Air quality measurements in urban green areas – a case study

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Abstract

The influence of traffic-induced pollutants (e.g. CO, NO, NO2 and O3) on the air quality of urban areas was investigated in the city of Essen, North Rhine-Westphalia (NRW), Germany. Twelve air hygiene profile measuring trips were made to analyse the trace gas distribution in the urban area with high spatial resolution and to compare the air hygiene situation of urban green areas with the overall situation of urban pollution. Seventeen measurements were made to determine the diurnal concentration courses within urban parks (summer conditions: 13 measurements, 530 30 min mean values, winter conditions: 4 measurements, 128 30 min mean values). The measurements were carried out during mainly calm wind and cloudless conditions between February 1995 and March 1996. It was possible to establish highly differentiated spatial concentration patterns within the urban area. These patterns were correlated with five general types of land use (motorway, main road, secondary road, residential area, green area) which were influenced to varying degrees by traffic emissions. Urban parks downwind from the main emission sources show the following typical temporal concentration courses: In summer rush-hour-dependent CO, NO and NO2 maxima only occurred in the morning. A high NO2/NO ratio was established during weather conditions with high global radiation intensities (K>800 W m−2), which may result in a high O3 formation potential. Some of the values measured found in one of the parks investigated (Gruga Park, Essen, area: 0.7 km2), which were as high as 275 μg m−3 O3 (30-min mean value) were significantly higher than the German air quality standard of 120 μg m−3 (30-min mean value, VDI Guideline 2310, 1996) which currently applies in Germany and about 20% above the maximum values measured on the same day by the network of the North Rhine–Westphalian State Environment Agency. In winter high CO and NO concentrations occur in the morning and during the afternoon rush-hour. The highest concentrations (CO=4.3 mg m−3, NO=368 μg m−3, 30-min mean values) coincide with the increase in the evening inversion. The maximum measured values for CO, NO and NO2 do not, however, exceed the German air quality standards in winter and summer.

Introduction

Changes in the composition of the urban atmosphere are caused largely by traffic-induced pollutants (Gorissen, 1990). These are mainly carbon monoxide (CO), nitrogen monoxide (NO), dust and soot as well as various types of non-methane hydrocarbons (NMHC), in particular benzene, toluene and xylene. Secondary trace gases which can be formed from these precursor substances in certain photochemical reaction conditions include nitrogen dioxide (NO2), ozone (O3) and other photooxidants, e.g., peroxiacetylnitrate (PAN).

The main centre of interest to date has been the distribution of traffic-induced trace gases directly at the roadside (e.g., Harrop et al., 1990; Kuhler et al., 1990; Laxen and Noordally, 1987; Bringfeld, 1987); less attention has been paid to analysing the air quality of urban areas with various types of land use and their interactions with high spatial resolution (Kuttler, 1996). This applies especially to the air hygiene situation in inner city green areas. It is well known that green areas have a significant recreational function for city-dwellers (Givoni, 1991; Horbert and Kirchgeorg, 1982). To date, only very little work has been done on the air quality of urban green areas near to roads. For example, an early publication (Wainwright and Wilson, 1962) describes SO2 horizontal concentration profiles in Hyde Park, London. PAH concentrations in the Tivoli Park, Copenhagen, are dealt with by Nielsen et al. (1996) and NO, NO2, O3, SO2 and CO concentrations in the Englischer Garten park in Munich are investigated by Mayer and Haustein (1994) on the basis of measurement trips.

This article analyses the extent to which busy roads influence the air quality of neighbouring urban green areas. The objective of the investigation was a worst-case analysis of the air hygiene situation of urban green areas during low-exchange high-radiation summer and winter weather conditions with a view to assessing typical diurnal concentration courses of CO, NO, NO2 and O3. The investigation was carried out in Essen (630 000 inhabitants, 210 km2), Germany, a city situated in the centre of the Rhine-Ruhr area, one of Europe's largest and most densely populated industrial agglomerations.

Section snippets

Experimental set up

Air hygiene and meteorological parameters were measured with a mobile laboratory during mainly calm and clear weather conditions between February 1995 and March 1996. The atmospheric pollutants CO, NO, NO2 and O3 were continuously recorded (4 m a.g.l.) by analysers using the following measuring methods:COIRabsorption(HORIBAAPMA−350E);NO,NO2Chemiluminescence(HORIBAAPNA−350E);O3UVabsorption(HORIBAAPOA−350E).The meteorological measurements made were wind velocity and wind direction (10 m a.g.l.),

Spatial air hygiene structure in different urban land-use types

Mobile air hygiene profile measurements were made to assess the air hygiene situation of urban green spaces within the overall situation of inner city pollutant concentrations. The measurements were made in a similar way to those reported by Luria et al. (1990) and Mayer et al. (1994). North–south trips (50 km, 6 trips) and west–east trips (60 km, 6 trips) were made in the city of Essen (Fig. 1 (a)) between 22.02.1995 and 12.10.1995. Concentrations were recorded at a sampling rate of 1 s−1. The

Conclusions

The investigations found similar patterns for the trace substances NO and NO2 to those reported by Mayer and Haustein (1994) for Munich, Germany. Unfortunately, the comprehensive results of Mayer and Haustein are not directly comparable with the results of this study as they are based on evaluation procedures not in accordance with the measurement methods used and must therefore be regarded critically. However, in low-emission areas of the city, Mayer and Haustein found O3 concentrations

Acknowledgements

This work was supported by the Forschungsvereinigung Automobiltechnik e. V. (FAT), Frankfurt/Main within the framework of the project “Analysis of automobile caused air pollutants in inner urban traffic and green areas”. The authors wish to thank A. Schmidt (laboratory technician) and R. Zimmermann (technical assistant) for their effective help with the field experiments.

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