Long-term Aerosol Instruments
(J. Propero and D. Savoie)
Bulk Aerosol Sample Flow Rates and Volumes: Air flow is measured by monitoring the differential pressure across a sharp-edged orifice plate in the outlet tube from the sampling pump. The flow rates are routinely calibrated using laminar flow elements traceable to NIST standards as well as with dry gas meters with subsets confirmed by calibrations at the DOE Environmental Measurements Laboratory. The oil manometers used for measuring the pressure drops are themselves primary differential-pressure measuring devices. Sample volumes are determined by multiplying the average flow rate by the total time that the air pumps operated. These times are provided by duplicate elapsed time meters wired in series with the pumps.
Sample Collection Efficiencies: For our bulk aerosol collection, we will use Whatman-41 filters. At our typical air volume flow rates of about 0.9 m3 min1, our past intercomparisons with two Whatman-41 filters in series have shown that a single Whatman filter collects greater than 95% of the sodium, chloride, nitrate and mineral dust and greater than 90% of the nonsea-salt sulfate. To check for losses of large particles, particularly mineral dust as a consequence of the presence of a rain cover over the sampler, 30 concurrent collections were made in sample systems with and without rain covers at Izana, Tenerife, Canary Islands during July 1995. There were no significant differences in the concentrations measured with the two systems; slopes of "uncovered" vs "covered" concentrations ranged from 0.90 to 1.06 with none differing significantly from 1.
Analytical Methods: For sample analysis, quarter sections of the sample filters are extracted in 20-ml of 18 Mohm cm-1 Milli-Q water in serial aliquots of 10, 5, and 5 mL using filtering centrifuge tubes with pre-washed 0.45 um polycarbonate filters. This procedure results in greater than 99% extraction efficiency for the major soluble ionic species on interest. The concentrations of chloride, sulfate, nitrate, and MSA in the extraction solutions are determined to within ±5% by ion chromatography using Dionex guard and analytical columns with suppressed conductivity detection. Sodium is determined to within ±2% by flame atomic absorption spectroscopy. Ammonium is determined to within ±5% using an Alpkem automated colorimeter with sequential 10 and 30 mm cells to cover the concentration range from 2 to 200 uM. For the IC and AA analyses, about 25% of an analysis stream is composed of standard solutions of diluted filtered, natural seawater spiked with nitric acid with final concentrations covering the full range of those in the samples. Standard solutions of ammonium chloride are used for the ammonium analyses with a frequency comparable to those used for the IC and AA. The UM group routinely determines method detection limits, and routinely analyses NIST standards, reagent blanks, field blanks, duplicates of samples, and check samples. The analytical results are also routinely cross-checked by sharing samples and standards with analytical teams from other institutions, most recently, with William Keene (Dept. of Environmental Science, University of Virginia), with Dr. Patricia Quinn (NOAA Pacific Marine and Environmental Laboratory) as part of a 6-institution intercalibration for IGAC Aerosol Characterization Experiment (ACE) 1, and with the European Commission Joint Research Center (Ispra, Italy) as part of a 7-institution international intercalibration for ACE-2. After extracting the soluble material, the sample and extraction filters are ashed at 500 C to destroy any organic matter including the filter matrices. The residual ash less that of the average blank is ascribed to mineral aerosol. Based on an average Al concentration of 8% in soils and an average ash Al ratio of 9.61 in about 1000 concurrent filter samples collected at Barbados, we estimate the actual mineral dust concentration to be 1.3 times that of the residual ash. For 24-hour samples, the standard error in the mineral aerosol concentration is ±10% for concentrations greater than about 1 ug m3. Below 1 µg m3, the standard error is essentially constant at ±0.1 ug m3.
