Spatial and seasonal distribution of Arctic aerosols observed by the CALIOP satellite instrument (2006–2012)

Abstract

We use retrievals of aerosol extinction from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard the CALIPSO satellite to examine the vertical, horizontal and temporal variability of tropospheric Arctic aerosols during the period 2006–2012. We develop an empirical method that takes into account the difference in sensitivity between daytime and nighttime retrievals over the Arctic. Comparisons of the retrieved aerosol extinction to in situ measurements at Barrow (Alaska) and Alert (Canada) show that CALIOP reproduces the observed seasonal cycle and magnitude of surface aerosols to within 25 %. In the free troposphere, we find that daytime CALIOP retrievals will only detect the strongest aerosol haze events, as demonstrated by a comparison to aircraft measurements obtained during NASA's ARCTAS mission during April 2008. This leads to a systematic underestimate of the column aerosol optical depth by a factor of 2–10. However, when the CALIOP sensitivity threshold is applied to aircraft observations, we find that CALIOP reproduces in situ observations to within 20% and captures the vertical profile of extinction over the Alaskan Arctic. Comparisons with the ground-based high spectral resolution lidar (HSRL) at Eureka, Canada, show that CALIOP and HSRL capture the evolution of the aerosol backscatter vertical distribution from winter to spring, but a quantitative comparison is inconclusive as the retrieved HSRL backscatter appears to overestimate in situ observations by a factor of 2 at all altitudes. In the High Arctic (>70° N) near the surface (<2 km), CALIOP aerosol extinctions reach a maximum in December–March (10–20 Mm⁻¹), followed by a sharp decline and a minimum in May–September (1–4 Mm⁻¹), thus providing the first pan-Arctic view of Arctic haze seasonality. The European and Asian Arctic sectors display the highest wintertime extinctions, while the Atlantic sector is the cleanest. Over the Low Arctic (60–70° N) near the surface, CALIOP extinctions reach a maximum over land in summer due to boreal forest fires. During summer, we find that smoke aerosols reach higher altitudes (up to 4 km) over eastern Siberia and North America than over northern Eurasia, where they remain mostly confined below 2 km. In the free troposphere, the extinction maximum over the Arctic occurs in March–April at 2–5 km altitude and April–May at 5–8 km. This is consistent with transport from the midlatitudes associated with the annual maximum in cyclonic activity and blocking patterns in the Northern Hemisphere. A strong gradient in aerosol extinction is observed between 60° N and 70° N in the summer. This is likely due to efficient stratocumulus wet scavenging at high latitudes combined with the poleward retreat of the polar front. Interannual variability in the middle and upper troposphere is associated with biomass burning events (high extinctions observed by CALIOP in spring 2008 and summer 2010) and volcanic eruptions (Kasatochi in August 2008 and Sarychev in June 2009). CALIOP displays below-a...

Arctic air pollution: Challenges and opportunities for the next decade

“…Sodemann et al, 2011). CALIOP has provided such vertical profile data on aerosol properties such as backscatter up to 80N over a number of years allowing examination of seasonal and inter-annual aerosol variability in the Arctic (Di Pierro et al, 2013). Emmons et al, (2015) used satellite observations to demonstrate consistent bias in simulated NO 2 among several models in high latitude regions dominated by fire emissions, where in-situ sampling was not available.…”

Aerosol indirect effects on the nighttime Arctic Ocean surface from thin, predominantly liquid clouds

“…Given these constraints, the backscatter aerosol detection limit for clean background clouds is as low as possible and should have only negligible variations based on detector noise and background molecular scattering and O 3 densities above cloud . Because CALIPSO cannot always detect dilute aerosols (Di Pierro et al, 2013;Kacenelenbogen et al, 2014;Rogers et al, 2014;Winker et al, 2013), particularly below cloud where the lidar signal has been reduced, clean background clouds were also required to have modeled above-and below-cloud FLEXPART (FLEXible TRAjectory model; Stohl et al, 1998Stohl et al, , 2005 black carbon concentrations of < 30 ng C m −3 (see Sects. 2.1.3 and 3.1 for further discussion).…”

Seasonal variation of aerosol water uptake and its impact on the direct radiative effect at Ny-Ålesund, Svalbard

“…Since no direct measurements on the vertical profiles of aerosol particle number size distributions were conducted in Ny-Ålesund, we assumed a vertical-scale factor that relates the aerosol number concentrations at a given altitude to the surface measurements (at 474 m a.s.l.). The vertical profile of the aerosol particle number size distribution was estimated based on mean extinction coefficient profiles obtained from observations with the spaceborne Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar over the Arctic (Di Pierro et al, 2013). Winker et al (2013) present Arctic extinction profiles that show an exponential decrease with height.…”