Secondary organic aerosol formation in haze particles and cloud droplets
Secondary organic aerosol (SOA) formation is usually described by condensation of (semi)volatile gas phase species onto the particle phase. However, large discrepancies between observed and modeled SOA mass loadings reveal that these processes are insufficient to explain ambient SOA growth rates. In addition, no current model can comprehensively explain the formation of oligomers and organosulfur and -nitrogen compounds that have been identified in ambient particles. Over the past decade, many laboratory, field and model studies have shown that chemical processes in dilute cloud droplets efficiently form SOA mass. Although these processes can represent a significant SOA source in cloudy conditions, they do not properly reflect chemical pathways in aqueous haze particles: Recent laboratory studies suggest that (i) the uptake of organics into such particles is much more efficient than into pure water and (ii) high concentrations of inorganic ions and water-soluble organics lead to unique products. However, to date there is no estimate of the efficiency of such processes in aqueous haze particles as compared to other SOA sources. Despite different experimental conditions in various recent laboratory experiments of glyoxal reactions on particles, all studies revealed (ir)reversible conversion of glyoxal in deliquesced particles into higher molecular weight organics. A parcel model has been modified to describe these glyoxal processes under atmospheric conditions (aerosol loadings, glyoxal mixing ratios). Results reveal that efficient uptake and subsequent reaction of glyoxal in sulfate particles can lead to considerable amounts of SOA mass in haze particles at RH >70%. The yields of individual products (oligomer, organosulfur, -nitrogen compounds) will be discussed. As previously shown, SOA(cloud) yields can be expressed as a simple function of liquid water content and reaction time in clouds. In a similar approach, SOA(haze) yields have been parametrized as a function of hygroscopicity and amount of preexisting aerosol mass and ambient relative humidity. Finally, the SOA efficiency of SOA(haze) and SOA(cloud) will be compared for various conditions (time scales, cloud types), and whether these sources can significantly help to close the gap between observed and modeled SOA mass.
Recent Studies Investigating Secondary Organic Aerosol Formation
The metropolitan areas of Mexico City and Atlanta have very different emissions and meteorology, yet in both cities secondary organic aerosol (SOA) comprises a significant fraction of fine particle mass. SOA in Mexico City is predominately from anthropogenic emissions and a number of studies have investigated the role of dicarbonyl partitioning to aerosol liquid water as a SOA formation route [Volkamer et al., 2006; 2007]. Hennigan et al.  noted a high correlation between SOA (measured as water-soluble organic carbon) and fine particle nitrate in Mexico City and used this to estimate the volatility of both species during periods of rapidly decreasing RH in late morning. Secondary aerosol may also form when particles are much drier. In Mexico City, both nitrate and SOA were also frequently observed and highly correlated in late afternoon when RH was below 30 percent. A thermodynamic model could reproduce the observed morning nitrate under high RH when equilibrium was between nitric acid and dissolved nitrate, whereas equilibrium between vapor and crystalline ammonium nitrate was predicted in the afternoon [Fountoukis et al., 2007]. By analogy, these results may suggest two different SOA partitioning mechanisms in Mexico City, occurring at different times of the day. In contrast, measurements suggest that SOA in the southeastern United States is largely from biogenic precursors, and there is evidence that liquid water also plays a role. The stability of dissolved organic aerosol in response to loss of liquid water is currently being investigated and preliminary data suggest that like Mexico City, there is some degree of volatility. Recent experiments comparing data from rural-urban sites shows that there are periods when anthropogenic emissions also substantially contribute to SOA in the Atlanta metropolitan region. However, the mechanisms, or organic precursors involved, are yet to be determined. Results from these various ongoing studies will be presented. Fountoukis, C., et. al., Atm. Chem. Phys. Discuss., 7, 9203-9233, 2007. Hennigan, C, et. al., Atm. Chem. Phys., 8, 3761-3768, 2008. Volkamer, R., et. al., Geophys. Res. Lett., 34, L19807, 2007. Volkamer, R., et. al., Geophys. Res. Lett., 33, L17811, 2006.
