Light scattering and extinction measurements combined with laser-induced
incandescence for the real-time determination of soot mass absorption
cross-section.
Abstract
An aerosol albedometer was combined with laser-induced incandescence
(LII) to achieve simultaneous measurements of aerosol scattering,
extinction coefficient, and soot mass concentration. Frequency doubling
of a Nd:YAG laser line resulted in a colinear beam of both λ = 532 and
1064 nm. The green beam was used to perform cavity ring-down
spectroscopy (CRDS), with simultaneous measurements of scattering
coefficient made through use of a reciprocal sphere nephelometer. The
1064 nm beam was selected and directed into a second integrating sphere
and used for LII of light-absorbing kerosene lamp soot. Thermal denuder
experiments showed the LII signals were not affected by the particle
mixing state when laser peak power was 1.5–2.5 MW. The combined
measurements of optical properties and soot mass concentration allowed
determination of mass absorption cross section (M.A.C., m2/g) with 1 min
time resolution when soot concentrations were in the low microgram per
cubic meter range. Fresh kerosene nanosphere soot (ns-soot) exhibited a
mean M.A.C and standard deviation of 9.3 ± 2.7 m2/g while limited
measurements on dry ambient aerosol yielded an average of 8.2 ± 5.9 m2/g
when soot was >0.25 μg/m3. The method also detected
increases in M.A.C. values associated with enhanced light absorption
when polydisperse, laboratory-generated ns-soot particles were embedded
within or coated with ammonium nitrate, ammonium sulfate, and glycerol.
Glycerol coatings produced the largest fractional increase in M.A.C.
(1.41-fold increase), while solid coatings of ammonium sulfate and
ammonium nitrate produced increases of 1.10 and 1.06, respectively.
Fresh, ns-soot did not exhibit increased M.A.C. at high relative
humidity (RH); however, lab-generated soot coated with ammonium nitrate
and held at 85% RH exhibited M.A.C. values nearly double the
low-humidity case. The hybrid instrument for simultaneously tracking
soot mass concentration and aerosol optical properties in real time is a
valuable tool for probing enhanced absorption by soot at atmospherically
relevant concentrations.