Radiance Research M903 Integrating Nephelometer: Prior to deployment a two point calibration will be performed. The nephelometer will be zeroed using particle free air and spanned with carbon dioxide gas. After deployment, the zero will be automatically checked every three hours with particle free air. The span will be automatically checked once each week with carbon dioxide gas. The flow rate of sample air will be defined by a critical orifice. A rotometer and needle valve will be used to set the carbon dioxide gas flow rate. The flow rates of sample air and carbon dioxide gas will be monitored by automatically measuring pressure differential across an orifice.
Radiance Research Particle/Soot Absorption Photometer: Since the optical portion of the aerosol absorption measurement is a differential measurement, it need not be routinely calibrated. We will verify the performance of the electronic optical sensors and light source by monitoring the raw digital output of the instrument. The instrument incorporates a mass flow meter to measure sample air flow. Properly protected, these devices typically operate reliably for continuous periods in excess of a year. To verify this flow rate measurement, we will deploy a simple bubble flow meter and stop watch as a primary measure of sample air flow rate. The bubble flow meter will be used on a weekly basis.
Performance All of the instruments are critical to the stated objectives. We have operated all of the sampling devices and instruments at remote stations for extended periods of time and all have proven to be extremely reliable. The bulk sampling systems have operated at about 30 coastal and islands stations for periods of 5 years or more without significant problems. Our long-term deployment of the aerosol instruments began more recently, but have already operated properly on a continuous basis for at least a year at three sites in the North Atlantic.
Long-term Broadband Solar Instruments
(D. Lubin, V. Ramanathan, W. Conant, A. Vogelmann)
Normal Incidence Pyrhelimoeter (NIP): Broadband (0.3 - 2.8 microns) radiometer with a 5.7 degree diameter field of view. Mounted on a sun-tracking motor so it measures direct solar radiation.
Calibration: When calibrated against a cavity radiometer, accurate within 2% is desirable. Calibration could be done either by Valero¹s Atmospheric Research Lab at SIO, or by the manufacturer before and after deployment.
Diffuse radiation pyranometer: Broadband (0.3 - 2.8 microns) radiometer with a 2pi sterradian field of view and cosine angular response. Physically subtracts the direct solar radiation from the global radiation through the operation of a shadowing arm mounted on a sun-tracking motor.
Calibration: Nominally accurate to 10 W m-2. Some instruments are worse than others due to problems with cosine-response characteristics. Must be calibrated against a standard radiometer at least once every 3 years for long-term accuracy. Must have cosine-response characterized in lab before deployed. Calibration could be done either by Valero¹s Atmospheric Research Lab at SIO, or by the manufacturer before and after deployment.
Total-Direct-Diffuse Multiple Spectral Channels Radiometer (TDDR): This is a ground- based version of an airborne instrument developed by F. Valero. Instrument information is provided in the RAMS section.
Long-term UV and Visible Irradiation Instruments
(D. Lubin, V. Ramanathan, W. Conant, A. Vogelmann, F. Valero)
BSI-GUV 511 Radiometer: 5 narrowband (10 nm half-width) UV channels (nominally 305, 320, 340, 380, and 400-700 nm) with a 2pi sterradian field of view and cosine angular response so as to measure irradiance. Temperature stabilized at 40 C. Calibration: To be done by the manufacturer before and after deployment.
Cimel 8 channel sun and sky photometer (Kaufman and Holben group, NASA-Goddard): The channels are 10 nm wide and sample the spectral range from ~348 to 1030 nm. The instrument measures the direct solar irradiance, and, when the solar zenith angle is around 60 degrees, it measures the sky radiance in the solar principle plane and in the solar almuncantar (ring through the sun at constant solar zenith angle). The atmospheric optical thickness is computed from the direct solar irradiance measurement (for 8 channels), and the aerosol size distribution is retrieved from its sky radiance measurement under clear sky conditions (from 3 channels). Angstrom exponent may also be computed from the aerosol optical thicknesses. The accuracy is +/- 0.01 to 0.02 in optical thickness, and the precision is supposed to be very high, of the order of 1.0 e-4. An almuncantar scan takes about 5 minutes per channel, and a full cycle of measurements takes about 30-40 minutes. The atmosphere must be relatively homogeneous for processing a given almuncantar scan, and screening for cloudy conditions is done automatically during data analysis. Data analysis requires surface pressure and ozone column abundance from other sources. The instrument is battery powered and is easily portable. The instrument plus auxiliary equipment can be fitted within a 4'x2'x3' box.
