Measurements at GSN
In situ measurements of a variety of aerosol optical properties are being made at GSN. 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:
Instrumentation at GSN
The instrumentation deployed at GSN is the most recent incarnation of the GMD aerosol monitoring system. The individual components are described below and there are links to pictures of each instrument. More information is available ont the aerosol instrumentation page.
The 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). One nephelometer is maintained at a humidity of 50%. A second nephelometer samples air at a variety of controlled humidities ranging from 20 to 90%.
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.
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.
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.
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.
The solar radiation instruments being sent to Kosan 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 GMD Global Radiation Group instrumentation webpage.