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The atmospheric humidity in the Polar Regions is an important factor for the global budget of water vapour, which is a significant indicator of Earth’s climate state and evolution. The Global Navigation Satellite System (GNSS) can make a valuable contribution in the calculation of the amount of Precipitable Water Vapour (PW). We focus on Polar Regions, especially Antarctica. 20-year GPS observations, acquired by more than 40 GNSS geodetic stations, were processed with the purpose of ensuring the utmost accuracy of the PW retrieval, adopting homogeneous, consistent, and up-to-date processing strategies. We also estimated PW from radio-sounding stations (RS), which operate Vaisala radiosondes, co-located with GNSS stations. The PW values from global atmospheric reanalysis model were used for validation and comparison, very high correlation coefficients between times series, have been highlighted both in the Arctic and Antarctica. A small dry bias of RS vs. GPS values was found in the Arctic, while no clear behaviour is present in Antarctica. The PWGPS and PWRS seasonal variations are consistent, as also confirmed by scatter plots. After validation, long-term trends, both for Arctic and Antarctic regions, were estimated using Hector scientific software, which allows the estimation of trends from time series with temporal correlated noise. We applied a function to estimate the linear trend plus the annual/semiannual signals, and autoregressive noise model AR(1) which best fits the residuals of all investigated PW time series. We investigated also on the choice of the most suitable noise model, this study was useful in determining the residuals of the time series, once the trend and seasonal signals were subtracted. Positive PWGPS trends dominate at Arctic sites near the borders of the Atlantic Ocean. Sites located at higher latitudes show no significant values. Negative PWGPS trends were observed in the Arctic region of Greenland and North America. A similar behaviour was found in the Arctic for PWRS trends. The stations in the West Antarctic sector show a general positive PWGPS trend, while the sites on the coastal area of East Antarctica exhibit some significant negative PWGPS trends, while in most cases, no significant PWRS trends were found. The present work confirms also that GPS is also able to provide reliable estimates of water vapour content in regions where data are sparse and not easy to collect as the Arctic and Antarctic regions are.

The atmospheric humidity in the Polar Regions is an important factor for the global budget of water vapour, which is a significant indicator of Earth’s climate state and evolution. The Global Navigation Satellite System (GNSS) can make a valuable contribution in the calculation of the amount of Precipitable Water Vapour (PW). The PW values retrieved from Global Positioning System (GPS), hereafter PWGPS, refer to 20-year observations acquired by more than 40 GNSS geodetic stations located in the polar regions. For GNSS stations co-located with radio-sounding stations (RS), which operate Vaisala radiosondes, we estimated the PW from RS observations (PWRS). The PW values from the ERA-Interim global atmospheric reanalysis were used for validation and comparison of the results for all the selected GPS and RS stations. The correlation coefficients between times series are very high: 0.96 for RS and GPS, 0.98 for RS and ERA in the Arctic; 0.89 for RS and GPS, 0.97 for RS and ERA in Antarctica. The Root-Mean-Square of the Error (RMSE) is 0.9 mm on average for both RS vs. GPS and RS vs. ERA in the Arctic, and 0.6 mm for RS vs. GPS and 0.4 mm for RS vs. ERA in Antarctica. After validation, long-term trends, both for Arctic and Antarctic regions, were estimated using Hector scientific software. Positive PWGPS trends dominate at Arctic sites near the borders of the Atlantic Ocean. Sites located at higher latitudes show no significant values (at 1σ level). Negative PWGPS trends were observed in the Arctic region of Greenland and North America. A similar behaviour was found in the Arctic for PWRS trends. The stations in the West Antarctic sector show a general positive PWGPS trend, while the sites on the coastal area of East Antarctica exhibit some significant negative PWGPS trends, but in most cases, no significant PWRS trends were found. The present work confirms that GPS is able to provide reliable estimates of water vapour content in Arctic and Antarctic regions too, where data are sparse and not easy to collect. These preliminary results can give a valid contribution to climate change studies.

In a such changing environment like the Arctic, improving the understanding of the thermodynamic state and processes of the atmosphere is critical for the development of accurate prediction and climatic models. This is fundamental for example for studies on sea-ice development as well as on cloud formation. Taking into account the above remarks, it is very important to know the pressure, temperature and moisture conditions of the Arctic atmosphere throughout the year and over the whole tropospheric and stratospheric altitude range. Furthermore, knowing these data is necessary to realistically evaluate the radiative effects involving both the short-wave and long-wave radiation fluxes, which regulate the energy balance of the Arctic surface-atmosphere system.

In this chapter remote sensing techniques as applied to studies of Arctic aerosol are surveyed. They include the analysis of ground and shipborne observations of atmospheric aerosol using sunphotometers and also airborne/satellite observations using optical instrumentation (lidars, imagers, radiometers).

Arctic surface temperature has been increasing at a rate 2-3 times that of the global average in the last half century. Enhanced warming of the Arctic, or Arctic Amplification, is a climatic response to external forcing. Despite good results obtained by climatic models for the globe, the largest intermodel differences in surface temperature warming are found in the Arctic. The magnitude of this warming drives many different processes and determines the evolution of many climatic parameters such as clouds, sea ice extent, and land ice sheet mass. The Arctic Amplification can be attributed to the peculiar feedback processes that are triggered in the Arctic. Most of these processes include radiation interaction with the atmosphere and with the surface, all of them contributing to the radiation budget. It is then mandatory to correctly evaluate this budget both at the surface and at the top of the atmosphere and in the solar and thermal spectra. This can be done using both direct observations, from ground and from space, and model simulation via radiation transfer codes. This last approach need many observed input parameters anyhow. In this contribution results on the evaluation of the radiation budget in the Arctic are first reviewed. Follows a detailed description of the effects of the most important atmospheric gases (carbon dioxide, methane, ozone etc.) on both shortwave and longwave radiation ranges. The same is illustrated for aerosol loading in the Arctic, based on a large dataset of aerosol radiative properties measured by means of sun-photometers in numerous Arctic stations. Finally, the effect of the surface reflectivity characteristics on the radiation budget is illustrated by means of albedo models specific for the Arctic.

