A43F-01 INVITED
Snow and Ice Photochemistry in Polar Regions, and Interactions With a Changing Climate
Research in the Arctic over the past couple decades has revealed that a wide range of processes initiated at the frozen surface result in emission of photochemically reactive gases, e.g. HCHO, HONO, NOx, Br2, and BrCl to the overlying atmosphere. The resultant changes to the normally stable boundary layer can be quite dramatic, including the near complete depletion of ozone and elemental Hg in that layer, followed by deposition of the Hg oxidation products to the surface. For some species, these processes can impact both the composition of the lower atmosphere, and that of the snow/ice. Superimposed on the discovery of these processes is the knowledge that snowcover in coastal regions is changing, and sea ice areal extent (in September) is decreasing at as much as ~8%/decade. The snow/ice mediated chemical processes require a combination of physical and chemical conditions that are subject to change as a result of a changing climate. In this talk I will discuss the wide range of important processes mediated by snow and ice, some of the current ideas regarding how these processes may change over the coming decades, and what feedbacks and interactions there may be with climate change.
A43F-02 INVITED
A Tale of Two Oxidants: Hydroxyl Radical and Singlet Molecular Oxygen on Ice
The lifetimes for most compounds in the troposphere are determined by reactions with oxidants such as the hydroxyl radical (OH). This paradigm likely also holds true for many trace contaminants on snow and ice, especially organic species. However, we cannot currently estimate the lifetimes of these frozen contaminants because we do not know the steady-state concentrations of OH or other oxidants in/on snow or ice. To address this gap, we are currently working to measure the concentrations of OH and singlet molecular oxygen (1O2*), another potentially important oxidant, in illuminated ice samples prepared from laboratory solutions as well as polar snows. Our results show some surprising differences between the photochemistries of OH and 1O2* on ice. In the case of hydroxyl radical, the steady-state concentration in an illuminated ice sample is very similar to the value in an illuminated aqueous solution made from the same sample. In contrast, the behavior of 1O2* is very different between ice and solution for the same sample: steady-state concentrations in/on the ice are several orders of magnitude larger than in solution. We will discuss how these behaviors in oxidant concentrations likely reflect differences in the oxidant kinetics (i.e., rates of formation and lifetimes) between ice and solution. Based on our measured steady-state concentrations, reactions with OH will be an important sink for relatively recalcitrant contaminants in snow, while 1O2* might be a very rapid sink for electron-rich compounds in snow, such as phenols, alkenes, and polycyclic aromatic hydrocarbons.
A43F-03
A Re-assessment of Nitrogen Recycling on the Antarctic Plateau
Since the initial finding some 8-9 years ago that polar regions are not simply chemical graveyards, it has become increasing apparent that the physical and chemical processes taking place in the snowpack are quite complex. With regard to the specific snow emission species NOx, nowhere have atmospheric levels been observed to be higher than those seen on the Antarctic plateau with median values of NO/NOx typically being one order of magnitude higher than at other sites investigated. Several field studies, particularly those focused on the South Pole (SP) region, have been carried out with the purpose of gaining an improved understanding why this is so. Factors that have thus far been identified as contributing to this large difference include: 1) 24 hours of continuous solar radiation during the spring/summer months while also frequently experiencing the onset of shallow planetary boundary layer (PBL) depths; 2) a geographical location that is near a convergence point for air drainage from the central high plateau; and 3) extremely low average temperatures (e.g., low H2O levels) on the plateau which lead to a very weak primary source of HOx radicals. The latter condition, in particular, leads to NOx being a major controller of HOx radical levels. Thus, when in excess of ~250 pptv, the NOx lifetime increases with further increases in NOx, leading to a further ramping up of the NOx concentration. What has not yet been established is whether there is still a fourth factor which is related to differences in snow emission rates of NOx at the different sites. To be presented are new field data, the results of which strongly suggest that the plateau has the unique ability to recycle reactive nitrogen more than once within a given spring/summer season, thus enhancing the overall nitrogen emission rate.
A43F-04
Isotopes of HNO3 in Air and Snow at Summit, Greenland
The ice core record of nitric acid (HNO3, or nitrate, NO3-), a final sink for atmospheric NO and NO2, potentially contains valuable information about past changes in our atmosphere. Through reactions with OH and ozone, NOx (NO and NO2) is closely linked to the oxidizing capacity of the atmosphere. However, the interpretation of nitrate ice core records is complicated by post-depositional processing of nitrate, whereby photolysis and volatilization can result in nitrate losses from the snow surface. Quantitatively understanding the air-to-snow transfer of nitric acid is a necessary step in unraveling ice core records of nitrate. We present isotopic measurements (15N/14N and 18O/16O) of HNO3 in air and snow from Summit, Greenland. Preliminary results show that the δ15N of HNO3 is similar in surface snow and in air sampled 1.5 meters above the snow surface. The δ18O of HNO3 is significantly higher in the surface snow versus the air samples. Previous studies of nitrate in precipitation have suggested that the δ15N of nitrate is influenced by sources of precursor NOx while the δ18O of nitrate is influenced by the chemical pathways of nitrate production. We discuss the possibility that the difference in δ18O between the air and snow samples may be an indication that the air samples contain nitrate that was photolyzed and recycled from the snow surface. The similarity between δ15N in air and snow samples suggests that, despite the possibility of nitrate recycling, there is no net loss of nitrate from the snow surface. The utility of comparisons between variations in the δ15N of nitrate and changes in the source regions of air masses reaching Summit will also be discussed.
