Lower tropospheric thermal structure greatly affects atmospheric boundary-layer (ABL) stability and mixing processes with the free troposphere. In particular, in polluted urban zones, ABL stratification becomes a key variable in air quality research. This study focuses on generating a climatology (1990–2017) of the seasonal variability of ABL thermal structure in Mexico City by way of radiosonde analysis. Thermal inversion intensity and frequency are shown to be greater during winter and spring, a behaviour which coincides with greater pollutant concentrations. Higher concentrations are found during the dry season (November to May) than during the rainy months. In addition, significantly higher than normal surface pollutant concentrations are found on days with simple thermal inversion layers as well as during multilayer inversion days. Furthermore, stable layers, determined by potential temperature, are found throughout the year but more frequently during winter, whereas stable layers based on the virtual potential temperature prevail all year. In regions of complex terrain, such multiple stable layers have also been identified by previous authors. Additionally, the most unstable surface layers (in which the bulk Richardson number (\({Ri}_{\mathrm {B}}\)) is small) develop during the rainy season, whereas during winter there are more levels in the vertical column with higher \({Ri}_{\mathrm {B}}\) values. Although the Mexico City ABL and pollution episodes have been widely studied, this represents the first long-term investigation to consider the thermal stability of the ABL. Therefore, the present study provides a baseline for further research employing different observational techniques and high-resolution numerical models.

对流层下部的热结构极大地影响了大气边界层（ABL）的稳定性以及与自由对流层的混合过程。特别是在污染严重的城市地区，ABL分层已成为空气质量研究的关键变量。这项研究的重点是通过无线电探空仪分析生成墨西哥城ABL热结构的季节变化的气候学（1990-2017）。在冬季和春季，热反演强度和频率显示较大，这种行为与较高的污染物浓度相吻合。在干旱季节（11月至5月）发现的浓度高于雨季。另外，在具有简单热反演层的日子以及多层反演的日子里，发现明显高于正常的表面污染物浓度。此外，全年都会发现由潜在温度决定的稳定层，但冬季更为频繁，而全年都基于虚拟潜在温度的稳定层占上风。在复杂地形的区域中，以前的作者也已经确定了这样的多个稳定层。此外，最不稳定的表面层（其中大的Richardson数（\（{Ri} _ {\ mathrm {B}} \））很小，在雨季发展，而在冬季，垂直列中的水平更高，\（{Ri} _ {\ mathrm {B} } \）值。尽管已经对墨西哥城的ABL和污染事件进行了广泛的研究，但这代表了考虑ABL的热稳定性的第一个长期研究。因此，本研究为采用不同观测技术和高分辨率数值模型的进一步研究提供了基线。