Analysis of the summer air quality over an artificial lake area near to a city using FTIR and DOAS

This paper considers the summer air quality of a recreation area near to a city taking the example of an artificial lake which is very popular for water sports in clear weather conditions. The measurements were made in the vicinity of the Baldeneysee (2,7 km²) in a valley about 50 m deep to the South of the city of Essen (j=51°27', l=7°00') in the central Ruhr area between July and October 1998. Because of the high recreation value, air quality near to the water surface is especially important. Stationary meteorological and air hygiene measurements were made at two stations facing each other on the northern and southern shores of the lake. Standard analysers were used for the air hygiene measurements. In addition, tracer propagation campaigns were made to investigate the cold air dynamics and SODAR and wire sonde measurements were made to determine the vertical structure of the near-surface atmosphere. The air quality above the water surface was analysed using op-FTIR and DOAS systems. The distance between the two stations was 390 m, and the height of the measurement line was 3 m above the water surface. NO, NO₂, N₂O, CO, O₃ and CH₄ concentrations were measured and the results were compared with those of a station near to the city. Special attention was paid to ozone concentrations, which showed a well-structured diurnal course and were up to 40 % higher than those of the comparison station in some cases. Concentrations of the other trace substances were low, comparable with those of unpolluted areas. However, diurnal courses similar to those typical of urban stations were observed. These were probably caused by mesocale transport processes and microscale cold air drainage, especially on the northern shore.

Comparisons were also made between the measurements of the FTIR and DOAS systems and between these systems and the standard analysers. With the two remote sensing methods, signal intensity variations were observed over the course of the day: these correlated with relative humidity in the case of op-FTIR and with air temperature in the case of DOAS. In general, there was good agreement between the results of the remote sensing methods and the standard analysers. There were some deviations in the evenings and mornings, when the standard analyser station on the northern shore was affected more severely by cold air flow than the measurement line above the lake.