A43F-05
Multiphase Modeling of Nitrate Photochemistry in the Quasi-Liquid Layer: Implications for NOx Release from the Arctic and coastal Antarctic Snowpack
We utilize a multiphase model, CON-AIR (Condense Phase to Air Transfer Model), to show that the photochemistry of nitrate ( ) in and on ice and snow surfaces, specifically the quasi-liquid layer (QLL), can account for the fluxes of gas phase NOx and the flux ratios measured above the Arctic and Antarctic snowpack. Maximum gas phase NOx fluxes and ratios simulated for spring and summer range from ~ 4.0 × 104 to ~ 5.0 × 105 molecules cm-3 s-1 and ~ 0.84 to 1.86, respectively. The model incorporates the appropriate actinic light spectrum, thereby properly weighting the different rates of photolysis of and . This is important since the immediate precursor for NO, for example, absorbs at wavelengths longer than nitrate itself. Finally, one-dimensional model simulations indicate that both gas phase and exhibit a negative concentration gradient as a function of height although [NO]:[NO2] are approximately constant.
A43F-06
Laboratory Studies of the Atmospheric Chemistry of Ice: Roles of Surface Disorder and Morphology
We will present an overview of our recent laboratory work on the surface chemistry of ice under a wide range of conditions relevant to atmospheric chemistry. The results of these studies provide new information on the complex dependence of the trace gas-ice interaction on the surface roughness, grain size, and surface disordering of the substrate. Using the surface-specific technique ellipsometry, we demonstrated that trace, atmospherically relevant quantities of hydrogen chloride (HCl) can induce surface disorder on ice at temperatures as low as -80°C. We then investigated the effects of surface disorder on the heterogeneous chemistry of ice by using a low- temperature coated-wall flow tube reactor with chemical ionization mass spectrometry (CIMS) detection of the gas phase composition. We found that surface disorder enhances the reaction of HCl with chlorine nitrate on ice, and is necessary to explain the levels of chlorine activation observed during PSC events. Additionally, we found that the affinity of acetic acid for the ice surface increases in the presence of surface disorder. We also explored the effect of ice morphology on ice-trace gas interactions using the coated wall flow tube-CIMS technique. We investigated the interaction of HCl with three different ice substrates, which allowed us to observe the effects of surface roughness and domain size at a range of conditions corresponding to three unique surface states. Perspective will be given on the role of surface disordering on ice in the atmospheric chemistry of snow in polar regions.
A43F-07 INVITED
An Overview of Polar HOx Chemistry and Potential Bromine Chemistry Impact.
It is now well recognized that the snow-air exchange is a critical component of polar photochemical cycles. Observed large snow to air gradients as well as elevated levels of many reactive species, e.g., NO, CH2O, have inspired intense studies on the snow/firn air chemical processes and the impact of snow emissions on the atmospheric chemistry above the snow covered surface. The first observations of OH at the South Pole (SP) revealed levels comparable to those recorded in the tropical marine boundary layer. Modeling interpretive analysis later attributed these high OH levels to snow emissions of CH2O, H2O2, and NO, leading to enhanced HOx (OH + HO2) sources. The enhanced oxidation capacity, in turn, can rapidly oxidize NOx into HNO3 and HO2NO2, which deposit back to the snow surface. During summer of 2003, the HO and HO2 were simultaneously measured at Summit, Greenland for the first time. This rich data set allowed a more comprehensive analysis of arctic HOx chemistry. Modeling analysis suggests that snow emissions are an important Summit HOx source, but unlike the SP case, they were not dominant one. While model can well reproduce the observed HOx levels and diurnal variations when constrained by measured precursors, they typically over-predicts the HO2/OH ratio by about a factor of 2. In an extreme case, model overprediction is up to a factor of 8. The latter case involved high wind speed. Trajectory analysis suggests that the airmass was brought from the coast region within 2 days. This raises the speculation about the role of BrO. In light of Summit BrO observations during the summer of 2007, two model simulation scenarios will be discussed as related to the sensitivity of Summit HOx and other important photochemical species to BrO levels. The first one involves a fixed BrO level, while the second simulates the effect of a pulse high BrO injection into the summertime Summit environment.