ESRL/GMD Aerosols Group

ESRL/GMD Aerosol Measurements

[ESRL/GMD Aerosol Sampling Station List]

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

Other parameters measured include:

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.

Instrumentation

Construction Schematics of ESRL/GMD's basic aerosol system are available to download.

The most recent version of the ESRL/GMD aerosol monitoring system was deployed at Trinidad Head, CA in Spring, 2002. Prior to that the system collected data for ~1 year in Kosan, Korea. Similar systems are in place at the other ESRL/GMD stations although each station is slightly different. The individual components are described below and there are links to descriptions and pictures of each instrument. You can take 'virtual tours' of the ESRL/GMD ground-based aerosol system and the ESRL/GMD small aircraft aerosol system or look at a clickable schematic of the aerosol system.

Nephelometers

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.

Particle Soot/Absorption Photometer (PSAP)

The PSAP (Radiance Research) is a filter-based method that measures light absorption by particles at a single wavelength: green (565 nm). Particles are collected on a filter and light transmission through the filter is monitored continuously.

Particle Counters

Construction Schematic of ESRL/GMD's Condensation Nuclei rack mounted unit are available to download.

A condensation nuclei counter (CNC) is used to count the number of particles with diameter greater than 0.01 micrometers. Particles are drawn into the CNC and vapor is condensed on them so that they grow to a size detectable by the instrument optics.
A passive cavity aerosol spectrometer probe (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.

Cloud condensation nuclei (CCN) counter

A 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.

Size Control

Construction Schematic of ESRL/GMD's rack mounted impactor drawer are available to download.

There are a 1 µm and 10 µm impactor 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 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.

Humidity control and scanning

The sample is heated, if necessary, to maintain the relative humidity in the reference nephelometer at ~50%. A scanning humidification/drying system is used to step the air stream through a range of humidities for sampling by the second 'humidified' nephelometer. The humidifier is an annular design consisting of a counterflow water stream around a water vapor permeable membrane stretched on a wire mesh tube. The aerosol air stream flows thru the wire mesh tube. The controlled temperature of the water determines the RH to which the airstream is exposed.

Temperature/Humidity Sensors

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 ~50%, 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.

Chemistry measurements

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.

Radiation measurements

Solar radiation instruments from ESRL/GMD's Solar and Thermal Atmospheric Radiation (STAR) group have accompanied the aerosol system on its deployments to Kosan and Trindad Head. Some of the solar radiation instruments include: 2 four channel sunphotometers, 3 precision spectral pyranometers (PSP) with solar trackers, 2 normal incidence pyrheliometers (NIP), and a multifilter rotating shadowband radiometer (MFRSR). Ambient pressure, temperature and relative humidity are also measured. More information about these measurements can be found at the ESRL/GMD Solar and Thermal Atmospheric Radiation (STAR) instrumentation webpage.

Counterflow Virtual Impactor

At a 2003 measurement campaign in Sweden to study interactions between aerosol particles and cloud/fog droplets a counterflow virtual impactor (CVI) was used to enhance the measurement of cloud droplets. A similar system will be deployed at various field projects between 2005-2007 to study aerosol cloud interactions as part of our DOE funded Aerosol Science Proposal (ASP).

Pumpbox

Click here for information about the pumpbox.

Sample Dilution System

Sample Dilution System Diagram

The sample dilution system mixes filtered, particle-free air that is has been dried to a low RH with the sample air. The primary benefit of this system is to reduce the relative humidity in environments with very high ambient dew points and to reduce the aerosol concentration to within the instrument measurement range for highly polluted or dusty atmospheres. In relatively clean environments you run the risk of lowering the signal level close to the instrument detection limit and reducing the signal quality. The dilution dryer is not recommended for environments with low aerosol loading. Another caveat of the system is that ample mixing time is needed to adequately mix the dilution and sample air. At an airflow of 30 lpm and 0.75” ID tubing the recommended length of tubing for mixing is 6-8 ft. At this length of sample tubing you run the risk of gravitational particle losses in the sample line. Care need be taken to minimize the horizontal component of the sample line. The sample-mixing device is a commercial unit fabricated by Mott Corporation, http://www.mottcorp.com/industry/oem/gassamp.htm, model number 7610. This should be placed at the outlet of the flow splitter at the bottom of the aerosol stack inlet. Dry, particle-free air is produced by an air compressor/membrane dryer system produced by Twin Tower Engineering, http://www.airdryers.com/prmd.htm. The part numbers are SWC-M15-100 for the membrane dryer and A-065 for the compressor/flow regulator system. The ratio of the dilution to total flow will depend on the desired dilution and sample RH.

Global Monitoring Division