Measurements of submicron particles vertical profiles by means of topographic relief in a typical valley city, China

To reveal PM1 vertical profiles and key affecting factors in a typical urban valley, daytime and nighttime PM1 (the particles with diameters less than 1 μm) samples were collected with medium volume air samplers during 26 December 2017 to 11 January 2018 at five different altitudes by means of high topographic relief at urban areas of Lanzhou. The synchronous boundary layer temperature and humidity profiles were observed by a microwave radiometer. Daytime PM1 concentrations reduced by about 3.86 μg m−3 when the height above the surface increased by 100 m, which was much lower than that for nighttime (5.68 μg m−3 100 m−1) as particles were easily accumulated near the surface when the air was stable during the nighttime. The three typical PM1 vertical profiles were identified by K-means clustering technique. The most frequent cluster with elevated PM1 concentrations near the surface was closely related to temperature inversion around the ground, while the cluster with relatively uniform PM1 within the boundary layer was mainly induced by unstable atmospheric stratification and thus relatively good vertical dispersion. About 50%–60% of PM1 variations could be attributed to atmospheric stratification near the surface in the valley city, which was much higher than that at the hilltop. The PM1 difference increased by 47.14 (36.91) μg m−3 when inversion layer thickness (intensity) increased by 100 m (1 °C 100 m−1). The newly calculated inversion index considering both inversion layer thickness and intensity explained about 87% of PM1 differences between near the surface and at the hilltop. The vertical dispersion had a more significant effect on PM1 than horizontal dispersion near the surface, while PM1 was more largely affected by horizontal dispersion at the hilltop, which was closely related to the valley terrain.