Carmine, C.; Campanelli, M.; Herber, A.; Treffeisen, R.; Stone, R. S.; Andrews, E.; Sharma, S.; Radionov, V.; Hoyningen-Huene, W.; Stebel, K.; Hansen, G. H.; Myhre, C. L.; Wehrli, C.; Aaltonen, V.; Lihavainen, H.; Virkkula, A.; Hillamo, R.; Stroem, J.; Toledano, C.; Cachorro, V. E.; Ortiz, P.; de Frutos, A. M.; Blindheim, S.; Frioud, M.; Gausa, M.; Zielinski, T.; Petelski, T.; Yamanouchi, T. J. Geophys. Res., [Atmos.] 2007, 112, D16205-1–D16205-28. (13) Artaxo, P.; Andrade, F.; Maenhaut, W. Nucl. Instrum. Methods Phys. Res. B 1990, B49, 383–387. (14) Artaxo, P.; Rabello, M. L. C.; Maenhaut, W.; van Grieken, R. Tellus B 1992, 44B, 318–334. (15) Teinila, K.; Kerminen, V. M.; Hillamo, R. J. Geophys. Res., [Atmos.] 2000, 105, 3893–3904. (16) Mazzera, D. M.; Lowenthal, D. H.; Chow, J. C.; Watson, J. G.; Grubisic, V. Atmos. Environ. 2001, 35, 1891–1902. (17) Truzzi, C.; Lambertucci, L.; Illuminati, S.; Annibaldi, A.; Scarponi, G. Ann. Chim. 2005, 95, 867–876. (18) Annibaldi, A.; Truzzi, C.; Illuminati, S.; Bassotti, E.; Scarponi, G. Anal. Bioanal. Chem. 2007, 387, 977–998. (19) Maenhaut, W.; Zoller, W. H. J. Radioanal. Chem. 1977, 37, 637–650. (20) Maenhaut, W.; Zoller, W. H.; Duce, R. A.; Hoffman, G. L. J. Geophys. Res. 1979, 84, 2421–2431. (21) Cunningham, W. C.; Zoller, W. H. J. Aerosol Sci. 1981, 12, 367–384. (22) Tuncel, G.; Aras, N. K.; Zoller, W. H. J. Geophys. Res., [Atmos.] 1989, 94, 13025–13038. (23) Wylie, D. J.; Harvey, M. J.; de Mora, S. J.; Boyd, I. S.; Liley, J. B. In Dimethylsulphide: Oceans, Atmosphere, and Climate; Restelli, G.; Angeletti, G., Eds.; Kluwer: Dordrecht, The Netherlands, 1993; pp 85-94.

