1 Motivation Particles are generally regarded as one of the most serious indoor air quality concerns Increasing concern about ultrafine particles Very high surface area/unit mass Direct transfer through cell walls Mechanism for respiratory disease “Asbestos-like” health effects 1

22 Arrestance describes how well an air filter removes larger particles such as dirt, lint, hair, and dust

Particle Measurement Distinguish between Particle counting Only counts number of particles, makes no distinction between sizes Particle sizing Counting and sizing information Particle mass Particle composition Viable and non-viable bioaerosol assessment Sampling issues

Particle Sensors Inexpensive (relatively) Gravimetric for particle mass Light scattering for large particle mass Condensation nucleus counter (CNC) for counting small particles Cascade impactor for size-resolved mass Mid-range Optical particle counters Expensive Aerodynamic particle sizing for large particles Differential mobility analyzer for small particles

Gravimetric (Mass-based) Techniques Particles have very low masses Need to collect many particles to have measurable mass Most mass based techniques are integrated samples

Gravimetric Sampling

Quantitiative Measure mass of clean filter Measure mass of filter after exposure Measure flow rate and exposure time Calculate concentration Corrections for blank filter Corrections for humidity

Optical Measurement Extinction What are limitations?

Mie Theory for Scattering Forward-scattering and back-scattering Functions of (λ, θ, d p, V p ) Often see size parameter, α = πd/λ

Photometers Typically relative instruments Sensitive to particle speed Nephelometer Measure scattering for aerosol sample (~ 1L) over wide range of angles (  ) Particle density is function of the light reflected into the detector –Scatered light depends on properties of the particles such as their shape, color, and reflectivity. Determines mass concentration much more accurately than photometer Often calibrated to single particle composition Measuring Particles Optically (Detection Sensor)

Condensation Nuclei Counter (CNC) Subject aerosol stream to alcohol (or water) vapor Cool air stream to cause condensation Count particles with an optical particle counter Closely related to a condensation particle counter (CPC)

Cascade Impactor

Cascade Impactor Curves

Optical Particle Counter Similar to photometer, but particles are isolated May require dilution – 20 µm Practically 0.1 – 5 µm Some devices just count

General Discussion of Accuracy For what size aerosol? For what concentration of aerosol? Even gravimetric For instruments that size Not counting particle vs. putting particle in wrong bin Manufacturer’s accuracy is not often useful Must calculate your own based on knowledge of instrument

Aerodynamic Particle Sizer One of many time-of-flight instruments Two laser beams separated by known distance Particle is accelerated between beams Time between beams being broken is calibrated to test aerosol um

22

APS Small particles move at the air velocity Large particles lag air velocity Problems Small particles not-Stokesian Bigger density sized as larger particle Shape also influences drag Multiple particles in sizing chamber (same as other devices)

APS air and particle flow diagram 24

25

Particle Mobility Analyzer Particle is subjected to careful (difusive) electric charging Charge on particle is proportional to diameter Electric mobility is known Particles are sorted by charge Particles are counted by other technique (mostly the condensation method) – 1 µm

Table 15.4

The SMPS Consists of 1. Electrostatic classifier (EC) 2. Differential mobility analyzer (DMA) 3. Condensation particle counter (CPC)

How the EC and DMA work EC Kr-85 bipolar charger DMA 2 laminar flows  Sheath and aerosol 2 concentric cylinders  Center negative voltage  Electric field + particles attracted through sheath air Location depends on electrical mobility, flow rate, and geometry  Cycles through different voltages to capture different size particles

How the CPC works Interface with EC and DMA to form the SMPS Particles are passed through a wick and grown with either water or butanol Aerosol stream saturated and temperature equilibrated Heterogeneous condensation on condensation nuclei (the particles) Grown to 2 to 3 micrometers Individual particles passed through light beam and scatter light onto a photodetector

SMPS Best for 2.5nm  m Can’t precisely classify larger particles b/c Fraction of +1 and +2 charged particles begin to converge Changing voltages begins to cause equal fractions of particles of the same size to fall in different bins Smaller particles Fraction of charged particles gets close to 0, so different voltages can’t control mobility

Control methods/devices No device works (well) for all particle sizes (a)(b) Efficiency as a function of particle diameter as measured with (a) Optical particle counter and (b) Aerodynamic particle sizer

Summary Wide variety of instruments available for particle measurement What size of aerosol are you interested in? Do you need sizing or is counting sufficient? Do you need real-time data? What type of aerosol are you trying to measure? How much accuracy do you need? How much money do you have?

34 Future Measurement Exercise Get manual and record data from: TSI Aerotrack optical handheld particle counter (4) P-Trak (2) DustTrak SidePak Colocate all instruments in a room and see concentrations that result from different sources Main purpose is to understand all instruments 34

Other Particle Measurement Issues Sampling line losses Sampling particles in moving air stream Particle composition Bioaerosol sampling

36 Sampling Line Losses Extensive literature on subject Generally an issue for large (>1 μm) and small (< 0.05 μm) particles What are mechanisms that cause loss and how do we minimize them? Calculating line loss Values from literature, software, or use equivalent lines Best approach is measurement 36

37 Isokinetic Sampling

38 Particle Composition Collect sample of particles on filter Analyze as you would for liquid or solid compounds Challenges? SMPS w/ mass spec. Very expensive and response time issues 38

39 Bioaerosol Sampling Many issues Fungi, bacteria, other stuff, metabolic byproducts Quantitiative or presence/absence Culturable, viable, DNA-based Inhibitors 39