Hygroscopic Growth of Aerosol Particles Measured at Bondville
The annual median f(RH) for the sub-1micrometer aerosol at 550 nm is ~1.96, while the annual median f(RH) total f(RH) value is ~1.98. (Total aerosol refers to particles with diameters less than 10 micrometers.)
Monthly time series box-whisker plots of the f(RH) values measured at BND between 1996-2000 show an annual cycle, with lower f(RH) values in February-June and higher f(RH) values July-January. Other aerosol optical properties measured at Bondvile (e.g., single scattering albedo, Angstrom exponent, hemispheric back scattering fraction) give no obvious indication of a cause for this observed cycle.
Sub-1micrometer f(RH) values tend to be higher than total f(RH) from June-December, while the opposite appears to be true March-May.
Comparison of f(RH) measurements
The value of the f(RH) parameter depends on several factors including particle size and composition. For example, dust tends to consist of large, non-hygroscopic particles. If we compare the hygroscopic properties of the Bondville aerosol over all time periods with times when the aerosol is dominated by super-micrometer particles (using the criterion that the sub-micron scattering fraction (Rsp) is less than 0.65), we see (below) that the larger aerosol tends to have a lower f(RH) value.
This is similar to what we see at Southern Great Plains, OK.
Different regions will have aerosol particles with different compositions, thus, f(RH) will vary by sampling location. The table below compares f(RH) values for several ESRL sampling sites.
|Sampling Site||f(RH) (total, 550 nm)||Comments|
| Southern Great Plains
Frequently exposed to field fires
| Kashidhoo Climate Observatory
High proportion of black carbon in aerosol