Since aerosols are an integral part of the Arctic climate system, understanding aerosol radiative properties and the relation of these properties to each other is important for constraining aerosol radiative forcing effects in this remote region where measurements are sparse. In situ measurements of aerosol size distribution, aerosol light scattering and absorption were taken near Eureka (80.05 degrees N, 86.42 degrees W), on Ellesmere Island, in the Canadian High Arctic over three consecutive years to provide insights into radiative properties of Arctic aerosols. During periods of Arctic haze, we find that the single scattering albedo (SSA) at 405 nm is generally higher and more stable than that determined at 870 nm, with values ranging between 0.90-0.99 and 0.79-0.97, respectively. Events with elevated absorption coefficients (B-abs) exhibit generally an absorption Angstrom exponent (AAE) of around 1 suggesting that black carbon (BC) is the dominant absorbing aerosol for the measurement period. AAE values close to 2 occurring with scattering Angstrom exponent (SAE) values near 0 and SAE values below 0 occasionally observed in December indicate a potential contribution from mineral dust aerosols in late fall and early winter. The apparent real and imaginary parts of the complex refractive index at 405 nm have been found to range between 1.6-1.9 and 0.002-0.02, respectively. The low imaginary component indicates very weak intrinsic absorption compared to BC-rich aerosols. Systematic variabilities between different aerosol optical and microphysical properties depend strongly on the given wavelength. SSA at 405 nm shows a strong inverse dependence with B-abs, because B-abs correlates positively with the imaginary component of the refractive index. On the other hand, SSA at 870 nm correlates with scattering coefficient (B-sca) and not with B-abs due to a greater sensitivity to the ambient particle size distribution for 870 nm scattering. Smaller particles with higher SAE that are prevalent during less polluted periods only weakly scatter at 870 nm leading to lower SSA when B-sca is also low. Lastly, FLEXPART back-trajectories show that lower aerosol SSA and higher B-abs correspond to air masses which are more influenced by Eurasian and Alaskan regions, including regions known to have important BC emissions. This work emphasizes the important variability in Arctic aerosol optical properties during winter and spring, which is likely due to changes in source regions.

The accelerated warming of the Arctic climate may alter the local and regional surface energy balances, for which changing land surface temperatures (LSTs) are a key indicator. Modeling current and anticipated changes in the surface energy balance tequires an understanding of the spatio-temporal interactions between LSTs and land cover, both of which can be monitored globally by measurements from space. This paper investigates the accuracy of the MODIS LST/Emissivity Daily L3 Global 1 km V005 product and its spatio-temporal sensitivity to land surface properties in a Canadian High Arctic permafrost landscape. The land cover ranged from fully vegetated wet sedge tundra to barren rock. MODIS LSTs were compared with in situ radiometer measurements from wet tundra areas collected over a 2-year period from July 2008 to July 2010 including both summer and winter conditions. The accuracy of the MODIS LSTs was -1.1 degrees C with a root mean square error of 3.9 degrees C over the entire observation period. Agreement was lowest during the freeze-back periods where MODIS 1ST showed a cold bias likely due to the overrepresentation of clear-sky conditions. A multi-year analysis of LST spatial anomalies, i.e., the difference between MODIS LSTs and the MODIS 1ST regional mean, revealed a robust spatiotemporal pattern. Highest variability in LST anomalies was found during freeze-up and thaw periods as well as for open water surface in early summer due to the presence or absence of snow or ice. The summer anomaly pattern was similar for all three years despite strong differences in precipitation, air temperature and net radiation. Summer periods with regional mean ISTs above 5.0 degrees C showed the greatest spatial diversity with four distinct 2.0 degrees C classes. Summer anomalies ranged from -4.5 degrees C to 2.6 degrees C with an average standard deviation of 1.8 degrees C. Dry ridge areas heated up the most, while wetland areas and dry areas of sparsely vegetated bedrock with a high albedo remained coolest. The observed summer LST anomalies can be used as a baseline against which to evaluate both past and future changes in land surface properties that relate to the surface energy balance. Summer anomaly classes mainly reflected a combination of albedo and surface wetness. The potential to use this tool to monitor surface drying and wetting in the Arctic should therefore be further explored. A multi-sensor approach combining thermal satellite measurements with optical and radar imagery promises to be an effective tool for a dynamic, process-based ecosystem monitoring scheme. (C) 2015 Elsevier Inc. All rights reserved.