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crustal, sea-salt, and sulfate)20,21,35,36 or again particle size distribution and density are considered,37 from all of which one can indirectly estimate (or reconstruct) the overall aerosol masses. The only measured data obtained by direct gravimetry refer to the five coastal areas of King George Island (Comandante Ferraz Station13,14), Queen Maud Land (Aboa15 and Maitri10 Stations), Ross Island (McMurdo Station16), and Victoria Land (Mario Zucchelli Station, formerly Terra Nova Bay17,18), where aerosol atmospheric concentrations were in the range of 1-7 µg m-3. These studies also demonstrated that the indirect estimates of the aerosol mass may be underestimated by up to two-thirds (-67%15 and -11%18 reported by authors, and -61%,13-48%,14 -15% to -22%16 computed by us from authors’ data; see section entitled Comparison with Literature Data). As regards the plateau of central Antarctica, no data have so far been reported for direct gravimetric measurements of aerosol atmospheric concentration, and the present work was designed to fill this gap with reference to the area of Dome C in East Antarctica. In this area the only information available on atmospheric particulates refers to the content of major chemical components, together with the (bimodal) size distribution of submicrometer particles28,32 and the aerosol optical depth,11,12 but no data on aerosol mass concentration has so far been reported. Exploiting the logistic support of the new French-Italian station Concordia at Dome C, one of the very few permanent stations located on the Antarctic plateau, we collected aerosol samples using high volume impactors (aerodynamic size e10 µm, the so-called PM10) and measured the mass of the aerosol by gravimetry at two sites: one “clean” site, located ∼800 m upwind from the station, and one site under the direct effect of the station (∼50 m downwind). We installed two impactors at the “clean” site, operating with differently timed sampling schedules to compare the results and to test the repeatability of measurements. (24) de Mora, S. J.; Wylie, D. J.; Dick, A. L. Antarct. Sci. 1997, 9, 46–55. (25) Hillamo, R.; Allegrini, I.; Sparapani, R.; Kerminen, V. M. Int. J. Environ. Anal. Chem. 1998, 71, 353–372. (26) Arimoto, R.; Nottingham, A. S.; Webb, J.; Schloesslin, C. A.; Davis, D. D. Geophys. Res. Lett. 2001, 28, 3645–3648. (27) Jourdain, B.; Legrand, M. J. Geophys. Res., [Atmos.] 2002, 107, ACH201–ACH20-13. (28) Udisti, R.; Becagli, S.; Benassai, S.; Castellano, E.; Fattori, I.; Innocenti, M.; Migliori, A.; Traversi, R. Ann. Glaciol. 2004, 39, 53–61. (29) Fattori, I.; Bellandi, S.; Benassai, S.; Innocenti, M.; Mannini, A.; Udisti, R. Conference Proceedings of 10th Workshop on Italian Research on Antarctic Atmosphere and SCAR Workshop on Oceanography, Oct 22-24, 2003, Roma, Italy;Colacino, M., Ed.; Italian Physical Society: Bologna, 2004; Vol. 89, pp 101-115. (30) Mishra, V. K.; Kim, K. H.; Hong, S.; Lee, K. Atmos. Environ. 2004, 38, 4069–4084. (31) Arimoto, R.; Hogan, A.; Grube, P.; Davis, D.; Webb, J.; Schloesslin, C.; Sage, S.; Raccah, F. Atmos. Environ. 2004, 38, 5473–5484. (32) Fattori, I.; Becagli, S.; Bellandi, S.; Castellano, E.; Innocenti, M.; Mannini, A.; Severi, M.; Vitale, V.; Udisti, R. J. Environ. Monit. 2005, 7, 1265–1274. (33) Weller, R.; Wagenbach, D. Tellus B 2007, 59B, 755–765. (34) Becagli, S.; Castellano, E.; Cerri, O.; Chiari, M.; Lucarelli, F.; Marino, F.; Morganti, A.; Nava, S.; Rugi, F.; Severi, M.; Traversi, R.; Vitale, V.; Udisti, R. Conference Proceedings of 11th Workshop on Italian Research on Antarctic Atmosphere, Apr 10-12, 2007, Roma, Italy; Colacino, M.; Rafanelli, C., Eds.; Italian Physical Society: Bologna, 2009; Vol. 97, pp 17-41. (35) Wagenbach, D.; Go ¨rlach, U.; Moser, K.; Mu ¨ nnich, K. O. Tellus B 1988, 40B, 426–436. (36) Dick, A. L. Geochim. Cosmochim. Acta 1991, 55, 1827–1836. (37) Harvey, M. J.; Fisher, G. W.; Lechner, I. S.; Isaac, P.; Flower, N. E.; Dick, A. L. Atmos. Environ., Part A: 1991, 25A, 569–580.

