The Role of Mountain-Valley Breeze on PM10 Concentrations with the Sudetes, Poland as an Example

INTRODUCTION AND DATA

The observed concentrations of air pollution in mountainous terrain are influenced by a series of factors like emission temporal and spatial structure, thermodynamic stability of atmosphere or direction and intensity of synoptic-scale circulation. A complex land relief is perceived chiefly as an obstacle for air flow contributing to deep thermal inversion episodes with accumulation of air pollutants trapped in concave landforms. However, in a complex terrain there is also a tendency for local diurnal-nocturnal air circulation to be formed. The diurnal valley breeze is directed along a valley axis upward, while the nocturnal breeze is reversed i.e. it blows down a valley. Thus, the aim of this study is to check if such local air circulation has an impact on PM10 concentration in the mountainous part of the Lower Silesia region in the south-west of Poland.

The present analysis is based on continuous PM10 hourly measurements made by the State Institute of Environmental Protection between January 2016 and March 2017.

RESULTS AND CONCLUSIONS

Data from five Lower-Silesian monitoring sites were taken into account: Dzierzoniow, Jelenia Gora, Klodzko, Nowa Ruda and Walbrzych. Three datasets have been analyzed: [1] all hourly data gathered during the 15 month time period; [2] hourly data measured during 8 months of the cold period i.e. January-March 2016 and November 2016-March 2017; [3] hourly data from 56 days belonging to the former time span and simultaneously characterized by strong thermodynamic stability of the Atmospheric Boundary Layer with the environmental lapse rate smaller than 0.3 K per 100 meters. The achieved results were then compared with data from the neighboring regions i.e. the Carpathians and the Upper Silesia in the southern part of Poland.

In each of the mentioned datasets this is Walbrzych, the largest of the analyzed towns in terms of population, with the cleanest air with average concentrations of PM10 respectively 31,8 [1], 34,8 [2] and 66,0 mg*m^-3 [3]. In case of Walbrzych the unique feature of the diurnal cycle of PM10 concentrations was the presence of the primary minimum in the late morning while all other stations had an afternoon minimum, which takes place when Atmospheric Boundary Layer is least stable.

Thorough analysis of local position of Walbrzych monitoring station leads to the conclusion that this site has best conditions for development of mountain-valley breeze – it is located in a short valley with significant inclination of the valley axis. That’s why this site supposedly experiences quite vigorous gravity-driven nocturnal mountain breeze, which reduces PM10 concentrations. In the second part of night this effect coincides with lower rate of particulates emission form low-level sources within residential areas. Particularly distinct effects are visible if the upper part of valley (in relation to a given monitoring site) is not densely populated. During the afternoon, when daily minimum concentration is experienced at the remaining stations, Walbrzych is under influence of diurnal valley breeze which transports more polluted air from lower parts of the valley. As the result PM10 concentrations in the afternoon are higher than in the morning being comparable or even higher than at other stations.

Similar conditions like in Walbrzych are observed at three monitoring sites in the Carpathians: Bielsko Biala, Ustron and Zakopane. All these stations are located at sites with favourable conditions for mountain-valley breeze circulation.

The achieved results let us state that gravity driven mountain-valley circulation is an important factor improving air quality at specific sites in a mountainous terrain. Taking this factor into account helps to select potentially best locations for new residential areas or for human activities that require good air quality.