Optical Extinction Analyzer
Fast, Accurate Measurements of Visibility (MOR) with No Calibration and No Consumables
Aerosol Optical Extinction Made Simple.
Nikira Labs’ Open-Path Optical Extinction Analyzer (OEA) is a next generation instrument for accurate measurements of visibility, meteorological optical range (MOR), and aerosol optical extinction. The OEA uses open-path cavity ringdown spectroscopy to measure the total optical extinction of aerosols from first principles. By periodically closing the cavity and purging it with filtered air, the OEA measures the optical loss with and without aerosols. Subtracting these measurements provides the absolute optical extinction due to ambient aerosols from first-principles with no calibration (or self-calibration, if you prefer).
Due to its high data rate (up to 10 Hz), the analyzer can be used for both routine monitoring and eddy covariance studies.
More Information about the Optical Extinction Analyzers
How does the OEA work?
Ambient air is pulled through a duct by fans at a speed of ~1 m/s.
Open-path cavity ringdown spectroscopy is used to make a direct measurement of the optical extinction coefficient (beta) in the sample.
The duct is periodically closed and the cell is purged with filtered air to a background measurement.
The difference between the open and closed duct values provides a direct, calibration-free measurement of the aerosol optical extinction.
What does the OEA Measure?
The OEA measures total optical extinction due to aerosols at a specific wavelength without sample handling losses or calibration.
Total optical extinction is directly related to the meteorological optical range (MOR) or visibility, and the OEA measured both quantities with high-accuracy.
The OEA can be used to estimate PM2.5 or PM10 for air quality monitoring.
What is Optical Extinction? Meteorological Optical Range? Visibility?
Optical extinction is the amount of light lost over a given distance. The OEA directly measures the optical extinction coefficient (beta in the equation below) in Mm-1.
The Meteorological Optical Range (MOR) is related to beta and is how far a person can actually see (called the image visual range).
Visibility is the same at MOR, but is a more commonly used term.
How do we verify that the OEA is accurate?
Since it is very difficult to generate an aerosol flow with precisely known characteristics (e.g. particle size, density, and index of refraction), each OEA is verified by measuring the Rayleigh scattering of helium, nitrogen, and carbon dioxide. The wavelength-dependent Rayleigh scattering cross-sections of these compounds have been measured and calculated by several groups and are well-established. As shown below, 6 different OEAs were used to measure the scattering and all provided accurate results with no calibration or adjustment. This highlights that the open-path cavity ringdown technology used in the OEA is a first-principles measurement of optical loss and shows that the self-calibration accounts for any other changes in the system.
How do we measure the OEA’s precision?
The OEA’s precision is determined by repeatedly measuring the same air sample over an extended period of time. The resulting Allan variance plot will then show how measurement precision scales with data averaging time. As shown in the figure below, the OEA provides a precision of better than 1 Mm-1 (1-sigma, 1s) and improves with averaging time. For a typical 15-minute average, the precision exceeds 0.1 Mm-1. This allows for accurate quantification of even very clean air (e.g. optical extinction coefficient ~10 Mm-1 and MOR ~300 km).
Figure 1: OEA-532 Measurement precision versus data averaging time.
Figure 2: OEA-532 Measurement stability of clean air over time.
How fast does the OEA provide data?
Air moves through the duct at ~1 m/s, giving the sample a residence time of only 0.025 s in the cavity. Thus, the instrument response time is limited by the data rate. Nikira Labs offers both 1 Hz and 10 Hz versions of the OEA, with the latter providing sufficient speed for eddy covariance studies. Note that the analyzer can be directly connected to a data logger (e.g. Campbell Scientific CR1000 or similar) to allow for simultaneous logging of a sonic anemometer and the OEA.
Can the OEA be used to measure highly polluted air?
Absolutely! The OEA can measure optical extinction values of up to 10,000 Mm-1, which corresponds to an MOR of 0.5 km. Note that the worst polluted days typically have MORs of >1km.
Has the OEA been compared to other aerosol optical measurements?
The 532 nm OEA was deployed in conjunction with an open-path nephelometer (Optec NGN-2a) at Christman Airfield in Fort Collins, Colorado. As shown below, the two instruments were very well correlated with the OEA providing slightly higher values as expected (e.g. aerosol optical extinction is greater than scattering alone). Moreover, the difference between the two measurements is a measure of aerosol optical absorption which scales with extinction, suggesting that the larger plumes contained more absorbing aerosols (e.g. black or brown carbon).
What wavelength does the OEA use?
The OEA comes in a variety of wavelengths ranging from the blue (485 nm) to the near-infrared (1650 nm). Our standard version operates at 520 nm to most closely match the conventional visibility measurement near 550 nm.
New research enabled by the OEA:
Eddy covariance measurements showing locations and fluxes of particulate matter
Accurate correlations between PM2.5 and meteorological optical range (MOR)
Mobile monitoring of MOR and PM2.5 (note that the OEA includes a GPS!)
Partitioning the components of extinction into gas-phase and aerosol contributions (the purge gas can be varied from filtered air to helium)
Real-time measurements of the Angstrom Extinction Coefficient (two OEAs can be operated in tandem to determine the aerosol component of the AEC)