A44B-01
Concentrations and Sources of Soot in Greenland Precipitation from 1788 to 2002: Implications for Radiative Forcing
Black carbon (BC) in the atmosphere results from biomass and fossil fuel combustion. It alters chemical and physical properties of the atmosphere and snow albedo, yet little is known about BC emission or deposition histories. Monthly resolved measurements of BC in an ice core indicate that concentrations in central Greenland precipitation varied greatly during the period of record from 1788 to 2002. Parallel measurements of vanillic acid and non-sea-salt sulfur in the same ice core suggest that BC in Greenland came from wildfires and industrial activities. Prior to 1850, BC concentrations were highest in late summer to autumn and resulted primarily from boreal forest fires. Beginning about 1850, industrial emissions resulted in a seven-fold increase in ice core BC concentrations, with most change occurring in winter. BC concentrations after about 1951 were lower, probably as a result of wildfire suppression policies and the shift from coal burning to oil and gas in North America. Late 20th century increases in BC, however, may be linked to coal combustion in the rapidly expanding economies of Asia. At its maximum from 1906 to 1910, estimated surface climate forcing in early summer from BC in Arctic snow was about 3 W per square meter, more than eight times typical pre-industrial forcing.
A44B-02
Uptake and Reactivity of Organic Compounds at the Air-Ice Interface
We have developed laser-induced fluorescence and Raman spectroscopic methods to probe the chemical environments of organic compounds in the quasi-liquid layer (QLL) at the air-surface boundary of ice and snow, as well as changes in the compounds' concentrations. The uptake and reactivity of the organic species to the QLL differ from those measured at the liquid water surface. Chemical reactions and physical interactions such as the self-association of aromatic hydrocarbons occur much more readily at the ice surface than at liquid water surfaces, even at temperatures close to the melting point. Pronounced differences in photochemical kinetics are observed in the QLL compared to those measured on liquid water surfaces. We will discuss these results, as well as experiments which probe ozonation rates in the QLL, as a function of temperature.
A44B-03
Adsorption and Uptake of Acetone on Ice Studied with Ambient Pressure Photoemission Spectroscopy
The interaction between small molecules and ice particles has broad implications in atmospheric chemistry. However, the influence of adsorbed molecules on the thickness and properties of the liquid-like layer at the ice surface is still not fully understood. We have used a combination of synchrotron-based Ambient Pressure Photoemission Spectroscopy (APPES) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) to investigate the adsorption and uptake of acetone on the ice surface at temperatures of 215-245 K. The combination of APPES with NEXAFS allows one to directly correlate the amount of adsorbed acetone with the state of the ice surface, in particular the presence of a liquid-like layer. Uptake measurements using the integrated C1s peak area as a function of acetone partial pressure indicate an adsorption energy of approximately 45 kJ/mol. In addition, high resolution C1s spectra as well as O and C K-edge NEXAFS both show little to no modification of the acetone molecule or the ice surface upon adsorption. The combined results indicate a weak interaction between the acetone molecule and ice surface, and the absence of a liquid-like layer both for clean and acetone covered ice at temperatures between 215-245 K.
A44B-04
Photochemical Transformations of Trace Persistent Organic Pollutants in Snow and Ice
Polar snowpacks have been established as active matrices for chemical processing of atmospheric species such as molecular halogens, volatile organics and ozone. It has previously been demonstrated that photochemical transformations of anthropogenic persistant organic pollutants (POPs) in ice occur under laboratory conditions, but there is little in the published literature regarding these processes. In the study described here, we consider the effect of near ultraviolet radiation on trace amounts of various organochlorine compounds in ice; the compounds selected are of interest because of their known presence in polar regions and their detrimental effect on organisms in polar ecosystems. Both direct photochemical transformations and indirect photo-oxidation by OH radical were investigated. To differentiate between pure liquid-phase, solid phase and interfacial processes, experiments were conducted on POPs in aqueous solution, ice and crushed ice/snow at various temperatures. In all cases, the degradation of the POP was monitored by the extraction of photolyzed samples followed by GC analysis. On the basis of these degradation studies, some reactive systems were selected for more detailed investigation. Reaction products were identified from irradiated samples by SPME with GCMS analysis. On the basis of the kinetic studies and identified reaction products, mechanisms for selected photochemical transformations are proposed. Potential implications of these transformations for environmental partitioning and interactions of the selected POPs will be discussed.
A44B-05
Measurements of light alkanes (C2-C4) in firn air at Summit, Greenland and West Antarctic Ice Sheet Divide, Antarctica: Is there evidence for a recent decline in polar tropospheric levels?
