Some quadrupole aerosol chemical speciation monitors (Q-ACSMs) have had issues with the quantification of particulate chloride, resulting in apparent negative chloride concentrations. We can show that this is due to the different behavior of Cl⁺ and HCl⁺, and we present a correction for the more accurate quantification of chloride. The correction can be applied to measurements in environments where the particulate chloride is dominated by NH₄Cl.