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Atmospheric Environment (v.42, #12)

Editorial board (pp. i).

Antarctic Tropospheric Chemistry Investigation (ANTCI) 2003 by Fred L. Eisele Guest editors; Douglas D. Davis (pp. 2747-2748).

Antarctic Tropospheric Chemistry Investigation (ANTCI) 2003 overview by Fred Eisele; Douglas D. Davis; Detlev Helmig; Samuel J. Oltmans; William Neff; Greg Huey; David Tanner; Gao Chen; Jim Crawford; Richard Arimoto; Martin Buhr; Lee Mauldin; Manuel Hutterli; Jack Dibb; D. Blake; Steven B. Brooks; Bryan Johnson; James M. Roberts; Yuhang Wang; David Tan; Frank Flocke (pp. 2749-2761).

The Antarctic Tropospheric Chemistry Investigation (ANTCI) was carried out from late November to December 2003 with both extended ground-based and tethered balloon studies at Amundsen Scott Station, South Pole. ANTCI 2003 was the first of two Antarctic field studies with the primary goal of further exploring the active photochemistry of the South Pole region that was first identified in the previous Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) program. Since ISCAT was fully ground-based, ANTCI 2003 goals included expanding chemical studies both vertically upward to investigate mixing and horizontally to better understand large-scale plateau NO _x production and transport. Thus, in addition to ground-based experiments at South Pole, Twin Otter aircraft sampling took place out to hundreds of kilometers in several directions from the South Pole. These were designed to specifically address the issue of how representative past South Pole chemical measurements are of the surrounding high plateau region. The Twin Otter was also used to make transects along the coast both north and south of McMurdo Station. The present paper summarizes the overall setting and results of this investigation and highlights the many new findings that were obtained.

Keywords: Antarctic photochemistry; South Pole chemistry; Nitric oxide; Hydroxyl radical; Ozone

A study of boundary layer behavior associated with high NO concentrations at the South Pole using a minisodar, tethered balloon, and sonic anemometer by W. Neff; D. Helmig; A. Grachev; D. Davis (pp. 2762-2779).

This paper focuses on the use of an acoustic sounder, or sodar, during the 2003 Antarctic Tropospheric Chemistry Investigation (ANTCI), to document the behavior of very shallow (<50m) stable boundary layers thought to be one of the critical factors for explaining the very high levels of nitric oxide (NO) found in past field experiments at the South Pole. The use of a tethered balloon, profiling wind, temperature, NO, and ozone provided for a detailed interpretation of sodar data for the period 12–30 December 2003. For the same period, sonic anemometer 2-m turbulence measurements, averaged to 0.5h, linked surface processes to the evolution of the boundary layer in response to changing radiative balance and synoptic weather changes. A mixing-layer detection method was developed and applied to half-hour average sodar amplitude profiles for the period 23 November–30 December 2003. These data also allowed for testing of simple diagnostic equations for the mixing-layer depth as well as estimates of vertical diffusion rates under stable conditions, the latter being important for the effective depth of the mixing layer vis-à-vis the nonlinear NO chemistry postulated from earlier analyses. With the extended sampling period, two sub-seasonal regimes were examined: (1) a late-December period, with the full suite of supporting measurements, where the earlier results that shallow mixing layers associated with light winds and strong surface stability can be among the dominant factors leading to high NO levels were repeated and (2) a late November period that revealed additional complexities with very high NO concentrations appearing at times in concert with higher winds, weaker surface stability, and deeper mixing layers. The latter results are only consistent with a more complicated picture of how NO can build to very high levels that involves invoking the previously expressed dependence of elevated NO levels on nonlinear NO _x (NO _x=NO+NO₂) chemistry, greater fluxes of NO _x from the snowpack than previously observed at the South Pole, and the potential for enhanced NO _x accumulation effects involving air parcels draining off the high plateau. The results of ANTCI from 2003 thus argue for more complete future observations of boundary layer conditions over the high Antarctic Plateau and determination of the spatial and temporal variability of snow nitrate concentrations over the high plateau and their relation to NO recycling and the snow accumulation/ablation/erosion cycle.

Keywords: South Pole; Polar chemistry and meteorology; Sodar; Boundary-layer height

Evaluation of ozone measurements from a tethered balloon-sampling platform at South Pole Station in December 2003 by Bryan J. Johnson; Detlev Helmig; Samuel J. Oltmans (pp. 2780-2787).