Light alkanes are an important part of the tropospheric photochemical system, acting as precursors for ozone (O3) and carbon monoxide (CO) and as a removal mechanism for the hydroxyl radical (OH). In this study, we report measurements of ethane (C2H6), propane (C3H8), and n-butane (n- C4H10) in firn air collected at Summit, Greenland (May-June 2006) and West Antarctic Ice Sheet Divide (WAIS-D) (Dec-Jan 2005-2006). C2H6, C3H8, and n-C4H10 levels in Summit firn were in the 1.5-2.0 ppb, 400-600 ppt, and 150-250 ppt range, respectively. These levels are within the range of modern mean annual levels in surface air. C2H6, C3H8, and n-C4H10 mixing ratios measured in the WAIS-D firn were much lower than the Summit values, ranging from 200-300 ppt, 20-40 ppt, and 10-20 ppt, respectively. This is consistent with expectations from the interhemispheric differences in the distribution of sources for these short-lived gases. The reliability of firn air as an archive for tropospheric levels of light alkanes was assessed by comparison of firn air records to surface air flask measurements. If the firn air alkane data are interpreted as atmospheric histories, the depth profiles suggest that there has been a decline on the order of 20-40 % in annual mean levels over Greenland during the last two decades of the 20th century. Interestingly, WAIS-D data also suggest a modest decline in C2H6 levels over the West Antarctic Ice Sheet around the same time period. Trends in C3H8 and n-C4H10 data from WAIS-D are harder to interpret due to higher noise in the measurements resulting from lower background levels.
A44B-06
Reconstructing Atmospheric Histories of Halogenated Compounds to Preindustrial Times Using Antarctic Firn Air
Atmospheric histories of many halogenated trace gases remain poorly known, hampering understanding of lifetimes and anthropogenic impacts. A profile of air samples dating back to the late 19th century was collected from the firn at the Megadunes site in central Antarctica (80.78° S, 124.5° E) in January 2004. A number of anthropogenic halogenated compounds were measured in these samples using the AGAGE Medusa gas chromatograph-mass spectrometer instrumentation (B. R. Miller et al., in preparation). A firn gas-diffusion forward model based on the work of Schwander et al. (1993) was tuned to CO2 and 15N observations from the same Megadunes site. The age distribution of CO2 in diffusively mixed air samples collected at each depth was approximated by running short pulses through the forward model. The atmospheric histories of a number of halogenated compounds were then reconstructed using the iterative dating technique developed by Trudinger et al. (2002). The modeled age spread at this site is relatively broad, but interstitial air at the close-off zone is comparatively old with a mean age of about 100 years. Reconstructed histories show good agreement with direct measurements, although rapid changes are not well resolved. The mixing ratios of the deepest layer are within the range of preindustrial estimates, most notably for tetrafluoromethane. Schwander, J., J. M. Barnola, C. Andrie, M. Leuenberger, A. Ludin, D. Raynaud, B. Stauffer (1993). The Age of the Air in the Firn and the Ice at Summit, Greenland. J. Geophys. Res. 98(D2): 2831-2838. Trudinger, C. M., D. M. Etheridge, G. A. Sturrock, P. J. Fraser, P. B. Krummel, and A. McCulloch (2004). Atmospheric histories of halocarbons from analysis of Antarctic firn air: Methyl bromide, methyl chloride, chloroform, and dichloromethane. J. Geophys. Res. 109(D22310): doi:10.1029/2004JD004932.
A44B-07 INVITED
Modeled Anthropogenic Halocarbon Trends and Budgets Constrained With Firn air Data
Firn air pumping allows to retrieve large amounts of air and thus to measure low concentration trace gases. These measurements provide information on atmospheric trends over longer periods than the atmospheric records. Within the FIRETRACC and CRYOSTAT EC project, halocarbon measurements in firn air were performed at five Arctic and Antarctic sites. The trends and budgets of 20 anthropogenic halocarbon species were modeled using a 1-D model of gas diffusion in firn and a 2-D model of tropospheric and stratospheric chemistry run over the last century. The chemistry model was constrained with best estimates of reported and non-reported emissions, and validated by comparison with atmospheric data sets. The modeled atmospheric concentration trends were then used to constrain the firn diffusion model and simulate firn air measurements. For most species, the predicted trends are consistent with both atmospheric and firn air measurements within the known uncertainty limits. The modeled halocarbon budgets were compared with the 2006 scientific assessment of ozone depletion, and uncertainties on emissions, lifetimes, measurement calibration, etc. are discussed. Strong temporal changes in the lifetime of halocarbons destroyed only in the stratosphere were obtained. They reflect changes in the vertical structure of their atmospheric concentrations. Firn diffusivity profile is a key parameter for modeling trace gas diffusion in polar firn. Systematic deviations, observed for several species, between measured and predicted concentrations in the firn at a specific site can be used to improve the firn diffusivity profile. Finally, a Green function approach was used to calculate the gas age probability density at a specific depth in the firn. This is used to discuss results at the bottom of the firn in terms of possible natural background concentrations of halocarbon species, and the impact of back diffusion to the atmosphere for species with recently decreasing trends.