Biogenic, urban, and wildfire influences on the molecular composition of dissolved organic compounds in cloud water
- 1Department of Chemistry, University of Michigan, Ann Arbor, MI, USA
- 2Michigan Society of Fellows, University of Michigan, Ann Arbor, MI, USA
- 3Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, USA
- 4Adirondack Lake Survey Corporation, Ray Brook, NY, USA
- 5Department of Earth & Environmental Sciences, University of Michigan, Ann Arbor, MI, USA
- anow at: Department of Environmental Sciences, University of California, Riverside, Riverside, CA, USA
- bnow at: Department of Chemistry, University of Washington, Seattle, WA, USA
- cnow at: College of Veterinary Medicine, Purdue University, West Lafayette, IN, USA
- dnow at: Department of Chemistry, Northwestern University, Evanston, IL, USA
- enow at: Department of Chemistry, Purdue University, West Lafayette, IN, USA
- *These authors contributed equally to this work.
Abstract. Organic aerosol formation and transformation occurs within aqueous aerosol and cloud droplets, yet little is known about the composition of high molecular weight organic compounds in cloud water. Cloud water samples collected at Whiteface Mountain, New York, during August–September 2014 were analyzed by ultra-high-resolution mass spectrometry to investigate the molecular composition of dissolved organic carbon, with a focus on sulfur- and nitrogen-containing compounds. Organic molecular composition was evaluated in the context of cloud water inorganic ion concentrations, pH, and total organic carbon concentrations to gain insights into the sources and aqueous-phase processes of the observed high molecular weight organic compounds. Cloud water acidity was positively correlated with the average oxygen : carbon ratio of the organic constituents, suggesting the possibility for aqueous acid-catalyzed (prior to cloud droplet activation or during/after cloud droplet evaporation) and/or radical (within cloud droplets) oxidation processes. Many tracer compounds recently identified in laboratory studies of bulk aqueous-phase reactions were identified in the cloud water. Organosulfate compounds, with both biogenic and anthropogenic volatile organic compound precursors, were detected for cloud water samples influenced by air masses that had traveled over forested and populated areas. Oxidation products of long-chain (C10−12) alkane precursors were detected during urban influence. Influence of Canadian wildfires resulted in increased numbers of identified sulfur-containing compounds and oligomeric species, including those formed through aqueous-phase reactions involving methylglyoxal. Light-absorbing aqueous-phase products of syringol and guaiacol oxidation were observed in the wildfire-influenced samples, and dinitroaromatic compounds were observed in all cloud water samples (wildfire, biogenic, and urban-influenced). Overall, the cloud water molecular composition depended on air mass source influence and reflected aqueous-phase reactions involving biogenic, urban, and biomass burning precursors.