Vertical boundary-layer ozone profiles were measured from a tethered balloon platform during the 2003 Antarctic Tropospheric Chemistry Investigation (ANTCI) at South Pole Station, Antarctica. Electrochemical concentration cell (ECC) ozonesondes were used in obtaining 128 ascent and descent profile measurements to about 500m height during 13–30 December 2003. Various data checks and intercomparisons were done to confirm the accuracy of the ozonesondes. The ozonesondes compared well to a surface ozone ultra-violet (UV) absorption monitor located next to the tether balloon site. During the 18-day period, ozonesonde measurement checks at the surface averaged 0.2±1.0ppbv higher than the continuous ozone measurements under ambient concentrations ranging from 18 to 51ppbv. This agreement was also consistent when compared to the nearby NOAA UV-monitor sampling at 17m above ground level during well-mixed conditions near the surface. In addition to the single ECC sonde profiles, five dual ECC ozonesondes were run on the tether platform. Four release balloon-borne ozonesondes were also launched during the project. Under very sharp ozone gradient events, the release ozonesonde (with a rise rate of ∼4–6ms⁻¹) passed through the gradient layer too quickly to capture the detail as measured by the controlled tethersonde at ∼0.3ms⁻¹ ascent/descent rate. Another method of ozone profiling was also done utilizing the UV monitor at the tether site and a 135-m-long Teflon sampling line with a sampling inlet mounted to and raised with the tethered balloon. The ECC ozonesonde averaged about 0.7±0.8ppbv lower than the long-line sampling method from eight profiles.

Keywords: Ozonesonde; Ozone measurements; Tropospheric ozone; Snow photochemistry

Elevated ozone in the boundary layer at South Pole by Detlev Helmig; Bryan Johnson; Samuel J. Oltmans; William Neff; Fred Eisele; Douglas D. Davis (pp. 2788-2803).

Vertical profile measurements of ozone, water vapor, and meteorological conditions, as well as surface and tower measurements of these parameters during the 2003 Antarctic Tropospheric Chemistry Investigation (ANTCI) yielded their vertical (between the surface and 500m) and temporal distribution in the boundary layer at South Pole (SP) during December 13–30, 2003. Ozone in the surface and lower planetary boundary layer above SP was frequently enhanced over lower free tropospheric levels. During stable atmospheric conditions (which typically existed during low wind and fair sky conditions) ozone accumulated in the surface layer to reach up to twice its background concentration. These conditions were correlated with air transport from the N–SE sector, when air flowed downslope from the Antarctic plateau towards the SP. These data provide further insight into the vigorous photochemistry and ozone production that result from the highly elevated levels of nitrogen oxides (NO _x) in the Antarctic surface layer.

Keywords: Antarctic plateau; Tropospheric ozone; Snowpack-atmosphere gas exchange; Snow photochemistry; Synoptic transport

Episodes of high surface-ozone amounts at South Pole during summer and their impact on the long-term surface-ozone variation by Samuel J. Oltmans; Bryan J. Johnson; Detlev Helmig (pp. 2804-2816).

Long-term surface-ozone and ozone-profile measurements are used to investigate the character of summertime ozone behavior at South Pole. Summer ozone profiles show a significant gradient more than 40% of the time in which mixing ratios at the surface are at least eight parts per billion by volume (ppbv) higher, and may exceed 20ppbv higher, than mixing ratios several hundred meters above the surface. These ozone gradients are linked to very stable conditions in the boundary layer. The frequency of occurrence of these surface-ozone enhancements has varied with time with the most recent 10-year period showing a greater number of occurrences. Although the summer enhancements have influenced the overall long-term pattern of change in surface ozone, they are not the only factor. The earlier decline in surface-ozone amounts that continued into the mid 1990s was influenced by changes in other seasons as well. Surface-ozone measurements from the 1960s show that summer enhancements were a significant feature of the record at South Pole during this period. Measurements at a lower elevation inland location (Byrd Station), not on the Antarctic Plateau, do not show large summer ozone chemical production events indicating that this phenomenon is primarily confined to the plateau.

Keywords: Tropospheric ozone; South Pole; Antarctica; ANTCI 2003

Nitric oxide in the boundary-layer at South Pole during the Antarctic Tropospheric Chemistry Investigation (ANTCI) by Detlev Helmig; Bryan J. Johnson; Matt Warshawsky; Thomas Morse; William D. Neff; Fred Eisele; Douglas D. Davis (pp. 2817-2830).

The vertical distribution of nitric oxide (NO) was investigated by profiling from a tethered balloon platform during the 2003 Antarctic Tropospheric Chemistry Investigation (ANTCI) at South Pole (SP), Antarctica. The lower atmosphere was probed between the surface and 120m height by pulling air from an inlet attached to the balloon through a thin-wall, 135m-long Teflon sampling line and by analyzing NO in this airflow with a ground-borne monitor. Losses and conversion of NO during the 2–4-min residence time in the sampling line were on average on the order of 6–16%, providing a feasible approach for the measurement of vertical NO profiles under SP conditions. NO was found to be highly variable within the lowest 100m of the atmosphere. Greatly enhanced NO mixing ratios were constrained to a shallow (20–50m height) air layer nearest to the surface, above which NO rapidly dropped to its mixed boundary layer background levels. Concurrent measurements of ozone and meteorological conditions provide insight into linkages between the ongoing snowpack and boundary layer nitrogen oxides (NO _x=NO+NO₂) and ozone chemistry. Since [OH] and [HO₂] are non-linearly coupled to absolute levels of NO _x, their concentrations and the rate of ozone production are expected to similarly show appreciable changes on small vertical scales during conditions with enhanced [NO _x].

