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UCI Aerosol Photochemistry Group   
University of California at Irvine   Department of Chemistry   
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Aerosol Photochemistry Group Research


Molecular Composition of Organic Aerosols

Photochemistry of Organic Aerosols

Organic Aerosol Aging Chemistry

Water Uptake by Nanoparticles

Indoor Air Chemistry

Research Archives

Browning Reactions in Organic Aerosols

Organic aerosols are generally colorless, and therefore they scatter incoming solar radiation back to space. This scattering prevents solar radiation from reaching the Earth’s surface thus leading to a climate cooling effect. Recent discovery of light-absorbing “brown carbon” aerosols has challenged this view and suggested that some types of organic aerosols actually absorb solar radiation thus warming the climate. The term “brown carbon” refers to organic aerosol capable of absorbing UV and visible light. Brown carbon decreases atmospheric visibility, deteriorates air quality in urban areas, and affects regional climate. The largest source of brown carbon in the atmosphere is the burning of biomass material, but recent studies show that the aging of SOA can also generate a significant amount of brown carbon. The aging processes responsible for this “browning” are still highly uncertain. We recently discovered [62] that SOA produced by ozonolysis of limonene and several other terpenes change from colorless to orange-brown when exposed to ammonia, a common atmospheric pollutant. The following images shows this transformation for an aerosol filter sample.

DESI imageThe highly colored compounds produced by aging reactions are generated when ammonia reacts with carbonyl compounds present in the SOA. Our DOE PNNL collaborators Julia Laskin and Alexander Laskin have been able to detect imine compounds that are likely responsible for the light absorption in aged limonene SOA using high resolution desorption ionization (HR-DESI) mass spectrometry [66]. These compounds are similar to the products of Maillard reactions between sugars and amino acids that cause browning in food like, for example, when toasting bread. Organic aerosols appear to get “toasted” on time scales of hours to days, which is a relevant atmospheric time scale. In collaboration with our DOE PNNL collaborators, we are currently investigating the colored compounds in aged SOA with techniques such as HR-ESIMS, coupled with liquid chromatography.  We are also investigating the kinetics of reactions between selected dicarbonyls and amines in order to better understand the reaction mechanism.

More recently, we determined that browning chemistry is not limited to SOA produced from limonene, and quantified the mass absorption coefficients of “brown carbon” material produced by this mechanism for a number of SOA types [77]. We also found that browning chemistry is accelerated quite dramatically by evaporative cloud/fog processing of aerosols; the “brown carbon” compounds are produced much faster during evaporation of droplets containing dissolved SOA and ammonium sulfate [72]. The fact that aging of SOA can dramatically change its optical properties has significant implications for direct forcing by aerosols. We continue to investigate the mechanistic details of this browning chemistry.



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