There is provided a cartridge (100) for use in an aerosol-generating system (10) for the generation of an aerosol comprising nicotine salt particles. The cartridge (100) comprises a first compartment (110) containing a nicotine source (210) comprising a first carrier material (211) impregnated with nicotine, the first compartment (110) having a first air inlet (132) and a first air outlet (133). The first compartment (110) defines a first airflow path (217) extending across a first surface (213) of the first carrier material (211) between the first air inlet (132) and the first air outlet (133). The cartridge (100) also comprises a second compartment (120) containing an acid source (220) comprising a second carrier material (221) impregnated with an acid, the second compartment (220) having a second air inlet (134) and a second air outlet (135). The second compartment (220) defines a second airflow path (227) extending across a first surface (223) of the second carrier material (221) between the second air inlet (134) and the second air outlet (135). The first compartment (110) and the second compartment (220) are arranged in parallel within the cartridge (100). A thickness of at least one of the first carrier material (211) and the second carrier material (221) varies in a direction along the first airflow path (217) or the second airflow path (227) respectively.