Measurement report : effect of wind shear on $PM_{10}$ concentration vertical structure in the urban boundary layer in a complex terrain

The paper shows wind shear impact on $PM_{10}$ vertical profiles in Kraków, southern Poland. The data used consist of background data for two cold seasons (September 2018 to April 2019 and September 2019 to April 2020) and data for several case studies from November 2019 to March 2020. The data are composed of $PM_{10}$ measurements, model data, and wind speed and direction data. The background model data come from operational forecast results of the AROME model. $PM_{10}$ concentration in the vertical profile was measured with a sightseeing balloon. Significant spatial variability of the wind field was found. The case studies represent the conditions with much lower wind speed and a much higher $PM_{10}$ level than the seasonal average. The inversions were much more frequent than on average too. Wind shear turned out to be the important factor in terms of $PM_{10}$ vertical profile modification. It is generated due to the relief impact, i.e. the presence of a large valley, blocked on one side with the hills. The analysis of $PM_{10}$ profiles from all flights allows us to distinguish three vertical zones of potential air pollution hazards within the valley (about 100 m deep) and the city of Kraków: (1) up to about 60 m a.g.l. - the zone where during periods of low wind speed, air pollution is potentially the highest and the duration of such high levels is the longest, i.e. the zone with the worst aerosanitary conditions; (2) about 60-100 m a.g.l. - transitional zone where the large decrease in $PM_{10}$ levels with height is observed; (3) above 100-120 m a.g.l. - the zone where air quality is significantly better than in zone 1, either due to the increase in the wind speed or due to the wind direction change and advection of different, clean air masses.