Uptake of methanol on mixed HNO 3 /H 2 O 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 (CH3 OH) on mixed nitric acid-water clusters (HNO3 )M (H2 O)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 (H2 O)n H+ series with m = 0-3 and n = 0-12. In addition, CH3 OH·(HNO3 )m (H2 O)n H+ series with very similar patterns appear in the spectra after the methanol pickup. The velocity measurements prove that the undoped (HNO3 )m (H2 O)n H+ mass peaks in the pickup spectra originate from the neutral (HNO3 )M (H2 O)N clusters which have not picked up any CH3 OH 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 Å2 . This is compared to the lower estimate of the mean geometrical cross section σg ¯≈ 60 Å2 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.