Surface-based nocturnal air temperature inversions in southern Poland and their influence on PM10 and PM2.5 concentrations in Upper Silesia

Abstract

The frequency, strength and seasonal variations of surface-based temperature inversions (SBTIs) in Upper Silesia (Southern Poland) were examined using data from January 2001 to September 2020. Based on the air temperatures recorded at the meteorological station of the Institute of Earth Sciences in Sosnowiec (263 m a.s.l.) at heights of 2 m and 88 m above the ground, the vertical temperature gradient of the 100-m layer (γSos100 m) was determined. A lapse rate of γSos100 m > 0.5 K was defined as a temperature inversion. The measurements for 00 UTC (midnight) and 12 UTC (midday) were compared with data from the upper air station in Wrocław (116 m a.s.l.) located in the Lower Silesia Lowland, approximately 170 km NW of Sosnowiec. Based on soundings from Wrocław, in addition to the temperature gradient in the lower 100-m layer of air (γWrc100), three other characteristics of SBTIs were calculated: inversion depth (ID) or thickness in metres, inversion strength (ΔTi) in K and vertical temperature gradients across the whole SBTI layer γi in K 100 m−1. On an annual basis, the frequency of nighttime SBTIs (γ > 0.5 K 100 m−1) ranged from 47% in Sosnowiec to almost 59% in Wrocław. At both stations, the fewest SBTIs occurred in winter (23–38%) and the most in summer (64–75%). Moreover, they were more frequent in spring (52–61%) than in autumn (49–59%). The SBTI frequency was very low during the midday hours, amounting to 0.6–0.7% days a year, and it increased to 1–2% only in winter. Annually, the depth of 81% of inversions ranged between 50 and 300 m, varying seasonally from almost 67% in winter to 87% in summer. The presented research shows that SBTIs in winter were among the main factors contributing to a high concentration of particulate matter pollutants in the ground-level atmosphere. During nights with temperature inversions, the annual mean PM10 concentration reached 125% of the mean value, ranging from 114% in summer to 189% in winter.

Relationships between Vertical Temperature Gradients and PM10 Concentrations during Selected Weather Conditions in Upper Silesia (Southern Poland)

“…Based on the average hourly air temperature (AT) data, the vertical gradients determining the state of the atmosphere were calculated as the differences between the AT measured at 2 m (AT2) and 88 m (AT88) above ground level. Next, the differences were converted to the standard meteorological gradient defined as the change per 100 m according to formula 1 [47]. Positive ATGs (γ) indicate surface-based temperature inversion (SBI), while negative ATGs represent strong or conditionally unstable conditions, i.e., a vertical decrease in AT below −0.5 K/100 m.…”

“…We adopted the threshold of γ ± 0.5 K for isothermal conditions to minimize measurement errors that might have not been detected [12,47]. Conditional instability depends on wet adiabatic lapse rate in the moist air.…”

“…On the other hand, when vertical gradient is lower than −0.98 K/100 m (dry adiabatic lapse rate), unstable conditions always occur [6]. In this study, as in the previous one [47], we took into account air temperature inversions when the vertical temperature gradient was greater than +0.5 K/100 m.…”

“…Anticyclonic conditions also lead to an increased SBI strength and thickness due to reductions in cloud cover, as can be seen in Greenland [41]. However, in Central Europe most SBI occurs during nights in summer season [42][43][44][45][46][47].…”

“…According to radiosounding in Wrocław (SW Poland) during the period 2001-2020, the average depth of the SBI layer at night is 207 m, changing from 186 m in spring to 249 m in winter. Most often (81%), the SBI thickness ranges from 50 to 300 m [47,60].…”