A measurement campaign in a street canyon in Helsinki and comparison of results with predictions of the OSPM model

In 1997, a measuring campaign was conducted in a street canyon (Runeberg St .) in Helsinki. Hourly mean concentrations of CO, NOx, NO2 and O-3 were mea sured at street and roof levels, the latter in order to determine the urban background concentrations. The relevant hourly meteorological parameters w ere measured at roof level; these included wind speed and direction, temper ature and solar radiation. Hourly street level measurements and on-site ele ctronic traffic counts were conducted throughout the whole of 1997; roof le vel measurements were conducted for approximately two months, from 3 March to 30 April in 1997. CO and NOx emissions from traffic were computed using measured hourly traffic volumes and evaluated emission factors. The Operati onal Street Pollution Model (OSPM) was used to calculate the street concent rations and the results were compared with the measurements. The overall ag reement between measured and predicted concentrations was good for CO and N Ox (fractional bias were - 4.2 and + 4.5%, respectively), but the model ove rpredicted the measured NO2 concentrations (fractional bias was + 22%). The agreement between the measured and predicted Values was also analysed in t erms of its dependence on wind speed and direction; the latter analysis was performed separately for two categories of wind velocity. The model qualit atively reproduces the observed behaviour very well. The database, which co ntains all measured and predicted data, is available for further testing of other street canyon dispersion models. The dataset contains a larger propo rtion of low wind speed cases, compared with other available street canyon measurement datasets. (C) 2000 Elsevier Science Ltd. All rights reserved.