MFRSR - Multi-filter Rotating Shadowband Radiometer: 6 narrowband (10 nm half- width; nominally centered at 415, 495, 610, 665, 862, and 940 nm) and 1 broadband (0.3 - 1.8 microns) channels that measure irradiance. The operation of a rotating shadowband allows the separation of the irradiance into direct and diffuse components. Narrowband channels are placed in NOx, O3, O2, and H2O absorption bands, as well as 2 window (aerosol) regions. Calibration: Direct solar radiation accurate to 5%. Temperature stabilized. Long-term calibration suspect for narrowband (interference filter) channels, particularly the 610 and 660 channels. Calibration to be done by the Valero group or by the manufacturer.
Long-term Trace Gases Instruments
Long-term surface ozone:
a. SIO/C4 Measurement (J. Lobert): Along with the mentioned CO measurements, there will be long-term surface ozone (O3) measurements carried out at the same site. These data will be acquired with a continuously operating instrument and will deliver half hour or more frequent averages of low level, ambient, surface ozone concentrations.
Frequent, automated zero checks of the instrument (at least once an hour) will provide a reliable means of drift analysis and correction. Internal calibration is provided in each instrument by means of a built-in calibration source and is used several times a day, alternating with regular ambient measurements and zero checks. Absolute calibration and comparison to NIST traceable standards of this instrument is necessary once a year and can be carried out on site. The importance of gound-based, surface ozone measurements was rated "essential". Performance is excellent and, again, is backed by many years of experience with similar type and brand instruments by many researchers all over the world, including EPA air quality programs.
b. University of Maryland (R. Dickerson) Measurement : See Shipborne Instruments. This instrument will also provide valuable in-situ calibration with the one from SIO.
Long-term surface CO:
a. SIO/C4 Measurement (J. Lobert): Continuous gas analyzer for surface carbon monoxide (CO). This instrument will deliver half-hour averages of surface CO throughout the year, which provides the intensive campaign with a valuable insight into annual trace gas patterns with respect to local and regional pollution and airmass transport.
Calibration of this instrument will be achieved by means of certified gas standards, calibrated at NOAA/CMDL, Boulder, CO, and ultimately traceable to NIST standard procedures. Frequent zero and calibration checks (zero: once per hour, calibration: once every day) will provide an excellent calibration, which is on par with more precise, but discontinuous instrumentation, which require more technical assistance.
Surface CO measurements were rated to be essential for the program. Performance of this type of instrument is excellent and backed by many years of experience of many researchers. These type and brand analyzers have been used throughout the world for continuous air quality monitoring, e.g., by the EPA and similar air quality programs. Maintenance of this and other instruments is carried out by local collaborators and by semi- annual site visits of C4 personnel.
b. University of Maryland (R. Dickerson) Measurement: See Shipborne Instruments. This instrument will also provide valuable in-situ calibration with the one from SIO.
Long-term surface SF6, CFCs and N2O: Discontinuous trace gas analyzer for conservative and semi-conservative tracers such as sulfur hexafluoride (SF6), chlorofluorocarbons (CFCs), and nitrous oxide (N2O) will be carried out along with the above mentioned CO measurements, over at least a two year time period prior to and following the INDOEX IFP. This instrument will measure low level, ambient concentrations of the mentioned gases at 20 to 60 minute intervals. Conservative gas tracers are needed to evaluate air mass transport, determine local and regional, industrial pollution and give insight into long-range transport including convective processes.
Calibration of this instrument will be carried out with gas standards that will be calibrated at NOAA/CMDL and SIO. Methods comparable to or better than NIST standard procedures will be used. Calibration is usually carried out alternating with ambient sample measurements, i.e., they are as frequent as the sampling itself. Precision is on the order of 1-5%, depending on the type of gas, and absolute accuracy is on the order of 5-10% of better.
