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Physical Sciences Division
Research Highlights

July 2013

The Changing Face of Aerosols

New study reveals chemical transformations of ambient organic aerosols

Aerosols and spectra lines
Changes in the chemical composition of organic aerosols throughout the day indicate the potential role of ammonium-carbonyl reaction chemistry in forming the nitrogen-containing organic constituents of aerosols. Enlarge Image.

Results: Throughout the day, the composition of organic aerosols that influence our climate changes: molecules containing carbon, hydrogen, and oxygen, or CHO, compounds dominate the composition of fresh aerosol during the day time, while nitrogen-containing organics increase at night, according to a new study by scientists at Pacific Northwest National Laboratory and two California universities. The formation of nitrogen-containing organic compounds is caused by reactions of CHO molecules with ammonium ions, a pervasive atmospheric constituent. To probe the compositional changes during a diurnal cycle and the distribution of unique species over different lengths of time, the team employed the Nanospray Desorption Electrospray Ionization (nano-DESI) coupled to high-resolution mass spectrometry (HR-MS) to characterize more than 850 molecular species.

Why It Matters: Deciphering the molecular composition of organic aerosols in the atmosphere is essential for understanding how these complex aerosols impact climate forcing. The chemical composition of organic aerosols influences how sunlight is absorbed. For example, nitrogen-containing organic compounds in the aerosols may greatly alter the particle's light-absorption properties, affecting how the aerosols influence regional haze and the Earth's climate. Organic aerosol sources, their atmospheric transformation, and radiative effects are key contributors of uncertainty in today's atmospheric climate models. At present, the understanding of organic aerosols composition is limited, hence their impact cannot be accurately predicted and mitigated.

Methods:  Using nano-DESI/HR-MS, the team performed sensitive direct analysis of minute amounts of organic aerosol samples from the National Oceanic and Atmospheric Administration's CalNex 2010 field study. The analysis accurately assesses the transforming reactions and their plausible causes.

The analyses revealed increased nitrogen-containing compounds in samples taken at night while organic aerosols containing only CHO were more common in the afternoon. High ozone concentrations correlated with the predominant CHO compounds, indicating they were secondary products of daytime oxidation, including ozonolysis and/or photochemical reactions. Meanwhile, the observed nighttime enhancement of nitrogen-containing compounds showed evidence of being formed by reactions that transform carbonyls into imines, which require aqueous-phase chemistry of ammonium and organic compounds in liquid-phase particles. Thus, the team hypothesized ammonium chemistry may be an important pathway for nitrogen-containing compound formation.

What's Next? In addition to providing an alternate technique for investigating organic aerosols, and potentially more precise models, this work is another step in the scientific community's understanding of climate change ramifications stemming from the chemical transformations of organic aerosols.

Acknowledgments:

Sponsors: California Air Resources Board; US Department of Energy's (DOE's) Office of Biological and Environmental Research Atmospheric System Research program; and DOE's Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division

User Facility: The nano-DESI/HR-MS experiments were performed at EMSL

Research Area: Climate and Earth Systems Sciences

Research Team: Rachel O'Brien, Robin Weber, Lynn Russell, and Allen Goldstein, University of California, Berkeley; Alexander Laskin and Julia Laskin, Pacific Northwest National Laboratory; Shang Liu and Lynn Russell, University of California, San Diego

Reference: O'Brien RE, A Laskin, J Laskin, S Liu, R Weber, LM Russell, and AH Goldstein. 2013. "Molecular Characterization of Organic Aerosol Using Nanospray Desorption/Electrospray Ionization Mass Spectrometry: CalNex 2010 Field Study." Atmospheric Environment 68:265-272. DOI: 10.1016/j.atmosenv.2012.11.056


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