Analysis of the air quality within the range of an urban green area

Urban green areas can contribute to the air exchange in cities and thus reduce thermal stress and air pollution in a human-biometeorological sense. Additionally if a connection exists to fresh air producing areas of the surrounding countryside, cool air of good quality can be supplied to the urban built-up areas especially at nights with appropriate weather conditions.

In order to determine qualitatively and quantitatively, to which extent an inner urban green area is affected air-hygienically by the emissions of directly adjacent roads, an extended measurement campaign was performed within the range of a large urban green area with a size of 140 hectares in Düsseldorf, (51°12'N/6°48'E; 570,000 inhabitants; North-Rhine-Westphalia, Germany). Measurements of selected atmospheric air pollutants (NO, NO2, O3, CO) were carried out from January to December 2001 continuously and simultaneously both at the south border of the green area at a high-traffic road (four-lane, 50.000 vehicles per day) and 200 m away from that location within the green area using optical remote measurement systems (OP-FTIR, UV-DOAS).

The trace gas concentrations show a clear variation at both locations in dependence of the season, the day of the week, the time of day, the emission situation and the atmospheric stability. The annual mean values for NO, NO2 and CO were found higher at the road location than within the green area, whereas O3 shows slightly higher mean values in the green area (ratios of annual mean values road/green area: NO 1.62, NO2 2.02, CO 1.21, O3 0.86). The annual variation for the primary emitted trace gases show clearly increased concentrations during winter months in relation to the summer at both locations (NO max. factor 5.4; CO max. factor 2.1). This variation can be attributed to different atmospheric stability during the main emission times (rush hours) in the different seasons. Wind directions which result in an direct air pollution input from the adjacent road into the green area were found at 56% of the year (ESE to SW). These 56% of the yearly hours cause 72% of the total mass flow density dose (in g/m²) within the range of the green area, from which the influence of the adjacent traffic emissions can be derived.

At the same time a reduction of the mass flow density dose for NO of 54% takes place from the road to the green area location for these wind directions, but the reduction is greater in the summer months with 64%, despite smaller wind velocities (photochemistry; stronger turbulence) than in the winter with 49%. The diurnal variations of the primary trace gases CO and NO correspond to the traffic-dependent emissions during the day. The secondary maximum during the afternoon rush hour is more weakly pronounced than the primary maximum in the morning as a result of a more unstable atmosphere. Higher concentrations within the green area in relation to the road location were found only for O3 during calm and clear weather conditions in summer.

In summary, the green area examined can be characterized as an favoured area in the urban structure from a air-hygienic point of view. Clearly reduced trace gas concentrations were found within the green area for NO, NO2 and CO, whereas increased O3 concentrations only occur, when those of the other trace gases are rather small. Altogether the reduction of the trace gas concentrations within the green area depends strongly on the atmospheric exchange conditions.