Abstract The diurnal evolution of the planetary boundary layer (PBL) is crucial to air quality studies as it impacts the exchange and distribution of pollutants close to the surface. This paper reports continuous detection of the daytime convective boundary layer height, the stable boundary layer height, and the residual layer height as estimated from the vertical profiles of virtual potential temperature, and moisture retrieved from a microwave radiometer (MWR) in Mexico City for the period 6–18 March 2016. This period included a severe smog episode. We analyzed the anatomy of this episode utilizing continuous air quality measurements recently deployed at elevated locations surrounding the basin of Mexico City, which were used to determine the impact of the background or residual pollutants during the severe smog episode in combination with back trajectory analysis and radar wind profiles data to track transport processes within the Mexico City basin. The first few days prior to the smog episode were impacted by the passage of a deep upper tropospheric trough and strong advection. Shortly before the smog episode, daytime maximum PBL height still reached ~2.5 km above ground level but then dropped to ~1.2–1.7 km above ground level for the most severe pollution days. During the first days with strong advection, the pollutant concentrations were flushed out from the basin and/or could not accumulate (maximum hourly ozone and carbon monoxide mixing ratios of ~50 ppbv and ~0.5 ppmv, respectively). At the departure of the storm, the winds became weaker, and a strong near surface temperature inversion was observed at nighttime increasing the nighttime mixing ratio of carbon monoxide to ~2.5 ppmv and daytime ozone mixing ratio to ~200 ppbv, which resulted in one of the most severe smog episodes in Mexico City over the last decade. Our results point to strong photochemical processes confined to the PBL within the Mexico City basin, whose maximum daytime convective boundary layer heights hardly surpassed the surrounding average mountain top heights.

中文翻译：

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2016 年 3 月墨西哥城严重臭氧烟雾事件的剖析

摘要 行星边界层 (PBL) 的日变化对空气质量研究至关重要，因为它会影响靠近地表的污染物的交换和分布。本文报告了连续检测白天对流边界层高度、稳定边界层高度和残余层高度的情况，这些高度是根据虚拟位温垂直剖面估计的，以及从墨西哥城的微波辐射计 (MWR) 中获取的水分。 2016 年 3 月 6 日至 18 日。这一时期包括严重的烟雾事件。我们利用最近在墨西哥城盆地周围高处部署的连续空气质量测量分析了这一事件的解剖结构，用于确定严重烟雾事件期间背景或残留污染物的影响，并结合反向轨迹分析和雷达风廓线数据来跟踪墨西哥城盆地内的运输过程。烟雾事件发生前的前几天受到对流层上层深槽和强平流的影响。在烟雾事件发生前不久，白天最大 PBL 高度仍达到地面以上约 2.5 公里，但在污染最严重的日子里降至地面以上约 1.2-1.7 公里。在强平流的前几天，污染物浓度从盆地中冲出和/或无法积累（最大每小时臭氧和一氧化碳混合比分别为~50 ppbv 和~0.5 ppmv）。风暴离开时，风变弱了，在夜间观察到强烈的近地表温度反转，夜间一氧化碳的混合比增加到~2.5 ppmv，白天臭氧混合比增加到~200 ppbv，这导致墨西哥城在过去十年中最严重的烟雾事件之一. 我们的结果表明强烈的光化学过程局限于墨西哥城盆地内的 PBL，其最大白天对流边界层高度几乎没有超过周围平均山顶高度。