Optical characterization of Polar winter aerosols and clouds

The Arctic region is particularly sensitive to climate change and has recently undergone major alterations including a dramatic decrease of sea-ice extent. Our ability to model and potentially mitigate climate change is limited, in part, by the uncertainties associated with radiative forcing due to direct and indirect aerosol effects which in turn are dependent on our understanding of aerosol and cloud processes. Aerosol loading can be characterized by aerosol optical depth (AOD) which is the most important (extensive or bulk) aerosol radiative parameter and arguably the most important regional indicator of aerosol behavior. One of the most important shortcomings in our understanding of Arctic aerosols is their behavior during the Polar winter. A major reason for this is the lack of night-time AOD measurements. In this work we use lidar and starphotometry instruments in the Arctic to obtain vertically resolved aerosol profiles and column integrated representations of those profiles (AODs) respectively. In addition, data from a space-borne lidar (CALIOP) is used to provide a pan-Arctic context and seasonal statistics in support of ground based measurements. The latter were obtained at the Eureka (80◦ N, 86◦ W) and Ny Alesund (79◦ N, 12◦ E) high Arctic stations during the Polar Winters of 2010-11 and 2011-12. The physical significance of the variation of the small-amplitude AODs that are typical of the Arctic Polar Winter, requires verification to ensure that artifactual contributions (such as incomplete cloud screening) do not contribute to these variations. A process-level event-based analysis (with a time resolution of ≈ minutes), is essential to ensure that extracted extensive (bulk) and intensive (per particle) optical and microphysical indicators are coherent and physically consistent. Using the starphotometry-lidar synergy we characterized several distinct events throughout the measurement period: these included aerosol, ice crystal, thin cloud and polar stratospheric cloud (PSC) events. In general fine ( 1μm) mode AODs from starphotometry (τf and τc) were coherent with their lidar analogues produced from integrated profiles : however several inconsistencies related to instrumental and environmental factors were also found. The division of starphotometer AODs into τf and τc components was further exploited to eliminate coarse mode cloud optical depths (spectral cloud screening) and subsequently compare τf with cloud-screened AODs using a traditional (temporal based) approach. While temporal and spectral cloud screening case studies at process level resolutions yielded good to moderate results in terms of the coherence between spectrally and temporally cloud screened data (both temporally screened starphotometer and lidar optical depths), seasonal results apparently still contained cloud contaminated data. Forcing an agreement using a more restrictive, second-pass, clear sky criterion ("minimal cloud envelope") produced mean 2010-11 AOD seasonal values of 0.08 and 0.04 for Eureka and Ny Alesund respectively. In 2011-12 these values were 0.12 and 0.09. Conversely, CALIOP AODs (0 to 8 km) for the high Arctic showed a slight decrease from 2010-2011 to 2011-2012 (0.04 vs 0.03).