Atmospheric organic matter in clouds: exact masses and molecular formula identification using ultrahigh resolution FT-ICR mass spectrometry
Author:
Zhao Y., Hallar A. G., Mazzoleni L. R.ORCID
Abstract
Abstract. Clouds alter the composition of atmospheric aerosol by acting as a medium for interactions between gaseous and particulate phase substances. To determine the cloud water atmospheric organic matter (AOM) composition and study the cloud processing of aerosols, two samples of supercooled clouds were collected at Storm Peak Laboratory near Steamboat Spring, Colorado (3220 m a.s.l.). Approximately 3000 molecular formulas were assigned to ultrahigh resolution mass spectra of the samples after using a reverse phase extraction procedure to isolate the AOM components from the cloud water. Nitrogen containing compounds (CHNO compounds), sulfur containing compounds (CHOS and CHNOS compounds) and other oxygen containing compounds (CHO compounds) with molecular weights up to 700 Da were observed. Average oxygen-to-carbon ratios of ~0.6 indicate a slightly more oxidized composition than most water-soluble organic carbon identified in aerosol studies, which may result from aqueous oxidation in the clouds. The AOM composition indicates significant influences from biogenic secondary organic aerosol (SOA) and residential wood combustion. We observed 60% of the cloud water CHO molecular formulas to be identical to SOA samples of α-pinene, β-pinene, d-limonene, and β-caryophyllene ozonolysis. CHNO compounds had the highest number frequency and relative abundances and are associated with residential wood combustion and NOx oxidation. We observed multiple nitrogen atoms in the assigned molecular formulas for the nighttime cloud sample composite indicating the significance of nighttime emissions or NOx oxidation on the AOM composition. Several CHOS and CHNOS compounds with reduced sulfur (in addition to the commonly observed oxidized sulfur containing compounds) were also observed, however further investigation is needed to determine the origin of the reduced sulfur containing compounds. Overall, the molecular composition determined using ultrahigh resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry provides an unambiguous identification of the cloud water organic composition in the Rocky Mountain area which could help to improve the understanding of aqueous phase processes.
Publisher
Copernicus GmbH
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