Aerosols play an important role in the climatic system through their direct and indirect effects on radiation. Beside this, they are also part of the complex chain of chemical reactions taking place in the atmosphere. Indirect effects involve aerosols acting as cloud and ice condensation nuclei, brightening of clouds, modification of precipitation capabilities etc. After deposition, they also change reflectivity properties of bright surfaces, particularly important in polar regions. In the Artic few natural aerosol sources exist, except oceans for sea-salt and soil for dust, both of them increasing in magnitude due to global warming. Beside this, anthropogenic aerosols are easily transported to the Arctic by atmospheric transport from middle latitudes, in particular during winter and early spring, forming the so-called Artic Haze. In this contribution the processes causing the development of Arctic Haze and its characteristics are introduced. Following, a review of the physical and optical properties as well as chemical composition of Arctic aerosols are reviewed using data obtained from numerous monitoring stations in the Arctic.

Arctic surface temperature has been increasing at a rate 2–3 times that of the global average in the last half century. Enhanced warming of the Arctic, or Arctic Amplification, is a climatic response to external forcing. Despite good results obtained by climatic models for the globe, the largest intermodel differences in surface temperature warming are found in the Arctic. The magnitude of this warming drives many different processes and determines the evolution of many climatic parameters such as clouds, sea ice extent, and land ice sheet mass. The Arctic Amplification can be attributed to the peculiar feedback processes that are triggered in the Arctic. Most of these processes include radiation interaction with the atmosphere and with the surface, all of them contributing to the radiation budget. It is then mandatory to correctly evaluate this budget both at the surface and at the top of the atmosphere and in the solar and thermal spectra. This can be done using both direct observations, from ground and from space, and model simulation via radiation transfer codes. This last approach need many observed input parameters anyhow.

The atmospheric water vapor is an important indicator of the Earth's climate state and evolution. We therefore aimed at calculating the content and long-term variation of the precipitable water vapor at five coastal Antarctic stations, i.e., Casey, Davis, Mawson, McMurdo, and Mario Zucchelli. To do that, we processed the 12-year time series of GPS and radiosounding (RS) observations acquired at those stations, with the purpose of ensuring the utmost accuracy of the results adopting homogeneous, consistent, and up-to-date processing strategies for both data sets. Using the two fully independent techniques, rather consistent contents and seasonal variations of precipitable water were detected, mainly ranging from 1 (Austral winter) to 10 mm (Austral summer). At each site, correlation coefficients varying from 0.86 to 0.91 were found between the GPS and RS time series, with mean discrepancies $\leq$0.75 mm. There is no clear indication regarding the possible dry or wet biases of one technique with respect to the other, with only a notable GPS wet bias identified at Mawson and a dry bias at Casey that, nevertheless, correspond to an average difference of $

A two-term algorithm is defined to evaluate the Rayleigh-scattering optical depth (ROD) in the Arctic and Antarctic atmospheres at ultraviolet, visible, and near-infrared wavelengths. The first term accounts for the tropospheric contribution up to 8 km altitude and the second for the upper troposphere and low stratosphere (UTLS) region and the upper atmosphere up to 120 km altitude. In these calculations, the vertical profiles of pressure p(z) and temperature T(z) are used, as derived from the multiyear sets of radiosounding measurements performed from 2000 to 2006 at four Arctic stations (Cambridge Bay, Resolute Bay, Danmarkshavn, and Alert) and four Antarctic stations (Neumayer, McMurdo, Dome C, and South Pole). By analyzing the radiosonde data sets collected at the eight sites, the daily average values of tropospheric ROD are calculated and their seasonal variations versus the mean tropospheric temperature and surface level temperature are determined in order to evaluate the temperature correction functions for tropospheric ROD. Similarly, the vertical profiles of p(z) and T(z) defined in the stratosphere and upper atmosphere are examined in order to (i) calculate the “stratospheric” ROD contribution, (ii) analyze its annual temporal variations, (iii) calculate the average temperature in the UTLS region, and (iv) determine the most suitable temperature anomaly corrections at the eight chosen polar sites. The spectral evaluations of the tropospheric and “stratospheric” ROD terms are given at 21 selected wavelengths from 0.30 to 4.00 µm for each polar site. The present two-term algorithm incorporates the tropospheric and “stratospheric” ROD terms, along with the correction factors derived for the actual pressure and temperature conditions, and can be used to calculate accurate overall ROD values at the given Arctic and Antarctic sites for the average annual thermal conditions of the atmosphere and the average seasonal variations characterizing the temperature conditions of the UTLS region. The ROD estimates obtained using the present method constitutes a nontrivial improvement over estimates made with the traditional method.

