Observations
Measurements of trace gases, sub-micron particle size distributions, total
particulate composition, and meteorological parameters were taken on Ross Island, in the
vicinity of McMurdo Station (77º51’ S, 166º40’ E) from 28 August through 28 October
2007. Measurements were taken at the cosmic ray facility (COSRAY) approximately 2
km south of McMurdo Station, and located 100 m away from and 20 m above multi-year
sea ice and the Ross Ice Shelf in McMurdo Sound.
Ozone mixing ratios were measured using a Thermo Environmental model 49C
ultraviolet (TEI 49) ozone analyzer. Additional ozone measurements were made at a sea
ice location 25 km north of McMurdo Station and data were compared with a similar TEI
49 operated by the National Oceanic and Atmospheric Administration (NOAA) located 3
km north of COSRAY at Arrival Heights (AH).
Particle size distributions were determined using an Ultra High Sensitivity
Aerosol Spectrometer (UHSAS) from Particle Measurements, Inc. (now distributed by
Droplet Measurement Technologies, Boulder, CO). This instrument was configured to
measure particle optical diameters at a wavelength of 1.06 m in 99 size bins ranging
from 55 to 1000 nm at a measurement rate of 0.1 Hz.
Aerosol filter measurements using Millipore 25 mm Aerosol Analysis Monitors
were collected intermittently along with the optical size measurements. The sealed filter
cartridges were returned to Colorado for ion chromatography (IC) analysis for bromide,
chloride and sulfate and inductively coupled plasma mass spectrometry (ICPMS) analysis
for bromide and iodide. Both UHSAS and filter samples were taken using ¼” ID
conductive copper tubing of length less than 3 meters with minimal bends and a flow rate
of 13.7 SLPM. While not quantified directly, calculations suggest that the inlet did not
transfer particles above approximately 10 µm diameter. The inlets for all instruments
were located on the windward side of COSRAY at a height of 2 meters above a mixed
rock and snow surface. The aerosol filters have a specified nominal pore size of 800 nm;
however analysis of air downstream of the filters using the UHSAS showed no particle
transmission down to optical diameters of 55 nm. Secondary filters taken behind the
primary filter also showed no significant variation from laboratory blanks, indicating
minimal passage of aerosol particles and minimal absorption of gas-phase species.
Detection limits for the ions of interest were determined by either the analytical limits of
the analysis instrumentation (I-), or the background levels present in blank filters (Cl-,
SO42-, Br-), whichever was greater. Mass mixing ratio detection limits for Cl-, Br-, SO42and I- were 0.16 ppm, 0.03 ppm, 0.15 ppm and 0.01 ppm respectively, with Cl- and Brdetection limits being higher due to high levels of these species in the blank filters.
Average mass loading per air volume for each species was calculated from filter
abundances, flow rate through the filters and run time for each filter. Run times for each
filter ranged from 2 to 24 hours dependent on logistical constraints and expected mass
loading inferred from optical sizing. For this analysis, only filters with a run time of
longer than 8 hours were considered, as the analytical uncertainty associated with shorter
runtime filters reduces the significance of these data points.
Bromide concentrations on the filters were below the detection limit of the IC
analysis, so results are based on analysis by ICPMS. Due to the low concentrations
relative to the detection limit, and variable bromide concentrations in the experimental
blanks, uncertainties for these measurements are estimated to be approximately 50%. In
all samples, the iodide concentration was below the detection limit of the ICPMS
analysis.
Carbon dioxide was measured using a Li-Cor 6200 IR gas analyzer running in
absolute mode. For this work, carbon dioxide is used to identify periods of local
anthropogenic influence, such that slow drift due to lack of frequent calibrations with
reference standards only presents a problem for interpretations of small, slowly varying
signals due to transport from distant sources. Here, we assume that any short-duration
(e.g., ~minutes), large positive excursions (> 10 ppm) are due to emissions from
combustion sources, such as building power sources and vehicles.
Meteorological parameters were collected from a variety of sources including a
local weather station at COSRAY (wind speed and temperature), the Willy Field
Automatic Weather Station located 5 km south of COSRAY, and the re-analysis of
Antarctic Mesoscale Prediction System (AMPS) model output.
Sub-micron particle volume (SMV) loading was calculated by summing the
volume of all size bins using the volume-weighted mean diameter for each bin and
assuming spherical particles. This quantity is used as a proxy for sub-micron aerosol
mass loading as particle composition, and therefore particle density, was unknown. For
some comparisons, the particle metric PM1.0 (1 µm mass loading) was estimated from the
sub-micron volume assuming a mean particle density of 1.4 g cm-3.
Many brief periods (~ hour or less) of ozone loss are seen in ozone measurements
near McMurdo Station, owing to local anthropogenic pollution. During these periods of
known anthropogenic influence, rapid fluctuations of up to 20 ppmv in the CO2 mixing
ratios over the course of minutes were observed. These were often accompanied by
ozone depletions of up to 10 ppbv. In order to distinguish such events from ‘natural’
events, the data were filtered by the derivative of the CO2 mixing ratio. Any period in
which CO2 was changing at a rate greater than 2 ppmv hr-1 was considered to be
contaminated by anthropogenic influence and was excluded from the analysis of nonanthropogenic events.
There is a statistically significant linear correlation between ozone depletion and
sub-micron particle enhancement, and 58% of the variability in ozone mixing ratio
correlates with changes in sub-micron particle volume (r2 = 0.58). This relationship is
statistically significant to greater than 95% confidence (p value < 0.05) based on an Ftest. This analysis excludes periods attributed to local anthropogenic pollution events
(see Figure S1).