Elevated ozone in the boundary layer at South Pole - Doug Davis

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ARTICLE IN PRESS D. Helmig et al. / Atmospheric Environment 42 (2008) 2788–2803 2791 simultaneously. The sampling flow rate was determined by the sampling pumps of these two analyzers and was 1.2 l min 1 (TEI) or 2.4 l min 1 (both instruments combined). Under these conditions the sample residence time in the sampling line was 4.2 min and 2.1 min, respectively. Between balloon flights a short sampling line (10 m) and the long line inlet were placed side by side on the 2- m tower and sample air was alternated between these two inlets every 5 min. The ozone loss rate in the long line was determined by comparing these two data series. This loss rate fluctuated slightly over nine days while this sampling line was used. A 6-h running mean was calculated and applied for correcting all long sampling line data. The mean ozone loss rate in the long sampling line over the nine-day period was 1.970.8%. A thorough intercomparison between the long sampling line data and concurrent ECC sonde measurements is presented by Johnson et al. (2007); further analytical details on the tethered balloon NO measurements are provided in Helmig et al. (2007b). Balloon data analysis: Ascent balloon heights were calculated by the radiosonde change in pressure referenced to the average ‘‘before launch’’ pressure, while descent balloon height calculations were referenced to the surface pressure measured after completion of the descent profile. All raw data were averaged to 1-m height intervals. Missing data points (fewer than 2% of 1-m interval data) at selected heights were interpolated from adjacent height measurements. The temporal and spatial distribution of atmospheric stability was determined by calculating 5-m interval bulk Richardson numbers using the vertical gradient temperature, wind speed and wind direction data from 2 m above and below the reference height. The averaged balloon and the time series surface data were combined for the color contour analysis plots. Back trajectories: The back trajectories to SP were computed from the NCEP/NCAR Reanalysis Data Set (Kalnay et al., 1996). The trajectory model (Harris et al., 2005) determines the vertical position of the air parcel explicitly using the vertical wind field in the analyzed data set (3D trajectories). 3. Results and discussion 3.1. Surface ozone Data from the two continuous ozone surface measurements (ARO and balloon launch site) are 55 50 Balloon Building ARO 45 Ozone (ppbv) 40 35 30 25 20 15 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 Fig. 2. Ozone during day of year 2003 measured from the roof (17 m) of the ARO building (hourly mean data, black solid line) in comparison to surface ozone (1-min data) measured from the roof (4 m above the surface, DOY 350.0–357.2) and a 2-m tower inlet (DOY 347.4–350.0, 357.2–364.1) adjacent of the balloon launch shelter.
2792 ARTICLE IN PRESS D. Helmig et al. / Atmospheric Environment 42 (2008) 2788–2803 shown in Fig. 2. During the period of this experiment, surface ozone at SP showed large variations, between minima of 18 ppbv (DOY 354) and maxima of 50 ppbv on DOY 358. Both measurements, even though 130 m separated by distance and 15 m by height show excellent agreement, typically within 1 ppbv during the first phase. A striking feature of these observations is that during the later part of DOY 354, a significant increase in surface ozone (almost doubling) was observed and that thereafter both measurements showed a 3–4 ppbv disagreement until ozone levels dropped back to below 30 ppbv on DOY 359. Upon closer inspection, it becomes apparent that generally high agreement between these two data series is seen at lower ozone levels and that the disagreement scales with the absolute ozone levels. The vertical balloon profile data, to be discussed in the following paragraphs, show that the differences in these two measurements do not stem from an analytical bias, but instead represent vertical ozone gradients in the shallow SP surface layer. 3.2. Vertical ozone profiles The vertical ozone distribution at SP showed strong variations during December 2003. Two examples that illustrate the extremes of these conditions are presented in Fig. 3. On December 24 a strong variability in ozone was seen in the lowest 500 m of the atmosphere. Near the surface, ozone levels were approaching 50 ppbv. Ozone mixing ratios declined steeply with altitude, dropping to 22 ppbv at 180 m. Several layers with 2–4 ppbv enhanced ozone were seen between 200 and 500 m height. Data from the balloon ascent and descent show a high degree of agreement, indicative that ozone profiles changed very little during the 58- min flight duration. It should be noted that due to the 25–30 s response time of the ECC sonde, the ozone readings are somewhat delayed causing a slight upwards/downwards shift of the ozone profile during ascent and descent, respectively (by 10 m at the 0.3 m s 1 ascent/descent rate). Correcting for this effect would further improve the agreement between the ascent and descent ozone profiles. Ozone mixing ratios measured near the surface generally agreed within 1–2 ppbv with the concurrent ARO and tower observations (Fig. 2) (Johnson et al., 2007). Much different conditions were encountered two days later, as shown in the pair of profiles on the right in Fig. 3. Ozone was homogenously distributed in the surface and boundary layer, showing less than a 2 ppbv gradient between the surface and 500 m. Again, both ascent and descent data follow each other closely and ozone data from the balloon instruments near the 500 450 Ascent Descent 500 450 Ascent Descent 400 400 350 350 Height (m) 300 250 200 Height (m) 300 250 200 150 150 100 100 50 50 0 15 20 25 30 35 40 45 50 Ozone (ppbv) 0 15 20 25 30 35 40 45 50 Ozone (ppbv) Fig. 3. Two examples of vertical ozone distribution at South Pole during December 2003. The profiles on the left were measured on December 24 (launch time DOY 358.82, flight duration 57 min). The profiles on the right were obtained on December 26 (launch time DOY 360.89, flight duration 47 min).
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Elevated ozone in the boundary layer at South Pole - Doug Davis

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