Uptake of methanol on mixed HNO₃/H₂O clusters: An absolute pickup cross section

The uptake of atmospheric oxidized organics on acid clusters is relevant for atmospheric new particle formation. We investigate the pickup of methanol (CH₃OH) on mixed nitric acid–water clusters (HNO₃)_M(H₂O)_N by a combination of mass spectrometry and cluster velocity measurements in a molecular beam. The mass spectra of the mixed clusters exhibit (HNO₃)_m(H₂O)_nH⁺ series with m = 0–3 and n = 0–12. In addition, CH₃OH·(HNO₃)_m(H₂O)_nH⁺ series with very similar patterns appear in the spectra after the methanol pickup. The velocity measurements prove that the undoped (HNO₃)_m(H₂O)_nH⁺ mass peaks in the pickup spectra originate from the neutral (HNO₃)_M(H₂O)_N clusters which have not picked up any CH₃OH molecule, i.e., methanol has not evaporated upon the ionization. Thus the fraction of the doped clusters can be determined and the mean pickup cross section can be estimated, yielding $σs¯≈$ 20 Ų. This is compared to the lower estimate of the mean geometrical cross section $σg¯≈$ 60 Ų obtained from the theoretical cluster geometries. Thus the “size” of the cluster corresponding to the methanol pickup is at least 3-times smaller than its geometrical size. We have introduced a method which can yield the absolute pickup cross sections relevant to the generation and growth of atmospheric aerosols, as illustrated in the example of methanol and nitric acid clusters.