NIRE Annual Report

1999

Objectives

Atmospheric aerosols including NO₃- and SO₄^2- are generated through gas-to-particle conversion processes of NOx and SO₂. The first step of aerosol generation is thought to be phase transition via hydration processes of gaseous HNO₃ and H₂SO₄ molecules . As one of the experimental approach to elucidate these hydration processes, we are studying molecular clustering in aqueous HNO₃, HCl and H₂SO₄ solutions.

Results

The electrospray mass spectrometer specially designed for the cluster in an electrolyte solution is schematically illustrated in Fig. 1. By injecting aqueous solutions including HNO₃, HCl and H₂SO₄ into the vacuum chamber as shown in Fig. 1, singly charged ionic clusters are isolated from the charged liquid droplet through adiabatic expansion and electrostatic repulsion. The resulting clusters were analyzed by a quadrupole mass filter.

Fig 1. Schematic illustration of an specially designed mass spectrometer

In a mass spectrum of clusters observed for an aqueous HNO₃ solution (1 mol/l), two series of clusters (H+(H₂O)n : n = 1, 2, 3, ···, 30, ···, and H+(HNO₃)(H₂O)m: m = 2, 3, 4, ···, 30, ···) are confirmed. The mass distribution of the clusters observed for aqueous HCl is similar to the case of aqueous HNO₃ solution. This indicates that the HNO₃ molecules do not obstruct the H₂O-H₂O hydrogen-bonding interaction in the aqueous solution,

On the other hand, in the mass spectrum for the aqueous H₂SO₄ solution, the water clusters larger than hexamer were hardly observed, but the H₂SO₄ self-association clusters accompanying with the small number of water molecules were clearly observed. In contrast with aqueous HNO₃ and HCl solutions, the H₂SO₄ molecules disintegrate the H₂O-H₂O hydrogen-bonding interaction, and gather several H₂O molecules around the H₂SO₄ molecules and the self-association clusters.

The observed difference in the cluster structures should be related to the atmospheric physicochemical reactions such as PSCs generation.

*This work was carried out in cooperation with Exited State Chemistry Division, Atmospheric Environmental Department.

Fig 2. Mass spectrum of clusters generated from aqueous H2SO4 solution (0.1 mol/l)