A novel analytical method is presented to estimate the total condensable vapor concentration in the ambient atmosphere. The method requires the aerosol size distribution as its only input and is thus straightforward enough to apply to atmospheric data. In order to estimate the condensable vapor concentrations over boreal forest regions, the method was applied to four years of new particle formation days at SMEAR II site in southern Finland. The growth rate of newly formed particles provided additional information of the concentration of low-volatile vapors. Hence the contribution of low-volatile and semivolatile vapors to particle growth can be separated. The analysis indicated that the median of the condensable vapor concentrations during the nucleation events was approximately 7.6 ¿ 107 cm-3, with values varying between 2.0 ¿ 107 and 1.8 ¿ 108 cm-3. Of the total concentration, the contribution of low-volatile compounds was 70% on average but ranged from 22% to over 100%. Sensitivity tests indicated that the obtained percentage of low volatiles is only slightly sensitive to chosen model parameters. When applying the method to ambient aerosol distribution measurements, the largest uncertainty is related to visually undetectable inhomogeneities in air mass and the boundary layer dynamics. These uncertainty factors together with the use of measured dry aerosol sizes instead of ambient sizes can lead the analysis to somewhat overestimate the importance of the low-volatile compounds to total aerosol growth. Even so, our results suggest that identification of submicron aerosol composition during new particle formation can provide information on the compounds that grow nucleation mode particles to larger sizes.