Dear Jakub,

In marine aerosols you will be mostly dealing with major NaCl bearing sea-salts with say Mg, etc salts as minor mixtures which may be fractionally crystallized at 50-60 % RH levels.

If you would like to scan fully crystallized sea-salt particle size its better to dry as low as possible.. below 4O% RH should be a necessity.

But then it obviously depends on whether you want the particle/droplet sizing in ambient conditions or only the dry-fully crystallized particle sizing. For measuring optical properties/particle sizing by SMPS, I guess one should measure as it is without changing ambient RH (or at RH and Temp. conditions likely to be existing at the sampling site...or the back trajectory conditions the particle experienced)

You may also want to consider the condition of your DMA, as arcing in the column becomes a concern at elevated RH.