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From an analytical point of view, the challenge concerned the ability to determine, with sufficient accuracy and precision, aerosol masses of a few milligrams by differential weighing of filters (exposed and not exposed) with a mass of a few grams. Problems in filter weighings38 arise mainly from the electrostatic charge of filters, which is high at the low humidities of Antarctica, and from the humidity of the weighing environment, which must not only be reproduced carefully in measurements before and after exposure but should also guarantee against possible water adsorption on the very dry aerosol. Finally, temperature stability during measurements is needed as usual. These problems were solved on site by creating a climatic chamber where air was taken directly from outside and brought to a stabilized inside temperature of 15.0 ± 0.5 °C (suggested as optimal to minimize volatilization and water adsorption/desorption biases38). A computerized microbalance was also used to obtain long cycles of semicontinuous measurements without the presence of an operator inside the room and at the same time to keep an automatic check on its temperature and humidity conditions. EXPERIMENTAL SECTION The Site. Dome C (Figure 1 and Figure S1 in Supporting Information) is located on the East Antarctic continental plateau at an altitude of 3233 m and more than 1000 km from the coast, 1100 km from the French research station Dumont d’Urville, and 1200 km from the Italian research station Mario Zucchelli. The Dome C summer camp (75° 06′ S, 123° 21′ E) has been fully operational for scientific work since the 1995-1996 season, and the most important research project carried out there has been the European Programme for Ice Coring in Antarctica, EPICA.39 Since 2005, the French-Italian permanent station Concordia (75° 06′ S, 123° 20′ E) has been open for the purpose of conducting year-round unique research and observation programs on glaciology, astronomy and astrophysics, atmospheric sciences, earth sciences, human biology and medicine, and remote sensing.40 The following main meteorological conditions characterize the site (average values): air temperature in summer -30 °C, in winter -60 °C (minimum -84.6 °C), wind speed 2.8 m s-1 (5.4 knots), maximum 17 m s-1 (33 knots), atmospheric pressure 645 hPa, annual precipitation 2 to 10 cm of snow, prevailing wind direction from the south in summer, and relative humidity 55%. Field Sampling. During the 2005-2006 austral summer, several samplings of atmospheric particulate matter (PM10) were carried out in central Antarctica at two sites of the Dome C area (Figure 1 and Figure S1) from December 7, 2005, to January 14, 2006, using three high-volume samplers (Tish TE-6070V-BL, see below). During this period, the station hosted 20-40 people, and ∼30 kL of Jet A1 fuel (special Antarctic blend) were used on-site. One logistic traverse arrived and ∼10 flights of a Twin Otter airplane were carried out. Two samplers were installed at a “clean” site, located ∼800 m upwind of both the main station Concordia and the Dome C summer camp (75° 06′ 26” S, 123° 19′ 38″ E, ∼150 m south of the unmanned Astrophysic tent, site 52 of the map in Figure S1). They were referred to as Astrophysic Tent 1 and Astrophysic Tent 2. (38) Chow, J. C. J. Air Waste Manage. Assoc. 1995, 45, 320–382. (39) EPICA, European Project for Ice Coring in Antarctica. Epica Community Members. Nature 2004, 429, 623–628. (40) Fossat, E. J. Astrophys. Astron. 2005, 26, 349–357.

Figure 1. Aerial view of the sampling area. The figure shows the locations of aerosol samplers and the prevailing wind direction for the 20052006 campaign.

Collections of samples at this site were provisionally scheduled for 12- and 20-day exposure times, respectively. This sampling strategy was adopted because we had no previous idea concerning the actual aerosol concentration and the real possibility of measuring the corresponding mass of collected particulate using low exposure times. In case of difficulty with mass measurements with the low exposure samples, we would still be able to use the 20-day exposure samples for this purpose, while having better temporal resolution for studies devoted to aerosol chemical characterization with the 12-day samples. Moreover, in general, long exposure times were also required to comply with the requirements of trace element determinations. The third sampler was installed directly downwind of Concordia station (75° 05′ 58” S, 123° 19′ 54” E, ∼50 m north, very close to the “water supply”, site 8 on the map in Figure S1, which was not being used at the time), in order to monitor the effect of the station itself. A 12-day exposure was applied here. To avoid the use of electric generators, the electric power at the two sites was obtained from the main station through electric cables. The actual sampling periods for each impactor, the meteorological conditions, the wind direction and the standard air volume sampled are shown in Table S1 in Supporting Information. Three samples were collected at each of the Concordia and Astrophysic Tent 1 sites, and two samples in the Astrophysic Tent 2 site. The wind direction was mostly as expected, i.e., from the south (or SW), except from December 27 to January 1, when it was reversed. The sampling was continued also under these unfavorable conditions to test whether or not there were contaminating effects at the “clean” site under such conditions. An average temperature of about -29 °C, a pressure of 656 hPa, and a relative humidity of 57% were recorded. The detailed meteorogical conditions during the sampling period are reported in Table S2, Supporting Information. The field activity was followed through from the beginning to the end by one of the authors (G.S.). Laboratory and Apparatus. A climatic chamber for filter conditioning and weighing was created inside the cold/warm

