A23D-01
Where Does the Carbon Go During the Atmospheric Oxidation of Hydrocarbons?
The evaluation of the impacts of secondary organics on pollution episodes, climate, and the tropospheric oxidizing capacity requires modelling tools that track the identity and reactivity of organic carbon in the various stages down to the ultimate oxidation products. The fully explicit representation of hydrocarbon oxidation, from the initial compounds to the final product CO2, requires a very large number of chemical reactions and intermediate species, far in excess of the number that can be reasonably written manually. We developed a "self generating approach" to explicitly describe (i) the gas phase oxidation schemes of organic compounds under general tropospheric conditions and (ii) the partitioning of secondary organics between gas and condensed phases. This approach was codified in a computer program, GECKO-A (Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere). The approach allows prediction of multiphase mass budget on the basis of first principles. Here, the VOC/NOx/aerosol system is studied for the oxidation of various hydrocarbons ranging from C6 to C20 to explore the behavior of organic matter during atmospheric oxidation. We present results showing the time evolution of major chemical characteristics (speciation, multiphase mass budget, oxidation state, volatility), with a focus on secondary VOC relevant for the formation of organic aerosols and cloud chemistry.
A23D-02
Representing Organic Aerosol Evolution with a Two-dimensional Volatility Basis Set
Organic aerosols comprise a complex mixture of thousands of organic compounds, but our understanding and representation of how this mixture evolves over the roughly one week lifetime (from emission to deposition) of organic material in the atmosphere is only beginning to mature. It is clear from ambient observations that the large majority of organic material in ambient particles is highly oxidized. This oxidized material exists in the accumulation mode, having almost certainly arrived on those particles principally via condensation. We have recently introduced a volatility basis set (VBS), in which organics are lumped into bins separated by one order of magnitude in saturation concentration, C* (at 300 K). The VBS allows us to concisely represent the phase partitioning of thousands of organic compounds using well-established Pankow partitioning theory and less than a dozen lumped species. However, the VBS does not contain any information about the oxidation state of organics, and very different compounds can reside in the same bin (for example, levoglucosan -- C6H10O5 and pentacosane -- C25H52, both have C* of around 1 μg m-3). Furthermore, with the advent of high-resolution aerosol mass spectrometry, it is now possible to measure the atomic composition of organic aerosol constituents with some precision. Consequently, we have developed a 2D-VBS with O:C as the second dimension in addition to C*. By adding O:C, we greatly reduce the chemical diversity in individual bins, and we can now describe the chemical evolution of organic mixtures as they age due to free-radical attack as well as other processes. We shall use several model systems constrained by chamber data and ambient measurements to describe how the 2D-VBS can address aging of organic aerosol, including both single-component systems as well as complex mixtures.
A23D-03
Glyoxal as a Model for Production and Fate of Organics in the Troposphere
Organics play an important role in the atmosphere, both in the gas and condensed phase. One of the challenges facing atmospheric chemistry today is to understand not only the gas-phase kinetics and photo- chemistry of organic compounds in the atmosphere, but partitioning to and reactions within the condensed phase such as clouds and aerosol. Glyoxal is one of the simplest multifunctional compounds found in the atmosphere, and its complex chemistry makes it suitable as a model system through which detailed insight into many of the processes of general importance to the atmospheric chemistry of organics can be gained. We present the results of studies aimed at elucidating the gas phase photochemistry of glyoxal, the parameters controlling partitioning to aerosol, and the condensed phase chemistry of glyoxal. In the gas phase we consider the dependence of glyoxal concentrations on meteorological conditions and concentrations of other atmospherically relevant organic compounds (many of which are oxidized to glyoxal). In the condensed phase, we examine photochemical and oligomerization reactions, and condensation reactions with compounds such as ammonium and sulfate that are typically found in aerosol.
A23D-04
Organic aerosol formation in cloud droplets and aqueous particles
Current parameterizations of secondary organic aerosol formation (SOA) include the condensation and subsequent absorption of low-volatility organic species formed upon oxidation of various volatile precursors. Models that use these approaches usually underpredict the amount of newly formed SOA mass. One reason for this discrepancy might be additional chemical/physical processes and precursors that contribute to SOA mass. Recently it has been suggested by both model and laboratory studies that the oxidation of water-soluble species, e.g. glyoxal and related compounds, in an aqueous phase might contribute considerably to SOA mass. Whereas SOA formation in dilute cloud droplets has been included in a state-of-the-art multiphase chemistry model, uptake and chemical processes that occur in wet aerosol particles that are comprised of highly concentrated ionic solutions had not yet been evaluated. We will present model results from an updated version of our previously applied multiphase cloud-chemistry model focusing on the SOA formation from glyoxal. The aim of the study is the comparison of the SOA formation efficiencies in cloud droplets vs aqueous particles based on most recent laboratory studies. We will discuss the differences in chemical/physical processes that occur in these different reaction media and show the extent to which these aqueous phase processes might help to explain the gap between predicted and observed SOA masses.
