NO2 Vertical Gradient Monitoring Along a Street Canyon by LP-DOAS in the Framework of the Escape Air Pollution and Health Study

Abstract

PP-30-045 Background/Aims: A major aim of the European study of cohorts for air pollution effects (ESCAPE) is to characterize the long-term spatial distribution of air pollutants. In all areas, NOx saturation monitoring was done at ∼40 locations over 3 seasons. In cities where only NOx (no PM) monitoring was done (eg, Heidelberg), passive Ogawa NOx samplers were mounted at approximately 3 meters above the street. Naturally, the main question is whether measurements made at this height are a good measure of exposure for individuals living and working at other heights. A second question is whether passive time-integrated samplers are adequate for capturing exposures or whether there are significant diurnal and weekday/weekend differences. Methods: To examine these questions, a long-path differential optical absorption spectrometer (LP-DOAS) was used in Heidelberg to monitor the vertical profile of NO2 along a street canyon in parallel with passive 2-week NOx measurements in April 2009. The real-time NO2 measurements were made by a blue-light LP-DOAS along 4 beampaths at heights ranging from 5.4 to 15.5 m above the street. The beampaths were ∼575 m long (1-way). Four NOx Ogawa samplers were located at 3-m height along 1 side of the street canyon. Results: Only 3 of the 4 Ogawa sampler data could be used. Two (on drain-pipes) indicated average NO2 concentrations around 22.6 ppb, while the third (at an open intersection) indicated a much lower concentration of 16.9 ppb. The average 2-week LP-DOAS NO2 concentrations at higher elevations ranged from 14.6 to 17.7 ppb (DL = 0.62 ppb, error = ±0.32 ppb) and showed an average vertical gradient of −0.43 ppb/m for the spring season. This vertical gradient estimated average NO2 concentrations to be around 20 ppb at 3 m. The 10-minute resolved vertical profile data showed clear diurnal levels of NO2 with peaks at 8:30 and 19:30 when the NO2 mixing ratio was 13–18 ppb above off-peak times. It was also observed that the vertical gradient increased at peak times. Conclusion: Along a street canyon, a significant 2-week NO2 vertical gradient was found. Therefore, further exploration of long-term vertical gradients of air pollutants is recommended to reliably use measured concentration data for exposure estimation.