New calculations of the relative optical air mass function are made over the 0°–87° range of apparent solar zenith angle θ, for various vertical profiles of background aerosol, diamond dust and thin cirrus cloud particle extinction coefficient in the Arctic and Antarctic atmospheres. The calculations were carried out by following the Tomasi and Petkov (2014) procedure, in which the above-mentioned vertical profiles derived from lidar observations were used as weighting functions. Different sets of lidar measurements were examined, recorded using: (i) the Koldewey-Aerosol-Raman Lidar (KARL) system (AWI, Germany) at Ny-Ålesund (Spitsbergen, Svalbard) in January, April, July and October 2013, (ii) the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite-based sensor over Barrow (Alaska), Eureka (Nunavut, Canada) and Sodankylä (northern Finland), and Neumayer III, Mario Zucchelli and Mirny coastal stations in Antarctica in the local summer months of the last two years, (iii) the National Institute of Optics (INO), National Council of Research (CNR) Antarctic lidar at Dome C on the Antarctic Plateau for a typical “diamond dust” case, and (iv) the KARL lidar at Ny-Ålesund and the University of Rome/National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) lidar at Thule (northwestern Greenland) for some cirrus cloud layers in the middle and upper troposphere. The relative optical air mass calculations are compared with those obtained by Tomasi and Petkov (2014) to define the seasonal changes produced by aerosol particles, diamond dust and cirrus clouds. The results indicate that the corresponding air mass functions generally decrease as angle θ increases with rates that are proportional to the increase in the pure aerosol, diamond dust and cirrus cloud particle optical thickness.

The book describes the morphological, physical and chemical properties of aerosols from various natural and anthropogenic sources to help the reader better understand the direct role of aerosol particles in scattering and absorbing short- and long-wave radiation.

The dependence functions of relative optical air mass on apparent solar zenith angle θ have been calculated over the θ

For most parts of the year the Antarctic Plateau has a surface temperature inversion with strength c. 20 K. Under such conditions the warmer air at the top of the inversion layer contributes more to the clear sky atmospheric longwave radiation at surface level than does the colder air near the ground. Hence, it is more appropriate to relate longwave irradiance (LWI) to the top of the inversion layer temperature (Tm) than to the ground level temperature (Tg). Analysis of radio soundings carried out at Dome C and South Pole during 2006–08 shows that the temperature at 400 m above the surface (T400) is a good proxy for Tm and is linearly related to Tg with correlation coefficients greater than 0.8. During summer, radiosonde measurements show almost isothermal conditions, hence T400 still remains a good proxy for the lower troposphere maximum temperature. A methodology is presented to parameterize the clear sky effective emissivity in terms of the troposphere maximum temperature, using ground temperature measurements. The predicted LWI values for both sites are comparable with those obtained using radiative transfer models, while for Dome C the bias of 0.8 W m-2 and the root mean square (RMS) of 6.2 W m-2 are lower than those calculated with previously published parametric equations.

Variations in total ozone column and sun exposures able to cause erythema and damage the DNA molecules were observed by the narrow-band filter radiometer UV-RAD in Bologna, Italy from 2005 to 2010. The ozone columns determined from the UV-RAD measurements were found to be close to those provided by the satellite Ozone Monitoring Instrument (OMI) showing an average discrepancy of 1% with standard deviation of ± 6%. Analysis of the data highlights a well-marked annual cycle of the ozone column variations while the oscillations with periods of 8, 18 and 34 months present much smaller amplitudes. The influence of the frequency of solar irradiance measurements on the accuracy of the evaluated daily exposure dose has been studied and it was found that time intervals no longer than 5-10 min between the measurements of erythema and DNA damage effective UV irradiances provide a satisfactory assessment of the corresponding daily exposures. The latter do not present significant year-to-year variations for the period under study, while their annual distributions show slight changes likely due to the specific cloud cover and ozone column variability for different years. The annual erythemal exposure dose for 2007-2010 varied between 603.7 and 638.1 kJ m(-2), while the corresponding sun exposure affecting DNA changed from 6.38 to 7.91 kJ m(-2).

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The reliability of astronomical observations at millimeter and sub-millimeter wavelengths closely depends on a low vertical content of water vapor as well as on high atmospheric emission stability. Although Concordia station at Dome C (Antarctica) enjoys good observing conditions in this atmospheric spectral windows, as shown by preliminary site-testing campaigns at different bands and in, not always, time overlapped periods, a dedicated instrument able to continuously determine atmospheric performance for a wide spectral range is not yet planned. In the absence of such measurements, in this paper we suggest a semi-empirical approach to perform an analysis of atmospheric transmission and emission at Dome C to compare the performance for 7 photometric bands ranging from 100 GHz to 2 THz. Radiosoundings data provided by the Routine Meteorological Observations (RMO) Research Project at Concordia station are corrected by temperature and humidity errors and dry biases and then employed to feed ATM (Atmospheric Transmission at Microwaves) code to generate synthetic spectra in the wide spectral range from 100 GHz to 2 THz. To quantify the atmospheric contribution in millimeter and sub-millimeter observations we are considering several photometric bands in which atmospheric quantities are integrated. The observational capabilities of this site at all the selected spectral bands are analyzed considering monthly averaged transmissions joined to the corresponding fluctuations. Transmission and pwv statistics at Dome C derived by our semi-empirical approach are consistent with previous works. It is evident the decreasing of the performance at high frequencies. We propose to introduce a new parameter to compare the quality of a site at different spectral bands, in terms of high transmission and emission stability, the Site Photometric Quality Factor.

A four-year set of vertical profiles of pressure, temperature and relative humidity derived from 1113 radiosoundings performed at Dome C (Antarctica) at 12h00 UT of each day, from late March 2005 to the end of March 2009, was examined by following a complex procedure for removing the most important lag errors and dry biases from the temperature and moisture data. The analysis provides evidence of annual cycles over the four years, characterizing the pressure and temperature conditions at the surface and at the various troposphere and low stratosphere levels, with maxima in summer and wide minima in winter for both parameters. Specific studies of the thermal parameters characterizing the ground layer and the tropopause region are also presented to describe their annual average variations. The analysis of moisture parameters indicates that absolute humidity varies regularly with season within the low troposphere, presenting well marked peaks in the summer months. Consequently, precipitable water was found to vary regularly during the year, from values of 0.2–0.4 mm in the winter to more than 0.6 mm in summer. The main year-to-year variations characterizing the monthly mean vertical profiles of pressure, temperature and moisture parameters are also described.

