The CIMEL (Model T) instruments at 0PAL and the Ridge Lab are (i) 9-band sunphotometers with a wavelength range between the ultraviolet and shortwave infra-red regions of the electromagnetic spectrum (340, 380, 440, 500, 675, 870, 1020, 1020 and 1640 nm) employed for the retrieval of aerosol optical depth (AOD) and (ii) 4-band sky radiometers (440, 675, 870, 1020 nm) employed for the retrieval of sky radiance in the solar almucantar (sky scans at a constant zenith angle starting at the center of the sun). The Model T CIMELs at Eureka include a moonphotometer mode that allows acquisition of AODs during the polar winter.
The AOD measurements are relatively high frequency (nominally a complete AOD spectrum every 5 minutes) while the sky radiance measurements are somewhat lower (nominally a complete almucantar scan about once an hour). The installation of both the 0PAL and Ridge Lab CIMELs occurred in the spring of 2007).
Most of the time, the CIMEL instruments are in sun-staring mode acquiring AODs in the 9 spectral bands. The rest of the time they are sky (almucantar) scanning mode. This AERONET video allows one to appreciate the CIMEL setup process and CIMEL measurements in general. The two collimators (see the figure below) of the CIMEL are employed for, respectively sun and sky measurements (fields of view of 1.26° and a combination of (near-sun) 0.1° and 1.1° respectively). The two collimators lead to a common 9-band filter wheel that preceeds a ultraviolet to near-infrared (silicon) and a near-infrared to short wave infrared (InGaAs) detector.
The AERONET processing chain incorporates 3 levels of AOD retrievals: Level 1.0 that includes a (triplet) check to exclude AODs that are highly variable in time and, thus, likely induced by clouds, Level 1.5 AODs that exclude cloud data (missed by the triplet criterion) using time-varying and spectral criteria and Level 2.0 AODs that includes post-measurement calibration and QA refinements that may have eluded the Level1.0 and 1.5 criteria.
The primary research goals of those who employ AERONET/AEROCAN data are to help characterize the properties of aerosols. In the high Arctic aerosols include fine mode1 organic-smoke, pollution-, biogenic- and volcanic-sulphates and sea-salt as well as coarse mode2 dust and seasalt (clouds are also coarse mode particles).
1 smaller than 1 μm (1 meter/1,000,000). 2 greater than 1 μm
Aerosols generally play a primary radiative forcing role (non-absorbing aerosols such as sulphates tend to oppose the warming effect of greenhouse gas radiative forcing while absorbing aerosols such as dust deposited on snow tend to reinforce warming effects). The characterization of aerosol properties and their radiative forcing effects (especially over the Arctic where average temperature increases are twice the global average) are critical to achieving more accurate climate model predictions.
The top graphs in the figure above show fine mode and coarse mode AOD (red and blue curves) retrieved using the Ridge Lab CIMEL (the red curves are sensitive to small-sized smoke particles while the blues curves are sensitive to large-sized cloud crystals). The middle and bottom graphs show altitude profiles of the time-synchronized AHSRL (Arctic High Spectral Resolution Lidar) beta and delta parameters at 532 nm (beta and delta are respectively, the backscatter coefficient which is sensitive to the number of particulates and the depolarization ratio which is sensitive to the size and shape of the particulates). Note, for example, that the red-coloured fine mode AOD on day 204 ( July event) increase) significantly while the beta values around 9 to 10 km altitude increase (to a light blue value) and the (somewhat noisy delta values) decrease to weak values typical of fine mode aerosols (and cloud water droplets but, the fact that the AOD is fine mode excludes coarse mode cloud droplets).
The coherence of the fine and coarse mode AODs and the beta and delta profiles is typical of this combination of sensors. Such data, coupled with aerosol and cloud computer models, leads to physical and optical insights into the identification and characterization of aerosols and clouds over the Arctic.