Monitoring of inorganic ions, carbonaceous matter and mass in ambient aerosol particles with online and offline methods

Abstract. Year-long high timeresolution measurements of major chemical components in atmospheric sub-micrometer particles were conducted at an urban background station in Finland 2006–2007. Ions were analyzed using a particle-into-liquid sampler combined with an ion chromatograph (PILS-IC), organic and elemental carbon (OC and EC) by using a semicontinuos OC/EC aerosol carbon analyzer (RT-OCEC), and PM_2.5 mass with a tapered element oscillating microbalance (TEOM). Long time series provides information on differences between the used measurement techniques as well as information about the diurnal and seasonal changes. Chemical mass closure was constructed by comparing the identified aerosol mass with the measured PM_2.5. The sum of all components measured online (ions, particulate organic matter (POM), EC) represented only 65% of the total PM_2.5 mass. The difference can be explained by the difference in cutoff sizes (PM₁ for online measurements, PM_2.5 for total mass) and by evaporation of the semivolatile/volatile components. In general, some differences in results were observed when the results of the continuous/semicontinuous instruments were compared with those of the conventional filter samplings. For non-volatile compounds, like sulfate and potassium, correlation between the filter samples and the PILS was good but greater differences were observed for the semivolatile compounds like nitrate and ammonium. For OC the results of the RT-OCEC were on average 10% larger than those of the filters. When compared to filter measurements, high resolution measurements provide important data on short pollution plumes as well as on diurnal changes. Clear seasonal and diurnal cycles were observed for nitrate and EC.