The Aerosol Chemical Speciation Monitor is a thermal vaporization, electron impact ionization mass spectrometer that measures bulk chemical composition of the non-refractory component of sub-micron aerosol particles in real time with typically 30-minute integration period. Standard derived quantities are the mass-concentrations in ug/m^3 of organics, sulfate, nitrate, ammonium, and chloride.
Translation: the ACSM provides information on the chemical composition of aerosols sampled with the AOS system.
As of now, please monitor the a0-level aosacsm datastream for all ACSM instruments. When submitting DQPRs for the ACSM, please use the ACSM instrument class under the AOS Group.
For more information, please see the ACSM Instrument web page.
The ACSM is a component instrument 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 metrics table below, the five primary measurements are listed below; all other measurements are diagnostic in nature. The primary fields are:
Note: there is often noise in these primary fields that will trip the automated QC tests, which is the case in the example metrics table above. While egregious QC failures should be noted, not all QC failures need to be noted. Values below 0 are common due to errors in the data logger when processing instrument voltages, and occurrences should only be noted if the values are far below 0. Total organics occasionally spike to unrealistic levels; occurrences of this should also only be mentioned if they last for an extended period.
The five primary measurements at the b1 level are the same as in a0 level. Note that these are the only five variables tracked in the metrics for b1 level, with the exception of 'acsm_vol_conc', which is the volume concentration for the ACSM.
It's okay for the 5 primary measurements to occasionally flag, as shown in the metrics table below. Just as in the a1 level, there is often noise in these primary fields, and values below 0 are common due to errors in the data logger. These only need to be noted when the values are FAR below zero. Total organics can sometimes spike to unrealistic levels as well, but only need to be mentioned if the spikes last for an extended, consistant period of time.
Currently, the acsm_vol_conc variable will flag as red when the ACSM volume concentration is less than 0.4 times the SMPS volume concentration or greater than 1.25 times the SMPS volume concentration. This is a way to track if the ACSM volume concentration is tracking with the SMPS volume concentration within a reasonable amount. Again, occasional flagging is okay (as the instrument sometimes gets spurious spikes in data), but a prolonged period of flagging might indicate that something is wrong and should be mentioned in a DQPR.
Daily plots of the mass concentration of total organics, ammonium, sulfate, nitrate, and chloride within the ambient aerosol in the air sample. In these time series plots, the data are plotted at the native 30-minute instrument resolution. A rain flag has been added to these plots to better distinguish common aerosol "wash-out" events (see example at ACSM Known Behaviors) that are not a problem with more serious instrument issues. Periods where rain was observed at a given site will be given a blue flag at the bottom of these mass concentration plots. There are also weekly versions of these plots.
There are also plots of daily diagnostic data associated with the ACSM. These include inlet pressures, flow rates of sample air through the instrument, and diagnostic currents and temperatures. Of particular note are odd behaviors exhibited within the inlet pressure fields, as these could be associated with common critical orifice clogs. There are also weekly versions of these plots.
The trends in backscatter near the surface from the CEIL has proven to compare relatively well with the trends in the total organics. This is likely because CEIL backscatter from aerosols in the boundary layer should roughly correlate with aerosol mass concentrations observed by the ACSM at the surface (though not always!). This plot can also be used to diagnose benign aerosol "wash out" events by rainfall (see example at ACSM Known Behaviors), similar to the rain flag included in the daily and weekly mass concentration plots.
Now that volume concentrations are available in the b1 level for many AOS instruments, volume concentration comparison plots can be seen when available. The ACSM, SMPS, nanoSMPS, and UHSAS volumes should track together, but not necessarily at the same amounts. As long as they trend well together, things should be okay. If you notice one trending completely differently than the others for extended periods of time, then please issue a DQPR (note: nanoSMPS will typically be lower in volume concentration amounts when compared to the volume concentrations of the other instruments).
The ACSM is currently located at SGP E13, ENA C1, AMF1, and AMF2. All locations operate similarly.
Precipitation and wind speed can have a noticeable affect on ACSM aerosol species mass concentrations. Precipitation can "wash out" aerosols from the atmosphere, causing mass concentrations to decrease sharply. In the example below, rain at SGP resulted in such a wash out event beginning just after 07:30 UTC, resulting in an abrupt decrease in aerosol total organic mass concentration. This wash out was evidenced by both the existence of the rain flag in the mass concentration plot and by the higher near-surface VCEIL backscatter magnitudes at that time.
You do not need to mention these washout events in your DQAs.
Shifts in wind direction or speed can also result in the advection of "cleaner" (less aerosol) or "dirtier" (more aerosol) air masses over a given site. Examples here would include the advection of aerosol-rich air from cities.
If you see these behaviors, please issue a DQPR.
The ACSM critical orifice can periodically become clogged with dirt/other aerosol deposits. These critical orifice clogs are evident by a decrease in the inlet pressure (inlet_closed field). The DQO has a test in place to flag such decreases in the QC for the inlet_closed field as “possible critical orifice clogs”. Inlet pressure drops can also be the result of a failed switching valve.
Condensation can form around the air inlet on the instrument causing spikes in the data. This typically occurs during periods where the humidity is high for several days. Spikes are noticed in the total organics mostly, but can also appear in some of the other aerosols.
If the vacuum pump used for pulling samples past the inlet of the ACSM fails, then the instrument would continue to be operational, but not produce any aerosol. If you notice the instrument not reading any aerosol values for several days, then a vacuum pump failure may be the culprit.