AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Diel Variations in the Phase State of Atmospheric Aerosol in a Mixed Forest: The Role of Molecular Composition and Liquid Water
JONATHAN SLADE, Alexander Bui, Ryan Cook, Amy Bondy, Sarah Desrochers, Rebecca Harvey, Jenna Ditto, Drew Gentner, Kerri Pratt, Andrew Ault, Robert Griffin, Brandon E. Boor, Giuseppe Petrucci, Paul Shepson, Purdue University
Abstract Number: 251 Working Group: Aerosol Chemistry
Abstract Phase state is a fundamental but poorly quantified physical property of atmospheric aerosol particles that influences reactive uptake and partitioning of trace reactive gases and semi-volatile compounds, ultimately affecting the lifetime, optical, and cloud condensation/ice nuclei activation properties of aerosol. Secondary organic aerosol (SOA) represents a significant fraction of the total global aerosol budget and exhibits different amorphous phase states that range from solid to semisolid and liquid particles, which change in response to the ambient relative humidity and temperature. While recent studies have shown that the phase state of SOA can vary regionally and as a function of altitude, whether particle phase state varies on a diurnal basis is currently unknown. Here we present an analysis of the nighttime and daytime particle phase state of aerosol particles sampled in a mixed deciduous/coniferous forest. To elucidate particle phase state, continuous one hertz measurements of particle bounce factors were made below the forest canopy during the summer of 2016 as part of the Atmospheric Measurements of Oxidants Study at the PROPHET research site in northern Michigan. A maximum in particle bounce was generally observed overnight and a minimum in the early afternoon and appears to be a strong function of both molecular composition and ISORROPIA-estimated aerosol liquid water (ALW). The morphology and phase state of individually impacted particles, determined by atomic force microscopy, show reasonable agreement with the measured bounce factors. During periods of significant biogenic influence, the mostly organic particles, determined by aerosol mass spectrometry (AMS), exhibited the largest bounce factors. In contrast, particles sampled from anthropogenically-impacted air masses generally contained more ALW and exhibited less bounce. No significant dependence on AMS-derived O:C was observed. In general, more ALW content corresponded to an overall decrease in particle bounce. However, while calculated ALW concentrations were generally greatest overnight, the overnight maximum and daytime minimum in particle bounce suggests a potentially significant dependence on particle molecular properties other than ALW. We will discuss the potential influence of variations in the average particle molar mass, based on a more detailed analysis of particle molecular composition using a suite of high-resolution mass spectrometric techniques, with preliminary results suggesting influence of predominantly larger molecular weight monoterpene-derived SOA products overnight and smaller isoprene-derived SOA products during the day.