Two spectroscopic techniques are often employed to examine the structure of atmospheric clusters: Anions composed of sulfuric acid and nitric acid have been detected in the atmosphere. While the composition of the smallest clusters was HSO - 4 (HNO 3 ) x (x = 1,2), field studies could not identify the charge-carrying species. The location of charge in atmospheric clusters influences their chemical reactivity and stability, making it important to determine the location of the proton in these clusters. IRMPD spectra of H 2 SO 4 /HNO 3 clusters (Figure ) reveal that the HSO - 4 peak (around 1190 cm -1 ) appears only in HSO - 4 (HNO 3 ) and HSO - 4 (H 2 SO 4 )(HNO 3 ), while nitrate was the charge carrier in (HNO 3 )(NO 3 -)(H 2 SO 4 ).