Comparison of the optical depth of total ozone and atmospheric aerosols in Poprad-Gánovce , Slovakia

The amount of ultraviolet solar radiation reaching the Earth’s surface is significantly affected by atmospheric ozone along with aerosols. The present paper is focused on a comparison of the total ozone and atmospheric aerosol optical depth in the area of Poprad-Gánovce, which is situated at the altitude of 706 ma.s.l. in the vicinity of the highest mountain in the Carpathian mountains. The direct solar ultraviolet radiation has been measured here continuously since August 1993 using a Brewer MKIV ozone spectrophotometer. These measurements have been used to calculate the total amount of atmospheric ozone and, subsequently, its optical depth. They have also been used to determine the atmospheric aerosol optical depth (AOD) using the Langley plot method. Results obtained by this method were verified by means of comparison with a method that is part of the Brewer operating software, as well as with measurements made by a Cimel sun photometer. Diffuse radiation, the stray-light effect and polarization corrections were applied to calculate the AOD using the Langley plot method. In this paper, two factors that substantially attenuate the flow of direct ultraviolet solar radiation to the Earth’s surface are compared. The paper presents results for 23 years of measurements, namely from 1994 to 2016. Values of optical depth were determined for the wavelengths of 306.3, 310, 313.5, 316.8 and 320 nm. A statistically significant decrease in the total optical depth of the atmosphere was observed with all examined wavelengths. Its root cause is the statistically significant decline in the optical depth of aerosols.


Instrument
Brewer ozone spectrophotometer is placed on the roof of Upper Air and Radiation Centre of Slovak hydrometeorological institute in Gánovce near town of Poprad. Its coordinates are 49.03°N. and 20.32°E. and the altitude is 706 m. n. m. The content of aerosols in the atmosphere, either the total amount or the species composition is determinated by the local sources and also by the atmospheric circulation, which can relocate the air mass together with the aerosols several thousand kilometres from its source. In rare cases, there may be for example the transport of Saharan dust. Among the major local resources include products of combustion of solid fuel in the surrounding villages and agriculture. Often is occurring the wind erosion of dry soil or the occurrence of plant products in the air because the region is quite windy. The proximity of the town of Poprad (about 1.5 kilometres) with about 53,000 inhabitants and various industrial activities also play a role.
Brewer ozone spectrophotometer (Model MKIV) works in the ultraviolet and visible range of the solar spectrum. Measurements of direct UV sun radiation is carried out at selected wavelengths. Based on the varying absorption of radiation after passing through the atmosphere, it is possible to derive the total amount of O3 and SO2. This measuring principle is known as the differential optical absorption spectroscopy (DOAS). Measurements of direct sunlight for wavelengths 306,3nm, 310.1 nm, 313.5 nm, 316.8 nm and 320.1 nm are also used to calculate the AOD.
Since the beginning of measurements (18.8.1993) device undergoing regular 2-year calibration and daily tests using internal lamps (mercury and standard lamp). The instrument is calibrated according to the global reference group (Brewer Triad), maintained at Environment Canada, through travel reference instrument n. 097. From a technical point of view measurements can be considered as homogeneous.

Method Cloud screening Basic equation
Beer-Bouguer-Lambert law:

Introduction
As it is widely known, anthropogenic changes in total ozone and atmospheric aerosols significantly affect the sun's UV radiation hitting the Earth surface. Depletion of the global ozone layer began gradually in the 1980s and reached a maximum of about 5% in the early 1990s. Nowadays the depletion has lessened and currently is about 3.5% averaged over the globe. Reduction of the ozone layer subsequently led to an increased amount of UV radiation released through the atmosphere. On the other hand, in the early 90s it was found that in non-urban areas of industrialized countries, solar UV -B radiation decreased since the industrial revolution by about 5-18% as a result of air pollution. In developed countries slowly occur the limitation of aerosols emissions and for some locations is observed a decrease of aerosols optical depth (AOD). This study shows the impact of the mentioned two factors to the attenuation of UV radiation for Gánovce, which is located near the High Tatras. Optical depth of ozone stagnates in the last 22 years, while on the other hand aerosols optical depth recorded a decline. Determining of AOD by Brewer spectrophotometer is not united. This study represents one of possible approach to determining of AOD.