¶ Aerosol Observing System Particle Soot Absorption Photometer (AOSPSAP3W)
¶ Instrument Description
A particle soot absorption photometer measures how strongly a sample of aerosol particles absorb light at three visible (red, green, blue) wavelengths. These PSAP measures these quantities, represented by absorption coefficients (an indicator of the impact of absorbing aerosols on radiative forcing) by collecting incoming aerosol particles on a filter and measuring the change in light transmission through that filter relative to a clean (not collecting aerosols) reference filter. The amount of light absorbed by aerosols on the collecting filter is given by Beers Law. Bulk particle absorption is derived after correcting for scattering effects.
The PSAP is part of the Aerosol Observing System (AOS).
- For more information see the ARM PSAP Instrument web page.
- For a nice general description of PSAP operations see PSAP Instrument Explanation
¶ Basic Measurement Concept
Imagine you're standing on the ground, staring up at smoke plumes from a wildfire. You should be panicked because you're in a wildfire, but for some reason you're rationally thinking about science. Coming out of your deep thoughts on the nature of the world around you, you notice that it's daytime and the sun is out. However, the smoke particles (aerosol) from the wildfire are absorbing and scattering the light from the sun, making the light from it appear "hazy" and less intense. You've probably more commonly noticed this effect when looking at smoke/pollution plumes from various sources (trucks, power plants, etc.).
Believe it or not, you have acted as the absorption coefficient measurement component of a large, natural "PSAP" in the above example. Light from a source (the sun) is absorbed (and scattered...) by aerosols (smoke) in a filter (the atmosphere above you), such that some of the initial light from the source has been extinguished by the time it reaches the detector (you). Great job!
¶ Instrument Operation Details
According to the Global Monitoring Division instrument page linked above a filter transmission for a good measurement should not drop below 0.7. When the transmission is below 0.7 the measurement is suspect. When the transmission is below 0.5 the measurement is considered invalid.
¶ Expected Operations
¶ Metrics
¶ Plots
¶ Daily plots
The primary PSAP measurements are the light absorbances for each of the three wavelengths (red, green, and blue). They are plotted below. As the PSAP is plumbed to the AOS Impactor, these absorbances are parsed according to the impactor_state setting (either 1- or 10-micrometers) for the daily light absorbance vs. time plot. For convenience, these two impactor settings are indicated by the different colors and the annotated text in the plotting window.
Note that between 1- and 10-um impactor settings once per hour that the absorbance may briefly drop towards 0 /Mm or become slightly negative. This is in-line with expected instrument behavior and does not need to be noted in your DQAs.
The middle panel shows the transmission of light through the current PSAP filter at each wavelength. As the filter collects more aerosol material, transmissions should generally decrease. When the transmission of light through the filter falls below 0.7, the PSAP filter is changed (it does not happen automatically as it does for CLAP). After filter changes, transmissions should return to a nominal value of 1.0 (all light passing through the clean filter). The example shown below shows a filter change happening at ~14:00 UTC.
The bottom panel shows the impactor setting, plotted in blue in this plot, while the mass flow rate is plotted in orange. It is extremely important to remember that the impactor setting corresponds with the axis on the left-hand side, while the mass flow rate corresponds with the right-hand axis. As the impactor switches from 1- to 10-um settings, the sample flow rate should toggle accordingly, with higher flow rates associated with a 10-um setting and lower flow rates associated with a 1-um setting.
¶ Site Specific Information
PSAPs are currently located at SGP E13, ENA C1, and the AMFs. Instruments at all locations operate similarly.
¶ Known Issues
¶ Known Behaviors
Known behaviors that may not require mention in DQAs or DQPRs are mentioned below.
¶ Lagged flow rate fluctuations
As the impactor setting on the PSAP fluctuates between its high (10um) and low (1um) settings, the PSAP flow rate should fluctuate in tandem. That is, a high impactor setting should be associated with a higher flow rate, and vice versa. In the below example plot of PSAP impactor setting and flow rates at MAO M1, this condition is generally met. However, it appears as though there is a slight time delay between the change in impactor setting and the corresponding change in flow rate (e.g., ~03:00 UTC). This is an indicator of NORMAL operation and is not a data quality concern.
From Instrument Mentor Anne Jefferson:
"What happens is that at the top of the hour when the system switches from sub-1um to sub-10um the nephelometer goes through a zero check. The psap is connected in parallel to the neph, so pressure changes in the neph also affect the psap. During the zero, air pass through a hepa filter which has a pressure drop across it. You don't see this behavior at the half hour (switching from 10 to 1 um) as there isn't a zero check. For this reason the neph data 3 minutes before and 3 minutes after the the hour have missing value codes."
¶ Noisy Coefficient Data
Sometimes you may notice some noisy absorption coefficient data, as shown below. This may not necessarily indicate a problem, however. In the case below, from Instrument Mentor Anne Jefferson:
"The noise seems to be related to low transmission. The tech changes the filter in the morning and perhaps by the end of the day the filter fills up with aerosol. Right now the site is seeing a lot of smoke aerosol from fires in Kansas, so that may contribute. This is one reason why we have the CLAP as the instrument cycles through 8 filter spots instead of one. This is normal instrument operation."
¶ Past Problems
Past problems that do require mention in DQAs or possibly DQPRs are mentioned below.
¶ Pump failure
A failed pump, as evident by the suddenly dropping mass flow rate below, can lead to incorrect light absorbance for all three channels.