Atmospheric Environment (v.38, #32)

An overview of ISCAT 2000 by D.D. Davis; F. Eisele; G. Chen; J. Crawford; G. Huey; D. Tanner; D. Slusher; L. Mauldin; S. Oncley; D. Lenschow; S. Semmer; R. Shetter; B. Lefer; R. Arimoto; A. Hogan; P. Grube; M. Lazzara; A. Bandy; D. Thornton; H. Berresheim; H. Bingemer; M. Hutterli; J. McConnell; R. Bales; J. Dibb; M. Buhr; J. Park; P. McMurry; A. Swanson; S. Meinardi; D. Blake (5363-5373).
The Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) took place over the timer period of 15 November to 31 December in the year 2000. The study location was the Amundsen Scott Station in Antarctica. ISCAT 2000 defines the second phase of a program designed to explore tropospheric chemistry in Antarctica. As in 1998, the 2000 ISCAT study revealed a strong oxidizing environment at South Pole (SP). During the 2000 investigation, however, the suite of measurements was greatly expanded. These new measurements established the recycling of reactive nitrogen as a critical component of this unique environment. This paper first presents the historical background leading up to the ISCAT 2000 observations; then it focuses on providing a summary of the year 2000 results and contrasts these with those recorded during 1998. Important developments made during the 2000 study included the recording of SP data for several species being emitted from the snowpack. These included NO, H2O2 and CH2O. In this context, eddy-diffusion flux measurements provided the first quantitative estimates of the SP NO and NO x snow-to-atmosphere fluxes. This study also revealed that HNO3 and HO2NO2 were major sink species for HO x and NO x radicals. And, it identified the critical factors responsible for SP NO levels exceeding those at other polar sites by nearly an order of magnitude. Finally, it reports on the levels of gas phase sulfur species and provides evidence indicating that the absence of DMS at SP is most likely due to its greatly shorten chemical lifetime in the near vicinity of the plateau. It is proposed that this is due to the influence of NO on the distribution of OH in the lower free troposphere over a region that extends well beyond the plateau itself. Details related to each of the above findings plus others can be found in the 11 accompanying Special Issue papers.
Keywords: Antarctica; South Pole; Photochemistry; Snow emissions; ISCAT; Overview;

South Pole NO x Chemistry: an assessment of factors controlling variability and absolute levels by D. Davis; G. Chen; M. Buhr; J. Crawford; D. Lenschow; B. Lefer; R. Shetter; F. Eisele; L. Mauldin; A. Hogan (5375-5388).
Several groups have now shown that snow covered polar areas can lead to the release of NO x to the atmosphere as a result of the UV photolysis of nitrate ions. Here we focus on a detailed examination of the NO observations recorded at South Pole (SP). Topics explored include: (1) why SP NO x levels greatly exceed those at other polar sites; (2) what processes are responsible for the observed large day to day NO concentration shifts at SP; and (3) possible explanations for the large variability in NO seen between SP studies in 1998 and 2000. As discussed in the main body of the text, the answer to all three questions lies in the uniqueness of the summertime SP environment. Among these characteristics is the presence of a large plateau region just to the east of SP. This region defines one of the world's largest air drainage fields, being nearly 1000 km across and having elevation of ∼ 3 km . In addition, summertime SP surface temperatures typically do not exceed - 25 ° C , leading to frequent cases where strong near surface temperature inversions occur. It experiences 24 h of continuous sunlight, giving rise to non-stop photochemical reactions both within the snowpack and in the atmosphere. The latter chemistry is unique at SP in that increasing levels of NO x lead to an enhanced lifetime for NO x , thereby producing non-linear increases in NO x . In addition, the rapid atmospheric oxidation of NO x , in conjunction with very rapid dry deposition of the products ( HNO 3 and HO 2 NO 2 ), results in a very efficient recycling of NO x back to the snowpack. Details concerning these unique SP characteristics and the extension of these findings to the greater plateau region are discussed. Finally, the relationship of NO x recycling and total nitrogen deposition to the plateau is explored.
Keywords: Antarctica; South Pole; Photochemistry; NO; NO x snow emissions; ISCAT;