An updated set of time series of derived aerosol optical depth (AOD) and Angstrom’s exponent a from a number of Arctic and Antarctic stations was analyzed to determine the long-termvariations of these two parameters. The Arctic measurements were performed at Ny-Alesund (1991e2010), Barrow (1977e2010) and some Siberian sites (1981e1991). The data were integrated with Level 2.0 AERONET sun-photometer measurements recorded at Hornsund, Svalbard, and Barrow for recent years, and at Tiksi for the summer 2010. The Antarctic data-set comprises sun-photometer measurements performed at Mirny (1982e2009), Neumayer (1991e2004), and Terra Nova Bay (1987e2005), and at South Pole (1977e2010). Analyses of

Measurements performed at polar latitudes by the MIPAS limb-scanning Fourier Transform spectrometer aboard ENVISAT, from July 2002 to March 2004 (FR mission) and January 2005 to April 2010 (OR mission) were analysed with the 2-D tomographic Geofit Multi-Target Retrieval (GMTR) procedure (Dinelli et al., 2010. The MIPAS2D database of MIPAS/ENVISAT measurements retrieved with a multitarget 2-dimensional tomographic approachAtmos. Meas. Techn. Discuss. (AMTD) 2, 2639–2688.) to obtain the MIPAS2D database, and extract more than 386,000 vertical profiles of pressure p, temperature T and water vapour volume mixing ratio Q. They were subdivided into 12 latitudinal classes selected in steps of 51 from 651 Nt o 901N and 651 St o 901S, each vertical profile consisting of values measured at 14 altitude levels from 12 to 60 km. Each latitudinal set was subdivided into 12 monthly sets to determine the multi-year monthly mean vertical profiles of the three parameters: those of p (monthly average standard deviations (SD) 5–15%) provide evidence of marked seasonal variations above 30 km; those of T (SD values of a few percent) show large seasonal variations, with summer maxima at all stratospheric levels; and those of Q (SD lower than 20% from 20 to 50 km) present values ranging in general between 2 and 6 ppmv at the 12–25 km levels and 4 and 7 ppmv at higher altitudes. To verify the reliability of the MIPAS results, the pressure profiles are compared with those obtained from radiosounding data-sets taken at Arctic and Antarctic sites from 2000 to 2003; those of T with both radiosounding measurements and MLS/Aura satellite data from 2005 to 2010; and those of Q with the MLS/Aura satellite data, finding a substantial agreement in all cases. Comparison of MIPAS pressure and temperature profiles with pre ozone-hole CIRA models at 701 and 801 latitudes highlights the variations occurring in the polar atmosphere over the last 3 decades, with relative pressure decreases of 5–10% on average, and overall average decreases in temperature of 0.4 and 2.0 K in the Arctic and Antarctica, respectively. Using the MIPAS profiles of p, T and Q, the monthly mean vertical profiles of absolute humidity were also calculated, from which the monthly values of stratospheric water vapour content from 12 to 50 km were determined, varying between 0.0047 and 0.0070 mm at Arctic latitudes and between 0.0026 and 0.0055 mm at Antarctic latitudes.

Growing attention has been paid over recent years to the possibility of monitoring surface particulate matter (PM) concentrations through the use of satellite observations. Satellite remote sensing of both aerosol and trace gas constituents can be usefully employed in air quality monitoring (AQ). The MODIS (Terra-Aqua/NASA platforms) aerosol optical properties were used here in a semi-empirical approach to estimate PM2.5 content at ground level. PM2.5 samplings were employed to convert aerosol optical depth AOD into PM estimates, considering meteorological fields simulated by MM5. Thus, daily maps of satellite-based PM2.5 concentrations over Northern Italy were derived. Comparison with daily PM2.5, sampled on the ground during the QUITSAT project over six validation sites of the Po valley, showed good agreement (R2 ? 0.68 and R2 ? 0.59 for MODIS/Terra and MODIS/Aqua, respectively), with the satellite-based concentrations tending to underestimate the values by at most ~20%. Monthly averaged values were also compared providing good agreement, with R2 ? 0.7 for each platform.

Within the framework of air quality monitoring, measurements by Earth-observing satellite sensors are combined here with regional meteorological and chemical transport models. Two satellite-derived products developed within the QUITSAT project, regarding significant pollutants including PM2.5 and NO2, are presented. Estimates of PM2.5 concentrations at ground level were obtained using moderate resolution imaging spectroradiometer (Terra-Aqua/NASA) aerosol optical properties. The semi-empirical approach adopted takes into account PM2.5 sampling and meteorological descriptions of the area studied, as simulated by MM5, to infer aerosol optical properties to PM projection coefficients. Daily maps of satellite-based PM2.5 concentrations over northern Italy are derived. Monthly average values were compared with in situ PM2.5 samplings showing good agreement. Ozone monitoring instrument (OMI) (Aura/NASA) NO2 tropospheric contents are merged using the GAMES chemical model simulations. The method employs a weighted rescaling of the model column in the troposphere according to the OMI observations. The weightings take into account measurement errors and model column variances within the satellite ground pixel. The obtained ground-level concentrations of NO2 show good agreement with the environmental agencies’ in situ. The capabilities of Earth observation satellites have greatly improved over the last few years, with gradual improvements in temporal and spatial resolutions and enhancements in radiometric accuracy, thus encouraging further studies on the use of satellite data to assess air quality (AQ). Within this context, the synoptic view and the daily repetition cycle of satellite-based measurements strengthen the potential for monitoring air pollution transport and directly evaluating the spatial distribution of various air pollutant concentrations. These evaluations are in compliance with the regulations of

