A cartridge for an aerosol-generating system is provided, including an elongate first compartment having a length L1, a width W1, and a height H1, the first compartment having a first air inlet and a first air outlet and containing a nicotine source; and an elongate second compartment having a length L2, a width W2, and a height H2, and having a second air inlet and a second air outlet and containing an acid source. The first and second compartments are arranged in parallel within the cartridge and ratios of L1 to W1 and L2 to W2 are between about 2:1 and about 4:1, and ratios of L1 to H1 and L2 to H2 are at least about 6:1. An aerosol-generating system is also provided, including the cartridge; and an aerosol-generating device including a housing defining a cavity configured to receive at least a portion of the cartridge.