Salmon Olivia E, Shepson Paul B, Ren Xinrong, Marquardt Collow Allison B, Miller Mark A, Carlton Annmarie G, Cambaliza Maria O L, Heimburger Alexie, Morgan Kristan L, Fuentes Jose D, Stirm Brian H, Grundman Robert, Dickerson Russell R
Department of Chemistry, Purdue University, West Lafayette, Indiana, USA.
Department of Earth, Atmospheric, and Planetary Sciences and Purdue Climate Change Research Center, Purdue University, West Lafayette, Indiana, USA.
J Geophys Res Atmos. 2017 Sep 16;122(17):9467-9484. doi: 10.1002/2016JD026074. Epub 2017 Sep 4.
Elevated water vapor (HO) mole fractions were occassionally observed downwind of Indianapolis, IN, and the Washington, D.C.-Baltimore, MD, area during airborne mass balance experiments conducted during winter months between 2012 and 2015. On days when an urban HO excess signal was observed, HO emissions estimates range between 1.6 × 10 and 1.7 × 10 kg s, and account for up to 8.4% of the total (background + urban excess) advected flow of atmospheric boundary layer HO from the urban study sites. Estimates of HO emissions from combustion sources and electricity generation facility cooling towers are 1-2 orders of magnitude smaller than the urban HO emission rates estimated from observations. Instances of urban HO enhancement could be a result of differences in snowmelt and evaporation rates within the urban area, due in part to larger wintertime anthropogenic heat flux and land cover differences, relative to surrounding rural areas. More study is needed to understand why the urban HO excess signal is observed on some days, and not others. Radiative transfer modeling indicates that the observed urban enhancements in HO and other greenhouse gas mole fractions contribute only 0.1°C day to the urban heat island at the surface. This integrated warming through the boundary layer is offset by longwave cooling by HO at the top of the boundary layer. While the radiative impacts of urban HO emissions do not meaningfully influence urban heat island intensity, urban HO emissions may have the potential to alter downwind aerosol and cloud properties.
在2012年至2015年冬季进行的机载质量平衡实验中，偶尔会在印第安纳州印第安纳波利斯市以及华盛顿特区 - 马里兰州巴尔的摩市地区的下风处观测到水汽（HO）摩尔分数升高。在观测到城市HO过量信号的日子里，HO排放估计值在1.6×10至1.7×10千克/秒之间，占城市研究站点大气边界层HO总（背景 + 城市过量）平流流量的8.4%。燃烧源和发电设施冷却塔的HO排放估计值比根据观测估计的城市HO排放率小1 - 2个数量级。城市HO增强的情况可能是城市区域内融雪和蒸发速率差异的结果，部分原因是相对于周围农村地区，冬季人为热通量更大以及土地覆盖存在差异。需要更多研究来理解为何在某些日子能观测到城市HO过量信号，而在其他日子却观测不到。辐射传输模型表明，观测到的城市中HO和其他温室气体摩尔分数的增强对地表城市热岛效应的贡献仅为0.1°C·天。这种通过边界层的综合变暖被边界层顶部HO的长波冷却所抵消。虽然城市HO排放的辐射影响对城市热岛强度没有显著影响，但城市HO排放可能有改变下风处气溶胶和云特性的潜力。
