Simplified mechanism for new particle formation from methanesulfonic acid, amines, and water via experiments and ab initio calculations

Abstract

Airborne particles affect human health and significantly influence visibility and climate. A major fraction of these particles result from the reactions of gaseous precursors to generate low-volatility products such as sulfuric acid and high-molecular weight organics that nucleate to form new particles. Ammonia and, more recently, amines, both of which are ubiquitous in the environment, have also been recognized as important contributors. However, accurately predicting new particle formation in both laboratory systems and in air has been problematic. During the oxidation of organosulfur compounds, gas-phase methanesulfonic acid is formed simultaneously with sulfuric acid, and both are found in particles in coastal regions as well as inland. We show here that: (i) Amines form particles on reaction with methanesulfonic acid, (ii) water vapor is required, and (iii) particle formation can be quantitatively reproduced by a semiempirical kinetics model supported by insights from quantum chemical calculations of likely intermediate clusters. Such an approach may be more broadly applicable in models of outdoor, indoor, and industrial settings where particles are formed, and where accurate modeling is essential for predicting their impact on health, visibility, and climate.

Fig. 1.

Total particle concentrations at 4.2-min reaction time for (A) TMA and (B) DMA with various precursor concentrations and relative humidities. Initial gas-phase concentrations ranged from 2–34 ppb MSA, 0–8 ppb TMA and DMA, and 0–21% RH. Specific experimental conditions are summarized in Table S1.

Fig. 2.

Structures for (A) MSA•H₂O, (B) MSA•TMA•H₂O, (C) MSA•TMA•(H₂O)₂, (D) MSA•TMA, (E) MSA•DMA•H₂O, (F) MSA•DMA•(H₂O)₂, and (G) MSA•DMA. MSA in red, TMA and DMA in green, H₂O in blue, and transferred proton in pink.

Fig. 3.

Calculated change in enthalpy for formation of (A) TMA and (B) DMA clusters relative to the separated molecules. Arrows indicate addition or loss of MSA (red); TMA or DMA (green); and water (blue).

Fig. 4.

Proposed reaction scheme for particle formation from the MSA/amine/H₂O system. Species labeled “Particle” are assumed to continue to grow to sizes detectable using our SMPS (≥3 nm).

Fig. 5.

Comparison of n modeled and measured particle concentrations at 4.2-min reaction time for the reaction of MSA, H₂O, and (A) TMA (n = 17) and (B) DMA (n = 25). Rate constants are presented in Table 1. The dashed line has a slope of 1 and is included for reference. Error bars indicate changes in modeled particle concentration with ± 25% of the initial gas-phase MSA and amine concentrations.