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Land cover change impacts on atmospheric chemistry: simulating projected large-scale tree mortality in the United States

机译:土地覆盖变化对大气化学的影响:模拟美国预计的大规模树木死亡

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Land use and land cover changes impact climate and air quality by altering the exchange of trace gases between the Earth's surface and atmosphere. Large-scale tree mortality that is projected to occur across the United States as a result of insect and disease may therefore have unexplored consequences for tropospheric chemistry. We develop a land use module for the GEOS-Chem global chemical transport model to facilitate simulations involving changes to the land surface, and to improve consistency across land–atmosphere exchange processes. The model is used to test the impact of projected national-scale tree mortality risk through 2027 estimated by the 2012 USDA Forest Service National Insect and Disease Risk Assessment. Changes in biogenic emissions alone decrease monthly mean Osub3/sub by up to 0.4?ppb, but reductions in deposition velocity compensate or exceed the effects of emissions yielding a net increase in Osub3/sub of more than 1?ppb in some areas. The Osub3/sub response to the projected change in emissions is affected by the ratio of baseline NOsubix/i/sub?:?VOC concentrations, suggesting that in addition to the degree of land cover change, tree mortality impacts depend on whether a region is NOsubix/i/sub-limited or NOsubix/i/sub-saturated. Consequently, air quality (as diagnosed by the number of days that 8?h average Osub3/sub exceeds 70?ppb) improves in polluted environments where changes in emissions are more important than changes to dry deposition, but worsens in clean environments where changes to dry deposition are the more important term. The influence of changes in dry deposition demonstrated here underscores the need to evaluate treatments of this physical process in models. Biogenic secondary organic aerosol loadings are significantly affected across the US, decreasing by 5–10?% across many regions, and by more than 25?% locally. Tree mortality could therefore impact background aerosol loadings by between 0.5 and 2?μg?msup?3/sup. Changes to reactive nitrogen oxide abundance and partitioning are also locally important. The regional effects simulated here are similar in magnitude to other scenarios that consider future biofuel cropping or natural succession, further demonstrating that biosphere–atmosphere exchange should be considered when predicting future air quality and climate. We point to important uncertainties and further development that should be addressed for a more robust understanding of land cover change feedbacks.
机译:土地利用和土地覆盖的变化通过改变地球表面与大气之间的微量气体交换,影响了气候和空气质量。因此,预计在美国范围内由于昆虫和疾病而导致的大规模树木死亡可能对流层化学产生无法探索的后果。我们为GEOS-Chem全球化学品运输模型开发了一个土地使用模块,以促进涉及土地表面变化的模拟,并提高陆地-大气交换过程的一致性。该模型用于测试由2012年美国农业部森林服务局国家昆虫和疾病风险评估所估计的预计到2027年的全国范围树木死亡风险的影响。仅生物排放量的变化就使每月平均O 3 降低高达0.4?ppb,但沉积速度的降低补偿或超过排放的影响,从而使O 3 净增加在某些地区超过1ppb。 O 3 对排放预计变化的响应受基准NO x ?:?VOC浓度之比的影响,这表明在土地覆盖变化的程度上,树木死亡率的影响取决于某个区域是NO x 限制区域还是NO x sub>-饱和。因此,空气质量(通过平均8?h O 3 超过70?ppb的天数诊断)在受污染的环境中有所改善,在此环境中排放的变化比干沉积的变化更为重要,但恶化在清洁环境中,干沉积的变化更为重要。此处展示的干沉降变化的影响强调了需要在模型中评估该物理过程的处理方法。在美国,生物二次有机气溶胶的载量受到很大影响,在许多地区下降了5-10%,在本地下降了25%以上。因此,树木死亡率可能对背景气溶胶负荷造成0.5至2?μg?m ?3 的影响。反应性氮氧化物丰度和分配的变化在当地也很重要。此处模拟的区域影响在规模上与考虑未来生物燃料种植或自然演替的其他情景相似,进一步表明在预测未来的空气质量和气候时应考虑生物圈-大气交换。我们指出了重要的不确定性和进一步的发展,应该对这些不确定性和进一步的发展进行更深入的了解,以更好地了解土地覆被变化反馈。

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