AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
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Observed Particle Phase Chemistry Deviates from Brown Carbon Formation in Bulk Solutions
HENSLEY JACK, Adam Birdsall, Valtierra Gregory, Frank Keutsch, Harvard University
Abstract Number: 480 Working Group: Aerosol Chemistry
Abstract Condensed-phase reactions involving carbonyls and amines have been shown to produce light-absorbing heterocycles that could affect aerosol radiative forcing. The proposed chemistry in aerosol particles is typically extrapolated from studies that characterize chemical reactions in bulk solutions. The underlying assumption is that the aerosol particle and a bulk solution serve as similar reaction media, but in reality, chemical mechanisms could differ in these two systems. Here, we study the reaction of butenedial (C4H4O2) and ammonia, and juxtapose their reaction in bulk solutions (~1-10 mL) with reaction in aerosol particles (~10 micron radius). We use NMR spectroscopy to characterize the formation of pyrrolinone (C4H5NO) in capped bulk solutions containing butenedial and ammonium sulfate, which darken as the reaction proceeds. To examine this chemistry over comparable timescales in actual aerosol, we apply a newly developed instrument: single particle levitation with an electrodynamic balance coupled to mass spectrometry (EDB-MS). We employ the EDB-MS technique to demonstrate that pyrrolinone formation in butenedial / ammonium sulfate particles is inhibited unless significant ammonia is fed into the surrounding gas phase. By contrast, in the capped bulk system, ammonia and reaction intermediates are not depleted from the condensed phase as they partition to the surrounding gas phase. This work illuminates the role of gas-particle partitioning as a critical control on reaction progress in aerosols, where surface area-to-volume ratios are large with respect to bulk solutions.