A21D-01 08:00h
Overview of the Antarctic Tropospheric Chemistry Investigation (ANTCI)
The first ANTCI campaign took place during the austral spring/early summer from the second half of November through December of 2003. It consisted of ground based measurements at the South Pole and Twin Otter aircraft studies over portions of the polar plateau, glacial valleys, and the coast of the Ross Sea. The study makes use of previous findings of the Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) and Sulfur Chemistry of the Antarctic Troposphere Experiment (SCATE) to define and address critical questions about what controls oxidation rates in the Antarctic troposphere and how these rates influence the evolution of reactive sulfur and nitrogen gases in the atmosphere prior to their incorporation into the ice core record. Ground based studies focused on vertical mixing and local photochemistry and sulfur chemistry. Boundary layer mixing heights were determined by several different methods including: acoustic sounder measurements up to 80 m, tethered balloon measurements of meteorological parameters and ozone to 500 m, and tethered balloon measurements of NO to 100m. These vertical measurements were designed to supplement a much larger suite of chemical measurements made from the South Pole ARO building in order to explore how chemical emissions from the snow mix into the lower troposphere and influence the photochemical oxidation process and its products. Aircraft measurements began the process of defining the horizontal extent of elevated NO concentrations over the plateau, which is presumed to result in enhanced photochemical oxidation rates. Flights also briefly investigated flow along large glacial valleys and the outflow of reactive nitrogen from the continent and the inflow of reactive sulfur gases like dimethyl sulfide.
A21D-02 08:15h
Boundary-Layer Ozone Production at South Pole, Antarctica
The distribution of ozone in the surface and planetary boundary layer was studied during the 2003/2004 Antarctic Tropospheric Chemistry Investigation (ANTCI) at South Pole, Antarctica from Dec. 13-30, 2003. A tethered balloon sampling platform was utilized for high spatial and temporal resolution measurements of ozone, temperature, wind speed, wind direction and water vapor between the surface and 500 m above ground. During several occasions ozone production of up to ~ 20 ppb was observed in the surface layer. These periods lasted several days and coincided with conditions of suppressed vertical mixing. The data further emphasize that ozone is produced in the Antarctic planetary boundary layer during the austral summer by photochemical reactions and due to the presence of large amounts of NO emitted from the snow. The tethered balloon data allow an in-depth analysis of the meteorological and chemical conditions that determine ozone production and transport at South Pole.
A21D-03 08:30h
The atmospheric boundary layer and its effect on NO concentrations during ANTCI-2003
The Antarctic Tropospheric Chemistry Investigation (ANTCI) field program, during November and December 2003 at the South Pole, deployed a number of measurement systems to document the behavior of the atmospheric boundary layer (ABL) and the associated chemical processes that lead to high values of NO. These measurements included an acoustic sounder to provide continuous measurements of the ABL through the entire experimental period. During a special observing period from December 13 to 30, a sonic anemometer was used to measure the surface fluxes of momentum and heat together with a tethered balloon that profiled NO, O3 and meteorological variables (see Helmig et al., this conference). Our analysis has focused on developing a comprehensive picture of the ABL and the meteorological processes that control its behavior. We used both meteorological and chemical data to trace the evolution of the ABL through transitions between well-mixed states (deeper than 200m) and ones with very shallow mixing layers (20 to 30 m) characterized by strongly suppressed turbulence and low-level jets (30 to 40 m above the surface).
A21D-04 08:45h
First Observations of Atmospheric Hydrogen Peroxide (H$_{2}$O$_{2}$) and Methylhydroperoxide (CH$_{3}$OOH) in West Antarctica: Comparison of Experiment and Model Results
Hydrogen peroxide (H$_{2}$O$_{2}$) and higher organic peroxides such as methylhydroperoxide MHP (CH$_{3}$OOH) are closely linked to chemical feedback mechanisms controlling the composition of the atmosphere. Recent findings in Central Greenland and at the South Pole show that a physical snowpack source is contributing significantly to boundary-layer H$_{2}$O$_{2}$. This has important implications for the current understanding of HO$_{x}$ radical chemistry above extended snow and ice surfaces as well as for the quantitative interpretation of the H$_{2}$O$_{2}$ record from ice cores. Here we report atmospheric measurements of hydroperoxides from three U.S. ITASE traverses, 2000-03. The wide spatial distribution of the 21 traverse sites between $75\deg$S and $90\deg$S, allows investigation of the peroxide photochemistry in the summer troposphere and the impact of the upper snowpack on the boundary layer in varying depositional environments, such as up to a 5-fold change in accumulation rate (8-44 cm SWE/yr) and a 30 K difference in mean annual temperature (-21.4 to $-49.3\deg$C). The only higher organic peroxide detected using a continuous-flow HPLC method is MHP, as expected in the remote atmosphere. Site averages of both, H$_{2}$O$_{2}$ and MHP showed a latitudinal gradient between $75\deg$S and $90\deg$S, decreasing from $803\pm150$ pptv to $230\pm56$ pptv and from $491\pm296$ pptv to $102\pm41$ pptv respectively. The MHP:H$_{2}$O$_{2}$ ratios varied between 0.4 and 2.5 with the higher values occuring during storm events as a consequence of the large difference in water solubility. Model runs using the NASA-Goddard Flight Center (GSFC) point photochemical model match observations of peroxides within the experimental uncertainties only by introducing source fluxes of H$_{2}$O$_{2}$ and HCHO, estimated from measured firn-ambient air ratios. The best model fits put some constraints on levels as well as snowpack fluxes of atmospheric NO$_{x}$ (not measured), ranging from 20 pptv at Byrd Surface Camp ($80\deg$S) to 30-40 pptv at the South Pole.
