Aging of Organic Aerosols

Exploring aqueous chemistry on atmospheric timescales (Adam, Amy, Greg, Jack)

Atmospheric particles typically live for several days aloft. During this time, they experience changing conditions, which drive the processes that shape the chemical composition and physical properties of these particles. Understanding how this "aging" takes place in laboratory surrogates that faithfully mimic the atmosphere on temporal and geometric axes is essential to our representations of particles in e.g., models. Levitated particles (with EDB-MS) offer an advantage as they can (a) stably reside in a chamber for minutes to days, (b) are authentic aqueous particles with radius ~ 10 um, (c) have controlled gas phase (with removal of volatilized gases possible) and no wall losses, and (d) have their chemical changes tracked at the molecular level. (See for more details.)

Using butenedial, a C4 dialdehyde observed in combustion plumes, as a model compound, we have explored gas-particle partitioning and condensed-phase reactions in levitated particles, and how these important processes can modulate the chemical composition of particles.

1. Gas-particle partitioning

Birdsall, et al. (2019) found that butenedial vapor pressures are not influenced by RH (over RH=5-75%). However, its effective Henry's Law (right) is reduced with dissolved sodium sulfate or chloride (i.e., butenedial "salts out"). Glyoxal and methylglyoxal (C2 and C3 dialdehydes) have increased vapor pressures at low RH. Glyoxal "salts in" while methyl glyoxal salts out with increasing ionic content. These findings suggest that even the small (<C4) dialdehydes do not all exhibit the same gas-particle partitioning behavior in conditions relevant to the atmosphere.

2. Competition between gas-particle partitioning and condensed-phase reactions (in prep.)

We compared butenedial/ammonia reactions in bulk solutions (as butenedial/ammonium sulfate) and in levitated particles (as butenedial/ammonium sulfate or butenedial/ammonia gas). We observed no pyrrolinone (product) formation unless ammonia was introduced to the gas phase. We illuminate the role that evaporation of volatile reagents plays in reducing apparent reaction rates in particles.

3. Multigeneration condensed-phase reactions in levitated particles: dependence on extreme pH and concentrations (in prep.)

Brown carbon is thought to form through amine/carbonyl chemistry in the atmosphere. The chromophores themselves are oligomeric and tend to be present in low abundance, making them difficult to analyze quantitatively.

We observed large, nitrogenated compounds in butenedial particles exposed to ammonia gas. We considered the role of pH and concentration on producing key brown carbon building blocks in atmospheric particles. We are able to recreate these compounds observed in levitated particles in bulk butenedial/ammonium hydroxide solutions at high pH. This indicates that extreme pH may play a larger role than extreme concentration (and possibly interfacial effects as well).

Calculated Butenedial effective Henry's Law coefficients (H*) as a function of ionic content (based on Birdsall, et al. (2019))

Pyrrolinone is clearly produced in bulk liquids containing butenedial/ammonium sulfate, but not in corresponding levitated particles. We found that ammonia (and possibly butenedial) evaporation prevented reaction from being observed in the particles.

Proposed chemical mechanism for formation of chromophores from butenedial/ammonia reactions. Diazepine and butenedial-pyrrolinone "dimer" are the building block molecules for oligomerization.