Two-channel sun photometers can be easily employed at Antarctic sites, where harsh environmental conditions prevail, to carry out measurements of precipitable water W. In the very dry air conditions observed in the Antarctic atmosphere, water vapor does not produce strong absorption features along the sun path. Therefore, these instruments need to be calibrated using analytical forms different from the square root regime, which can be determined by simulating the output voltages measured at Antarctic sites, for the spectral near-IR curves of extraterrestrial solar irradiance, instrumental responsivity parameters, and atmospheric transmittance, relative to various measurement periods. For this purpose, average models of the Antarctic atmosphere from the ground level up to the 30-km altitude were considered for different solar zenith angles and relative humidity conditions. The ratios between the output voltages simulated in the band and window channels were plotted as a function of total water vapor content Cw, for each site and each period, to define the best-fit calibration curves, which were subsequently normalized to the field measurements to take into account the aging effects on the filter transmission characteristics. Each of the five calibration curves was found to present a slope coefficient decreasing gradually with Cw from values higher than 0.8 to about 0.6. Using these curves, measurements of W were obtained, which differ appreciably at both sea level and high-altitude sites from those given by the square root calibration curves, avoiding large overestimation errors of 10%–40% at the high-altitude sites and underestimation errors of 5%–15% at the sea level site.

Carbon dioxide (CO 2 ) emissions from industrial sources is one of the main anthropogenic contributors to the greenhouse effect. We investigated the potentiality of satellite hyperspectral sensors, as the Hyperspectral Precursor of the Application Mission (PRISMA) — an optical Earth Observation satellite mission funded by the Italian Space Agency (ASI) — to evaluate significant amounts of CO 2 concentrations in the lower troposphere. Using synthetic hyperspectra at top of atmosphere, sensitivity analysis to the retrieval of CO 2 concentrations was performed in the weak (1610 and 1570 nm) and strong (2010 and 2050 nm) absorption bands. Considering the designed spectral and radiometric features of PRISMA, results of the analysis show a relative uncertainty between 3 and 15 % on the retrieval of CO 2 concentrations between 390 to 5100 ppmv.

Multi-year sets of ground-based sun-photometer measurements conducted at 12 Arctic sites and 9 Antarctic sites were examined to determine daily mean values of aerosol optical thickness tau(lambda) at visible and near-infrared wavelengths, from which best-fit values of Angstrom's exponent alpha were calculated. Analyzing these data, the monthly mean values of tau(0.50 micrometers) and alpha and the relative frequency histograms of the daily mean values of both parameters were determined for winter-spring and summer-autumn in the Arctic and for austral summer in Antarctica. The Arctic and Antarctic covariance plots of the seasonal median values of alpha versus tau(0.50 micrometers) showed: (i) a considerable increase in tau(0.50 micrometers) for the Arctic aerosol from summer to winter-spring, without marked changes in alpha; and (ii) a marked increase in tau(0.50 micrometer) passing from the Antarctic Plateau to coastal sites, whereas alpha decreased considerably due to the larger fraction of sea-salt aerosol. Good agreement was found when comparing ground-based sun-photometer measurements of tau(lambda) and alpha at Arctic and Antarctic coastal sites with Microtops measurements conducted during numerous AERONET/MAN cruises from 2006 to 2013 in three Arctic Ocean sectors and in coastal and off-shore regions of the Southern Atlantic, Pacific, and Indian Oceans, and the Antarctic Peninsula. Lidar measurements were also examined to characterize vertical profiles of the aerosol backscattering coefficient measured throughout the year at Ny-Alesund. Satellite-based MODIS, MISR, and AATSR retrievals of tau(lambda) over large parts of the oceanic polar regions during spring and summer were in close agreement with ship-borne and coastal ground-based sun-photometer measurements. An overview of the chemical composition of mode particles is also presented, based on in-situ measurements at Arctic and Antarctic sites. Fourteen log-normal aerosol number size-distributions were defined to represent the average features of nuclei, accumulation and coarse mode particles for Arctic haze, summer background aerosol, Asian dust and boreal forest fire smoke, and for various background austral summer aerosol types at coastal and high-altitude Antarctic sites. The main columnar aerosol optical characteristics were determined for all 14 particle modes, based on in-situ measurements of the scattering and absorption coefficients. Diurnally averaged direct aerosol-induced radiative forcing and efficiency were calculated for a set of multimodal aerosol extinction models, using various Bidirectional Reflectance Distribution Function models over vegetation-covered, oceanic and snow-covered surfaces. These gave a reliable measure of the pronounced effects of aerosols on the radiation balance of the surface-atmosphere system over polar regions.

A set of 1330 radiosoundings was performed at the Italian station of Terra Nova Bay in Antarctica during 12 measurement campaigns from 1987 to 1998. These measurements were separately analyzed for the various 10-day periods from mid-October to mid-February, to determine the mean vertical profiles of air pressure and temperature in the troposphere and lower stratosphere, and those of moisture parameters in the troposphere. The temperature data were corrected for the errors due to radiation and heat exchange processes and for the lag errors of the sensor. Due to temperature dependence and other dry bias effects, the humidity errors were also taken into account. The tropospheric temperature was found to present average values of its vertical gradient varying between � 5.4 and � 6.3 K/km, while its minimum height associated with the tropopause gradually lowered from 13.7 to 7.9 km. Total water vapor content increased correspondingly from 0.12 to 0.36 gc m � 2 during the first two months and decreased to 0.26 g cm � 2 in the following period. Stratospheric temperature was observed to increase during the first 2 months by about 15 K in the region below the 24-km height and to appreciably decrease at upper levels, maintaining almost stable features during the subsequent months. D 2004 Elsevier B.V. All rights reserved.

