Vito Vitale

Aerosols in polar regions: A historical overview based on optical depth and in situ observations

Large sets of filtered actinometer, filtered pyrheliometer and Sun photometer measurements have been carried out over the past 30 years by various groups at different Arctic and Antarctic sites and ...

Improved algorithm for calculations of Rayleigh-scattering optical depth in standard atmospheres

Precise calculations of the total Rayleigh-scattering optical depth have been performed at 88 wavelengths ranging from 0.20 to 4.00 microm for the six well-known standard atmosphere models by integrating the volume Rayleigh-scattering coefficient along the vertical atmospheric path from sea level to a 120-km height. The coefficient was determined by use of an improved algorithm based on the Ciddor algorithm [Appl. Opt. 35, 1566 (1996)], extended by us over the 0.20-0.23-microm wavelength range to evaluate the moist air refractive index as a function of wavelength, air pressure, temperature, water-vapor partial pressure, and CO2 volume concentration. The King depolarization factor was also defined taking into account the moisture conditions of air. The results indicate that the influence of water vapor on Rayleigh scattering cannot be neglected at tropospheric altitudes: for standard atmospheric conditions represented in terms of the U.S. Standard Atmosphere (1976) model, the relative variations produced by water vapor in the Rayleigh scattering parameters at a 0.50-microm wavelength turn out to be equal to -0.10% in the moist air refractivity at sea level (where the water-vapor partial pressure is equal to approximately 7.8 hPa), -0.04% in the sea-level King factor, -0.24% in the sea-level Rayleigh-scattering cross section, and -0.06% in the Rayleigh-scattering optical depth.

Study of the Relationship between Ångström's Wavelength Exponent and Junge Particle Size Distribution Exponent

Abstract The volume extinction coefficient produced by aerosol particles at wavelength λ is generally proportional to λ−α as found by Anstrom. On the other hand, particle size spectra frequently give a particle number density approximately proportional to the inverse of the radius power with exponent ν, as found by Junge. On the basis of computations made using a very accurate Mie extinction algorithm, a study of the relationship curve between exponents α and ν is made for aerosol particle polydispersions of different radius intervals and for different refractive index values. Moreover, a set of relationship curves is obtained for realistic particulate extinction models based on different size distribution curves and refractive index models. The results show that the linear relationship α = ν − 2 is not valid in most cases. In particular, exponent α assumes appreciably lower values than ν − 2 in the range ν > 3 for all the particulate extinction models.

Characterization of the atmospheric temperature and moisture conditions above Dome C (Antarctica) during austral summer and fall months

Two sets of radiosounding measurements were taken at Dome C (Antarctica) in December 2003 and January 2003 and 2004, using RS80-A, RS80-H, and RS90 Vaisala radiosondes, and from March to May 2005, employing the RS92 model. They were examined following accurate correction procedures to remove the main relative humidity dry bias and the temperature and humidity lag errors. The results showed that a strong cooling usually characterizes the thermal conditions of the whole troposphere from December/January to April/May, with an average temperature decrease from 245 to 220 K at the ground, of around 10 K at upper tropospheric levels, and of more than 15 K in the lower stratosphere. The relative humidity data were found to be affected by dry bias of 5-10%, on average, for the RS80-A and RS80-H Humicap sensors and by smaller percentages for the other sensors. The mean monthly vertical profiles of absolute humidity were found to decrease sharply throughout the troposphere, especially within the first 3 km, and to diminish considerably passing from December/January to March/April/ May, with average values of precipitable water decreasing from 0.75 to 0.28 mm, median values from 0.69 to 0.25 mm, and first and third quartiles from 0.60 to 0.22 mm and from 0.87 to 0.34 mm, respectively. The results demonstrate that Dome C (where a permanent scientific station has been open for winter operations since austral winter 2005) is a site of comparable quality to the South Pole for both validation of satellite radiance measurements and astronomic observations in the infrared, submillimetric, and millimetric wavelength range, performed with large telescopes that cannot be carried on satellites.

Regional aerosol optical depth characteristics from satellite observations: ACE-1, TARFOX and ACE-2 results

Analysis of the aerosol properties during 3 recent international field campaigns (ACE-1, TARFOX and ACE-2) are described using satellite retrievals from NOAA AVHRR data. Validation of the satellite retrieval procedure is performed with airborne, shipboard, and land-based sunphotometry during ACE-2. The intercomparison between satellite and surface optical depths has a correlation coefficient of 0.93 for 630 nm wavelength and 0.92 for 860 nm wavelength. The standard error of estimate is 0.025 for 630 nm wavelength and 0.023 for 860 nm wavelength. Regional aerosol properties are examined in composite analysis of aerosol optical properties from the ACE-1, TARFOX and ACE-2 regions. ACE-1 and ACE-2 regions have strong modes in the distribution of optical depth around 0.1, but the ACE-2 tails toward higher values yielding an average of 0.16 consistent with pollution and dust aerosol intrusions. The TARFOX region has a noticeable mode of 0.2, but has significant spread of aerosol optical depth values consistent with the varied continental aerosol constituents off the eastern North American Coast.

Shipboard Sunphotometer Measurements of Aerosol Optical Depth Spectra and Columnar Water Vapor During ACE-2, and Comparison with Selected Land, Ship, Aircraft, and Satellite Measurements

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.

