Biogenic, urban, and wildfire influences on the molecular composition of dissolved organic compounds in cloud water

Abstract

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 AugustSeptember 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 (C 10−12 ) 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.

Molecular insights on aging and aqueous-phase processing from ambient biomass burning emissions-influenced Po Valley fog and aerosol

“…Laboratory studies focusing on simplified systems of only one or two precursor components have successfully recreated some of the complexity of ambient atmospheric samples (De Haan et al, 2011;Lee et al, 2013;Nguyen et al, 2013;Hawkins et al, 2016;Yu et al, 2016). A number of recent studies focusing on the molecular composition of cloud (Lee et al, 2012;Desyaterik et al, 2013;Pratt et al, 2013;Zhao et al, 2013;Boone et al, 2015;Cook et al, 2017) and fog (Mazzoleni et al, 2010;LeClair et al, 2012;Xu et al, 2017) chemistry have been reported. Together these studies indicate a clear importance of aqueous-phase reactions for the production of aqueous SOA, including the formation of organonitrates, organosulfates, and nitrooxy organosulfates.…”

“…Along with organonitrates, organosulfates are susceptible to hydrolysis in the aqueous phase, though high kinetic barriers under atmospheric conditions often slow these reactions and allow for the observation of these species in ambient samples (Darer et al, 2011;Hu et al, 2011). Organosulfates are often described in the liter-ature as the products of acid-catalyzed oxidation of biogenic terpenoids (Surratt et al, 2008;Pratt et al, 2013;Schindelka et al, 2013), but they have also been observed in biomasscombustion-influenced cloud water Cook et al, 2017). The formation of aqueous-phase products in aerosol, fog, and cloud waters greatly increases the complexity of organic aerosol.…”

Is there an aerosol signature of cloud processing?

“…As the smallest activated particles are smaller than 100 nm and thus the activated fraction is substantial, this change in O/C ratio is also reflected in the bulk O/C ratio in as the strong increase in mass is mostly due to sulfate which does not affect the O/C ratio. It can be expected that in biomass burning scenarios, cloud water might contain highly oxidized organics, and thus a high O/C ratio (Gilardoni -13 -et al, 2016;Cook et al, 2017). However, as the dissolved mass only comprises a small fraction of the total particle number, this oxidation might not affect bulk aerosol properties to a large extent.…”

“…by (Cook et al, 2017), in 360 cloud samples that were affected by biomass burning plumes. However, such analyses do not reveal the extent to which the total aerosol population might have been altered in the cloud.…”

Is there an aerosol signature of chemical cloud processing?

“…While in "hill cloud" experiments more continuous datasets can be collected, they are limited in their geographical coverage and their interpretation is complicated by variable advection of various sources and air masses. Many studies show enhanced concentrations of sulfate, oxalate, and related organics in cloud-processed air compared to cloud-free air (Crahan et al, 2004;Sorooshian et al, 2006a;2007a;Wonaschuetz et al, 2012). It has been recognized for several decades that globally a major fraction of sulfate is formed in clouds (Roelofs et al, 1998;Barth et al, 2000) and to a smaller extent also in deliquesced aerosol particles (Sievering et al, 1991;Alexander et al, 2005;Zheng et al, 2015).…”