Kinetics and Spectroscopy of Atmospherically Important Molecules

Aerosols are solid or liquid particles suspended in air.They affect visibility, human health, and climate.¹ Primary aerosols are released directly into the atmosphere from both biogenic and anthropogenic sources.¹ Secondary aerosols form in the atmosphere via physical and chemical processes.The formation of secondary aerosol particles is frequently modeled with classical nucleation theory(CNT),^2,3, as outlined in Figure 1. The first step in CNT is nucleation, in which molecular clusters form and then grow in size until they reach the critical cluster size. The critical cluster size is defined as the cluster size with the maximum Gibbs free energy. The second step in CNT is the growth of the critical cluster through coagulation or condensation. Current CNT predictions of atmospheric aerosol contents underestimate the measured concentrations of aerosols.^4-11

Recent studies have demonstrated that the inclusion of ppb concentrations of amines in a gaseous mixture of H₂SO₄ and water vapor increases the rate of particle formation by 10–1,000 times.^19-24 When modeled by CNT, new particle formation requires a high relative humidity for particle growth to occur. In a surprise finding, the introduction of ppb concentrations of amines into a reaction vessel containing H 2 SO 4 coupled with relative humidities as low as 20% resulted in particle formation.¹⁹ These findings suggest that amines can play a critical role in new particle formation under otherwise unfavorable conditions.

Amines are the most abundant bases emitted into the atmosphere, with estimated global emissions in excess of 5,000 Gg∙N∙yr⁻¹ and concentrations measured in the atmosphere ranging from 1 ppt to 10 ppb.^{25, 26} The many biogenic and anthropogenic sources of amines include the decomposition of plants, animal husbandry, and biomass burning. Trimethylamine (TMA) and ammonia (NH₃) are the most abundant amines in the atmosphere, with emission rates of 108 and 5,000 Gg∙N∙yr⁻¹, respectively.^{25, 26}

Sulfuric acid (H₂SO₄) has been extensively studied and serves as a model for particle formation.^{4, 12-14} Curtius et al.,² Becker and Doring et al.,¹⁵ and others ^16-18 have proposed particle formation via CNT in which H₂SO₄ interacts with water vapor to form a nucleating seed (i.e.,H₂SO₄–H₂O complex). Additional water molecules, volatile organic compounds, semivolatile secondary organic gases, or other substituents in the atmosphere can adhere to the complex to overcome the surface free energy barrier, resulting in the formation of the critical cluster. Once the critical cluster size (~1 nm) has been reached, cluster growth becomes thermodynamically favorable.

Carboxylic acids have been detected in particulates from various parts of the world.^{5, 27} Formic acid is the most abundant carboxylic acid in the atmosphere. Its strong binding energy with water makes it an ideal candidate for studying its ability to serve as a nucleating seed along with water vapor for particle formation. Recent work by Shaw et al.²⁹ demonstrated that acetaldehyde can undergo photo-tautomerization into formic acid. Figure 2 shows the influence of this previously unknown reaction on global concentrations of formic acid, which we hypothesize may serve as nucleating seeds for particle formation. The higher formic acid concentrations, indicated by warm colors in Figure 2, are typically centered over the oceans and indicate that between 39% and 50% of the formic acid found in these regions can be attributed to the photo-tautomerization of acetaldehyde.

In a collaboration with the Francisco group at the University of Pennsylvania, the thermodynamics of new particle formation by formic acid, water vapor, and TMA were investigated using high-level ab initio and Born–Oppenheimer molecular dynamics (BOMD) simulations.³⁰ These computational studies were done in support of experimental work performed in the Hansen Laboratory. The results of the ab initio alculations indicate that a strongly hydrogen-bonded complex forms between formic acid, water vapor, and TMA with a binding energy of 77.9 kJ∙mol⁻¹ (relative to the energy of the separated monomers, see the geometry illustrated in Figure 3).³⁰