Narrowband filter radiometer for ground-based measurements of global ultraviolet solar irradiance and total ozone

The ultraviolet narrowband filter radiometer (UV-RAD) designed by the authors to take ground-based measurements of UV solar irradiance, total ozone, and biological dose rate is described, together with the main characteristics of the seven blocked filters mounted on it, all of which have full widths at half maxima that range 0.67 to 0.98 nm. We have analyzed the causes of cosine response and calibration errors carefully to define the corresponding correction terms, paying particular attention to those that are due to the spectral displacements of the filter transmittance peaks from the integer wavelength values. The influence of the ozone profile on the retrieved ozone at large solar zenith angles has also been examined by means of field measurements. The opportunity of carrying out nearly monochromatic irradiance measurements offered by the UV-RAD allowed us to improve the procedure usually followed to reconstruct the solar spectrum at the surface by fitting the computed results, using radiative transfer models with field measurements of irradiance. Two long-term comparison campaigns took place, showing that a mean discrepancy of +0.3% exists between the UV-RAD total ozone values and those given by the Brewer #63 spectroradiometer and that mean differences of +0.3% and -0.9% exist between the erythemal dose rates determined with the UV-RAD and those obtained with the Brewer #63 and the Brewer #104 spectroradiometers, respectively.

Insights on nitrate sources at Dome C (East Antarctic Plateau) from multi-year aerosol and snow records

Here we present the first multi-year record of nitrate in the atmospheric aerosol (2005–2008) and surface snow (2006–08) from central Antarctica. PM10 and size-segregated aerosol, together with superficial snow, have been collected all year-round at high resolution (daily for all the snow samples and for most of aerosol samples) at Dome C since the 2004/05 field season and analysed for main and trace ionic markers. The suitability of the sampling location in terms of possible contamination from the base is shown in detail. In spite of the relevance of nitrate in Antarctic atmosphere, both for better understanding the chemistry of N cycle in the plateau boundary layer and for improving the interpretation of long-term nitrate records from deep ice core records, nitrate sources in Antarctica are not well constrained yet, neither in extent nor in timing. A recurring seasonal pattern was pointed out in both aerosol and snow records, showing summer maxima and winter minima, although aerosol maxima lead the snow ones of 1–2 months, possibly due to a higher acidity in the atmosphere in mid-summer, favouring the repartition of nitrate as nitric acid and thus its uptake by the surface snow layers. On the basis of a meteorological analysis of one major nitrate event, of data related to PSC I extent and of irradiance values, we propose that the high nitrate summer levels in aerosol and snow are likely due to a synergy of enhanced source of nitrate and/or its precursors (such as the stratospheric inputs), higher solar irradiance and higher oxidation rates in this season. Moreover, we show here a further evidence of the substantial contribution of HNO3/NOx re-emission from the snowpack, already shown in previous works, and which can explain a significant fraction of atmospheric nitrate, maintaining the same seasonal pattern in the snow. As concerning snow specifically, the presented data suggest that nitrate is likely to be controlled mainly by atmospheric processes, not on the daily timescale but rather on the seasonal one.

Atmospheric observations at the Amundsen-Nobile Climate Change Tower in Ny-Ålesund, Svalbard

The Amundsen-Nobile Climate Change Tower (CCT) is one of the important scientific platforms operating in Ny-Ålesund, Svalbard. The CCT is equipped with a consistent set of meteorological sensors installed at different heights to provide continuous measurements of the atmospheric parameters that affect the climate and its variability. In this paper, some features of the main meteorological parameters observed during the 6 years of measurements since November 2009 are presented in order to describe the thermodynamic characteristic of the lower layers of the atmosphere and the peculiarities of CCT. Monthly and seasonal behavior of temperature, humidity and wind as well as radiation budget and albedo variability are also shown. Such preliminary statistical description aims to provide an overview of the phenomenology occurring in the Kongsfjord area, useful to proceed with further analysis of the arctic climatic system. Even if the time series are not long enough to consider the parameters variability on a climatological time scale, useful assumptions can be made for detailed analysis concerning turbulence studies, data intercomparison at different time and space scales, validation of theory and numerical model results. CCT dataset is stored in a dedicated built-in digital infrastructure that allows other users, in the frame of international cooperations, to visualize, access and download the data and contribute to strengthen the collaboration within the scientific community operating in Svalbard.

Quality assurance of solar UV irradiance in the Arctic.

The first Arctic intercomparison of three solar ultraviolet (UV) spectroradiometers and two multifilter radiometers was held in May and June 2009 at Ny-Alesund, Svalbard, Norway. The transportable reference spectroradiometer QASUME acted as reference instrument for this intercomparison. The measurement period extended over eleven days, comprising clear sky and overcast weather conditions. Due to the high latitude, measurements could be performed throughout the day during this period. The intercomparison demonstrated that the solar UV measurements from all instruments agreed to within +/-15% during the whole measurement period, while the spectroradiometer from the Alfred-Wegener Institute agreed to better than +/-5%. This intercomparison has demonstrated that solar UV measurements can be performed reliably in the high-latitude Arctic environment with uncertainties comparable to mid-latitude sites.