Multispectral sun-photometric measurements of aerosol optical depth were performed at five stations located at different altitudes from about 500 to 2165 m amsl in the Leo valley in the Apennines (Italy), on some clear-sky autumn days of 1981 and 1982. These measurements showed that the aerosol optical depth greatly increased during the morning at all stations due to the convective transport of particulate matter induced by the local upslope breezes. To evaluate the corresponding variations of the vertical mass loading of particulate matter, two continental and rural aerosol extinction models were determined: the first provides the spectral series of both volume and mass aerosol extinction coefficients at nine visible and near-infrared wavelengths for 27 aerosol particle size-distribution curves of Junge-type with values of the Angstrom exponent α varying between 0.15 and 1.99; the second is based on linear combinations of three monomodal particle size-distribution curves determined to give form to a nuclei mode, an accumulation mode and a coarse particle mode, respectively, whose particle number concentrations were defined to fit the values of α characterising the spectral series of aerosol optical depth. These aerosol extinction models were used to obtain estimates of the columnar aerosol mass content at the five stations, finding values mostly ranging between 0.03 and 0.12 g m −2 at the two lower stations, 0.02 and 0.08 g m −2 at the two intermediate stations and between 0.01 and 0.03 g m −2 at the high-altitude Mt. Cimone station. The corresponding hourly mean percentage variations of the vertical aerosol mass loading were found to vary between a few percentages and nearly 100%, with median values ranging from 14% to 29% at the various stations. From these estimates, evaluations of the aerosol mass concentration were obtained within the four layers, varying from 15 to 160 μg m −3 in the first layer, from 15 to 95 μg m −3 in the second, from 10 to 60 μg m −3 in the third and from 5 to 20 μg m −3 in the fourth. Correspondingly, the hourly mean percentage variations were found to range between −26% and +124% within the four layers, with median values gradually increasing from +4% to +20%, passing from the first to the fourth layer.

Simultaneous measurements of downwelling short-wave solar irradiance and incoming total radiation flux were performed at the Reeves Neve glacier station (1200 m MSL) in Antarctica on 41 days from late November 1994 to early January 1995, employing the upward sensors of an albedometer and a pyrradiometer. The downwelling short-wave radiation measurements were analysed following the Duchon and O'Malley [J. Appl. Meteorol. 38 (1999) 132] procedure for classifying clouds, using the 50-min running mean values of standard deviation and the ratio of scaled observed to scaled clear-sky irradiance. Comparing these measurements with the Duchon and O'Malley rectangular boundaries and the local human observations of clouds collected on 17 days of the campaign, we found that the Duchon and O'Malley classification method obtained a success rate of 93% for cirrus and only 25% for cumulus. New decision criteria were established for some polar cloud classes providing success rates of 94% for cirrus, 67% for cirrostratus and altostratus, and 33% for cumulus and altocumulus. The ratios of the downwelling short-wave irradiance measured for cloudy-sky conditions to that calculated for clear-sky conditions were analysed in terms of the Kasten and Czeplak [Sol. Energy 24 (1980) 177] formula together with simultaneous human observations of cloudiness, to determine the empirical relationship curves providing reliable estimates of cloudiness for each of the three above-mentioned cloud classes. Using these cloudiness estimates, the downwelling long-wave radiation measurements (obtained as differences between the downward fluxes of total and short-wave radiation) were examined to evaluate the downwelling long-wave radiation flux normalised to totally overcast sky conditions. Calculations of the long-wave radiation flux were performed with the MODTRAN 3.7 code [Kneizys, F.X., Abreu, L.W., Anderson, G.P., Chetwynd, J.H., Shettle, E.P., Berk, A., Bernstein, L.S., Robertson, D.C., Acharya, P., Rothman, L.S., Selby, J.E.A., Gallery, W.O., Clough, S.A., 1996. In: Abreu, L.W., Anderson, G.P. (Eds.), The MODTRAN 2/3 Report and LOWTRAN 7 MODEL. Contract F19628-91-C.0132, Phillips Laboratory, Geophysics Directorate, PL/GPOS, Hanscom AFB, MA, 261 pp.] for both clear-sky and cloudy-sky conditions, considering various cloud types characterised by different cloud base altitudes and vertical thicknesses. From these evaluations, best-fit curves of the downwelling long-wave radiation flux were defined as a function of the cloud base height for the three polar cloud classes. Using these relationship curves, average estimates of the cloud base height were obtained from the three corresponding sub-sets of long-wave radiation measurements. The relative frequency histograms of the cloud base height defined by examining these three sub-sets were found to present median values of 4.7, 1.7 and 3.6 km for cirrus, cirrostratus/altostratus and cumulus/altocumulus, respectively, while median values of 6.5, 1.8 and 2.9 km were correspondingly determined by analysing only the measurements taken together with simultaneous cloud observations.

