Meteorological evaluation of a severe air pollution episode in Helsinki on 27–29 December 1995

This paper describes an evaluation and meteorological analysis of a severe air quality episode that occurred in the Helsinki Metropolitan Area during 27–29 December 1995. The pollutants originated predominantly from local sources. The episode was formed in anticyclonic high pressure conditions that lead to low wind speeds and the formation of an extremely strong ground-based radiation inversion. We utilised the 24 hourly forecasts of the Finnish version of the numerical weather forecasting model HIRLAM (High Resolution Limited Area Model, operational from 1999 to 2003). The HIRLAM model under-predicted both the inversion strengths (°C) and inversion temperature gradients (°C m–1), compared with the corresponding measured data. We also compared the temperature profiles predicted by HIRLAM with those predicted by the non-hydrostatic meteorological model MM5. The pollutant concentrations during the episode have been predicted using an urban dispersion modelling system, and evaluated against measured data.