Micrographs also clearly show some irregularly shaped
particles. These particles are mixed in composition and are
assumed to be the result of chemical reactions between preexisting
particles, liquid or gaseous phase. Li and Shao
(2009b) have also suggested that alkaline mineral particles
react with gaseous species (such as SO2, NO2 and HNO3)
and form the coating on the pre-existing particles of
minerals.