Characterization of transport regimes and the polar dome during Arctic spring and summer using in situ aircraft measurements
Characterization of transport regimes and the polar dome during Arctic spring and summer using in situ aircraft measurements
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DOI：
10.5194/acp-19-15049-2019
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发表时间：
2019-12-13
影响因子：
6.3
通讯作者：
Abbatt, Jonathan
中科院分区：
文献类型：
--
作者：
Bozem, Heiko;Hoor, Peter;Abbatt, Jonathan
春季北极对流层下层的组成在很大程度上受中纬度气团进入北极的输送控制。在夏季，降水和自然资源是最重要的。在北极地区，倾斜的等熵线形成了水平输送的屏障，称为极地穹窿。极穹的时空变化对中纬度气团的输送有很强的影响，冬季增强输送，夏季抑制输送。2014年7月和2015年4月，我们利用阿尔弗雷德·韦格纳研究所极地6号飞机，分析了两次NETCARE空中实地行动（2014年7月和2015年4月）在北极地区基于飞机的痕量气体测量，覆盖了从斯匹次卑尔根岛到阿拉斯加（西经134度至17度，北纬68度至83度）的区域。利用这些数据，我们基于CO和CO2测量以及运动学10 d后轨迹，描述了中纬度气团前往北极高海拔地区的运输制度。我们发现，北极高纬度对流层低层的动力隔离导致化学示踪剂的梯度，反映了不同的局部化学寿命、源和汇。特别是，CO和CO2的梯度允许在两个测量期间对极穹边界进行基于微量气体的定义，这显示出明显的季节差异。我们从两个战役中得出了一个过渡区，而不是一个明确的边界。2014年7月，极穹边界位于北纬73.5度，位温299 ~ 303.5 K。2015年4月，极穹边界平均位于北纬66 ~ 68.5度，283.5 ~ 287.5 K。示踪-示踪散点图证实了春季和夏季极穹内外不同的气团性质。此外，我们还探讨了控制极地穹窿内外气团近期运输历史的过程。春季极地穹内的气团在经过寒冷表面时主要经历绝热冷却。相反，夏季极地穹窿中的气团由于日晒而被绝热加热。在这两个季节中，极地穹外的气团通过辐射冷却从上方缓慢下降到北极对流层下层。上升到对流层中高层主要发生在北极以外，随后向北移动。极地穹窿内外的气团也可以通过微量气体和气溶胶颗粒的不同化学成分来区分。我们发现，来自北极海洋生物源的含胺颗粒的比例在极地穹丘内增加。相比之下，难熔黑碳的浓度在极穹外最高，表明污染源较远。天气尺度的天气系统经常干扰由极穹形成的运输屏障，并促进中纬度和极地气团之间的交换。在NETCARE 2014测量的第二阶段，Resolute湾以南明显的低压系统带来了来自南纬地区的流入，将极地穹丘向北推进，并显著影响了测量区域的微量气体混合比。在这两种情况下，平均二氧化碳混合比从77.9 +/- 2.5增加到84.9 +/- 4.7 ppbv。同时，CO2混合比从398.16 +/- 1.01显著降低到393.81 +/- 2.25 ppmv。我们的结果证明了应用基于示踪剂的诊断来确定极地穹顶边界的效用，以解释在运输历史背景下大气成分的观测结果。
The springtime composition of the Arctic lower troposphere is to a large extent controlled by the transport of midlatitude air masses into the Arctic. In contrast, precipitation and natural sources play the most important role during summer. Within the Arctic region sloping isentropes create a barrier to horizontal transport, known as the polar dome. The polar dome varies in space and time and exhibits a strong influence on the transport of air masses from midlatitudes, enhancing transport during winter and inhibiting transport during summer.We analyzed aircraft-based trace gas measurements in the Arctic from two NETCARE airborne field campaigns (July 2014 and April 2015) with the Alfred Wegener Institute Polar 6 aircraft, covering an area from Spitsbergen to Alaska (134 to 17 degrees W and 68 to 83 degrees N). Using these data we characterized the transport regimes of midlatitude air masses traveling to the high Arctic based on CO and CO2 measurements as well as kinematic 10 d back trajectories. We found that dynamical isolation of the high Arctic lower troposphere leads to gradients of chemical tracers reflecting different local chemical lifetimes, sources, and sinks. In particular, gradients of CO and CO2 allowed for a trace-gas-based definition of the polar dome boundary for the two measurement periods, which showed pronounced seasonal differences. Rather than a sharp boundary, we derived a transition zone from both campaigns. In July 2014 the polar dome boundary was at 73.5 degrees N latitude and 299-303.5 K potential temperature. During April 2015 the polar dome boundary was on average located at 66-68.5 degrees N and 283.5-287.5 K. Tracer-tracer scatter plots confirm different air mass properties inside and outside the polar dome in both spring and summer.Further, we explored the processes controlling the recent transport history of air masses within and outside the polar dome. Air masses within the springtime polar dome mainly experienced diabatic cooling while traveling 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 through radiative cooling. Ascent to the middle and upper troposphere mainly took place outside the Arctic, followed by a northward motion. Air masses inside and outside the polar dome were also distinguished by different chemical compositions of both trace gases and aerosol particles. We found that the fraction of amine-containing particles, originating from Arctic marine biogenic sources, is enhanced inside the polar dome. In contrast, concentrations of refractory black carbon are highest outside the polar dome, indicating remote pollution sources.Synoptic-scale weather systems frequently disturb the transport barrier formed by the polar dome 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, which pushed the polar dome northward and significantly affected trace gas mixing ratios in the measurement region. Mean CO2 mixing ratios increased from 77.9 +/- 2.5 to 84.9 +/- 4.7 ppbv between these two regimes. At the same time CO2 mixing ratios significantly decreased from 398.16 +/- 1.01 to 393.81 +/- 2.25 ppmv. Our results demonstrate the utility of applying a tracer-based diagnostic to determine the polar dome boundary for interpreting observations of atmospheric composition in the context of transport history.