Long‐term and seasonal variability of the aerosol optical depth at Mount Kasprowy Wierch (Poland)

Abstract

This paper presents the results of long-term observations of direct solar radiation, to determine aerosol optical depth (AOD), made with a Linke-Feussner actinometer at the Tatra Mountain Meteorological Observatory on Mount Kasprowy Wierch (1991 m above sea level, 49.233°N, 19.982°E). To this end, broadband direct solar flux (0.29-2.9 μm) and wideband solar radiation measured with OG530 and RG630 filters are used to estimate the broadband and wideband (0.53-0.63 μm) AOD. The inversion algorithm used is based on the MODTRAN (MODerate resolution atmospheric TRANsmission) radiative transfer model applied to estimate direct flux for aerosol-free atmosphere. Total water vapor content, which accounts for the largest extinction of clear-sky direct flux, was obtained by radio sounding from the Poprad-Ganovce station (33 km from Mount Kasprowy Wierch) and from water vapor pressure measurements at the Observatory. The almost 900 clear-sky observations, performed close to noon time, found a significant long-term reduction of AOD. AOD decadal trends were À0.006 (À8 ± 4% [2σ]) with a 95% confidence interval of ± 0.003 and À0.014 (À13 ± 4% [2σ]) with a 95% confidence interval of ± 0.004 for broadband and wideband, respectively. Similar trends, but for years with negligible contamination of volcanic aerosol, are À0.012 (À16 ± 6% [2σ]) and À0.018 (À17 ± 6% [2σ]) with a 95% confidence interval of ± 0.003 and ± 0.004. However, positive AOD trends (from 0 to 0.04 per decade) were found between 1964 and 1983 and negative AOD trends (from À0.016 to À0.035 per decade) were found between 1984 and 2003. Changes of the AOD trends between both periods are associated with global dimming and brightening phenomenon, which took place in the second half of the twentieth century and at the beginning of the 21st century. The long-term mean broadband and wideband AOD were 0.07 ± 0.01 and 0.11 ± 0.02, respectively. Both quantities show a significant annual cycle, with maximum AOD during spring and summer. Very low values during the autumn and winter (about 0.03 for broadband and 0.05-0.06 for wideband) are the result of planetary boundary layer height (defined for the lower elevations), which during this period lies below Mount Kasprowy Wierch itself. The other explanation of low AOD values can be the high frequency (more than 50%) of inversion occurrence below the top of the mountain.

Trends in sunshine duration in Poland (1971–2018)

“…In Poland, the growing trend in SD since the early 1980s can be explained not only by changes in cloud cover but also by a decrease in the amount of aerosols in the atmosphere, the highest rate of which occurred in the years 1992–1997 (Figure 10). This is confirmed by the results of studies of other authors who analysed changes in atmosphere transparency using ground data (Weller and Gericke, 2005; Ruckstuhl et al ., 2008; Ohvril et al ., 2009; Markowicz and Uscka‐Kowalkowska, 2015). The period of strong industrialization, especially in south‐west Poland (Lower and Upper Silesia, Krakow agglomeration) contributed to high concentrations of air pollution that are visible in the first half of the 1980s (Figure 11a).…”

Decadal changes in radiative fluxes at land and ocean surfaces and their relevance for global warming

“…However, on a longer period of 30 years (1980 to 2010) Nabat et al suggested that aerosol changes can explain around 80% of the brightening over Europe, based on simulations with a regional aerosol climate model driven with reanalysis data, with the direct aerosol effect dominating the magnitude of the simulated brightening. From long‐term records of direct solar radiation at a high altitude site in the Tatra Mountains (Poland) away from local sources, increases in AOD from 1964 to 1983 and subsequent declines from 1984–2003 were inferred, that fit to the SSR dimming and brightening, respectively . Dimming and brightening has also been consistently detected at the Izana station on the Canary Islands both under all sky and clear sky conditions, indicative of aerosol as a major contributor .…”

Long‐Term Variability of Aerosol Optical Depth in the Tatra Mountain Region of Central Europe

“…BAOD is estimated on the basis of broadband direct shortwave radiation measurements according to Markowicz and Uscka-Kowalkowska (2015) methodology. BAOD is defined by means of the following equation (Qiu, 2001):…”

“…Mean absolute errors change from ±0.01 to ±0.03 for WAOD and from ±0.01 to ±0.04 for BAOD. As part of the BAOD conversion to an air mass of 2, the AOD at 500 nm (τ 500 ) is estimated as follows: (1) simulation of aerosol-free spectral flux based on MODTRAN, (2) estimation of the Angstrom exponent from long-term mean at Belsk AERONET station (see Markowicz & Uscka-Kowalkowska, 2015), (3) computation of spectral aerosol transmission…”