laboratory container previously used for the EPICA project. This laboratory is totally isolated from the other tents, shelters, and buildings of the station, and it was shared with only one other scientist (a chemist). A wooden partition wall with a door was built to obtain a ∼20 m2-room containing only an ISO 14644-1 Class 5 laminar flow cabinet (Gelaire, Australia, Mod. Twin 30) for filter conditioning, and a bench with the balance for filter weighing and the data logger for continuous temperature/ humidity monitoring (see below). The heating system was set to 15 °C while two holes (L about 10 cm2) pierced in the external wall of the laboratory guaranteed almost equal water vapor concentration levels inside and outside. The balance readings were obtained from outside the chamber, in the second part of the laboratory, using a personal computer connected to the balance. No one remained inside the chamber during filter conditioning and weighing, in order not to increase the inner humidity. The personnel entered the chamber only to introduce filters and to change filters for weighing and storage, always wearing clean room garments, masks, and gloves and strictly following clean room procedures. With this arrangement quite stable temperature (15.0 ± 0.5 °C) and relative humidity RH% (2.5% ± 0.5%) were observed throughout the period of extended measurements (∼50 days). Note that similar very low relative humidity conditions (values approaching 0%) are reported in the literature for external air sampled between -20 and -80 °C and brought to an inside temperature of about 20 °C.41 Of course, more stable humidities (between ±0.2% and ±0.3%) were generally observed during single cycles of measurements. It should be stressed that the conditions in terms of water vapor concentration inside the climatic room correspond approximately to the external values. In fact, from the average external temperature (-28.9 °C), pressure (655.9 hPa), and relative humidity (56.5%), and their variations (see Table S2 in Supporting Information), a water vapor concentration of 0.28 ± (41) Bodhaine, B. A.; Deluisi, J. J.; Harris, J. M.; Houmere, P.; Bauman, S. Tellus B 1986, 38B, 223–235.

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0.08 g m-3 can be calculated while the value for the climatic room conditions is 0.32 ± 0.07 g m-3. Given the practical impossibility of performing measurements under climatic conditions close to the external ones, the present laboratory procedure represents a good compromise in which water is certainly not added to the sample (an important problem when working with the driest of Antarctic aerosols). Temperature and humidity measurements inside the climatic room were carried out using a data logger (Escort Data Logging Systems Ltd., Auckland, NZ, Mod. Junior 10D16) which was specifically calibrated for low humidities (certification October 12, 2005). Data were then downloaded into the personal computer outside the climatic room at the end of each weighing stage. Three Teflon-coated (inner and outer), brushless volumetric flow-controlled, high-volume air samplers (impactors model Tish TE-6070V-BL, Tish Environmental Inc., Village of Cleves, OH, Serial #P6586BLX, #P6786BL, and #P6116XBL) were used to collect aerosol samples with aerodynamic size e10 µm and air flow 1.13 m3 min-1 (±10%). A Tish TE-5028A calibration orifice (serial #0943) was available for field calibration. The calibrator and the impactors had been factory calibrated on August 23, 2005. The primary standard positive displacement volume meter was a Roots meter Model 5M175 HVC high volume serial #9833620, Dresser Inc., Houston, TX, which is directly traceable to NIST. The samplers comply with the USA42-45 and European46-48 regulations, and the air inlet is located 1.50 m above the ground. Before aerosol collections, the samplers were cleaned inside and outside by repeated washings using ultrapure water (Milli-Q, Millipore, Bedford, MA) with particular care for the filter support. The impactors were calibrated in the field upon installation and checked at the end of the sampling period. A heating system for the water manometer used for calibration was arranged on-site using an electric bulb inserted inside a wooden box containing the manometer. The container was fitted with a transparent plastic shield for the manometer readings. Details of field calibration procedure and results are reported in Supporting Information. Acid-cleaned18 8 in. × 10 in. cellulose filters (Whatman 41) specifically prepared and tested for trace element determinations were used to collect the particulate matter from the high-volume aerosol samplers. Blank filters were also collected in the field (42) US EPA. Monitoring PM10 in Ambient Air Using a High-volume Sampler. In Quality assurance handbook for air pollution measurement systems. Quality Assurance Guidance Document, Vol. II, Part. II, Chapt. 2.11, U.S. Environmental Protection Agency, Washington, DC; September 1997. (43) US EPA. In Compendium of methods for the determination of inorganic compounds in ambient air, EPA/625/R-96/010a, Compendium method IO2.1; U.S. Environmental Protection Agency: Cincinnati, OH; 1999. (44) US EPA. National Ambient Air Quality Standards for Particulate Matter, Federal Register, 71. U.S. Environmental Protection Agency: Washington, DC, October 17, 2006, pp 61144-61233. (45) US EPA. National Primary and Secondary Ambient Air Quality Standards for PM10 Code of Federal Regulations; Title 40: Protection of environment. Chapt. I., Part 50.6, Appendix J; U.S. Environmental Protection Agency: Washington, DC, 2008. (46) European Directive 96/62/EC of 27 September 1996 on ambient air quality assessment and management. (47) European Directive 1999/30/EC of 22 April 1999 relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air. (48) European Norm 12341: 1998 Air quality - Determination of the PM10 fraction of suspended particulate matter. Reference method and field test procedure to demonstrate reference equivalence of measurements methods, 1998.