A21D-05 09:00h
HO$_{x}$ at the South Pole: Measurements and Model Predictions
Recent studies have revealed an unexpected and complex chemistry in the lower atmosphere at the South Pole. The combination of low sun angle and low water concentrations would be thought to produce minimal OH concentrations. However during ISCAT-1 and ISCAT-2000, consistently high levels OH were observed and found to be a result of elevated NO levels cycling HO$_{2}$ back into OH. The Antarctic Tropospheric Chemistry Investigation (ANTCI) was conducted at the South Pole during the austral spring of 2003 (22 November to 29 December). This program builds upon the previous ISCAT studies conducted at the South Pole in November-December of 1998 and 2000. Once again elevated levels of OH were observed (peak values of ~4-6 x 10$^{6}$ molecule cm$^{-3}$) and found to correlate with NO concentrations. HO$_{2}$ (HO$_{2}$ + RO$_{2}$) measurements which were interleaved with those of OH, were found to vary as expected in an anti-correlated manner with NO concentrations. Results of measurements and model simulations of OH and HO$_{2}$ will be presented. Special attention will be paid to the occurrence of a partial solar eclipse (86%), during which the concentrations of OH, HO$_{2}$, and NO all responded to the rapid changes in sunlight.
A21D-06 INVITED 09:15h
Modeling and analysis of atmospheric chemistry over the South Pole during ANTCI
The Antarctic Tropospheric Chemistry Investigation (ANTCI) was conducted at the South Pole during the austral spring of 2003 (22 November to 29 December). This program builds upon the Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) study also conducted at the South Pole in November-December of 1998 and 2000. Although the ISCAT results revealed that very little gas phase sulfur reaches the South Pole, observations of unexpectedly high levels of NOx (routinely reaching a few hundred pptv) were found due to snowpack emissions into the shallow boundary layer of the polar plateau. These NOx emissions in the presence of 24-hr sunlit conditions were also critical to the enhanced OH levels that were observed. Here, the latest measurements from the South Pole are examined in the context of an expanded array of deployed instrumentation. Particular attention will be paid to the interdependence between HOx and NOx with NOx values encompassing a broader range (<10 pptv to ~1 ppbv) than previously observed during the ISCAT experiments. Also of special interest will be observations during the partial (86%) eclipse on 23 November, 2003 which provided a natural setting for testing the internal consistency of the measurements through their response to rapid changes in sunlight conditions. Taking advantage of measurements of HNO3, HO2NO2, and HONO; discussion of the NOy budget will focus on the influence of boundary layer chemical cycling on post-depositional losses of nitrate observed in ice cores from across the polar plateau.
A21D-07 INVITED 09:30h
Airborne Observations of NO, NOy, Particles, and Other Trace Gases Over the Antarctic Continent During ANTCI 2003
The Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) field studies in 1998 and 2000 generated several surprising atmospheric-chemistry findings. Based on observations at South Pole's (SP) Atmospheric Research Observatory, very little gas phase sulfur was measured during these studies, but unexpectedly high levels of NO were recorded (e.g., 650 pptv). Although it has now been convincingly demonstrated that the source of this NO is its emission from the snowpack due to the photolysis of nitrate ions, major unanswered questions persist. Among the more significant of these has been the issue of "the spatial extent of this phenomenon". Do these highly elevated levels of NO at SP occur only in the immediate region surrounding SP? If not, how far out do they range over the 1000+ km wide Antarctic plateau? Equally important is the question: what does the altitudinal profile of NO look like over the plateau, and does the presence of elevated NO lead to the formation of an OH oxidizing canopy over this region? There also remain the pivotal issues of identifying the primary source of nitrogen to the plateau as well as determining the major pathways by which this reactive nitrogen is lost from near surface snow (~ 1m). During the Antarctic Tropospheric Chemistry Investigation (ANTCI), an airborne platform was added to extend the sampling already taking place at SP. The Twin Otter aircraft provided the opportunity to examine several of the issues raised above requiring three-dimensional sampling. Over the time period of 27 November to 6 December 2003, a total of 10 data collection flights were flown. Major species sampled were NO, NOy and CN particles. The NO and NOy were measured using a chemiluminescence instrument equipped with a Molybdenum converter, and particles were measured using a standard CN counter. More limited observations were made of DMS, CHBr3, CH3I, CH3ONO2, and other low molecular weight NMHC's using evacuated canisters in conjunction with grab-sampling. Details concerning the results from these flights will be presented.
A21D-08 09:45h
Interannual variability of surface NOx at the South Pole
Three field measurements (ISCAT 1998, 2000 and ANTCI 2003) have shown high concentrations of surface NOx at the South Pole. They also show a large interannual variability in the occurrence of high NOx events. We have used the polar version of NCAR/Penn State MM5 to assimilate the meteorological fields for the three field experiments using the ECMWF meteorological products and surface and rawinsonde observations. 1-D column and 3-D chemical transport models, driven by polar MM5 assimilation fields, are applied to simulate the evolution of surface NOx and other observed trace gases at the South Pole during the three experiments. We show that meteorological variability is a major contributor to the observed interannual variability. The 3-D model results imply a photochemical oxidizing canopy over Antarctica due to surface NOx emissions.