3.1.2. Experimental Methods

A number of changes were made to CMDL's aerosol sampling network during 1996-1997. Two noteworthy changes occurred as a result of collaboration with the Atmospheric Radiation Measurements (ARM) program of the U.S. Department of Energy (DOE): CMDL assumed responsibility for the aerosol observing system at ARM's site in Lamont, Oklahoma, and the aerosol sampling system at the Barrow, Alaska (BRW) baseline observatory was upgraded to use the same sampling protocols and instrumentation as at the CMDL regional monitoring sites. These changes substantially increase the number of sites where comparable measurements of aerosol intensive properties are made routinely. Routine aerosol measurements at Cheeka Peak, Washington, and Niwot Ridge, Colorado, were terminated during 1996 as a result of funding limitations, although intermittent field campaigns at Cheeka Peak have continued.

Extensive aerosol properties monitored by CMDL include the condensation nucleus (CN) concentration, aerosol optical depth (d), and components of the aerosol extinction coefficient at one or more wavelengths (total scattering s_sp, backwards hemispheric scattering sb_sp, and absorption s_ap). At the regional sites, size-resolved impactor and filter samples (submicrometer and supermicrometer size fractions) are obtained for gravimetric and chemical (ion chromatograph) analyses. All size-selective sampling, as well as the measurements of the components of the aerosol extinction coefficient at the regional stations, is performed at a low, controlled relative humidity (<40%) to eliminate confounding effects due to changes in ambient relative humidity. Data from the continuous sensors are screened to eliminate contamination from local pollution sources. At the regional stations, the screening algorithms use measured wind speed, direction, and total particle number concentration in real-time to prevent contamination of the chemical samples. Algorithms for the baseline stations use measured wind speed and direction to exclude data that are likely to have been locally contaminated.

Prior to 1995, data from the baseline stations were manually edited to remove spikes from local contamination. Since 1995 an automatic editing algorithm has been applied to the baseline data in addition to manual editing of local contamination spikes. For the baseline stations (BRW, Mauna Loa, Hawaii (MLO), American Samoa (SMO), and South Pole, Antarctica (SPO), as well as Sable Island (WSA)), data are automatically removed when the wind direction is from local sources of pollution (such as generators and buildings) as well as when the wind speed is less than a threshold value (0.5^-1 m s^-1). In addition, at MLO data for upslope conditions (1800-1000 UTC) are excluded since the airmasses do not represent “background” free tropospheric air for this case. A summary of the data editing criteria is given in Table 3.1.

TABLE 3.1. Data Editing Summary for NOAA Baseline and Regional Stations

a: Manual removal of local contamination spikes;

b: Automatic removal of data not in clean sector;

c: Automatic removal of data for low wind speeds;

d: Removal of data for upslope wind conditions;

WD: Wind direction.

Integrating nephelometers are used to determine the light scattering coefficient of the aerosol. These instruments operate by illuminating a fixed sample volume from the side and observing the amount of light that is scattered by particles and gas molecules in the direction of a photomultiplier tube. The instrument integrates over scattering angles of 7-170°. Depending on the station, measurements are performed at one, three, or four wavelengths in the visible and near-infrared. Newer instruments allow determination of the hemispheric backscattering coefficient by using a shutter to prevent illumination of the portion of the instrument that yields scattering angles less than 90°. A particle filter is inserted periodically into the sample stream to measure the light scattered by gas molecules; this is subtracted from the total scattered signal to determine the contribution from the particles alone. The instruments are calibrated by filling the sample volume with a gas of known scattering coefficient; CO₂ is used for high sensitivity instruments, while CFC-12 is used for the few single-wavelength, lower sensitivity instruments still in use.

The aerosol light absorption coefficient is determined with a continuous light absorption photometer. This instrument continuously measures the amount of light transmitted through a quartz filter, while particles are being deposited on the filter. The rate of decrease of transmissivity, divided by the sample flow rate, is directly proportional to the light absorption coefficient of the particles. Newer instruments were calibrated in terms of the difference of light extinction and scattering in a long-path extinction cell, for laboratory test aerosols. Instruments at the baseline stations (aethalometers, Magee Scientific, Berkley, California) were calibrated by the manufacturer in terms of the equivalent amount of black carbon from which the light absorption coefficient is calculated assuming a mass absorption efficiency of the calibration aerosols of 10 m² g^-1. Preliminary results from side-by-side operation of the two methods at BRW reveal a systematic difference (see below).

Particle number concentration is determined with a CN counter that exposes the particles to a high supersaturation of butanol vapor. This causes the particles to grow to a size where they can be detected optically and counted. The instruments in use have lower particle-size detection limits of 10-20 nm diameter.

Summaries of the extensive measurements obtained at each site are given in Tables 3.2 and 3.3. Table 3.4 lists the intensive aerosol properties that can be determined from the directly-measured extensive properties. These properties are used in chemical transport models to determine the radiative effects of the aerosol concentrations calculated by the models. Inversely, these properties are used by algorithms for interpreting satellite remote-sensing data to determine aerosol amounts based on measurements of the radiative effects of the aerosol.

TABLE 3.2. CMDL Baseline Aerosol Monitoring Stations (Status as of December 1997)

TABLE 3.3. CMDL Regional Aerosol Monitoring Sites (Status as of December 1997)

TABLE 3.4. Intensive Aerosol Properties Derived From CMDL Network

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