Multispectral measurements of direct solar irradiance were simultaneously taken employing various sun-photometers at five stations located at different altitudes along the western slope of the Leo valley, in the Apennines (Italy), on a number of clear-sky autumn days in 1981 and 1982. Using precise estimates of the calibration constants of the various sun-photometers, obtained through a rigorous intercomparison procedure, the measurements were examined following the most up-to-date criteria of the sun-photometry method to determine daily homogeneous sets of aerosol optical depths at three window-wavelengths, at all five stations. The values of aerosol optical depth obtained from the 1982 measurements were corrected for the extinction effects produced by the stratospheric cloud of El Chichon volcanic aerosols, following a procedure based on the multimodal aerosol extinction model proposed by Pueschel et al. (Pueschel, R.F., Kinne, S.A., Russell, P.B., Snetsinger, K.G., 1993. Effects of the 1991 Pinatubo volcanic eruption on the physical and radiative properties of stratospheric aerosols. Keevallik, S. Karner, O. (Eds.), IRS '92: Current Problems in Atmospheric Radiation. Proceedings of the International Radiation Symposium, A. Deepak Publ., Hampton, Va., 183–186.) for 6-month-old volcanic particles (see Appendix). The results show that the tropospheric aerosol optical depth at 500-nm wavelength usually assumed low values in the early morning, most generally decreasing from 0.08 to 0.01 with the station altitude, and considerably increased during the morning, especially at the lower stations, as a result of the vertical transport of particulate matter due to the upslope breezes. The corresponding daily percentage increases varied between 60% and 370%, presenting a median value of 175% and quartiles of 140% and 230%. The time-variations of the volume aerosol extinction coefficient at the 500-nm wavelength were evaluated within the four atmospheric layers defined by the station altitudes, presenting early morning values mainly varying from 0.02 to 0.20 km−1 in the first layer, from 0.02 to 0.11 km−1 in both second and third layers and from 0.01 to 0.03 km−1 in the fourth. Therefore, greater increases were found to occur within the three lower layers situated below 1500 m height, with hourly mean percentage variations ranging between 10% and 420%, with a median value of 230% and quartiles of 70% and 270%.

Aerosol particles suspended in the atmosphere may originate from either natural or anthropic sources, or through mixed processes involving their variable combinations. Among the primary natural emissions, the most important are those leading to the formation of (i) mineral dust through wind erosion of natural soil and (ii) sea-salt particles from the ocean surface forced by winds. In addition, significant emission processes include biological particles released by plants and animals, combustion particles forming in forest fires and biomass-burning smokes, and volcanic debris ejections.

International audience; The strong ozone depletion event that occurred in Arctic during spring 2011 was found to cause appreciable reduction in the ozone column (OC) in Europe, even at lower latitudes. The features of this episode have been analysed using the data recorded at 34 ground-based stations located in the European area and compared with the similar events in 2000 and 2005. The results provided evidence that OC as far south as 40°N latitude was considerably influenced by the Arctic ozone loss in spring 2011. The reduction of OC at the northernmost sites was about 40% with respect to the mean value calculated over the previous six-year period, while a similar decrease at the southern extreme ranged between 15 and 18%, and were delayed by nearly two weeks compared to the Arctic region. The ozone distributions reconstructed for the West Europe sector show that the decline of OC lasted from late March to late April 2011. The echo of the Arctic ozone depletion on mid-latitude UV irradiance has been analysed trough model computations that show an increase of the midday erythemal dose by 3-4 SED (1 SED = 100 J m-2) that was slightly higher than at polar regions. On the other hand it was assessed that the biosystems in the northernmost regions were a subject of about 4 times higher UV stress than those at mid-latitudes. Despite indications of an OC recovery, the event examined here shows that the issue of ozone depletion episodes cannot be belittled.

During the Antarctic summer 1994/95 the values of downward and upward flux densities of both solar and terrestrial radiation were recorded at 1200 m for a period of 1 month on the Reeves Neve Glacier (lat 74°39′S, long 161°35′E), near the Italian base of Terra Nova Bay. The relations proposed by Swinbank [Swinbank, W.C., 1963. Long-wave radiation from clear skies. Q. J. R. Meteorol. Soc. 89, 339–348], Idso and Jackson [Idso, S.B., Jackson, R.D., 1969. Thermal radiation from the atmosphere. J. Geophys. Res. 74, 5397–5403] and Deacon [Deacon, E.L., 1970. The derivation of Swinbank's long-wave radiation formula. Q. J. R. Meteorol. Soc. 96, 313–319] associating the long-wave atmospheric radiation flux only to the air temperature at screen level were tested in extreme Antarctic climatological condition. A relation between the long-wave radiation flux and both screen air temperature and cloud cover fraction in accordance to the height of the cloud base was defined using the Kasten and Czeplak relationship that relates the solar radiation flux and the cloud cover index. The study of the incoming short-wave radiation flux from the clear sky and that reflected by the surrounding snow cover allowed for highlighting the role of surface geometry on the albedo measurements.

During the CLEARCOLUMN campaign which took place at Sagres (Portugal) from 16 June to 25 July 1997, more than 2000 spectral measurements of direct solar irradiance were performed at thirteen window-wavelengths in the 0.4–3.7 μm range, on 27 days. The measurements were performed using the IR-RAD sun-radiometer designed and manufactured at the Institute ISAO (FISBAT), Bologna (Italy), and carefully calibrated by applying the Langley plot method to the measurements performed on 24 October 1996, at the Schneefernerhaus Observatory on the Zugspitze (Germany). From these measurements, the values of the total atmospheric optical depth were obtained at the various wavelengths, from which the corresponding spectral values of aerosol optical depth were determined through accurate corrections for Rayleigh scattering and gaseous absorption. Such values were found to be mostly smaller than 0.1 during June and early July, presenting almost neutral spectral dependence characteristics, closely related to the prevailing oceanic origins of particulate matter. Higher values of aerosol optical thickness, mainly ranging between 0.1 and 0.5 at visible wavelengths and sharply decreasing with wavelength, were found during the rest of July in the presence of predominant contents of continental and anthropogenic aerosol particles arriving from polluted European regions. The King inversion method was applied to the spectral series of aerosol optical depth to determine the particle size-distribution curves in the 0.2–20 μm diameter range. For 2 of these cases (one chosen for a low atmospheric loading of marine aerosols and the other for a mean content of continental/anthropogenic aerosols), the changes in the outgoing solar radiation flux produced by aerosol particles were calculated, considering different surface albedo conditions. The results obtained in the second case show that changes of opposite sign can be caused by the same atmospheric aerosol loading when passing from oceanic to continental areas.

