Objectives This study attempted to develop a simple statistical model for assessing the contribution of aerosols transported regionally and those transported long range to the concentrations of fine particulate matter (PM_2.5) in urban air in Helsinki.

Methods The construction and testing of the linear regression model was based on PM_2.5 measurement data from two locations in the City of Helsinki (Vallila & Kallio) and on ion concentration data obtained from the three nearest monitoring stations of The Co-operative Programme for Monitoring and Evaluating of the Long-range Transmission of Air Pollutants in Europe (EMEP). The "ion sum" was calculated on the basis of the following daily measured EMEP parameters in 1998-2000: (i) sulfate (SO₄^2-), (ii) the sum of nitrate (NO₃^-) and nitrogen acid (HNO₃), and (iii) the sum of ammonium (NH₄⁺) and ammonia (NH₃). The ion sum was compared with sulfate as the proxy variable for long-range transmission.

Results The correlation of the daily average PM_2.5 concentration with the ion sum (R² = 0.59-0.61) was higher than that with sulfate (R² = 0.48-0.50). The regression estimates showed relatively small year-to-year variation. The contribution of long-range transportation to the measured PM_2.5 concentration in urban air in Helsinki was estimated to be 64-76%.

Conclusions The results showed a strong association between the ion sum interpolated from the EMEP data and the PM_2.5 concentration measured at urban sites in Helsinki. This association can be utilized in local dispersion modeling of PM_2.5 concentrations in urban air.

Key terms air quality; fine particles; fine particulate matter; ion concentration; long-range transport; modeling; particulate matter; secondary inorganic ion; statistical model; The Co-operative Programme for Monitoring and Evaluating of the Long-range Transmission of Air Pollutants in Europe; transport; urban air