BOMD simulations provide useful information about the time scale and molecular mechanism of ion-pair formation along with the dynamic behavior of the ion pair formed on the aqueous surface. We performed BOMD simulations to probe the nature of the interaction between HCOOH and N(CH₃)₃ on a water cluster containing 191 H₂O molecules. The HCOOH..N(CH₃)₃ interaction was found to follow a typical trajectory of acid–base chemistry and involves proton transfer between HCOOH and N(CH₃)₃ without the direct involvement of surface water molecules. This process, shown in Figure 4, results in the formation of a HCOO_-...NH(CH₃)₃⁺ ion pair on a picosecond (ps) time scale. The water cluster stabilizes the ion-pair particle by forming a hydration shell around it. These findings are consistent with field measurements conducted in Riverside, California³¹ and the Central Valley region of California,³² which indicated that ammonium salts can form in aged organic carbon particles. A laboratory study of the reactive uptake of NH 3 onto slightly soluble organic acid particles also found that this process can significantly enhance the cloud condensation nuclei activity and hygroscopic growth of these particles.³³ For the reaction between HCOOH and N(CH₃)₃ , a transition state-like complex is formed at 5.18 ps. In this complex, the hydroxyl proton of HCOOH is partially dissociated and transferred toward N(CH₃)₃; the resulting O1–H1 bond length is 1.33 Å, whereas the H1–N1 bond length is 1.29 Å. This complex is converted into the HCOO − ...NH(CH₃)₃ + ion pair at 5.23 ps. In this ion pair, the O1–H1 bond is lengthened to 1.70 Å, indicative of a hydrogen-bonding interaction, whereas the H1–N1 bond has become a true covalent bond (H1–N = 1.06 Å).

Figure 4: Snapshots of structures of BOMD simulation of the reaction of formic acid with TMA [N(CH₃)₃] which illustrate the formation of a HCOO^-NH(CH₃)₃⁺ ion pair on a water droplet.

The Hansen lab has investigated the kinetics of particle formation using a slow-flow reaction cell equipped with a sliding injector and coupled to two mobility particle sizers as detectors for measuring the size distribution and absolute number of particles formed (Figure 5). Gaseous mixtures of formic acid, water vapor, and TMA were introduced into the cell, and the particle size distribution and total number of particles were measured as a function of reaction time. One new element of this study was the introduction of trace concentrations of TMA into the gaseous formic acid/water vapor reaction mixture. The rate of particle formation was observed to increase by a factor of 2 when ppb quantities of TMA were incorporated into the formic acid/water vapor mixtures. Figure 6 shows the total particle concentration and size distribution for reaction times of 8 and 48 s. In the absence of TMA, flowing formic acid and water vapor resulted in a total of ~3.5×10⁶ particles∙cm⁻³ with a size distribution of 0.2–75 nm. Adding 200 ppb TMA to the reaction mixture resulted in the generation of ~4×10,⁶ particles∙cm⁻³ (8 s reaction time) with a size distribution of 14–200 nm. Upon increasing the reaction time to 48 s, the total concentration of particles increased to ~7.5×10⁶ particles∙cm⁻³, and the size distribution shifted toward larger particle diameter. Based on these results, we hypothesize that the HC(O)OH–H₂O–N(CH₃)₃ complex (along with other carboxylic acid–water–amine complexes) can serve as nucleating seeds for particle formation.

The strong hydrogen-bonding interactions between carboxylic, water vapor, and amines may play an important role in controlling the abundance and chemical reactivity of particles in the atmosphere.³⁴ Our previous computational work^{30, 35, 36} demonstrated that in the presence of amine, formic and acetic acids react to form an ion pair solvated by a cage of water molecules. Due to the strong polarizing effect of the carboxylic acid–amine ion pair in water clusters, we theorize that carboxylic acids may serve as nucleating agents for particle formation in the presence of trace amounts of amines.

In the Hansen lab we experimentally test the above hypothesis using the method/setup described previously. The apparatus shown in Figure 5 allows us to probe the effects of temperature, concentrations of carboxylic acid, water vapor, and amine on aerosol particle growth in terms of size distribution and concentration.