In this talk I will present laboratory and theoretical studies that explore how relative humidity (RH) affects the chemical mechanisms of atmospheric new particle formation. Such studies not only provide insights into the most important steps in the formation of new atmospheric particles, but also more accurately represent the conditions of the real atmosphere. In one set of experiments, we explore the role of water vapor in the formation of highly oxidized molecules (HOMs), which are key components of new particle formation and initial growth from oxidized organics. Selected monoterpenes were oxidized under low NOx conditions and 3 - 90% RH in a temperature-controlled flow tube. A novel transverse-ionization chemical ionization mass spectrometer detected HOMs and number-size distributions of generated particles were measured with SMPS. A major finding from this work is that that all of the detected gas-phase reaction products did not change with relative humidity, even though particle concentration clearly decreased as humidity increased. This observation rules out the role of autoxidation of organic peroxy radicals (the reaction pathway that we observe through these reaction products) in the initial steps of new particle formation. A second study explores the formation and growth of nanoparticles from inorganic and organic acid-base chemistry. Ammonia and amines were combined with organic and inorganic acids in a flow tube reactor under dry conditions and at ~60% RH. Number-size distributions and the size-dependent composition of the formed nanoparticles were measured by SMPS and a Thermal Desorption Chemical Ionization Mass Spectrometer, respectively. Measurements were supplemented with quantum chemical calculations that explore the stability of growing acid, base, and water clusters. A major finding of this work is that, for many systems, both new particle formation rates and size-resolved nanoparticle composition are highly dependent on the strength and volatility of the acid and base as well as RH. Quantum chemical modeling of stability of growing molecular clusters provides insights into the underlying chemistry behind these observations.

It’s the water: Exploring the impacts of water vapor on new particle formation mechanisms

2019 Fall National Meeting

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