African dust outbreaks over the Mediterranean Basin during 2001-2011: PM10 concentrations, phenomenology and trends, and its relation with synoptic and mesoscale meteorology
African dust outbreaks over the Mediterranean Basin during 2001-2011: PM10 concentrations, phenomenology and trends, and its relation with synoptic and mesoscale meteorology
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DOI：
10.5194/acp-13-1395-2013
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发表时间：
2013-01-01
影响因子：
6.3
通讯作者：
Stafoggia, M.
中科院分区：
文献类型：
--
作者：
Pey, J.;Querol, X.;Stafoggia, M.
对整个地中海盆地发生的非洲沙尘暴发进行了长达11年的研究(2001-2011年)。为了评估这种矿物粉尘暴发对周围环境颗粒物浓度的影响，汇编了地中海地区区域和郊区背景地点的PM10数据。在确定了非洲沙尘的每日影响后，应用了一种方法来估计自然沙尘对每日PM10浓度的贡献。我们的研究结果指出，非洲沙尘暴发在地中海南部地区更为频繁，从每年30%到37%的天数，而在北部地点，它们发生的天数不到全年的20%。地中海中部作为一个过渡区出现，与盆地西侧和东侧类似的纬度位置相比，其下端的沙尘事件频率略高。在整个地中海，受监测点的纬度位置的驱动，非洲粉尘对PM10的贡献从南到北的梯度在整个地中海是明显的，至少向西穿过盆地东经25度。结果，得到了根据纬度位置估算非洲尘埃年贡献量的经验公式。从东经25度向东，由于严重的沙尘年发生次数增加，但也因为来自内盖夫和中东沙漠的输入，每年的沙尘贡献较高。在地中海盆地中部南部地点观察到的非洲沙尘事件频率略高，但强度相对较低，这一点得到了补偿。在季节格局和强度特征方面，西部夏季盛行明显，强降水少(PM10日均沙尘超过100微克(-3))；中部无季节变化趋势，多为中等强降水；东部秋季贡献较大，常年出现各种强降水。总体而言，非洲沙尘是地中海南部区域背景中PM10的最大来源(占PM10的35%-50%)，对PM10的季节性峰值贡献高达总质量的80%。对非洲沙尘事件及其对PM10浓度的贡献的多年研究表明，2006/2007至2011年期间，17个研究区域中有4个区域的PM10浓度持续下降，所有这些区域都位于地中海西北部。这种下降几乎与夏季NAO(北大西洋涛动)指数的下降平行，自2006年以来，NAO指数的降幅逐渐变得更负。因此，过去5年大气环流的急剧变化(在1950年代出现了类似的负NAO期间)影响了非洲沙尘事件的数量，从而影响了地中海西北部观测到的每年输入到PM10的沙尘。通过调查850百帕的平均温度和位势高度图，可以明显看出，2007-2008两年期非洲沙尘向地中海中部移动，2009-2011年三年期向非洲西北部海岸和加那利群岛移动。
The occurrence of African dust outbreaks over the whole Mediterranean Basin has been studied on an 11-yr period (2001-2011). In order to evaluate the impact of such mineral dust outbreaks on ambient concentrations of particulate matter, PM10 data from regional and suburban background sites across the Mediterranean area were compiled. After identifying the daily influence of African dust, a methodology for the estimation of the natural dust contributions on daily PM10 concentrations was applied.Our findings point out that African dust outbreaks are sensibly more frequent in southern sites across the Mediterranean, from 30 to 37% of the annual days, whereas they occur less than 20% of the annual days in northern sites. The central Mediterranean emerges as a transitional area, with slightly higher frequency of dust episodes in its lower extreme when compared to similar latitudinal positions in western and eastern sides of the Basin. A decreasing south to north gradient of African dust contribution to PM10, driven by the latitudinal position of the monitoring sites at least 25 degrees E westwards across the Basin, is patent across the Mediterranean. As a result of this, an experimental equation for the estimation of annual African dust contributions based on the latitudinal position was obtained. From 25 degrees E eastwards, higher annual dust contributions are encountered due to the elevated annual occurrence of severe episodes of dust but also because of inputs from Negev and Middle Eastern deserts. The slightly higher frequency of African dust episodes observed over southern sites in the central Mediterranean Basin is compensated by its moderately lower intensity. Concerning seasonality patterns and intensity characteristics, a clear summer prevalence is observed in the western part, with low occurrence of severe episodes (daily dust averages over 100 mu gm(-3) in PM10); no seasonal trend is detected in the central region, with moderate-intensity episodes; and significantly higher contributions are common in autumnspring in the eastern side, with occurrence of various severe episodes throughout the year. Overall, African dust emerges as the largest PM10 source in regional background southern sites of the Mediterranean (35-50% of PM10), with seasonal peak contributions to PM10 up to 80% of the total mass.The multi-year study of African dust episodes and their contributions to PM10 concentrations reveals a consistent decreasing trend in the period 2006/2007 to 2011 in 4 of the 17 studied regions, all of them located in the NW of the Mediterranean. Such decrease is almost parallel to that of the NAO (North Atlantic Oscillation) index for the summer period, progressively more negative since 2006. Therefore, a sharp change in the atmospheric circulation over the last 5 yr (a similar negative NAO period occurred in the 1950 decade) have affected the number of African dust episodes and consequently the annual dust inputs to PM10 observed in the NW part of the Mediterranean. By investigating mean temperatures and geopotential height maps at 850 hPa it is evident a displacement of warm air masses accomplishing African dust towards the central Mediterranean in the 2007-2008 biennium, and towards the NW African coast and the Canary Islands in the 2009-2011 triennium.