Ion-Molecule Rate Constants for Reactions of Sulfuric Acid with Acetate and Nitrate Ions

Abstract

Atmospheric nucleation from precursor gases is a significant source of cloud condensation nuclei in the troposphere and thus can affect the Earth's radiative balance. Sulfuric acid, ammonia, and amines have been identified as key nucleation precursors in the atmosphere. Studies have also shown that atmospheric ions can react with sulfuric acid to form stable clusters in a process referred to as ion-induced nucleation (IIN). IIN follows similar reaction pathways as chemical ionization, which is used to detect and measure nucleation precursors via atmospheric pressure chemical ionization mass spectrometers. The rate at which ions form clusters depends on the ion-molecule rate constant. However, the rate constant varies based on the ion composition, which is often not known in the atmosphere. Previous studies have examined ion-molecule rate constants for sulfuric acid and nitrate ions but not for other atmospherically relevant ions like acetate. We report the relative rate constants of ion-molecule reactions between nitrate and acetate ions reacting with sulfuric acid. The ion-molecule rate constant for acetate and sulfuric acid is estimated to be a factor of 1.9-2.4 times higher than that of the known rate constant for nitrate and sulfuric acid. Using quantum chemistry, we find that acetate has a higher dipole moment and polarizability than nitrate. This may contribute to an increase in the collision cross-sectional area between acetate and sulfuric acid and lead to a greater reaction rate constant than nitrate. The ion-molecule rate constant for acetate with sulfuric acid will help quantify the contribution of acetate ions to atmospheric ion-induced new particle formation.

Figure 1

Plot of the sulfuric acid monomer to reagent signal ratios for acetate (S₉₇/S_acetate) vs the monomer to reagent signal ratio for nitrate (S₉₇₊₁₆₀/S_nitrate). A linear fit of the MTE-corrected signal ratios reveals slopes of 2.4 and 1.9 respectively for the PCC (solid red line) and MCC (solid black line). The PCC and MCC signal ratios not corrected for MTE (red and black dashed lines respectively) have a slope of 2.4.

Figure 2

Calculated sulfuric acid monomer concentration [H₂SO₄] vs flow rate of sulfuric acid injected in two different flow reactors for acetate (black) and nitrate (red). (A) Squares show [H₂SO₄] calculated with k_1,acetate= k_1,nitrate = 1.9 × 10^–9 cm³ s^–1 for PCC measurements. Triangles are [H₂SO₄] calculated with k₁,_acetate = 4.6 × 10^–9 cm³ s^–1. (B) Squares show [H₂SO₄] calculated with k_1,acetate= k_1,nitrate = 1.9 × 10^–9 cm³ s^–1 for MCC measurements. Triangles are [H₂SO₄] calculated with k₁,_acetate = 3.6 × 10^–9 cm³ s^–1. The error bars show the extent to which the sulfuric acid concentrations varied for a given sulfuric acid flow rate over the 6-day and 3-day measurement periods for the PCC and MCC, respectively.

Figure 3

Panels a and c show MTE-corrected signal ratios of sulfuric acid monomer to the reagent (S₉₇/S_acetate and S₉₇₊₁₆₀/S_nitrate) vs t_CI at constant sulfuric acid concentration for acetate (black) and nitrate (red) for PCC and MCC respectively. Panels b and d show MTE-corrected signal ratios of the sulfuric acid dimer to the monomer (S₁₉₅/S₉₇ and S₁₉₅/S₉₇₊₁₆₀) vs t_CI for acetate (black) and nitrate (red) for PCC and MCC, respectively. The lines are linear fits, and the error bars show the extent to which the signal ratios at a given t_CI varied over the measurement period.