Multiple-scattering correction factor of quartz filters and the effect of filtering particles mixed in water: implications for analyses of light absorption in snow samples
Abstract. The deposition of light-absorbing aerosol (LAA) onto snow initiates
processes that lead to increased snowmelt. Measurements of LAA, such as
black carbon (BC) and mineral dust, have been observed globally to darken
snow. Several measurement techniques of LAA in snow collect the
particulates on filters for analysis. Here we investigate micro-quartz
filters' optical response to BC experiments in which the particles are initially
suspended in air or in a liquid. With particle soot absorption photometers
(PSAPs) we observed a 20 % scattering enhancement for quartz filters
compared to the standard PSAP Pallflex filters. The multiple-scattering
correction factor ( Cref ) of the quartz filters for airborne soot
aerosol is estimated to be ∼3.4 . In the next stage correction
factors were determined for BC particles mixed in water and also for BC
particles both mixed in water and further treated in an ultrasonic bath.
Comparison of BC collected from airborne particles with BC mixed in water
filters indicated a higher mass absorption cross
section by approximately a factor of 2 for the liquid-based filters, which is probably due to the BC particles
penetrating deeper in the filter matrix. The ultrasonic bath increased
absorption still further, roughly by a factor of 1.5, compared to only mixing
in water. Application of the correction functions to earlier published field
data from the Himalaya and Finnish Lapland yielded mass absorption coefficient (MAC) values of
 ∼7 –10 m 2  g −1 at λ=550  nm, which is
in the range of the published MAC of airborne BC aerosol.