Analysis of Nitrogen Containing Organic Compounds in Biomass Burning Aerosols Using High Resolution Mass Spectrometry
Chemical characterization of atmospheric aerosols presents a serious analytical challenge because of the complexity of particulate matter analyte composed of a large number of compounds with a wide range of molecular structures, physico-chemical properties, and reactivity. In this study the chemical composition of the nitrogen containing organic (NOC) constituents of biomass burning aerosol (BBA) samples is characterized by high-resolution electrospray ionization mass spectrometry (ESI/MS). Accurate mass measurements combined with MS/MS fragmentation experiments of selected ions were used to assign molecular structures to individual NOC species. Our results indicate that N-heterocyclic alkaloid compounds - species naturally produced by plants and living organisms - comprise a substantial fraction of NOC in BBA samples collected from test burns of five biomass fuels. High abundance of alkaloids in test burns of ponderosa pine - a widespread tree in the western U.S. areas frequently affected by large scale fires - suggests that N-heterocyclic alkaloids in BBA may play a significant role in dry and wet deposition of fixed nitrogen in this region. Atmospheric processing and chemical transformations of alkaloids in the particulate phase will be discussed.
Effect of Slow Aging Reactions on Optical Properties of Secondary Organic Aerosol Prepared by Oxidation of Selected Monoterpenes
Organic particulate matter (PM) has a major impact on atmospheric chemistry, climate, and human health. Secondary organic aerosol (SOA) accounts for a rather significant fraction of organic PM; this includes SOA produced by oxidation of biogenically emitted monoterpenes. Once such SOA is formed, it is believed to undergo slow aging processes, which may have large effects on the physical and chemical properties of the particles. This presentation focuses on the effect of slow chemical aging on optical properties of SOA formed from the ozone-induced oxidation of limonene, myrcene, and other selected monoterpenes. Several complementary techniques including high resolution electrospray ionization mass spectrometry, FTIR spectroscopy, UV/vis spectroscopy, NMR spectroscopy, 3D-fluorescence spectroscopy, and photodissociation spectroscopy are used to probe the aging-induced changes in physical properties and chemical composition of laboratory generated SOA. Limonene SOA appears to undergo a dramatic change in its absorption spectrum on a time scale of hours; it develops strong visible bands in the 400-500 nm region, and becomes fluorescent. This transformation is catalyzed by ammonium sulfate and certain amino acids. This rather unusual aging process can potentially contribute to the formation of brown carbon in biogenic SOA.
Organic Aerosol Production from Methylglyoxal
Recent modeling suggests that methylglyoxal may form 27 percent of atmospheric SOA (8 Tg C/yr) if it is irreversibly taken up by clouds and aerosol with an uptake coefficient of 0.0029 (Fu et al. 2008 JGR 113 D15303), less than that measured in two lab studies. Once in a cloud, methylglyoxal may be chemically transformed via oxidation, self-reaction, or reaction with other compounds. All of these processes can combine to prevent re-evaporation. We describe the ability of methylglyoxal to form oligomers with itself, with methylamine, and with ammonium salts in evaporating droplets in lab simulations of cloud processing. Products and reaction kinetics are analyzed by high-resolution time-of-flight aerosol mass spectrometry (HR- ToF-AMS), electrospray ionization mass spectrometry (ESI-MS) and proton nuclear magnetic resonance (1H- NMR). Product molecules are non-volatile, and their formation is irreversible and accompanied by browning. These reactions suggest that SOA formation by methylglyoxal may be very significant.
Sources and Properties of Organic Material in Aqueous Atmospheric Aerosol Particles
We have observed the formation of high-molecular-weight and light-absorbing secondary organic aerosol material in mildly acidic aqueous mixtures of 1,2-dicarbonyls and inorganic salts meant to mimic the composition of aqueous aerosols. The organics studied include the VOC oxidation products glyoxal, methylglyoxal, oxalic acid, and mixtures thereof. The solutions studied were saturated in the salt of interest, and the concentration of organics in the mixture was maintained at 1-10% of the solute mass, consistent with ambient aerosol composition. The product mixtures were characterized using UV-Vis spectrophotometry, MALDI-MS, and pendant drop tensiometry. The results suggest a mechanism involving the participation of the ammonium ion. Kinetics studies show that substantial product formation occurs within hours. If these products form in atmospheric aerosol particles, they could change the radiative properties of the seed aerosol over its lifetime in the atmosphere.