First, a little short history might help.  The HV sampler was developed in the 50s by General Metal Works and the first legislation was the Air Pollution Control Act of 1955, which gave federal money to monitor air pollution.  Then in 1963 it established a program that gave the federal government the ability to control air pollution.  Then things progressed to where the USEPA was formed, the clean air act was adopted, etc. 

https://www.epa.gov/clean-air-act-overview/evolution-clean-air-act

The early method for monitoring air pollution was the HV sampler.  The early systems were vacuum sweeper motors connected to a filter holder to collect particulate onto an 8×10” piece of filter paper.  The first legislation was for TSP – total suspended particulate.  The flow rate range was 35cfm to 60cfm.  The filter is weighed in a lab prior to sampling, then a 24-hour sample is run through the filter.  The filter is then weighed again, and the difference in weight is the weight of the particulate on the filter.  Since the sampler ran at a constant flow rate for 24 hours, you can calculate how much air passed through the filter and you can then calculate a concentration based on the (weight) / (volume).  This will be in micrograms per cubic meters of air.  This is called a gravimetric determination or method. 

The early samplers were outfitted with what are called mass flow controllers and these are still sold today.  They work on the principle of heating up an element to a specific temperature then flowing air across it and measuring the temperature downstream.  The temperature drop between the two is proportional to the rate of flow.  So, the user could control the speed of the motor based on a flow setpoint and maintain a flow rate.  Most users of HV samplers will run them around 40cfm.   This is above the 35cfm limit to give them a little cushion.  The reason to run at the low end is to prevent motor brush wear, pulls less current, less wear, etc. 40cfm = ~1200 lpm.    

Then the volumetric device was invented next which employed what is called a ‘critical orifice’.  The critical orifice will only allow a certain flow rate to pass and will keep that flow rate consistent all the time.  So you can try to pull as much air as you want through the orifice, but only a fixed amount will come through and it will be the same consistent flow rate forever.  These are set around 40cfm by drilling a hole through the VFC device  – one might be 41.7, one might be 42.3 – you get the idea – each one if a little different but the flow rate will be the same all the time at around 40cfm.

 After TSP, in the 1980s Dr. Wedding developed the HV PM10 inlet.  PM10 particles have an aerodynamic area of 10 micron and below.  The studies at the time showed that these particulates were problematic as they entered into the lungs and caused damage, especially to the elderly and health compromised populations.  PM10 HV samplers employ the same mass flow control or volumetric flow control methods of controlling flow rate as the TSP predecessor samplers.

The in the 1990s low-volume (LV) samplers were developed and they ran at 1 cubic meter per hour.  1 m3/hr = 16.67 lpm.  The reason for this radical shift from a HV sampler that runs at 1200 lpm (40cfm) to the LV sampler was to simulate human respiration.  Humans breathe at rest 0.5 m3/hr.  When active the rate is 1.5-3 m3/hr.  So 1m3/hr was set as the sampling rate and was a good round number.  

So now the USEPA was concerned with PM2.5 particulate.  These are particles equal or less than 2.5 microns in aerodynamic size.  The WINS (well impactor ninety six) impactor was developed in 1996 by the USEPA to fractionate PM2.5 particulate.  Then Dr. Lee Kenny in the UK developed the cyclone which improved on the WINS design.  So in the 90s the PM2.5 laws were put into place and the sampler used to collect PM2.5 particulate was the low-volume sampler running at 1m3/hr or 16.67 lpm.  The filter used for this collection was a circular 46.2mm diameter filter.  With such a low flow rate, they couldn’t use the big 8×10 filters of course.   The same gravimetric approach was taken – weigh the filter prior and after sampling for 24 hours and determine the concentration in micrograms per cubic meter of air.   All the LV sampler manufacturers use mass flow meters to determine the flow rate.  By measuring the barometric pressure and temperature of the air, you can then calculate the volumetric flow rate through the sampler. 

Then some developments were made for continuous PM2.5 and PM10 analyzers like the beta attenuation monitor (BAM).  The BAM uses a radioactive source to put gamma rays across a piece of filter tape.  The amount of rays that are absorbed by the particulate on the tape is proportional to the amount of concentration of the particulate.  These monitors are used to give real-time updates to particulate levels to alert the public that there are air quality alerts and to stay indoors, etc. 

The reason the continuous monitors are popular is that they do not require the filter to weigh and results can be obtained quickly – the first version of the BAM was hourly, but now is every few minutes. 

