Estimating the contribution of strong daily export events to total pollutant export from the United States in summer
Estimating the contribution of strong daily export events to total pollutant export from the United States in summer
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估计夏季强劲的日常出口事件对美国污染物出口总量的贡献
DOI：
10.1029/2008jd010946
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发表时间：
2009
影响因子：
--
通讯作者：
A. Russell
中科院分区：
文献类型：
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作者：
Yuanyuan Fang;A. Fiore;L. Horowitz;A. Gnanadesikan;H. Levy;Yongtao Hu;A. Russell
[1]虽然美国的污染物出口每天都有显著的变化，而且往往被认为是“偶发性的”，但每天大量出口事件对总出口的贡献尚未量化。我们使用一氧化碳（CO）作为示踪剂的人为污染物的模型OZone和相关示踪剂（MOZART）来估计这一贡献。我们首先确定了美国的主要输出途径是通过东北边界（24-48°N沿着67.5°W和80-67.5°W沿着48°N），然后分析了15个夏季通过该边界的每日CO输出通量。这些每日CO输出通量具有近似高斯分布，平均值为1100 Gg CO day−1，标准差为490 Gg CO day−1。为了关注天气变化，我们定义“天气背景”输出通量等于15天移动平均输出通量，并根据相对于该背景的通量对强输出日进行分类。正如高斯统计所预期的那样，16%的夏季天数是“强输出日”，即CO输出通量超过天气背景一个标准差或更多的天数。强出口天数占总出口的25%，该值由CO通量分布的相对标准差确定。回归异常的CO出口通量通过美国东北部边界相对于天气背景的每日异常的表面压力场（也相对于15天的运行平均值）表明，强大的每日出口通量与通道的中纬度气旋在圣劳伦斯湾。相关的气旋环流和温暖的输送带（WCBs），解除美国东北部的表面污染物已被证明与远程传输事件。从2004年INTEX-NA现场活动的观测结果比较证实，我们的模型捕捉到所观察到的增强CO流出，并解决了强出口日与气旋通道的过程。“中等输出天数”，定义为通过东北边界的CO通量超过15天运行平均值小于一个标准差的天数，代表额外的34%的夏季天数和40%的总输出。这些日子也与迁移中纬度气旋有关。其余35%的总输出发生在“弱输出日”（50%的夏季），此时圣劳伦斯湾上空出现高压异常。我们对夏季的研究结果也适用于春季，此时美国污染物出口通常最强，在强、中、弱出口日对总出口和相关气象的贡献相似。虽然气旋通道是每日强输出事件的主要驱动力，但在没有气旋通道的日子里的输出也对总输出做出了相当大的贡献，从而对全球污染物预算做出了贡献。
[1] While the export of pollutants from the United States exhibits notable variability from day to day and is often considered to be “episodic,” the contribution of strong daily export events to total export has not been quantified. We use carbon monoxide (CO) as a tracer of anthropogenic pollutants in the Model of OZone And Related Tracers (MOZART) to estimate this contribution. We first identify the major export pathway from the United States to be through the northeast boundary (24–48°N along 67.5°W and 80–67.5°W along 48°N), and then analyze 15 summers of daily CO export fluxes through this boundary. These daily CO export fluxes have a nearly Gaussian distribution with a mean of 1100 Gg CO day−1 and a standard deviation of 490 Gg CO day−1. To focus on the synoptic variability, we define a “synoptic background” export flux equal to the 15 day moving average export flux and classify strong export days according to their fluxes relative to this background. As expected from Gaussian statistics, 16% of summer days are “strong export days,” classified as those days when the CO export flux exceeds the synoptic background by one standard deviation or more. Strong export days contributes 25% to the total export, a value determined by the relative standard deviation of the CO flux distribution. Regressing the anomalies of the CO export flux through the northeast U.S. boundary relative to the synoptic background on the daily anomalies in the surface pressure field (also relative to a 15 day running mean) suggests that strong daily export fluxes are correlated with passages of midlatitude cyclones over the Gulf of Saint Lawrence. The associated cyclonic circulation and Warm Conveyor Belts (WCBs) that lift surface pollutants over the northeastern United States have been shown previously to be associated with long-range transport events. Comparison with observations from the 2004 INTEX-NA field campaign confirms that our model captures the observed enhancements in CO outflow and resolves the processes associated with cyclone passages on strong export days. “Moderate export days,” defined as days when the CO flux through the northeast boundary exceeds the 15 day running mean by less than one standard deviation, represent an additional 34% of summer days and 40% of total export. These days are also associated with migratory midlatitude cyclones. The remaining 35% of total export occurs on “weak export days” (50% of summer days) when high pressure anomalies occur over the Gulf of Saint Lawrence. Our findings for summer also apply to spring, when the U.S. pollutant export is typically strongest, with similar contributions to total export and associated meteorology on strong, moderate and weak export days. Although cyclone passages are the primary driver for strong daily export events, export during days without cyclone passages also makes a considerable contribution to the total export and thereby to the global pollutant budget.