Ultraviolet transmissions measurements along a horizontal near ground path in Nordic environment

Abstract

Results are reported from a study of propagation near-ground within the solar blind ultraviolet wavelength range, including measurements of ultraviolet transmission along a horizontal path. In addition a discussion is presented about ozone data measured at 41 stations spread across Sweden, collected and stored by the Swedish Meteorological and Hydrological Institute (SMHI) and the Swedish Environmental Protection Agency (SEPA). In connection with this a simple statistical analysis of occurrence of the ground ozone is done. The transmission measurements were made along slightly inclined path of 1830 m during the period from mid- November 2018 to mid-January 2019. The transmission values were collected together with weather data including measurement of ozone content in the atmosphere. A simple model for ultraviolet transmission that takes into account both visibility and measured ozone content has been developed. This model shows a difference to the model found in the Modtran program, often used to predict atmospheric transmissions in several wavelength bands including ultraviolet wavelengths. Overall, the new model shows a weaker connection between ultraviolet extinction, ozone content and visibility compared to Modtran. Data from the countrywide ozone measurements as well as data collected in connection with transmission measurement showed variations in the atmospheric ground-level ozone content that are not easily explained from weather parameters alone. Reports, see for instance [2], state that it is volatile hydrocarbons and other molecules, both natural and anthropogenic, along with solar radiation that influence the amount of ozone. The variation of both volatile hydrocarbons and solar radiation together with a variation in wind direction and speed have an impact of varying transmission. In practice all measured ozone values during conditions with low relative humidity were relatively high. There was also a high correlation between high ozone values and high wind velocities. The relation between ultraviolet extinction deduced from measurements and the corresponding values for the visual wavelength range and corresponding values calculated using Modtran show different relationships. The transmission measurements in the ultraviolet wavelengths range showed that the amount of aerosols has no major significance for the ultraviolet extinction at visual ranges above 15 km. Due to the short period of data collection, mostly fall-like weather conditions, the conclusions that can be drawn from the measurements are limited. Furthermore, the ground-level horizontal measurements make it very hard to draw conclusions about the ultraviolet transmission at higher air layers.