Activation Barriers in the Growth of Molecular Clusters Derived from Sulfuric Acid and Ammonia

Abstract

Unraveling the chemical mechanism of atmospheric new particle formation (NPF) has important implications for the broader understanding of the role of aerosols in global climate. We present computational results of the transition states and activation barriers for growth of atmospherically relevant positively charged molecular clusters containing ammonia and sulfuric acid. Sulfuric acid uptake onto the investigated clusters has a small activation free-energy barrier, consistent with nearly collision-limited uptake. Ammonia uptake requires significant reorganization of ions in the preexisting cluster, which yields an activation barrier on the order of 29−53 kJ/mol for the investigated clusters. For this reason, ammonia uptake onto positively charged clusters may be too slow for cluster growth to proceed by the currently accepted mechanism of stepwise addition of sulfuric acid followed by ammonia. The results presented here may have important implications for modeling atmospheric NPF and nanoparticle growth, which typically does not consider an activation barrier along the growth pathway and usually assumes collision-limited molecular uptake. ■ INTRODUCTIONAtmospheric new particle formation (NPF) is a gas-to-particle conversion process that influences the concentration of ambient particulate matter. 1 While the chemical mechanisms of NPF have not been fully elucidated, important species include sulfuric acid, 2−4 water, bases 5−8 (such as ammonia and amines), and carbonaceous matter. 9−12 The structures and reactivities of the molecular clusters driving NPF have been the focus of experimental and theoretical investigations, 13−28 and both approaches have provided valuable insight into the chemical mechanisms underlying particle formation. In order to effectively model both the correct formation pathway as well as the rate of particle formation, energetic barriers must be considered. In the few modeling studies of cluster growth energetics, 28,29 the barriers explored have been only thermodynamic barriers, that is, the Gibbs free-energy difference between a product cluster and the reactant species. To our knowledge, the direct transition states (TSs) between reactant and product clusters have not been explored. If an activation barrier must be traversed when going from reactant to product, it will impact both the pathways and rates of cluster growth.Our research group has studied the formation and reactivity of clusters between sulfuric acid and a base (ammonia or amine) using a combination of experimental and computational approaches. 30−36 In recent experimental work, we explored the kinetics and energetics of the fragmentation of charged ammonium bisulfate clusters using Fourier transform ion cyclotron resonance mass spectrometry in order to elucidate possible cluster growth mechanisms. 31 On the basis of the assumption that cluster growth is the reverse of cluster fragmentation, activation barriers were found to exist for the addition of ammonia to charged clusters but not for the addition of sulfuric acid (i.e.,...

Role of base strength, cluster structure and charge in sulfuric-acid-driven particle formation

“…In addition, the cluster growth might be hindered due to kinetic barriers. The addition of a monomer or a cluster to a pre-existing cluster might require cluster reorientation which in turn may lead to the breaking of intermolecular bonds, and thus non-negligible kinetic barriers (DePalma et al, 2014;Bzdek et al, 2017;Xu et al, 2017). The subsequent growth as well as the evaporation may be slower than our calculations assume, especially in the case of strongly bound cage-like clusters.…”

Theoretical Studies on Reactions of OH with H2SO4…NH3Complex and NH2with H2SO4in the Presence of Water

“…These results could be of great interest in simulating one to consider nucleation processes from the kinetic point of view, whereas previous investigations on nucleation processes mainly concentrated on the thermodynamic processes and considered the collision rate in the isomerization of molecular clusters . Very recently, experimental and theoretical results have indicated that there is an energy barrier for the addition of ammonia to molecular clusters …”

Mechanisms of Atmospherically Relevant Cluster Growth

“…Finally, the relationship between growth mechanisms for molecular clusters and those for larger nanoparticles remains to be fully resolved. For example, we have shown that the same (barrierless) mechanism for sulfuric acid addition exists for both clusters (<1.5 nm diameter) , and nanoparticles (>10 nm diameter). − On the other hand, amines are not necessarily major components of nanoparticles, so the mechanisms discussed above for small molecular clusters are likely not applicable to nanoparticles. In fact, our recent field measurements show that carbonaceous matter accounts for more than half of the mass growth of 10–20 nm diameter nanoparticles produced during new particle formation. − Resolving the relationship between clusters and nanoparticles will require laboratory and field measurements along with complementary computational simulations in the 3–5 nm diameter size regime.…”

“…Computational chemistry helps to explain why a large activation barrier exists for the neutralization step but the barrier is minimal for the acidification step Figure presents initial, preassociation complex, transition state, and product structures for the reactions described in eqs and . …”