Keywords: Antarctic plateau; Snowpack–atmosphere gas exchange; Snow photochemistry; Tethered balloon profiling; Nitric oxide; Ozone

A reassessment of Antarctic plateau reactive nitrogen based on ANTCI 2003 airborne and ground based measurements by Douglas D. Davis; Jon Seelig; Greg Huey; Jim Crawford; Gao Chen; Yuhang Wang; Marty Buhr; Detlev Helmig; William Neff; Don Blake; Rich Arimoto; Fred Eisele (pp. 2831-2848).

The first airborne measurements of nitric oxide (NO) on the Antarctic plateau have demonstrated that the previously reported elevated levels of this species extend well beyond the immediate vicinity of South Pole. Although the current database is still relatively weak and critical laboratory experiments are still needed, the findings here suggest that the chemical uniqueness of the plateau may be substantially greater than first reported. For example, South Pole ground-based findings have provided new evidence showing that the dominant process driving the release of nitrogen from the snowpack during the spring/summer season (post-depositional loss) is photochemical in nature with evaporative processes playing a lesser role. There is also new evidence suggesting that nitrogen, in the form of nitrate, may undergo multiple recycling within a given photochemical season. Speculation here is that this may be a unique property of the plateau and much related to its having persistent cold temperatures even during summer. These conditions promote the efficient adsorption of molecules like HNO₃ (and very likely HO₂NO₂) onto snow-pack surface ice where we have hypothesized enhanced photochemical processing can occur, leading to the efficient release of NO _x to the atmosphere. In addition, to these process-oriented tentative conclusions, the findings from the airborne studies, in conjunction with modeling exercises suggest a new paradigm for the plateau atmosphere. The near-surface atmosphere over this massive region can be viewed as serving as much more than a temporary reservoir or holding tank for imported chemical species. It defines an immense atmospheric chemical reactor which is capable of modifying the chemical characteristics of select atmospheric constituents. This reactor has most likely been in place over geological time, and may have led to the chemical modulation of some trace species now found in ice cores. Reactive nitrogen has played a critical role in both establishing and in maintaining this reactor.

Keywords: Antarctic plateau; Reactive nitrogen; Nitric oxide; Airborne profiles; Nitrate; Recycling; Hydroxyl radicals; Oxidizing canopy; Ice core chemical proxies

Assessing the photochemical impact of snowNOx emissions over Antarctica during ANTCI 2003 by Yuhang Wang; Yunsoo Choi; Tao Zeng; Douglas Davis; Martin Buhr; L. Gregory Huey; William Neff (pp. 2849-2863).

Surface and aircraft measurements show large amounts of reactive nitrogen over the Antarctic plateau during the ANTCI 2003 experiment. We make use of 1-D and 3-D chemical transport model simulations to analyze these measurements and assess the photochemical impact of snowNOx emissions. Boundary layer heights measured by SODAR at the South Pole were simulated reasonably well by the polar version of MM5 after a modification of ETA turbulence scheme. The average of model-derived snowNOx emissions(3.2–4.2×108moleccm-2s-1) at the South Pole is similar to the measured flux of3.9×108moleccm-2s-1 during ISCAT 2000. Daytime snowNOx emission is parameterized as a function of temperature and wind speed. Surface measurements of NO,HNO3 andHNO4, and balloon measurements of NO at the South Pole are reasonably simulated by 1-D and 3-D models. Compared to Twin Otter measurements of NO over plateau regions, 3-D model simulated NO concentrations are at the low end of the observations, suggesting either that the parameterization based on surface measurements at the South Pole underestimates emissions at higher-elevation plateau regions or that the limited aircraft database may not be totally representative for the season of the year sampled. However, the spatial variability of near-surface NO measured by the aircraft is captured by the model to a large extent, indicating that snowNOx emissions are through a common mechanism. An average emission flux of0.25kgNkm-2month-1 is calculated for December 2003 over the plateau (elevation above 2.5km). About 50% of reactive nitrogen is lost by deposition and the other 50% by transport. The 3-D model results indicate a shallow but highly photochemically active oxidizing “canopy” enshrouding the entire Antarctic plateau due to snowNOx emissions.

