Chemical nature and molecular weight distribution of the water-soluble fine and ultrafine PM fractions collected in a rural environment

► Water-soluble organic carbon (WSOC) in different size PM fractions. ► WSOC in the PM1-2.5 fraction ranged from 37% to 47%. ► PM<0.1 contribute the least to the WSOC content (19%-31%). ► Average molecular weights ranged from 560 Da to 700 Da. ► Tandem MS found carboxylic, polycarboxylic acids, organosulfates, and -nitrates.