Eddy-covariance heat flux as well as temperature and NO concentration gradients were measured during the ISCAT 2000 (Investigation of Sulfur Chemistry in the Antarctic Troposphere) field study at the South Pole (SP). These quantities allowed for the use of the modified Bowen ratio technique to estimate the surface flux of NO and, from photochemical considerations, the NO x flux. The meteorological measurement package employed in these experiments consisted of sonic anemometer/thermometers (ATI K-probes) and temperature/humidity sensors (NCAR). A chemiluminescent analyzer housed in an environmental enclosure was used to measure NO. All sampling took place on a 22 m meteorological tower. The time period over which flux measurements were recorded was 26 November through 30 November 2000. The average value of the NO flux was estimated to be 2.6 ± 0.3 × 10 8 molec cm - 2 s - 1 ; whereas, for NO x the average flux was 3.9 ± 0.4 × 10 8 molec cm - 2 s - 1 . To assist in the interpretation of these results, the height of the atmospheric boundary-layer at the SP from 23 November to 28 December 2000 was also estimated.
Keywords: Polar chemistry and meteorology; NO x flux; Boundary-layer height; Modified Bowen ratio;

Soluble reactive nitrogen oxides at South Pole during ISCAT 2000 by Jack E. Dibb; L. Gregory Huey; Darlene L. Slusher; David J. Tanner (5399-5409).
The mist chamber/ion chromatography technique was used to measure soluble gaseous nitrite (NO2 ) and nitrate (NO3 ) within the lowermost atmosphere (2 cm up to 10 m above the snow) and in the firn air at South Pole during the last 2 weeks of December 2000. Collected NO2 and NO3 are attributed to nitrous (HONO) and nitric (HNO3) acids, respectively. Firn air mixing ratios of HONO were more than five times higher than those just above the snow. A single day of measurements showed HONO mixing ratios to be significantly lower at 10 m than 85 cm above the snow. These gradients suggest that HONO is produced in the snowpack and fluxes out into the overlying air. A strong and persistent local source of HONO is required to sustain mean mixing ratios of 30 ppt against very fast loss by photolysis under the intense 24 h sunlight at South Pole. For HNO3, measurements in the atmosphere were made with two independent techniques. A CIMS technique provided data at 1 min resolution, at a sampling height of 10 m. The mist chamber/ion chromatography technique integrated over nominally 30 min intervals, with most measurements made 85 cm above the snow. Mixing ratios of HNO3 were highest (mean 38 ppt) at 85 cm above the snow, compared to those both in the firn air and at 10 m above the snow. Comparing HNO3 mixing ratios between 85 cm and firn air suggests flux into the snow, while the comparisons between 85 cm and 10 m suggest fluxes in the opposite direction. We speculate that the maximum just above the snow surface reflects in situ production of HNO3, supported by fluxes of NO x and OH precursors (e.g., HONO, HCHO, HOOH) out of the snow. Comparison of NO3 concentrations in snow immediately upwind of the Atmospheric Research Observatory, 50 m away from the building and 15 km away from the station revealed no significant differences. This finding indicates that the very high NO mixing ratios observed during ISCAT 1998 and 2000 cannot be attributed to NO3 contamination of the snow surrounding the sampling location.
Keywords: Photochemistry in snow; South Pole; Nitric acid; Nitrous acid; ISCAT 2000;

CIMS measurements of HNO3 and SO2 at the South Pole during ISCAT 2000 by L.G. Huey; D.J. Tanner; D.L. Slusher; J.E. Dibb; R. Arimoto; G. Chen; D. Davis; M.P. Buhr; J.B. Nowak; R.L. Mauldin; F.L. Eisele; E. Kosciuch (5411-5421).
HNO3 and SO2 were measured by chemical ionization mass spectrometry at the South Pole (SP) during ISCAT 2000 (December, 2000). HNO3 mixing ratios averaged 22 pptv and ranged from less than 5 to 68 pptv. A simple steady state photochemical analysis indicates that most of the time HNO3 is deposited to the snow with a lifetime of the order of a few hours. Periods of relatively high levels of HNO3 and low levels of NO were observed when air from aloft was mixed downward, but the source of this HNO3 is uncertain. One explanation for these observations is that free tropospheric air, enriched with NO x at lower latitudes, descends to the surface at SP; this process could be an important source of nitrate to the Antarctic Plateau. Another explanation is that these descending air parcels were previously in contact with the surface and enriched with snowpack emissions of NO x upwind of SP. The measured SO2 mixing ratio was found to be less than 20 pptv on average. However, a simple steady state analysis of OH and H2SO4 observations indicates that average SO2 levels are most likely less than a few pptv.
Keywords: Nitric acid; Sulfur dioxide; Snowpack photochemistry; Pernitric acid; Tropospheric chemistry;