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(called “field blanks”) for each sampler at both sampling sites; these filters were simply installed onto the switched off samplers for a few tens of minutes and then taken without any exposure to air flux. These “field blank” filters were subjected to the same treatments as the sample-filters as regards the weighing. Filters were transported to and from the sampling sites inside acidcleaned polyethylene bags. A computerized Mettler AT261 electronic microbalance (readability 0.01 mg, repeatability SD ) 0.015 mg) was used. In Antarctica accuracy tests for the balance were obtained by two certified reference “weights” (OIML class E1) of 10 mg (certified mass 0.0100005 g, 2SD ) 0.0020 mg) and 100 mg (certified mass 0.0999979 g, 2SD ) 0.0020 mg), respectively (certification October 4, 2005). Repeated tests carried out inside the climatic chamber always gave accurate results within the balance repeatability. Given the size of the filters, they were rolled up along the longest side (with the exposed part inside) and placed vertically on the balance pan. After gravimetric measurements, the filters were stored at -20 °C in acid-cleaned 500 mL low density polyethylene bottles (see cleaning details elsewhere18) and transported frozen to Italy for subsequent analytical determinations of trace elements. Filter Weighing. To ensure the complete conditioning of filters to the measurement environment, they were left inside the climatic chamber under the ISO 14644-1 Class 5 laminar flow cabinet, for at least 48 h before weighing. Generally, filter mass measurements were carried out in long stages of several hours of readings (e.g., from 8 a.m. to 6 p.m. and from 7 p.m. to 7 a.m.) with automatic recording of temperature/humidity, every 5 min, and mass, every 20 s (see Figure 2). This experimental scheme allowed us to select from all the measurements carried out before and after exposure (up to 2 thousand), those which were taken under the most stable humidity conditions and, even more important, as close to each other as possible. Finally the automatic reading of the mass allowed us in any case to obtain hundreds of measurements from which to calculate average values with uncertainties sufficiently low to comply with requirements in the differential weighing procedure. Repeatability of Filter Mass Measurements. To highlight the repeatability of mass determinations obtained using the above procedure, Figure 2 shows two typical measurements in which the filter masses are plotted against time together with the laboratory temperature and humidity. In both cases it can be seen that temperature and humidity stabilizes, after initial changes, due to the entrance of personnel to within ±0.4 °C and ±0.3%, respectively. The masses, which show variations related to the humidity changes (but delayed by about 2-3 h) and possibly to electrostatic charges, also reach stable values within ±0.2 mg. On average, a repeatability of ±0.3 mg was obtained. The effect of the heating pulses of the electric heater is clearly visible, too. No problem arises from the effect of the electrostatic charge on the filter since this effect, if present, disappears within about 0.5 h17 and does not interfere in these long-term measurements. Aerosol Mass by Differential Weighings. A differential weighing procedure was adopted to determine the collected aerosol mass directly from the difference between the mass of the exposed filters and their mass measured before exposure. The

Figure 2. Change of the mass, temperature (T), and relative humidity (RH) during a weighing period of 12 h. Two examples of measurements carried out during typical cycles of filter weighings. (a) Weighing of December 21-22, 2005. (b) Weighing of December 23-24, 2005. Highlighted are the periods of stable conditions of temperature and humidity and the related mass measurements. Averages ( SD are reported together with the related number of measurements.

general procedure is shown in Figure 3, where the proper data selection according to stability of mass and comparability of humidity is highlighted. The figure also shows all the results for average masses and repeatabilities (as standard deviation), relative humidities in both measurements, and the final aerosol mass computed by difference together with its uncertainty obtained as standard deviation of a difference. Given the field experimental conditions, and from all the measurements carried out on site, this uncertainty was generally within ±0.5 mg. When the difference in RH% during weighings before and after exposure, as average values from hundreds of measurements, was higher than 0.2%, a correction was applied according to a humidity coefficient obtained from measurements carried out at different humidities (see below).