Analyses of aerosol optical depth (AOD) and columnar water vapor (CWV) measurements acquired with NASA Ames Research Center's 6-channel Airborne Tracking Sunphotometer (AATS-6) operated aboard the R/V Professor Vodyanitskiy during the 2nd Aerosol Characterization Experiment (ACE-2) are discussed. Data are compared with various in situ and remote measurements for selected cases. The focus is on 10 July, when the Pelican airplane flew within 70 km of the ship near the time of a NOAA-14/AVHRR satellite overpass and AOD measurements with the 14–channel Ames Airborne Tracking Sunphotometer (AATS-14) above the marine boundary layer (MBL) permitted calculation of AOD within the MBL from the AATS-6 measurements. A detailed column closure test is performed for MBL AOD on 10 July by comparing the AATS-6 MBL AODs with corresponding values calculated by combining shipboard particle size distribution measurements with models of hygroscopic growth and radiosonde humidity profiles (plus assumptions on the vertical profile of the dry particle size distribution and composition). Large differences (30–80% in the mid-visible) between measured and reconstructed AODs are obtained, in large part because of the high sensitivity of the closure methodology to hygroscopic growth models, which vary considerably and have not been validated over the necessary range of particle size/composition distributions. The wavelength dependence of AATS-6 AODs is compared with the corresponding dependence of aerosol extinction calculated from shipboard measurements of aerosol size distribution and of total scattering measured by a shipboard integrating nephelometer for several days. Results are highly variable, illustrating further the great difficulty of deriving column values from point measurements. AATS-6 CWV values are shown to agree well with corresponding values derived from radiosonde measurements during 8 soundings on 7 days and also with values calculated from measurements taken on 10 July with the AATS-14 and the University of Washington Passive Humidigraph aboard the Pelican. DOI: 10.1034/j.1600-0889.2000.00045.x

Since 1998, the Italian Antarctic Programme has been funding space geodetic activities based on the use of episodic and permanent global positioning system (GPS) observations. As well as their exploitation in geodynamics, these data can be used to sense the atmosphere and to retrieve and monitor its water vapor content and variations. The surface pressure p and temperature Ts at the GPS tracking sites are necessary to compute the zenith hydrostatic delay (ZHD), and consequently, the precipitable water. At sites where no surface information is recorded, the p and Ts values can be retrieved from, e.g., global numerical weather prediction models. Alternatively, the site-specific ZHD values can be computed by interpolation of the ZHD values provided in a grid model (2.5° × 2.0°). We have processed the data series of the permanent GPS site TNB1 (Mario Zucchelli Station, Antarctica) from 1998 to 2010, with the purpose of comparing the use of grid ZHD values as an alternative to the use of real surface records. With these approaches, we estimate almost 7 × 104 hourly values of precipitable water over 13 years, and we find discrepancies that vary between 1.8 (±0.2) mm in summer and 3.3 (±0.5) mm in winter. In addition, the discrepancies of the two solutions show a clear seasonal dependency. Radiosounding measurements were used to derive an independent series of precipitable water. These agree better with the GPS precipitable water derived from real surface data. However, the GPS precipitable water time series is dry biased, as it is ca. 77% of the total moisture measured by the radiosoundings. Both the GPS and radiosounding observations are processed through the most up-to-date strategies, to reduce known systematic errors.

Airborne aerosol is a suspension of solid particulate matter and/or liquid particles in air, which are often observed as dust, haze and smoke. They present an overall number concentration usually varying between a few hundred per cubic centimeter of air in the remote areas of the planet and more than 104 cm-3 in the most polluted urban areas, with sizes ranging mainly between 0.01 and no more than 100 μm, and therefore varying by more than four orders of magnitude (Heintzenberg, 1994). Aerosol particles are present in the atmosphere as a result of primary emissions or are formed through secondary processes involving both natural and anthropogenic gaseous species.

Twenty-three spectral curves of extraterrestrial solar irradiance in the (180–400) nm wavelength range, derived from measurements taken on rockets and airborne platforms by various research groups, were examined in order to determine the average spectral curves of extraterrestrial solar irradiance. Such curves can be more reliably used in computational programmes prepared for simulating the radiative-transfer processes occurring in the terrestrial atmosphere. For each spectral curve, a careful examination was performed of the errors due to instrumental characteristics, calibration procedures, atmospheric extinction, solar limb-darkening effects, as well as to the use of normalization procedures to zero-air-mass and the mean Earth-Sun distance. These error estimates were used to obtain realistic evaluations of the standard deviations at the various wavelengths. The resulting data sets were employed to calculate the weighted mean values of the full-disk solar irradiance outside the atmosphere, for spectral resolution values of 0.4 and 1 nm. The values are presented in two tables. A further analysis was performed in order to evaluate the variability of the extraterrestrial solar irradiance as a function of solar activity, but indistinct and contradictory results were found, presumably because the experimental errors were greater than the periodical changes in the density flux of extraterrestrial solar radiation.