Measurements of OH, HO2+RO2, H2SO4, and MSA at the South Pole during ISCAT 2000 by R.L. Mauldin; E. Kosciuch; B. Henry; F.L. Eisele; R. Shetter; B. Lefer; G. Chen; D. Davis; G. Huey; D. Tanner (5423-5437).
Measurements of hydroxyl radical (OH), sulfuric acid (H2SO4) and methane sulfonic acid (MSA) together with the first HO2+RO2 observations at the South Pole are presented. These results were recorded as part of the Investigation of Sulfur Chemistry in the Antarctic Troposphere 2000 (ISCAT 2000) study. OH concentrations were found to be highly elevated, having a mean value over the time period of 15 November–30 December 2000 of 2.5×106 molecule cm−3, thus confirming the results from ISCAT 1998. Although data were more limited for the sum of HO2+RO2, a mean value of ∼7×107 molecule cm−3 was estimated. Typically, OH and HO2+RO2 both showed large day-to-day variability. Box model simulations suggest that most of this variability was a direct result of elevated and highly variable levels of nitric oxide. Comparisons of OH with overhead O3 column density measurements revealed that for certain time periods as much as a 30% enhancement occurred in OH as a result of decreases in column O3 levels. Like ISCAT 1998, the observational data for H2SO4 and MSA generally showed very low concentrations with mean values of 2.7×105 and 8×104 molecule cm−3, respectively. When compared against measured levels of particulate sulfate and methane sulfonate, these low gas phase concentrations indicate, as suggested by the more limited data from the ISCAT 1998 study, that local production of gas phase sulfur species contributes little to particle composition.
Keywords: Hydroxyl; Sulfuric; OH; HO2; South Pole;

Formaldehyde and hydrogen peroxide in air, snow and interstitial air at South Pole by Manuel A. Hutterli; Joseph R. McConnell; Gao Chen; Roger C. Bales; Douglas D. Davis; Donald H. Lenschow (5439-5450).
Average H2O2 (HCHO) mixing ratios measured above the snowpack at South Pole (SP) were 278 pptv (103 pptv) in December 2000 and between 4 and 43 times (1.4–2.6 times) the value estimated from gas-phase photostationary state (PSS) model calculations. The larger difference is realized if dry deposition of both species is included in the model. H2O2 and HCHO fluxes from the snowpack were independently determined from gradient measurements in the air above the snow surface, from firn-air measurements and from the temporal concentration changes in near-surface snow. On average, the snowpack at SP was releasing on the order of 1×1013 and 2×1012 molecules m−2  s−1 of H2O2 and HCHO, respectively, in December 2000. This is consistent with the volumetric fluxes needed for the PSS model to reproduce the observed atmospheric mixing ratios of both H2O2 and HCHO. The highly elevated levels of both species found in firn air further support the above estimates. In the case of HCHO, it was also shown that there was good agreement between the measured flux and the physical air–snow exchange model as driven by changes in snow temperature from winter to summer. Shading experiments suggest that the net production of HCHO within the snow by heterogeneous photochemical processes may have exceeded photochemical destruction by no more than 20% of the measured fluxes. The very rapid changes observed in atmospheric HCHO, which are also seen in NO and OH, can be understood in terms of dynamical processes that lead to rapid changes in the atmospheric mixing depth.
Keywords: Exchange air snow; Flux snow atmosphere; Snow chemistry; Antarctica; ISCAT 2000;

