A. Royer

Bimodal size distribution influences on the variation of Angstrom derivatives in spectral and optical depth space

The variation of the aerosol optical depth and its first and second spectral derivatives (α and α′) can be largely described in terms of the spectral interaction between the individual optical components of a bimodal size distribution. Simple analytical expressions involving the separate optical components of each mode explain virtually all the features seen in spectra of the aerosol optical depth and its derivatives. Illustrations are given for a variety of measured optical depth spectra; these include comparative simulations of the diurnal behavior of α and α′ spectra as well as the diurnal and general statistical behavior of α and α′ as a function of optical depth (optical depth space). Each mode acts as a fixed “basis vector” from which much of the behavior in spectral and optical depth space can be generated by varying the extensive (number density dependent) contributions of fine and coarse mode optical depths. Departures from these basis vectors are caused by changes in aerosol type (average size and refractive index) and thus are associated with differing synoptical air masses, source trajectories or humidity conditions. Spectral parameters are very sensitive to interband errors in measured optical depth data. Third-order polynomial fits within the visible-NIR spectral region effectively filter such errors while representing the limit of useful extractable information.

Aerosol optical depth over the oceans: Analysis in terms of synoptic air mass types

The results of spectral aerosol optical depth measurements in the Pacific Ocean, Baltic Sea, and North Atlantic are considered with regard to air mass types. It is found that the optical properties of continental and maritime air mass types differ significantly for the data employed in this study. A synoptical air mass context was also employed in demonstrating the correlation between near infrared aerosol optical depth τa(1640 nm) and wind speed as well as for investigations into the relationship between deck level relative humidity and the aerosol optical depth at 550 nm. Simulations, employing well-known aerosol parameterization models, of the aerosol optical depth spectra for various air mass types show good agreement with the experimental results in the visible and near infrared range.

Optical properties of boreal forest fire smoke derived from Sun photometry

[1] Aerosol optical properties derived from Sun photometry were investigated in terms of climatological trends at two Sun photometer sites significantly affected by western Canadian boreal forest fire smoke and in terms of a 2-week series of smoke events observed at stations near and distant from boreal forest fires. Aerosol optical depth (ta) statistics for Waskesiu, Saskatchewan, and Thompson, Manitoba, were analyzed for summer data acquired between 1994 and 1999. A significant correlation between the geometric mean and the forest fire frequency indices (hot spots) was found; on the average, 80% of summertime optical depth variation in western Canada can be linked to forest fire sources. The average geometric mean and geometric standard deviation at 500 nm was observed to be 0.074 and 1.7 for the clearest, relatively smoke-free summer and 0.23 and 3.0 for the summer most influenced by smoke. A systematic decrease of fine mode Angstrom exponent (af) was noted (daf /d log ta �� 0.6). This decrease roughly corresponds to an increase in the fine mode effective radius (reff) from 0.09 to 0.15 mm and an abundance (A) to size rate increase near 2.0 (d log A / d log reff). A 1998 series of forest fire events was tracked using TOMS, AVHRR, and GOES imagery, back trajectories, and data from six Sun photometer sites in Canada and eastern United States. The results showed rates of decrease of af with increasing ta which were similar to the climatological data. An analysis in terms of source to station distance showed a decrease in af and an increase in reff with increasing distance. This observation was coherent with previous observations on the particle growth effects of aging. INDEX TERMS: 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 4801 Oceanography: Biological and Chemical: Aerosols (0305); KEYWORDS: aerosols, forest fire smoke, Sun photometry, optics

A study of the link between synoptic air mass type and atmospheric optical parameters

The paper deals with air mass influence on atmospheric spectral transmittance in a rural, pollution-free area (Sherbrooke, Quebec, in eastern Canada). A statistical analysis of aerosol optical depths (0.04 ≤ τa≤0.60) and Angstrom parameters (0.20 ≤ α ≤ 2.1) derived from measurements of direct spectral solar radiation in the period from January 1989 up to August 1991 is presented. The analysis incorporates investigations into correlations with air mass type (obtained from synoptic maps) and source type (deduced from back trajectory analysis). Mean monthly values of aerosol optical depth and corresponding information about frequency of air mass occurrence are also presented. The temporal comparison of these two ensembles of data helps to explain the similarities and differences observed in the month to month variations of aerosol optical depth during the 3-year measurement period. The data arrays are then partitioned in such a way as to facilitate the interpretation in terms of determining the optical mechanisms which influence the aerosol optical depths. For air masses of arctic origin the discrimination of seasonal variations of aerosol optical depth is consistent with independent measurements of turbidity made in Alaska. The role of air mass source in defining aerosol optical depth is evaluated in terms of its being potentially a more fundamental influence than air mass type.

