The water-soluble part of airborne fine white (graphite poor) particles is usually with referense to its ionic composition in equilibrium with air. This has been checked by means of the phase diagram for the system (NH4)2SO4-H2SO4-H2O which is now avialable. In the same way is has been shown that the ionic concentration in precipitaion probably represents equilibrium condition near the cloud base and not at the ground level. Finallly the disagreement between teh composition of the water-soluble part of black (graphite rich) particles and the air according to the phase diagram may be due to an analytical bias caused by a sorbed alkaline layer on the graphite particles