Aerosol Distribution in The Planetary Boundary Layer Aloft a Residential Area

Abstract

Atmospheric aerosol is an omnipresent component of the Earth atmosphere. Aerosol particle of diameters < 100 nm or > 1 μm defines ultrafine or coarse aerosol particles, respectively. Aerosol particle concentrations within the planetary boundary layer - PBL are measured at the ground level while their vertical profiles in the PBL are usually estimated by modelling. The aim of this study was to construct vertical concentration profiles of ultrafine and coarse aerosol particles from airborne and ground measurements conducted in an urban airshed. Airborne measurements were done by an unmanned airship, remotely controlled with GPS 10 Hz position tracking, and electrically powered with propulsion vectoring, which allows average cruising speed of 6 m.s-1. The airship carried three aerosol monitors and a temperature sensor. The monitors acquired 1 Hz data on mass concentration of coarse and number concentration of ultrafine particles. Four flight sequences were conducted on the 2nd of March 2014 above Plesna village, up-wind suburb of Ostrava in the Moravian-Silesian region of the Czech Republic. The region is a European air pollution hot-spot. Repeated flights were carried out in several height levels up to 570 m above ground level - a.g.l. Early morning flight revealed a temperature inversion in the PBL up to 70 m a.g.l. This lead to coarse particle concentrations of 50 μgm-3 below the inversion layer and 10 μgm-3 above it. Concurrently, air masses at 90-120 m a.g.l. were enriched with ultrafine particles up to 2.5x104 cm-3, which may indicate a fanning plume from a distant emission source with high emission height. During the course of the day, concentrations of ultrafine and coarse particle gradually decreased. Nevertheless, a sudden increase of ultrafine particle concentrations up to 3.7x104 cm-3 was registered at 400 m a.g.l. at noon and also after a lag of 20 min at the ground. This may indicate formation of new aerosol particles at higher altitudes, which are then transported downward by evolved convective mixing. Detailed information acquired by the airship measurements allow us to better understand processes resulting in the increase of aerosol particle concentrations at ground level in urban air.