A reassessment of HO x South Pole chemistry based on observations recorded during ISCAT 2000 by G. Chen; D. Davis; J. Crawford; L.M. Hutterli; L.G. Huey; D. Slusher; L. Mauldin; F. Eisele; D. Tanner; J. Dibb; M. Buhr; J. McConnell; B. Lefer; R. Shetter; D. Blake; C.H. Song; K. Lombardi; J. Arnoldy (5451-5461).
Reported here are modeling results based on ISCAT (Investigation of Sulfur Chemistry of Antarctic Troposphere) 2000 observations recorded at the South Pole (SP) during the Austral Summer of 2000. The observations included a comprehensive set of photochemical parameters, e.g., NO, O3, and CO. It is worthy to note that not only were OH and HO2 observed, but also HO x precursor species CH2O, H2O2, and HONO were measured. Previous studies have suggested that HONO is the major source of OH/HO x in the Arctic; however, observed HONO levels at SP induced dramatic model overprediction of both HO x and NO x when used to constrain the model calculations. In contrast, model predictions constrained by observed values of CH2O and H2O2 are consistent with the observations of OH and HO2 (i.e., within 20%) for more than half of the data. Significant model overpredictions of OH, however, were seen at the NO levels lower than 50 pptv or higher than 150 pptv. An analysis of HO x budget at the median NO level suggests that snow emissions of H2O2 and CH2O are the single most important primary source of SP HO x , contributing 46% to the total source. Major sinks for HO x are found to be dry deposition of HO2NO2 and HNO3 as well as their reactions with OH. Although ISCAT 2000 led to a major progress in our understanding of SP HO x chemistry, critical aspects of this chemistry are still in need of further investigation.
Keywords: Antarctica; South Pole; Photochemistry; HO x ; Snow emissions; ISCAT;

Organic trace gases of oceanic origin observed at South Pole during ISCAT 2000 by Aaron L. Swanson; Douglas D. Davis; Richard Arimoto; Pauline Roberts; Elliot L. Atlas; Frank Flocke; Simone Meinardi; F. Sherwood Rowland; Donald R. Blake (5463-5472).
Volatile organic compounds (VOCs) were measured at the South Pole (SP) from late Austral spring to mid-summer 2000 as part of the Investigation of Sulfur Chemistry in the Antarctic Troposphere Program (ISCAT-2000). This paper focuses on VOCs that are directly emitted from the ocean, specifically dimethyl sulfide (DMS), methyl nitrate (CH3ONO2), methyl iodide (CH3I) and bromoform (CHBr3). A partial seasonal cycle of these gases was also recorded during the year following ISCAT-2000. During the summer, the SP periodically receives relatively fresh marine air containing short-lived oceanic trace gases, such as DMS (τ≈1 day). However, DMS was not detected at the SP until January even though DMS emissions from the Southern Ocean typically start peaking in November and elevated levels of other ocean-derived VOCs, including CH3ONO2 and CHBr3, were observed in mid-November. We speculate that in November and December most of the DMS is oxidized before it reaches the SP: a strong correlation between CH3ONO2 and methane sulfonate (MSA), an oxidation product of DMS, supports this hypothesis. Based on a limited number of samples taken over the course of one year, CH3ONO2 apparently accumulates to a quasi-steady-state level over the SP in winter, most likely due to continuing emissions of the compound coupled with a lower rate of photochemical destruction. Oceanic emissions were concluded to be the dominant source of alkyl nitrates at the SP; this is in sharp contrast to northern high latitudes where total alkyl nitrate mixing ratios are dominated by urban sources.
Keywords: Oceanic emissions; DMS; South pole; Methyl iodide; Bromoform; Alkyl nitrates; Photochemistry;

