Processing and Ageing in the Atmosphere - Mineral Dust: A Key Player in the Earth System - page 75

Fig. 4.1 Pictorial representation of changes in a composite mineral dust particle as it is processed
in the atmosphere (Reprinted with permission from Usher et al. ( 2003 ). Copyright (2003) American
Chemical Society)

transport may alter hygroscopicity (and hence removal mechanisms), reactivity of
dust constituents and the composition of the atmosphere, as illustrated schematically
in Fig. 4.1 . In this chapter, we examine these processes using data from field
observations and laboratory measurements. Field studies allow us to assess the net
effect of the various complex processes involved, while laboratory work provides
the means to study fundamental chemical and physical mechanisms under controlled
conditions.

4.2 Physical Processing

As discussed in Chaps. 8 and 17 , winnowing leads to shifts in the size spectrum
of suspended dust during transport away from source areas (Maring et al. 2003 ;
Mahowald et al. 2005 ). Large particles (>10 m) are removed rather rapidly and do
not contribute appreciably to long-range transport (Formenti et al. 2011 ). This shift
also potentially affects the mineralogy of transported dust, as large quartz grains are
removed close to source (Glaccum and Prospero 1980 ).
The modal size of long-range transported dust is generally considered to be
1-3 m (Arimoto et al. 1997 ; Schulz et al. 1998 ;Reidetal. 2003 ), but there are
reports of giant particles (>75 m) being transported many thousands of kilometres
away from sources (see Chap. 17 , Betzer et al. 1988 ).