J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences, Dolejškova 3, 18223 Prague, Czech Republic.
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstraβe 25, 6020 Innsbruck, Austria.
J Chem Phys. 2018 Apr 21;148(15):154301. doi: 10.1063/1.5021471.
The uptake of atmospheric oxidized organics on acid clusters is relevant for atmospheric new particle formation. We investigate the pickup of methanol (CHOH) on mixed nitric acid-water clusters (HNO)(HO) by a combination of mass spectrometry and cluster velocity measurements in a molecular beam. The mass spectra of the mixed clusters exhibit (HNO)(HO)H series with m = 0-3 and n = 0-12. In addition, CHOH·(HNO)(HO)H series with very similar patterns appear in the spectra after the methanol pickup. The velocity measurements prove that the undoped (HNO)(HO)H mass peaks in the pickup spectra originate from the neutral (HNO)(HO) clusters which have not picked up any CHOH 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 σ¯≈ 20 Å. This is compared to the lower estimate of the mean geometrical cross section σ¯≈ 60 Å 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.
大气氧化有机物在酸簇上的吸收对于大气新粒子形成很重要。我们通过质谱和分子束中簇速度测量相结合的方法研究了甲醇（CHOH）在混合硝酸-水簇（HNO）（HO）上的吸收。混合簇的质谱表现出（HNO）（HO）H 系列，m = 0-3，n = 0-12。此外，在甲醇吸收后，光谱中出现了非常相似模式的 CHOH·（HNO）（HO）H 系列。速度测量证明，在吸收光谱中未掺杂的（HNO）（HO）H 质荷比峰源自未吸收任何 CHOH 分子的中性（HNO）（HO）簇，即甲醇在电离时没有蒸发。因此，可以确定掺杂簇的分数，并估算平均吸收截面，得到 σ¯≈20 Å。这与从理论簇几何形状得出的平均几何截面 σ¯≈60 Å 的较低估计值进行了比较。因此，与甲醇吸收相对应的簇的“大小”至少比其几何尺寸小 3 倍。我们已经引入了一种方法，可以得到与大气气溶胶生成和增长相关的绝对吸收截面，如甲醇和硝酸簇的例子所示。