Major ions and radionuclides in aerosol particles from the South Pole during ISCAT-2000 by R. Arimoto; A. Hogan; P. Grube; D. Davis; J. Webb; C. Schloesslin; S. Sage; F. Raccah (5473-5484).
As part of ISCAT 2000, bulk, high-volume, aerosol samples were collected at the South Pole (SP) nominally over 24-h intervals, and they were analyzed for major ions, several trace elements, and three naturally occurring radionuclides. The mean concentrations of Na (<17 ng m−3), sulfate (98 ng m−3), and methanesulfonate (MSA, 4.4 ng m−3) all were lower in ISCAT 2000 compared with ISCAT 1998, suggesting weaker marine influences during the latter study. In contrast, the 210Pb activity (0.20 mBq m−3) was more than two-times higher in ISCAT 2000 than in 1998, and nitrate concentrations (150 ng m−3) were approximately four-times higher, suggesting stronger continental influences in the second study. These differences between experiments are consistent with an analysis of meteorological transport and exchange. 7Be activities were generally comparable for ISCAT-1998 and 2000, suggesting that there were, on average, similar upper tropospheric/lower stratospheric influences on surface air during the two experiments: long-term records of 7Be, however, show pronounced annual and lower-frequency cycles. The concentration ratios of MSA to nss-sulfate (R) were similar in the two campaigns, and a regression analysis suggests that a non-biogenic source or sources account for up to ∼30% of the nonsea-salt sulfate. Various possible explanations for the low values of R (=0.08) relative to other Antarctic sites are discussed, including differences in R due to where the oxidation of DMS takes place (that is, in the marine boundary layer or in the buffer layer/free troposphere), chemical fractionation during transport, and the transport of sulfur compounds from lower latitudes and possibly from Mt. Erebus.
Keywords: Sulfur cycle; Major ions; Trace elements,Radionuclides; Aerosols; Antarctica; Geochemistry;

Lead and mercury in aerosol particles collected over the South Pole during ISCAT-2000 by R. Arimoto; C. Schloesslin; D. Davis; A. Hogan; P. Grube; W. Fitzgerald; C. Lamborg (5485-5491).
As part of the ISCAT-2000 field study, aerosol particle samples were collected over ∼24 hr intervals at the South Pole (SP), and they were analyzed for trace elements and for selected ions important for Antarctic sulfur and nitrogen chemistry. This paper focuses on lead and mercury while a companion paper in this issue presents the results of the aerosol ion studies. The results showed that most trace metals were below their limits of detection. Two that were not were Pb and Hg. Lead was only quantified in 15 of 53 samples, and based on those data, the mean concentration of Pb would be <0.032 ng m−3. This is substantially lower than the aerosol Pb concentrations reported for the SP in the early to mid-1970s, suggesting that the decrease may be related to controls on pollution emissions; however, contamination of the earlier samples also would explain the difference. The arithmetic mean concentration of filterable Hg was 0.04 ng m−3; thus approaching levels reported for the Arctic. In contrast to the Arctic, however, filterable Hg at the SP was not associated with low ozone mixing ratios; rather the reactions driving the cycling of Hg at the SP appear to involve HO x radicals, reactive nitrogen, and more than likely other substances through complex and not fully understood pathways.
Keywords: Trace metals; Aerosols; Antarctica; Geochemistry; Pollution;

Aerosol size distributions measured at the South Pole during ISCAT by Jongsup Park; Hiromu Sakurai; Karl Vollmers; Peter H. McMurry (5493-5500).
Aerosol physical size distributions were measured at the South Pole during December 1998 and December 2000 as part of the ISCAT program. The size ranges covered by these measurements were 3 to 250 nm in 1998 and 3 nm–2 μm in 2000. “Typical background aerosols” measured during both periods were similar. Total aerosol number concentrations ranged from 100 to 300 cm−3 with occasional spikes as high as 10,000 cm−3. We believe the spikes were due to local emissions. The number mean size of background aerosols ranged from 50 to 70 nm, and total aerosol surface area concentrations were 2.8±0.4 μm2  cm−3. Aerosols measured in December 2000 were cleanly separated into “low volume” and “high volume” periods. During the low-volume periods, volume concentrations were 0.07±0.01 μm3cm−3 with a volume mean diameter of 0.27±0.05 μm, and these volume concentrations were mostly within a factor two of values that would be expected based on reconstructed mass from particulate chemical composition. Volume concentrations during the “high volume” periods exceed levels that can be explained from aerosol chemistry and calculated light-scattering coefficients exceed values that have been recorded historically. We have been unable to identify why this might have occurred. We observed one 4-h event on December 15, 2000 during which nanoparticles grew slowly from ∼3.0 to 3.6 nm.We believe these particles had recently been formed by nucleation. Because this occurred during a period of stagnation, it is possible that this event was associated with local emissions.
Keywords: South Pole; Aerosol Size Distributions; Atmospheric Aerosol; Particle growth rate;