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...

Vertical profiles of aerosol extinction obtained with the CALIOP lidar onboard CALIPSO are used in conjunction with the GEOS-Chem chemical transport model and NOAA's HYSPLIT trajectory model to document three aerosol export events from East Asia to the Arctic in the year 2007. During each of these events CALIOP sampled the pollution plumes multiple times over periods of five to seven days. Midlatitude cyclones lifted the pollution to the free troposphere with net diabatic heating of ~5 °C day^&minus;1 and precipitation in this initial ascending stage. Rapid meridional transport to the Arctic took place at 3–7 km altitude, and was mediated by either a blocking high pressure system in the NW Pacific or a trough-ridge configuration. Once in the Arctic transport was nearly isentropic with slow subsidence and radiative cooling at a rate of 1–1.5 °C day^&minus;1. We find good agreement between modeled and observed plumes in terms of length, altitude, thickness and, within the measurement uncertainties, extinction coefficient. In one event the satellite algorithm misclassifies the aerosol layer as ice clouds as a result of the relatively high depolarization ratio (0.06), likely caused by a high dust component in the aerosol mixture. Using 500 hPa geopotential height anomalies for these three events along with eight other export events observed by CALIOP in 2007–2009, we develop a meteorological index that captures 40–60% of the variance of Asian transport events to the Arctic in winter and spring. Simulations with the GEOS-Chem model show that 6 major export events from Asia to the Arctic occur each year, on average. The maximum probability for such events is during March–June, with a secondary maximum in October–November. During these events, Asian pollution and dust aerosols account for 50–70% of the aerosol optical depth over the Siberian sector of the Arctic, compared to a mean background contribution of 33%

Vertical profiles of aerosols obtained with the CALIOP lidar onboard CALIPSO are used in conjunction with the GEOS-Chem chemical transport model and NOAA's HYSPLIT trajectory model to document three aerosol export events from East Asia to the Arctic that occurred in the year 2007. During each of these events CALIOP sampled the pollution plumes multiple times over periods of five to seven days. Meridional transport to the Arctic was rapid, taking 3–4 days and was accompanied by net diabatic heating of ~5 °C/day and precipitation in its ascending stage. Once in the Arctic transport was nearly isentropic with slow subsidence and radiative cooling at a rate of 1–1.5 °C/day. We find close agreement between modeled and observed plume in terms of length, altitude, thickness and, within the measurement uncertainties, extinction coefficient. In one event the satellite algorithm misclassifies the aerosol layer as ice clouds as a result of the relatively high depolarization ratio (0.06), likely caused by a somewhat high dust component in the aerosol mixture. The misclassification is more severe at daytime (67% of layers are misclassified) than at nighttime (32%). The two most intense export events occurred in early spring within a three-week time span and are strongly related to a persisting blocking anticyclone that was located in the NW Pacific. Using 500 hPa geopotential height anomalies of these two events along with several others in 2007–2009 we develop a meteorological index that captures 40–60% of the variance of Asian transport events to the Arctic in winter and spring

We use retrievals of aerosol extinction from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on board the CALIPSO satellite to examine the vertical, horizontal and temporal variability of tropospheric Arctic aerosols during 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 HSRL Lidar 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 factor of 2 at all altitudes. In the High Arctic (> 70&deg; N) near the surface (< 2 km), CALIOP aerosol extinctions reach a maximum in December-March (10–20 Mm^&minus;1), followed by a sharp decline and a minimum in May–September (1–4 Mm^&minus;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&deg; 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 it remains 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 mid-latitudes 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&deg; N and 70&deg; 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...