 Now there is a new light-scatter principle that Teledyne put together using a light scatter engine from a company called Pallas.  The light-scatter approach uses a laser diode to essentially shine through an aerosol (dust) and the amount of light that is refracted and absorbed is proportional to the concentration of the aerosol.  The nice thing with light-scatter is you can use different wavelengths of light to get different particle sizes – so you can do PM10, PM2.5, PM1 – whatever you want by ‘tuning’ the laser.  These light scatter principles are what is being used in the low-cost sensors – they somewhat work, but tend to read high mostly because of humidity.  Humidity particles can ‘look’ like PM2.5 particles and get counted like they were an actual dust particle. 

  • Sampling Rate
    • How is the airflow monitored in each of these samplers? What are the flow rate ranges? 

HV samplers use either mass flow control or volumetric flow control.  TSP and PM10 run at 36-60 cfm but most users typically run at 40cfm. 

LV samplers must run at 16.67lpm, there is no range per se.  The sampler can deviate from this by up to 10% for 10 minutes, but if it deviates from this more than that over a 24 hour sample, the sample must be stopped and flagged as invalid.  All LV samplers use mass flow meters to measure the flow, then measure the barometric pressure and temperature of the air to determine the volumetric flow rate. 

  • Particulate Size Range
    • What type of air particulates do each measure (i.e. pollen, dust, & larger aerosols)?

HV samplers were first developed to measure TSP – total suspended particulate.  This is everything in the air. 

HV samplers then in the 80s were measuring PM10 by use of the PM10 ‘head’ to fractionate particles 10 micron or less. 

So HV approved methods are TSP and PM10 particle sizes.

LV samplers were developed to measure PM2.5 – particles that are aerodynamically 2.5 micron and less. 

LV samplers can also measure PM10 with the use of the LV PM10 head. 

NOTE: We do have a HV sampler that can fractionate PM2.5 but it is not an approved method.  It works, but has not gotten USEPA approval.    

NOTE: LV samplers can also measure PM1 using a PM1 cyclone.  This is also not an approved method (yet), but probably will be where the USEPA will be heading next.  

  • Applications
    • In what types of situations are each of these used?

Both HV and LV samplers are regulatory products.  They would be used by air agencies on the federal, state and local levels here in the US.  They are used to make sure the states are meeting the requirements set forth in the clean air act. 

Worldwide, they are used for the same purpose, to measure what is in the air and to gauge progress of control actions or laws that are put in place to reduce emissions. 

There are other, not so common situations where HV samplers mostly would be used and that is called ‘speciation’.  So take the HV PM2.5 sampler for example.  It is not an approved method for measuring PM2.5, but if you want to collect a lot of PM2.5 particulate on a filter to determine what it is (speciation) it’s a great sampler to use. 

Another application we’ve seen is in superfund cleanup sites that were old nuclear depositories or manufacturing facilities.  HV samplers are used to bring in a lot of material from the remediation / construction process to make sure there is not radioactive material being released during the remediation.  

There are probably a lot of other applications I’m not mentioning here, but for the most part, the majority of samplers are used for regulatory purposes, to determine the amount of TSP, PM10 or PM2.5 that is in the air. 

  • Sampling Duration
    • Does one sampler take longer or shorter timeframe to collect data?

Both HV and LV samplers run for 24 hours for a valid sample.  This is set forth by the USEPA. 

  • Equipment Size & Portability
    • Are low volume samplers more portable and better suited for remote monitoring applications?

LV samplers aren’t very portable, but can be.  They weigh about 40lbs and can be carried around to different locations.  It’s not very common however.  Most samplers end up in one location and stay there.  The LV sampler can run on a solar installation since it only pulls about 2 amps when running.  The HV sampler is not suited for solar installations as they pull about 8 amps.  We have had several instances where users would run HVs on generators if they are doing some site studies or something, but not very common.   

  • Data Analysis
    • Are there different analysis methods used for low vs high volume samplers?

The data analysis is the same, it’s a gravimetric determination.  Weigh the filter before and weigh after.  

  • Are there any benefits to using a low vs high volume sampler or does it depend on what is being measured?

 If someone needs to measure TSP – the only approved method for that is a HV sampler.  If they want to measure PM10 they can use a HV or a LV.  If they need to measure PM2.5 they must use a LV only. 

A lot of agencies have switched to LV samplers for PM2.5 and PM10.  They will have what’s called a ‘collocated pair’ of samplers.  One configured for PM10 and one configured for PM2.5.  Then they only have one type of filter to keep on hand and weigh.  This makes sense as they only have one type of sampler to maintain and report data from.