The Effect of (and correction for) Humidity. In cases of differences in actual relative humidity registered during mass measurements carried out before and after exposure a correction had to be carried out for the mass measured after exposure to compute the value predicted at the same RH% of the measurement carried out before exposure (or vice versa). To do this, a humidity coefficient was determined from mass measurements of the same filter at several humidities (from 2 RH% to 16 RH%). Results are reported in Figure S2 (in Supporting Information) from which a straight line is obtained for the filter mass as a function of the RH%. The correlation coefficient of 0.9882 indicates a relatively strong relationship between the variables which is statistically significant (P < 0.0001) at the 99% confidence level. The line slope (humidity coefficient) and its uncertainty (as a 95% confidence Analytical Chemistry, Vol. 83, No. 1, January 1, 2011

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Figure 3. The differential weighing procedure without correction for humidity. Change of the mass and RH during weighing periods before and after filter exposure. One of the measurements carried out from data of the Concordia no. 2 sample (a typical case without correction for humidity). Filter weighings: not exposed December 14, 2005, exposed January 4-5, 2006. See the proper selection of the exposed filter weighing set carried out at very similar humidity to that of the nonexposed filter.

Figure 4. The differential weighing procedure with correction for humidity. Change of the mass and RH during filter weighing periods before (with estimate of the mass at a different humidity) and after exposure. The second measurement carried out from data of the Concordia no. 2 sample (a typical case with correction for humidity). Filter weighings: not exposed December 14, 2005, exposed January 4-5, 2006. Given the different humidities during weighings carried out before and after exposure a correction for humidity was applied to the data set referred to the tare in order to estimate the values corresponding to the humidity of the gross measurements.

interval) are 3.0 ± 0.5 mg (RH%)-1. Corrections according to this coefficient were therefore applied when the difference in humidity was higher than ±0.2% RH (about one-third of the experimental measurements). 148

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As an example of measurements carried out with correction for humidity, consider again the case displayed in Figure 3 in which the whole set of weighings after exposure shows two stable values for two different average humidities, one of these practically

Table 1. Analytical Results and PM10 Aerosol Mass Concentration at Dome C during the Summer 2005-06a PM10 aerosol mass concentration site Concordia Astrophysic Tent 1 Astrophysic Tent 2 a

sample

actual air volume (m3)

standard air volumeb (m3)

measured aerosol mass (mg)

actual air (µg m-3)

standard airb (µg m-3)

1 2 3 1 2 3 1 2

20683 20103 17958 19413 19428 20037 32752 27727

16484 15741 14115 15453 15212 15749 26005 21793

[n ) 4] 9.20 ± 1.43 (±16%) [n ) 10] 5.65 ± 0.52 (±9%) [n ) 4] 7.04 ± 0.42 (±6%) [n ) 4] 2.06 ± 0.16 (±8%) [n ) 8] 3.67 ± 0.31 (±8%) [n ) 4] 1.78 ± 0.26 (±15%) [n ) 8] 3.82 ± 0.25 (±7%) [n ) 4] 4.35 ± 0.52 (±12%)

0.455 ± 0.069(±15%) 0.281 ± 0.026(±9%) 0.392 ± 0.024(±6%) 0.106 ± 0.008(±8%) 0.189 ± 0.016(±8%) 0.089 ± 0.013(±15%) 0.117 ± 0.008(±7%) 0.157 ± 0.019(±12%)

0.558 ± 0.087 (±16%) 0.359 ± 0.033 (±9%) 0.499 ± 0.030 (±6%) 0.133 ± 0.010 (±8%) 0.241 ± 0.020 (±8%) 0.113 ± 0.017 (±15%) 0.147 ± 0.010 (±7%) 0.200 ± 0.024 (±12%)