A23D-05
HCHO Activity Gauges Ozone Production and Aerosol Production Rates in Both Urban and Far-Downwind Atmospheres
We have found a surprisingly informative decomposition of the complex question of smoggy ozone production
in a set of of expanding investigations starting from modestly smoggy Eastern North America (by NASA
aircraft, INTEX, July 2004) to rather polluted Flushing, NYC (Queens College, CAPTEX, July, 2001). In both
rural and very polluted situations, we find that a simple "contour graph" parameterization of the local
principal ozone production rate can be estimated using only the variables [NO] and jrads [HCHO]:
Po(O3) = c (jrads [HCHO])a [NO]b. The method immediately suggests a local
interpretation for concepts of VOC limitation and NOx limitation. We believe that the product jrads
[HCHO] gauges the oxidation rate of observed VOC mixtures in a way that also provides [HO2] useful for
the principle ozone production rate k [HO2] [NO], Mechanisms suggest that ozone production due to
RO2 is proportional to the HO2 process, hence we may capture all ozone chemical production. The
success of the method suggests that dominant urban primary-HCHO sources may transition to secondary
plume-HCHO sources, so that HCHO is never too far away from an evolving steady state with VOC reactivity.
Are there other, simple, near-terminal oxidized VOC's which help gauge ozone production and aerosol
particle formation? Regarding particles, we report on suggestive relationships between far-downwind (Atlantic
PBL) HCHO and very fine aerosol. Since jrads [HCHO] provides a reactive-flux rate, we may
understand distant-plume particle production in a more quantitative manner. Additionally, we report on a
statistical search in the nearer field for relationships between glyoxals (important penultimate aromatic and
isoprene reaction products) with ozone and aerosol production, looking for VOC's that might be most
implicated, e.g., aromatics and biogenics.
Note that all three of our variables jrads, [HCHO], and [NO] are relatively easily measured in
widespread air pollution networks, and all are deducible form space-borne observations. Estimation of [NO]
from [NO2] (the species observable from space) may require care. Wider measurement of HCHO and its
reactivity should be key environmental measurements, justifying relatively greater expeditures. Fortunately,
the use of the 3.58 micron microwindow for HCHO remote sensing allows more economical resolution of
HCHO than other techniques.
http://geo.arc.nasa.gov/sgg/chatfield/
A23D-06
Common Inorganic Salts Catalyze the Transformations of Organic Compounds in Atmospheric Aerosols
This presentation reports the discovery that inorganic salts that are ubiquitous in atmospheric aerosols are
efficient catalysts for the transformations of organic compounds in these aerosols, by reactions such as aldol
condensation or acetal formation.1 For some of these salts, these catalytic properties were not even
known in chemistry.2 Kinetic and product studies of these reactions will be presented for carbonyl
compounds such as acetaldehyde, acetone, and glyoxal,1,3 and compared with previously known
catalysts such as the recently discovered amino acids.4,5 These studies show that these salts make the
reactions as fast in typical tropospheric aerosols as in concentrated sulfuric acid. These reactions produce
secondary "fulvic" compounds that absorb light in the near UV and visible and would affect the optical
properties of aerosols.1,5 They would also account for the depletion of glyoxal recently reported in
Mexico city.3 Thus, while acid catalysis is several orders of magnitudes too slow to be significant in
tropospheric aerosols, this work identifies new processes that should be ubiquitous in these aerosols and
important for atmospheric chemistry. Refs.
1Noziere, B., Dziedzic, P., Cordova, A., Common inorganic
ions catalyze chemical reactions of organic compounds in atmospheric aerosols, Submitted, 2008. 2
Noziere, B., Cordova, A., A novel catalyst for aldol condensation reaction, patent pending 02/10/2007.
3Noziere, B., Dziedzic, P., Cordova, A., Products and kinetics of the liquid-phase reaction of glyoxal
catalyzed by inorganic ions, Submitted to J. Phys. Chem. A, 2008. 4Noziere, B., and Cordova, A., A
Kinetic and Mechanistic Study of the Amino Acid-Catalyzed Aldol Condensation of Acetaldehyde in Aqueous
and Salt Solutions, J. Phys. Chem. A, 112, 2827, 2008. 5Noziere, B., Dziedzic, P., and Cordova, A., The
Formation of Secondary Light-Absorbing "fulvic-like" Oligomers: A Common Process in Aqueous and Ionic
Atmospheric Particles?, Geophys. Res. Lett., 34, L21812, doi:10.1029/ 2007GL031300, 2007.
A23D-07
Atmospheric HULIS enhances pollutant degradation by promoting the Dark Fenton Reaction
Humic-like substances (HULIS) in the atmosphere are ubiquitous macromolecular substances that comprise a major fraction of the organic component of atmospheric aerosols. In this study we report first results showing that HULIS extracted from collected wood burning and pollution atmospheric particles enhances aqueous phase oxidation of model organic contaminants (pyrene and phenol) by promoting the dark Fenton reaction under atmospherically relevant conditions. The paucity of radical sources at night makes this reaction, which is not accounted for in cloud chemistry models, potentially quite important for understanding and quantifying in-cloud degradation of organic pollutants, and for understanding Fe oxidation state speciation in atmospheric waters.