The importance of this instrument was rated ³highly desirable.² Performance is excellent and backed by many years of experience at similar ground-based measurements in equivalent environments at NOAA/CMDL and SIO. This type of instrument has been operating since the 1970s at many locations throughout the world with equivalent operating and calibration techniques.
Intensive Field Phase Aerosol and Trace Gases Instruments
Fourier Transform Spectrometer (FTS) System (D. Lubin and V. Ramanathan): a Bomem DA-8 Michleson Interferometer for visible and near IR; using a heliostat to permit sun tracking for solar spectra and angular distribution of sky radiance with a wavelength resolution of about 0.12 cm-1.
Calibration: BSI will assist C4 Interferometry Lab in calibrating of this instrument. A true radiometric calibration procedure must place the standard of spectral irradiance on the outside of all entrance optics, including the heliostat. A roof-mounted fxture similar to those constrcuted by BSI in the Antarctic program will be xonstrcuted, with the following componnets: A 1 kW lamp that is fully characterized by NIST; and several uncharacterized 1 kA lamps will be used as transfer standards. Also needed is a high precision constant current source to power the calibration lamps. A BSI GUV-531 flux radimoter syustem will be used to constrain estimates of downwelling flux from the FTS radiances.
Performance: Bomem has a great reputation in making reliable Fourier Transform radiometers. This high spectral resolution instrument will be the workhorse for validating atmospheric solar absoprtion models and indentifying the magnitude and spectral location of the unexplained (by models) absorption by aerosols and clouds.
NO/NOy (R. Dickerson): Nitric oxide (NO) plays a central role in the photochemical production of ozone, and therefore is critical to Objective 3. It will be measured with O3 chemiluminescence using a modified commercial detector that provides a detection limit of about 10 ppt for 20 s integration times (Dickerson et al. 1984). Total reactive nitrogen (NOy) included the reservoirs of NO, and also figures prominently in Objective 3. NOy will be measured by conversion to NO on 375 C Mo (Fehsenfeld et al., 1987; Nunnermacker, 1990).
Calibration: NO will be calibrated with NO in compressed N2 standards traceable to NIST. NOy will be calibrated with NO2 produced by gas phase titration and from permeation tubes. The conversion will be further checked with HNO3 (Nunnermacker et al., 1989). Ultra zero air will be employed to test for artifact. Both systems will be zeroed every 15 min and calibrated twice a day. The techniques for calibration and analysis of NO and NOy have been thoroughly evaluated and intercompared (Fehsenfeld et al., 1987; Fried et al., 1990).
Performance: NO and NOy has have been measured successfully on several island experiments and ship cruises (e.g., Carsey et al., 1997; Dickerson et al., 1995; Rhoads et al., 1997). Seasalt does not appear to interfere with NOy conversion efficiency for periods of one month. Recently, the selectivity of total reactive nitrogen measurements has been questioned, but the hot molybdenum technique, when used with an appropriate inlet, does not suffer interferences from species such as HCN (Nunnermacker, 1990).
SO2 (R. Dickerson): Sulfur dioxide is a precursor to sulfate aerosol, and its measurement is critical to Objectives 1 and 2. As a tracer of certain kinds of combustion it is useful for Objective 3 as well. We will measure SO2 with a modified commercial pulsed- fluorescence instrument (Thermoenvironmental Model 43S) modified for high sensitivity and zero stability. This instrument provides a detection limit of about 20 ppt for 30-min integration times.
Calibration: standard additions with SO2 in compressed air will be compared to a NIST standard. The span and zero efficiency will be checked daily.
Performance: This is a relatively new technique, but performed remarkably well in a recent rigorous intercomparison (Luke, 1997).
NCAR Integrated Sounding System: One system will be requested for the intensive field phase for mapping the local vertical profiles.