They are suspended in the atmosphere as atmospheric aerosol, a term which refers to the particulate/air mixture, as opposed to the particulate matter alone.
Four more are seriously stressed: freshwater use, ocean acidification, atmospheric aerosol pollution, and the introduction of exotic chemicals and modified organisms.
They show that vapor traces, assumed until now to account for aerosol formation in the lower atmosphere can explain only a tiny fraction of the observed atmospheric aerosol production.
Thus, ngstrm exponent is a useful quantity to assess the particle size of atmospheric aerosols or clouds, and the wavelength dependence of the aerosol/cloud optical properties.