AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Characterizing Brown Carbon Formation and Persistence in Various Conditions: Cloudwater pH, Evaporation, and Photodegradation
LELIA HAWKINS, Hannah G. Welsh, Matthew V. Alexander, Harvey Mudd College
Abstract Number: 628 Working Group: There Must be Something in the Water: Cloud, Fog and Aerosol Aqueous Chemistry for Aerosol Production
Abstract Secondary organic aerosol (SOA) composes a non-negligible fraction of brown carbon (BrC), and typically appears as small, nitrated aromatics or larger, highly functionalized humic-like substances (HULIS). Both nitrated aromatics and HULIS contain nitrogen, indicating the importance of nitrogen to light-absorbing aerosol. It is therefore unsurprising that BrC, when generated in aqueous phase reactions (aqBrC) between amines and small aldehydes, often resembles atmospheric HULIS. The effects of atmospheric aging on absorptivity and composition were simulated using bulk (microliter) samples under a variety of experimental conditions, including evaporation, pH control, and hydroxyl radical degradation. The system of amines and small aldehydes included methylamine, ammonium sulfate, glyoxal, and methylglyoxal. Chemical composition of these products was characterized using an Aerosol Chemical Speciation Monitor (ACSM). The results of this study indicate that methylamine and methylglyoxal form the most absorptive BrC, cloud processing serves to increase BrC absorptivity, and the generated BrC is highly persistent to oxidative and photodegradation. Results of this research also show that evaporation increased formation of large molecular fragments (m/z > 100). Furthermore, the mass spectra showed significant formation of these larger fragments in methylamine systems with little evidence for similar compounds in ammonium sulfate systems. Systems with methylglyoxal had higher absorptivity than all other systems, although in both methylamine and ammonium sulfate systems, glyoxal appeared to result in a higher percentage of large fragments than methylglyoxal. Lastly, hydroxyl radical degradation seemed to have a minimal effect on absorptivity and composition, although longer reaction time may produce a larger effect on both properties. These results may simplify some aspects of atmospheric models (like negligible degradation) but may complicate others (highly variable absorptivity between glyoxal and methylglyoxal).