Temperature and Relative Humidity Vertical Profiles within Planetary Boundary Layer in Winter Urban Airshed
Abstract
The planetary boundary layer is a dynamic system with turbulent flow where air mixing depends mainly on the weather conditions and geomorphology. Temperature inversion situation may occur, mainly during winter.
Pollutant dispersion is poor during inversions so air pollutant concentration can quickly rise, especially in urban closed valleys. Air pollution was evaluated by WHO as a human carcinogen mostly by polycyclic aromatic hydrocarbons.
Knowledge about inversion layer height is important for estimation of the pollution impact and it can give us also information about the air pollution sources. The Svermov, residential district of the city of Kladno is an air pollution hot-spot for long time benzo[a]pyrene (B[a]P) limit exceedances, reaching the highest B[a]P annual concentration in Bohemia - west part of the Czech Republic.
Since the Svermov sits in a shallow valley, frequent vertical temperature inversion in winter and low emission heights of pollution sources prevent pollutant dispersal off the valley. Such orography is common to numerous small settlements in the Czech Republic.
Ground measurements in the Svermov were complemented by temperature and humidity vertical profiles acquired by a Vaisala radiosonde positioned at tethered He-filled balloon. Total number of 53 series of vertical profiles up to the height of 300 m was conducted.
Meteorology parameters were acquired with 4 Hz frequency. The measurements confirmed frequent early-morning and night formation of temperature inversion within boundary layer up to the height of 50 m.
This rather shallow inversion had significant influence on air quality due to inversion cap over the valley. Nevertheless, formation of an inversion showed strong diurnal variability.
For example, on the 18th Feb. early morning shallow inversion quickly disappeared within less than 2 hours. According to this study tethered balloon measurements has proved to be a good tool for completion comprehensive ground air quality measurements.