The Influence of Monoterpene and Isoprene Nitrates on the Chemistry and Phase State of Secondary Organic Aerosol in a Low-NOx Mixed Deciduous/Coniferous Forest
Abstract
Particle phase state plays a key role in secondary organic aerosol (SOA) growth, reactive uptake of gas-phase radicals, and condensed phase reactions, influencing atmospheric composition and clouds. While biogenic SOA derived from isoprene and monoterpene oxidation can exhibit glassy or highly viscous semisolid phase states depending on relative humidity, the lifetimes of important SOA precursors including organic nitrates (ON) can depend on the liquid water content (LWC) and particle acidity. Reactions such as acid-catalyzed hydrolysis of particulate ON (pON) are especially sensitive to changes in aerosol LWC. However, the role of particle phase state on the chemistry of pON, and the chemical transformations of pON on the phase of SOA are poorly understood. During the summer of 2016, as part of the PROPHET - Atmospheric Measurements of Oxidants in Summer (AMOS) field study in a northern Michigan mixed deciduous/coniferous forest, we investigated the production of isoprene (IN) and monoterpene nitrates (MTN) at the ground and canopy levels and their role in particle formation, growth, and phase using a combination of high-resolution chemical ionization and aerosol mass spectrometry techniques, size distribution measurements, and particle bounce factors (BF) using two electrical low pressure impactors operating in parallel. Preliminary derived BF suggest the particles may be in a liquid-like state similar to those measured in other high-humidity and isoprene-rich regions such as the Amazon and Southeastern US. However, maxima in BF are observed during periods of high pON, NO, and IN during the day and high NO2 and MTN at night. Some periods were marked with low pON followed by an increase in particulate organic sulfate (pOS), and decrease in BF. Based on these observations, we hypothesize that acid-catalyzed hydrolysis of pON from IN- and MTN-derived SOA may be driving the transition of pON to pOS, leading to a change in the phase state of SOA. Preliminary results also indicate that the smallest particles can be acidic. We will further explore the role of LWC on the phase state of the particles applying ISORROPIA analysis. Although the PROPHET forest is generally NOx-limited, the work presented here highlights the potential importance of NOx and pON chemistry in dictating the phase state of SOA.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.A41M..04S
- Keywords:
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- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSESDE: 3355 Regional modeling;
- ATMOSPHERIC PROCESSESDE: 0426 Biosphere/atmosphere interactions;
- BIOGEOSCIENCES