Average values ± SD (± RSD%). b 298 K, 760 mmHg.

coincident with that of the weighing before exposure (directly used for the differential measurement), and the other ∼1% RH higher. Figure 4 shows the correction for humidity applied in this second case to the mass of the unexposed filter and the overall procedure of aerosol mass calculation, the result of which (5.37 ± 0.41 mg) is consistent with the previous one (5.75 ± 0.44 mg, see Figure 2) within the data variability. Blank and Accuracy. As regards accuracy, apart from the check on the balance readings (see above), tests were carried out on the three field-blank filters measuring the mass before and after their simple installation/disinstallation on the impactors (left turned off). The results, reported in Table S5, Supporting Information, show that if one operates within ±0.2% RH, the filter masses measured before and after installation on the sampler agree to within ±0.4 mg, i.e., they are practically coincident within the limits of the experimental error calculated for the subtracting procedure (as the square root of the sum of variances).

Figure 5. Temporal evolution of aerosol mass concentration at Dome C during the 2005-2006 austral summer. Error bars ( SD.

RESULTS AND DISCUSSION Aerosol Atmospheric Concentration at Dome C. In Table 1 we report a summary of the analytical results of the measured aerosol masses and the corresponding data of the aerosol atmospheric concentrations referred to both actual and standard air (298 K, 760 mmHg). Figure 5 shows the data obtained for PM10 in µg m-3 actual air during the austral summer 2005-2006 at Dome C. First of all we note that due to the combined effects of low temperature and atmospheric pressure, results in terms of standard air are generally significantly different, about 25% higher, than those referred to actual air. This fact is to be taken into account when comparisons are attempted, especially with coastal data, where the difference is much smaller. As an example, the correction computed for standard air in our previous measurements carried out at Mario Zucchelli Station (formerly Terra Nova Bay) in summer 2000-200117,18 (average conditions in mid-January, temperature 271 K and pressure 745 mmHg) was negligible (8%) compared to precision in the volume sampled (±10%). As expected, the minimum values are observed for the “clean” site of Astrophysic Tent in the periods when the wind was blowing from the south (the prevailing direction at Dome C), i.e., for samples #1 and #3 from the Astrophysic Tent 1 sampler and sample #1 from the Astrophysic Tent 2 sampler. From these samples, the volume-weighted average (±weighted SD) is 0.134 (±0.012) µg m-3 (with reference to standard air) and 0.116 (±0.008) µg m-3 (with reference to actual air). Although only Analytical Chemistry, Vol. 83, No. 1, January 1, 2011

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Table 2. Comparison with Literature Data: Summer Data and Actual Air unless Otherwise Stateda

a Values as average ± SD or ranges. Results in italics were computed by us from authors’ data. b TSP ) total suspended particulate. Reported by authors after summing of major aerosol components. d Obtained by summing authors’ data of aerosol elemental concentrations after any transformation of S to SO4. e Obtained by summing authors’ data of the major aerosol components. f Obtained by summing authors’ data of aerosol ionic composition. g STP ) reference to standard temperature and pressure (273 K, 1 atm). h Baseline calculated by excluding major episodes from the average. i SCM ) reference to standard cubic meters (298 K, 1 atm). j Al is not included. k Obtained by applying the fraction of nonidentified particulate matter in measured ionic mass from weighed particulate mass (67% for fractions 6 µm). l Median. m Computed from size distribution assuming spherical particles of uniform density of 2 g cm-3. n w ) Winter. c

three useful measurements are presented, these refer to and cover a total period of about one month of the maximum two months available for summer activity at Dome C. These data suggest a lower limit for the summer aerosol concentration at Dome C of the order of 0.1 µg m-3. Such results are useful as references for subsequent measurements to validate the 150

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reconstruction of atmospheric load obtained by indirect methods, such as optical particle counter (OPC), aerosol optical depth (AOD), and sum of ionic content. Concentrations increased about 4-5 times at the site very close to and downwind of the Concordia Station (wind direction from the south; see samples #1 and #3).