Keywords: Antarctica; Photochemistry and transport; Snow emissions; Nitrogen flux; ANTCI

Concentrations and sources of aerosol ions and trace elements during ANTCI-2003 by R. Arimoto; T. Zeng; D. Davis; Y. Wang; H. Khaing; C. Nesbit; G. Huey (pp. 2864-2876).

As part of the Antarctic Tropospheric Chemistry Investigation (ANTCI), bulk aerosol-particle samples collected at the South Pole were analyzed for nitrate, sulfate, methanesulfonate (MSA), selected trace elements and radionuclides. The samples were collected in the same manner as in the Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) campaigns of 1998 and 2000. The ANTCI mean sulfate (124ngm⁻³) and MSA (9.1ngm⁻³) concentrations were comparable to those during ISCAT, but high MSA and sodium and high MSA/sulfate in late November/early December indicated pervasive maritime influences during that time. Trajectory analyses indicate that the Weddell Sea and the Southern Ocean near Wilkes Land were probable sources for the ocean-derived sulfate. The transport of marine air occurs mainly in the buffer layer or free troposphere, and the rapid oxidation of biogenic sulfur to SO₂ appears to be the basis for the observed low MSA/sulfate ratios. Elements typically associated with mineral dust (Al, Fe, K) and other elements with continental sources (Pb, Sb, Zn) had higher concentrations during ANTCI than ISCAT. The mean filterable nitrate (f-NO₃⁻) concentration (280ngm⁻³) also was conspicuously higher than during ISCAT (39 and 150ngm⁻³). Several peaks in f-NO₃⁻ were synchronous with those for MSA and sulfate, but some samples had high f-NO₃⁻ but neither high MSA nor sulfate. While there is some evidence that nitrate or nitric acid is transported to SP from distant sources, local emissions of nitrogen oxides from the snow are a far more important source overall.

Keywords: Sulfur; Aerosol ions; Elements; Radionuclides; South Pole atmosphere

Antarctic polar plateau snow surface conversion of deposited oxidized mercury to gaseous elemental mercury with fractional long-term burial by Steven Brooks; Richard Arimoto; Steven Lindberg; George Southworth (pp. 2877-2884).

The role of the vast Antarctic polar plateau in the global mercury cycle was previously relatively unknown. Here, for the first time, mercury concentrations in snow and air, combined with vertical flux measurements at the South Pole (November–December 2003 and November 2005) have provided considerable insight into the cycling of this element through the Antarctic environment. These insights include observations showing atmospheric oxidized mercury depositing to the snow pack, subsequent photoreduction, and emissions of gaseous elemental mercury from the snow pack. Oxidized mercury (e.g., reactive gaseous mercury and fine particulate mercury) showed high concentrations (100–1000pgm⁻³) in the near-surface air, with these concentrations strongly correlated with vertical mixing rates and showing rapid surface deposition. This suggests that the troposphere over Antarctica is enhanced in oxidized mercury, with mercury cycling between elemental and oxidized states, and between the atmosphere and snow pack. Based on these limited measurements at South Pole, we estimate that the Antarctic polar plateau could sequester as much as 60 metric tons of Hg annually. These data also suggest that there could be a seasonal cycling of atmospheric mercury oxidation, deposition, and re-emission via photoreduction of 490 metric tons annually. This cycling is restricted to the annual sunlit period and peaks 3–4 weeks after solar maximum. To our knowledge, these provisional values represent the first estimates of the mercury balance and cycling for the extensive Antarctic polar plateau.

Keywords: Antarctica; Mercury; Snow

Springtime atmospheric mercury speciation in the McMurdo, Antarctica coastal region by Steven Brooks; Steven Lindberg; George Southworth; Richard Arimoto (pp. 2885-2893).

This paper describes springtime atmospheric mercury (Hg) speciation and snow pack mercury concentration measurements in the McMurdo/Ross Island sea ice region of Antarctica. Near-surface gaseous elemental mercury (GEM) depletions (to concentrations below our detection limit, <0.01ngm⁻³), similar to those shown to occur in the springtime Arctic, were observed and reactive gaseous mercury (RGM) and fine particulate mercury (FPM) were produced in significant quantities (average 116 and 49pg(Hg)m⁻³, respectively). GEM concentrations in the near-surface air were significantly enhanced during brief afternoon terrestrial snowmelt events. Snow pack total mercury was significantly elevated (40–430ngl⁻¹), with a maximum at the northern extent of the fast-ice (adjacent to the grease ice/freezing ocean surface), and lesser values towards the coast and on Ross Island, suggesting that, similarly again to recent Arctic results, marine halogens, released by the freezing sea surface, induce localized mercury depletion events. A possible secondary contributing source of local halogens and mercury are direct emissions from the active Ross Island volcano, Mt. Erebus.

Keywords: Antarctica; Mercury; Snow; Halogens; Bromine

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Atmospheric Environment (v.42, #12)