In situ measurements of a variety of aerosol optical properties are being made at ESRL/GMD stations. The measurement suite enables calculation of direct aerosol climate forcing. The measured values relevant for climate forcing calculations are: light absorption, total scattering and backscattering. These measurements are used to derive parameters required in the forcing calculation:
- optical depth which is a function of the amount of absorbing and scattering aerosol present
- single scattering albedo which is the ratio of total scattering to extinction (extinction = absorption + scattering)
- upscatter fraction which is derived from the ratio of backscatter to total scattering
- Ångström exponent which gives the wavelength dependence of scattering and is an indication of the shape of the particle size distribution.
- f(RH) which gives the humidity dependence of light scattering. f(RH) provides an indication of the hygroscopic nature of the particles and is strongly related to particle chemical composition.
- chemical composition of the particles which provides information on possible sources of the aerosol and its behavior in the atmosphere.
Other parameters measured include:
|Photo Tours||Schematic Tours|
|Basic Aerosol System||Basic Aerosol System|
|Expanded Aerosol System||Expanded Aerosol System|
|Aircraft Aerosol System|
We use two types of nephelometers. The TSI nephelometer (TSI Model#3563) measures total scattering (between 7 and 170 degrees) and backscattering (between 90 and 170 degrees) by aerosol particles at three wavelengths: blue (450 nm), green (550 nm) and red (700 nm). The Radiance Research nephelometer (RR Model#903) is a smaller, lighter instrument. It measures total scattering at ~530 nm or ~545nm.
The PSAP (Radiance Research) is a filter-based method that measures light absorption by particles at a single wavelength: green (565 nm) or three wavelengths (467, 530, 660 nm). Particles are collected on a filter and light transmission through the filter is monitored continuously.
The CLAP is a NOAA/GMD developed, filter-based method that measures light absorption by particles at three wavelengths (467, 528, 652 nm). The CLAP is similar to the PSAP in that particles are collected on a filter and light transmission through the filter is monitored continuously. The CLAP differs from the PSAP in that instead of a single sample spot, it has 8 sample spots. Solonoids are used to switch to the next sample spot once the transmittance reaches 0.7. Thus, CLAP can run 8x as long as the PSAP before requiring a filter change, ideal for remote sites which aren't visited daily.
The aerosol systems in our collaborative network use various types of CNCs to measure particle number concentrations. Particles are drawn into the CNC and vapor is condensed on the particles so that they grow to a size detectable by the instrument optics and thus can be counted. Different CNCs have different minimum cut size (i.e., smallest diameter of particle that can be counted.
|Model Number||Lower Cut Size||Working Fluid|
|TSI#3010||Diameter > 10nm||Butanol|
|TSI#3022A||Diameter > 7nm||Butanol|
|TSI#3760||Diameter > 14nm||Butanol|
|TSI#3775||Diameter > 4nm||Butanol|
|TSI#3781||Diameter > 6nm||Water|
For many years a passive cavity aerosol spectrometer probe (PCASP) was used at SGP. The PCASP is an optical particle counter which passes individual particles through a beam of light and uses the intensity of light scattered by the particle to size the particle, resulting in size distributions between 0.1 and 30 um. More recently we have collaborated with scientists outside NOAA to deploy size distribution instruments which classify particles based on their mobility diameter. These instruments scan through a range of sub-micron mobility diameters (typically between 0.01-0.7 um) to measure the particle size distribution.
A DMT cloud condensation nuclei counter (CCN) is used to count the number of particles which activate at a specific super-saturation. A wetted column provides the super-saturations and the particles are counted by an optical particle counter.
We define hygroscopic growth as the change in light scattering as a function of relative humidity (in contrast to other researchers who look at changes in particle size or mass with RH.)
We use two types of tandem nephelometer systems to investigate the hygroscopicity of atmospheric aerosol particles.
The humidograph system scans through various relative humidities, typically between 40 - 90%. (link to humid.html)
A simpler non-scanning humidifier system typically conditions the air stream to a single RH, typically ~85%. (link to humid_simple.html)
At several sites, an automated filter carousel is used to collect filters samples every 24 hours. There are 8 filters on the carousel - one for each day of the week and a field blank. The filters are changed once a week in a glove box and sent to the Pacific Marine Environmental Labs for major ion analysis and total gravimetric mass by Patricia Quinn's group. At one site (BRW) the filter carousel was modified for organic aerosol sampling. This was a collaboration with Lynn Russell at Scripps
NOAA has designed a rack-mounted impactor drawer to control size cut switching of the aerosol sample. There are a 1 µm and 10 µm within the rack-mounted impactor drawer upstream of the nephelometer and PSAP. The impactors ensure that only particles smaller than the impactor size cut, are measured. The 10 µm impactor is always in line, while every 6 minutes (for humidograph systems, the impactor switching occurs every 30 minutes) the 1 µm impactor is switched in line and the submicrometer aerosol are measured for 6 minutes. Particles smaller than 1 µm are important as they are the dominant contributor to light scattering. Measuring light scattering by particles smaller than 10 µm is useful as that tends to encompass all the scattering by particles.
Construction Schematic of ESRL/GMD's rack-mounted impactor drawer is available to download.
Basic measurements of the aerosol are made at low RH (<40%) conditions. If necessary, the sample is heated using heaters upstream of the aerosol instrumentation to maintain the relative humidity in the reference nephelometer at less than 40%. Construction Schematic of ESRL/GMD's stack heater is available for download.
A number of calibrated Vaisala T/RH sensors are used to monitor temperature and RH throughout the system. These measurements are also used to ensure the reference measurements are at the correct humidity ~40%, and they provide a check that there are no leaks in the system, i.e., dewpoint should remain constant unless water vapor is being added or diluted in the aerosol air stream. The nephelometers also have built in T/RH sensors.
At some sites with high humidity and/or high concentrations of aerosol, a dilution system is used to lower humidity and/or aerosol concentrations to prevent damage to instruments by water condensation or particle deposition.
ESRL/GMD have designed an aerosol stack and inlet system that is relatively inexpensive to build and provides excellent sampling efficiency for sub-10um aerosol. It is less effective for ultra-fine aerosol. The high sample flow (150 lpm) allows for multiple instruments to get their sample from the same inlet.
Construction Schematic of ESRL/GMD's inlet stack is available for download.
ESRL/GMD pumpbox design includes the actual box containing the pumps as well as pumps, a filter for pump exhaust and various electronic and analog indicators for diagnosing pump status. Click here for information about the pumpbox.
Various field campaigns have used a separate, co-located CVI inlet to study interactions between aerosol particles and cloud/fog droplets. The CVI preferentially enhances the measurement of cloud droplets, while repelling smaller interstitial aerosol from the sample air stream.
Related Links:ESRL/GMD's General Operations Manual for our Aerosol Systems
WMO/GAW guidelines and recommendations for aerosol measurement procedures the WMO website