Multisensor analysis of integrated atmospheric water vapor over Canada and Alaska

[1] Atmospheric water vapor is a key parameter for the analysis of climatic systems (greenhouse gas effect), in particular over high latitudes where water vapor displays an important seasonal variability. The sparse spatial and temporal sampling of atmospheric water vapor observations across Canada needs to be improved. A series of instruments and methods including a 940-nm solar absorption band radiometer (R) and radiosonde (S) analysis from a numerical weather prediction model and a ground-based bi-frequency Global Positioning System (GPS) were used to evaluate the integrated atmospheric water vapor (IWV) at various sites in Canada and Alaska from a multiyear database. The IWV-R measurements were collected within the framework of the North American Sun Radiometry network (AERONET/AEROCAN). Intercomparisons between [IWV-GPS and IWV-S], [IWV-R and IWV-GPS], and [IWV-R and IWV-S] show root mean square (RMS) differences of 1.8, 1.9, and 2.2 kg m � 2 , respectively. GPS meteorology appears to be the easiest approach to calibrate the solar radiometer water vapor band owing to its flexibility, and it allows us to overcome the Sun radiometry limitation in high-latitude areas like the Arctic. The sensitivity of the GPS retrieval to various parameters like GPS satellite constellation and meteorological data are discussed. The classical linear relationship between the surface temperature and the integrated weighted mean temperature profile needed for IWV-GPS retrieval may be significantly different for Arctic air masses compared with midlatitude air masses in the case of tropospheric temperature profile inversion. An ever-expanding multiyear (1994–2001) North American summer water vapor climatology, derived from AERONET/Canadian Sun Radiometer Network, is presented and analyzed, showing a mean value of 19.8 ± 6.1 kg m � 2 and variations from 17 kg m � 2 in Alaska to 23 kg m � 2 in southeastern Canada. The results in Bonanza Creek, Alaska, show significant interannual variations with a peak in 1997, which may

Aerosol optical depth over Canada and the link with synoptic air mass types

Aerosol optical depth measurements acquired through the Canadian sunphotometer network were statistically analyzed for the 1987-1992 period in order to investigate spatial and temporal commonalities between the member stations. Four stations were chosen to yield a spatially representative sampling of atmospheric optical conditions across Canada (East Coast, eastern continental, western continental, and West Coast). All the stations were located in rural, local pollution free areas. The results of aerosol optical depth measurements showed significant differences between eastern and western Canadian stations. The effect of the Pinatubo volcanic eruption was clearly seen in the measurements acquired at Sable Island, Nova Scotia. Air mass relationships for the four stations sampled demonstrated the relevance of applying air mass classification criteria to the analysis and discrimination of atmospheric optical depth. Knowledge of the seasonal trend combined with information concerning air mass type enables a coarse a priori estimation of aerosol optical depth in the absence of traditional optical data. Synoptical air mass analysis facilitates the understanding of the mechanisms involved in the seasonal variations of aerosol optical depth and yields useful information about the atmospheric optical state. The relevance of the synoptical air mass approach was demonstrated in one particular case : a seasonal aerosol optical depth trend for Arctic air masses was observed for the three sunphotometer stations which regularily experience this type of air mass.

Variability analysis of the transitory climate regime as defined by the NDVI/T/sub s/ relationship derived from NOAA-AVHRR over Canada

This research work outlines an original method for climate observation by remote sensing based on the local combination of normalized difference vegetation index (NDVI) and land surface temperature (T/sub s/) measurements acquired by the NOAA-AVHRR sensor. It explores the phenomenon of linearity observed between T/sub s/ and the NDVI, which varies from positive to negative according to the conditions of the land surface energy budget regime and the vegetation type. Over vegetation, the decreasing relationship of T/sub s/ in relation to the NDVI (negative regression) due to vegetation cover transpiration is well known. However, over soils with sparse vegetation, bare soil, lichens or tundra, the relationship is reversed (positive regression) due to the high surface albedo which influences T/sub s/ values. The method is first demonstrate using full spatial and temporal resolution HRPT images over the BOREAS area corrected for atmospheric effects and screened for cloud cover in comparison with temperature and precipitation data. The method is then applied to composite images from the PAL multi-annual database at a resolution of 8 km and for Canada overall. It permits the determination of the ecotone position separating the forest from the tundra and the monitoring of the inter-annual fluctuations related to climatic variations and global warming.

Modelling of aerosol optical depth variability at regional scale

Monitoring of aerosol optical depth (AOD) is of particular importance due to the significant role of aerosols in the atmospheric radiative budget. AOD is a key parameter in studies related to global climatology, atmospheric pollutants, forest fires, and for performing atmospheric corrections on remotely sensed imagery of surface scenes. Up to now the two standard techniques used for retrieving AOD are; (i) sun photometry which provides measurements of high temporal frequency and sparse spatial frequency, and (ii) approaches such as DDV (Dense Dark Vegetation) inversion algorithms which use dark targets in remotely sensed imagery. Although the latter techniques allow AOD retrieval over appreciable spatial domains, the irregular spatial pattern of dark targets and the typically low repeat frequencies of imaging satellites exclude the acquisition of AOD databases on a continuous spatio-temporal basis. In order to overcome this drawback, the authors adopted a modelling approach based on a passive atmospheric circulation model and an AOD data assimilation technique, which ties AOD computations from the circulation model to available AOD measurements. The authors present in this paper the first validation results for this new model applied to mid-latitude eastern North America during June 1997. The results show the potential of this approach especially when used with remotely sensed AOD.