A nephelometer is an instrument designed to measure the scattering of light by atmospheric aerosol particles at red, blue and green wavelengths in the visible portion of the electromagnetic spectrum. The primary nephelometer measurements are those of aerosol scattering and hemispheric backscattering coefficients at these three wavelengths. Some factors which control the scattering characteristics of aerosol particles include their shape, color and reflectivity.
Nephelometers are usually and increasingly deployed in pairs, with one of them measuring scattering under ambient conditions (AOSNEPHDRY) and the other measuring scattering as a function of slowly increasing or decreasing instrument-controlled relative humidity (AOSNEPHWET). The combination can then be used to derive the hygroscopic growth factor of the sample aerosol as a function of RH: f(RH), a metric that is of critical importance for cloud physics studies.
As of now, please monitor the b1-level aosnephdry and aosnephwet datastreams for all nephelometer DQA assignments (you can ignore the aosnephdry1m and aosnephwet1m datastreams). When submitting DQPRs for either wet or dry nephelometers, please use the NEPHELOMETER instrument class under the AOS Group.
For more information, please see the NEPH Instrument web page. You can also find a detailed description of a nephelometer (with a very nice description of the backing measurement theory) courtesy of the NOAA Earth System Research Laboratory's Global Monitoring Division. Some additional information can also be found in the vendor's operations manual.
For a quick reference on how the nephelometers scan through different wavelengths of visible light, watch the demonstration below:
The nephelometers are component instruments within the Aerosol Observing System (AOS). For a more complete overview the AOS system and its general backing measurement theory, please see the AOS DQ Wiki page.
In the dry nephelometer metrics table below, the primary measurements are outlined in red; all other measurements are diagnostic in nature. The primary fields are:
In the wet nephelometer metrics table below, the primary measurements are outlined in red; all other measurements are diagnostic in nature. The primary fields are:
The primary wet and dry nephelometer total light scattering and backscattering coefficients for blue, green and red wavelengths are plotted on daily and weekly timescales. While these coefficients should be similar for both ambient ("dry") and humidified ("wet") nephelometers, you may note that wet nephelometer scattering coefficients scale with the ramping of instrument relative humidity through time.
Note that all nephelometers are dependent upon AOS Impactors, a component within the AOS systems that effectively controls the size of aerosol particles reaching the nephelometers. In the daily scattering and backscattering coefficient plots, the data are parsed by the associated impactor setting (either 1- or 10-microns) and colored accordingly (lighter colors for 1-micron and darker colors for 10-microns); see key on right-hand-side of these plots for reference. Note that scattering and backscattering coefficients associated with a 10-micron impactor setting often exceed those associated with a 1-micron impactor setting. Physically, why would we expect this to be the case?
For monitoring general dry nephelometer instrument status, a number of diagnostic fields are also plotted on daily and weekly timescales. These include diagnostic pressures, temperatures, relative humidities, and lamp voltage/lamp current.
Note that nephelometer pressures should track with impactor setting, with higher pressures associated with a 10-micron impactor setting and lower pressures associated with a 1-micron impactor setting. Note also that for the dry nephelometer, instrument relative humidities (bottom panel) should remain more constant through time than those associated with the wet nephelometer.
When nephelometers are collocated with each other or other instruments which measure the scattering of light by aerosol particles, we produce a comparison plot of the unique measurements on daily and weekly timescales. These comparisons are done for blue, green and red visible light wavelengths, and, if available, aerosol number concentrations from collocated CPC instruments are plotted for reference (grey; right-hand y-axis).
This plot is meant to provide some context for the individual nephelometer scattering coefficient measurements. It could, for instance, be used to identify trend differences between two collocated nephelometers. Note that you may often see small-magnitude differences in wet and dry nephelometer scattering coefficients (see example below); this is related to the ramping of relative humidity within the wet nephelometer. That said, you should often find that the collocated scattering measurements trend well with one another.
Nephelometers - either wet, dry or both - are deployed as a component instrument within all of the AOS systems ARM deploys. All instrument systems operate in a similar manner.
Instrument behaviors that do not require DQAs or DQPRs.
Every day at ~00:00 UTC, the wet and dry nephelometers operated by Brookhaven National Laboratory (i.e., all those not at SGP C1 or NSA X1) perform an automatic "zero" check for ~5 minutes as part its operational deployment strategy. This manifests in the associated QC metrics as a short period flagged as "bad" (see example above). As scattering coefficients during this auto-zero are often quite a bit higher than and not representative of the physical atmospheric signal, they are screened from the DQO's diagnostic plots.
You can simply ignore these auto-zeros when monitoring the BNL nephelometers. There's no need to make note of this behavior in your DQAs.
When environmental RH is very low, the RH plots may drop to zero frequently and for long periods of time. Both wet and dry nephelometers show this behavior, although the dry neph plots tend to show noticeably more severe flatlining. This is most likely due to inherent limitations on the sensitivity of the RH sensors. As long as the wet and dry neph plots correlate well otherwise and RH values are consistently very low, long periods of flatlined RH are not indicative of an instrument problem.
This also shows up as missing data in the metrics. This is not a cause for concern. When the air is very dry and cold, and subsequently brought into the warm observatory, the RH can occasionally fall to zero. The RH neph inlet is a calculated parameter based on the measured RH and temperature inside the nephelometer and the measured temperature neph inlet. So, if any of those other parameters (RH inside neph, T inside neph, T neph inlet) are missing, then that's when there should be concern.
Past problems that do need to be mentioned in DQAs and possibly DQPRs.
From time to time, nephelometer (either wet or dry) scattering and backscattering coefficients will drop negative and flatline for many hours at a time. When this happens, it is likely that a bulb critical to proper nephelometer operation has burned out.
NOTE (from the mentor): Remember that the nephelometers automatically zero themselves every midnight UTC to compensate for several things (including lamp brightness). The numbers from zeroing are then used to correct the data until the next zeroing. So, for instances where the bulb gets replaced, the lamp brightness is different and won't be properly accounted for until the next zeroing. An example of this is shown below:
As you can see, the differences in data from before the bulb change and after the bulb change may seem negligible to the eye, but it is important to remember that the data quality is still affected until the next zeroing at midnight UTC.
If nafion cells between the dry and wet nephelometer become obstructed or flooded, the air flow can become partially or fully blocked, leading to the wet and/or dry nephelometer to suck in cabin air. This can cause the wet and/or dry nephelometer Bs and Bbs measurements to drift higher than those from the unaffected nephelometer.
Action: If you see this behavior, please issue a DQPR.