Review and uncertainty assessment of size-resolved scavenging coefficient formulations for below-cloud snow scavenging of atmospheric aerosols L. Zhang1, X. Wang2, M. D. Moran1, and J. Feng3 1Air Quality Research Division, Science and Technology Branch, Environment Canada, 4905 Dufferin St, Toronto, Ontario, M3H 5T4, Canada 2Kellys Environmental Services, Toronto, Ontario, Canada 3Canadian Meteorological Centre, Meteorological Service of Canada, Environment Canada, 2121 Trans-Canada Highway, Dorval, Quebec, H9P 1J3, Canada
Abstract. Theoretical parameterizations for the size-resolved scavenging coefficient
for atmospheric aerosol particles scavenged by snow (Λsnow)
need assumptions regarding (i) snow particle–aerosol particle collection
efficiency E, (ii) snow-particle size distribution N(Dp), (iii) snow-particle
terminal velocity VD, and (iv) snow-particle cross-sectional
area A. Existing formulas for these parameters are reviewed in the present
study, and uncertainties in Λsnow caused by various
combinations of these parameters are assessed. Different formulations of E
can cause uncertainties in Λsnow of more than one order of
magnitude for all aerosol sizes for typical snowfall intensities. E is the
largest source of uncertainty among all the input parameters, similar to
rain scavenging of atmospheric aerosols (Λrain) as was found
in a previous study by Wang et al. (2010). However, other parameters can
also cause significant uncertainties in Λsnow, and the
uncertainties from these parameters are much larger than for Λrain.
Specifically, different N(Dp) formulations can cause
one-order-of-magnitude uncertainties in Λsnow for all aerosol
sizes, as is also the case for a combination of uncertainties from both
VD and A. Assumptions about dominant snow-particle shape (and thus
different VD and A) will cause an uncertainty of up to one order of
magnitude in the calculated scavenging coefficient. In comparison,
uncertainties in Λrain from N(Dp) are smaller than a factor
of 5, and those from VD are smaller than a factor of 2. As expected,
Λsnow estimated from empirical formulas generated from field
measurements falls in the upper range of, or is higher than, the
theoretically estimated values, which can be explained by additional
processes/mechanisms that influence field-derived Λsnow but
that are not considered in the theoretical Λsnow formulas.
Predicted aerosol concentrations obtained by using upper range vs.
lower range of Λsnow values (a difference of around two orders
of magnitude in Λsnow) can differ by a factor of 2 for just a
one-centimetre snowfall (liquid water equivalent of approximately 1 mm).
Based on the median and upper range of theoretically generated Λsnow
and Λsnow values, it is likely that, for typical
rain and snow events, the removal of atmospheric aerosol particles by snow is
more effective than removal by rain for equivalent precipitation amounts,
although a firm conclusion requires much more evidence.
Citation: Zhang, L., Wang, X., Moran, M. D., and Feng, J.: Review and uncertainty assessment of size-resolved scavenging coefficient formulations for below-cloud snow scavenging of atmospheric aerosols, Atmos. Chem. Phys., 13, 10005-10025, doi:10.5194/acp-13-10005-2013, 2013.