Characterization of Transport Regimes and the Polar Dome during Arctic Spring and Summer using in-situ Aircraft Measurements

Abstract

<p><strong>Abstract.</strong> The springtime composition of the Arctic lower troposphere is to a large extent controlled by transport of mid-latitude air masses into the Arctic, whereas during the summer precipitation and natural sources play the most important role. Within the Arctic region, there exists a transport barrier, known as the polar dome, which results from sloping isentropes. The polar dome, which varies in space and time, exhibits a strong influence on the transport of air masses from mid-latitudes, enhancing it during winter and inhibiting it during summer. Furthermore, a definition for the location of the polar dome boundary itself is quite sparse in the literature.</p> <p>We analyzed aircraft based trace gas measurements in the Arctic during two NETCARE airborne field camapigns (July 2014 and April 2015) with the Polar 6 aircraft of Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI), Bremerhaven, Germany, covering an area from Spitsbergen to Alaska (134° W to 17° W and 68° N to 83° N). For the spring (April 2015) and summer (July 2014) season we analyzed transport regimes of mid-latitude air masses travelling to the high Arctic based on CO and CO<sub>2</sub> measurements as well as kinematic 10-day back trajectories. The dynamical isolation of the high Arctic lower troposphere caused by the transport barrier leads to gradients of chemical tracers reflecting different local chemical life times and sources and sinks. Particularly gradients of CO and CO<sub>2</sub> allowed for a trace gas based definition of the polar dome boundary for the two measurement periods with pronounced seasonal differences. For both campaigns a transition zone rather than a sharp boundary was derived. For July 2014 the polar dome boundary was determined to be 73.5° N latitude and 299–303.5 K potential temperature, respectively. During April 2015 the polar dome boundary was on average located at 66–68.5° N and 283.5–287.5 K. Tracer-tracer scatter plots and probability density functions confirm different air mass properties inside and outside of the polar dome for the July 2014 and April 2015 data set. Using the tracer derived polar dome boundaries the analysis of aerosol data indicates secondary aerosol formation events in the clean summertime polar dome.</p> <p>Synoptic-scale weather systems frequently disturb this transport barrier and foster exchange between air masses from midlatitudes and polar regions. During the second phase of the NETCARE 2014 measurements a pronounced low pressure system south of Resolute Bay brought inflow from southern latitudes that pushed the polar dome northward and significantly affected trace gas mixing ratios in the measurement region. Mean CO mixing ratios increased from 77.9 ± 2.5 ppbv to 84.9 ± 4.7 ppbv from the first period to the second period. At the same time CO<sub>2</sub> mixing ratios significantly dropped from 398.16 ± 1.01 ppmv to 393.81 ± 2.25 ppmv.</p> <p>We further analysed processes controlling the recent transport history of air masses within and outside the polar dome. Air masses within the spring time polar dome mainly experienced diabatic cooling while travelling over cold surfaces. In contrast air masses in the summertime polar dome were diabatically heated due to insolation. During both seasons air masses outside the polar dome slowly descended into the Arctic lower troposphere from above caused by radiative cooling. The ascent to the middle and upper troposphere mainly took place outside the Arctic, followed by a northward motion. Our results demonstrate the successful application of a tracer based diagnostic to determine the location of the polar dome boundary.</p>

Overview paper: New insights into aerosol and climate in the Arctic

“…Through the 1990s and beyond, concentrations of Arctic haze components declined at the northernmost observatories: Alert, Nunavut; Barrow, Alaska; Mount Zeppelin, Svalbard; and Station Nord, Greenland (Heidam et al, 1999;Hirdman et al, 2010;Quinn et al, 2009;Sharma et al, 2004Sharma et al, , 2006Sinha et al, 2017;Sirois and Barrie, 1999). Recent measurements (Fisher et al, 2011;Frossard et al, 2011;Massling et al, 2015;Sharma et al, 2017;Sinha et al, 2017) have found surface mass concentrations of sulfate, organic material and black carbon (BC) 3-10 times lower than those estimated from studies conducted prior to 1981 (Rahn and Heidam, 1981), but the total Arctic column burden of BC may have increased (Koch and Hansen, 2005;Sharma et al, 2013) with implications for climate forcing efficiency (Breider et al, 2017). The turn of the century saw renewed interest in Arctic haze with concern for the role of BC in Arctic warming (Flanner et al, 2007;Hansen and Nazarenko, 2004;Law and Stohl, 2007;McConnell et al, 2007;Quinn et al, 2008;Shindell and Faluvegi, 2009).…”

Chemical composition and source attribution of submicron aerosol particles in the summertime Arctic lower troposphere

“…Meteorological conditions combined with air mass history play a major role when understanding Arctic aerosol composition influenced by local as well as distant sources and transport. The synoptic situations during NETCARE 2014 have been discussed in previous publications (see Bozem et al, 2019;Burkart et al, 2017;Köllner et al, 2017;Leaitch et al, 2016). changed over the course of NETCARE 2014 from an initial Arctic air mass period (July 4-12) to a southern air mass period (July 17-21) with a transition in between (Burkart et al, 2017;Bozem et al, 2019).…”

“…Carbon monoxide (CO) mixing ratios were measured by an Aero-Laser ultra-fast CO monitor (model AL 5002), based on the fluorescence of CO in the vacuum ultraviolet (VUV) at 150 nm (Scharffe et al, 2012;Aero-Laser GmbH, 2013;Wandel, 2015;Bozem et al, 2019). Here, CO is used as an indicator of air mass influenced by combustion sources, including fossil fuel, and biomass burning combustion (Andreae and Merlet, 2001).…”

“…Here, CO is used as an indicator of air mass influenced by combustion sources, including fossil fuel, and biomass burning combustion (Andreae and Merlet, 2001). Further details can be found in Bozem et al (2019) and Table S3.…”

“…The synoptic situations during NETCARE 2014 have been discussed in previous publications (see Bozem et al, 2019;Burkart et al, 2017;Köllner et al, 2017;Leaitch et al, 2016). changed over the course of NETCARE 2014 from an initial Arctic air mass period (July 4-12) to a southern air mass period (July 17-21) with a transition in between (Burkart et al, 2017;Bozem et al, 2019). Since this has a significant impact on the distributions of the observed quantities, we analyze the measurements for each period individually.…”