When, in the intermediate period of the campaign, the wind direction was reversed for several days with respect to the prevailing direction, i.e., from the north instead of from the south, and thus blew from the station toward the “clean” site of the Astrophysic Tent, the PM10 was approximately doubled at the “clean” site (and this fact was also observed visually in the filter itself), while the concentration recorded at the Concordia site exposed to the emissions from the Station diminished by about one third, the change in wind direction being responsible in both cases. The data we report are the first available for aerosol atmospheric concentration measured by direct gravimetry at Dome C and, as far as we know, for any other site on the Antarctic plateau, and the values obtained at the “clean” site are the lowest ever observed in Antarctica (and in the world). They reveal that the aerosol concentration in central Antarctica is about 1/10 of the measured values of coastal areas (1-7 µg m-3)10,13-18 and ∼500 times lower than the European legal limit of 50 µg m-3. Comparison with Literature Data. Comparisons of the present results with other Antarctic data obtained both by direct and indirect methods (indirect methods from major aerosol components, from elemental or ionic composition, and from size distribution and particle density) are reported in Table 2. For the Antarctic plateau, our measured data can be compared only with indirect results (see Table 2, almost all data, except one, computed by us from authors’ measurements), which give values that are two to three times higher for the South Pole (up to 0.35 µg m-3)19-22,26,31 and Dronning Maud Land (0.20 µg m-3),33 and similar to ours for an inner site of the East Antarctic Plateau (0.118 µg m-3)24 and Dome C (0.098-0.15 µg m-3 and 0.074 ± 0.037 µg m-3 obtained in different years).28,29,32,34 From this comparison, the two procedures of direct and indirect evaluation of aerosol mass appear, in general, to validate each other for data reliability. CONCLUSIONS Accurate and precise aerosol mass concentrations have been obtained by direct gravimetry in central Antarctica using a carefully modified differential weighing procedure for use in extreme conditions. The PM10 measurements carried out at Dome C, in the “clean” site and in upwind conditions with respect to the station, show for the first time that the background aerosol concentration in the Antarctic plateau is of the order of 0.1 µg m-3. The contribution of local activity was evident for the site close to the station (values increased by a factor of 4-5). The present data can be compared with literature data obtained using indirect methods which show the same order of magnitude but very high variability and values up to 2-3 times higher.

We anticipate that our aerosol mass measurements will be useful as references for subsequent measurements to validate the reconstruction of atmospheric load by indirect methods (OPC, AOD, sum of ionic content, etc.). They will also become a starting point for measuring major components and impurity concentrations in the aerosol with reference to the mass of the particulate matter instead of to the usual air volume, even in central Antarctica. Aerosol content and substance concentrations are both important per se and of crucial importance in calculations of the radiative forcing of aerosols present on the vast Antarctic plateau. As regards possible future developments, the next challenge is to measure the aerosol mass in size-fractionated samples, which will require still more stable environmental weighing conditions. Reduction of the exposure time will be considered for study of the seasonal evolution of the aerosol content. Other improvements should involve increasing the distance from the Station, using solar panel energy, and installing a remote system, triggered by wind direction, which turns the sampler on or off. Finally it remains to consider the possibility of collecting aerosol samples during the winter, which will require prior solution of problems affecting impactors’ functionality and calibration down to a temperature of about -80 °C. ACKNOWLEDGMENT We thank all the logistic personnel of the Concordia Station of the summer 2005-2006 (led by C. Malagoli and G. Venturi) for their cooperation in all phases of the field scientific activity, in particular to G. Bonanno for the repair on site of the microbalance, and to L. Colturi and L. Bonetti for setting up the climatic chamber, the installation of aerosol samplers, and the construction on-site of the heating system for the manometer of the calibration kit. Thanks are also due to M. Scaleggi (HSP srl, Porto Recanati, Italy) for the hardware/software, set up in Italy, of the procedure of data transmission from the microbalance to the PC. This work was supported by financial support from the Italian Programma Nazionale di Ricerche in Antartide under the projects on “Physics and Chemistry of the Atmosphere” (line 6.4/ 2004, Climatic Effects of Aerosol Particles and Thin Clouds over the East Antarctic Plateau), “Chemical Contamination”, and “Study of Sources and Transfer Processes of the Antarctic Aerosol”. SUPPORTING INFORMATION AVAILABLE Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review August 3, 2010. Accepted October 25, 2010. AC102026W

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