Differential Mobility Particle Sizer - Institute for Atmospheric Science
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1. room temperature the mean free path is amp 66 5 nm and the viscosity 4 1 83 10 5Nsm 2 1 1 Neutralizer Before the particles enter the DMA they pass through a neutralizer The neutralizer does not neutralize charges but rather brings the particles into a well known charge distribution with a radioactive polonium source The result is a Boltzmann distribution of negative and positive charged particles In an electric field a particle with n charges experiences an electric force causing it to move through the gas in which it is suspended 2 2 Condensation Particle Counter CPC The CPC counts the aerosol particles that pass through the DMA Inside a CPC incoming particles are enlarged due to vapor condensation so that they are big enough for later detection This is done in a heated saturator where alcohol vapor condenses onto the particles causing them to grow into droplets These droplets are then detected with a laser beam Figure 3 The working fluid in the CPC TSI Model 3010 used in this experiment is n butanol The instrument detects particles between 10 nm and 3 um Single particles are detected for concentrations lower then 10 cm For higher concentrations the particle number is estimated from the scattered laser light The sample flow of the instrument is 1 1 min Page 6 Differential Mobility Particle Sizer Figure 3 Condensation Particle Counter Figure from TSI 2 3 Differential Mobility Particle Siz2. sheath air in the DMA system must be set to 5 1 min in the labview program The flow is then measured using the Gilibrator Flowmeter The DMA sheath flow system is a closed loop Air is sucked by a blower from the lower exit of the DMA excess air out and passes a filter A mass flow controller upstream of the blower regulates the flows and is controlled by a labview program After passing the two 3 way valves the particle free air enters the DMA again sheath air in Before starting disconnect the CPC First block the DMA polydisperse aerosol in line as well as the monodisperse aerosol out line Turn both 3 way valves so that they point outwards Start the labview program and set the sheath flow to 5 1 min The blower will now suck air from the 3 way valve V1 then through the DMA and finally release it at V2 Measure with the Gilibrator Flowmeter if the inlet flow at V1 equals the exhaust flow at V2 You should see only little differences Next measure the flow of the CPC by starting its pump and connecting the flowmeter right at its inlet Finally in order to check if leaks or blockages are present in the system measure the aerosol flow upstream of the neutralizer For that unblock the DMA aerosol in and out lines and connect the CPC to the aerosol out line Switch on the CPC pump and measure the aerosol flow with the flowmeter You should not see a strong deviation from 1 1 min 3 2 Delay time Particles need some time to p
3. ETH Eidgen ssische Technische Hochschule Z rich Swiss Federal Institute of Technology Zurich Institute for Atmospheric and Climate Science IACETH Atmospheric Physics Lab Work Differential Mobility Particle Sizer Aerosol measurements Abstract A differential mobility particle sizer DMPS is a standard tool in atmospheric science to measure the particle size distribution in the sub micron size range In this experiment a DMPS is used to measure the aerosol size distribution of two particle sources A candle and urban air Both size distributions are analyzed and compared to each other Questions to be answered during the reading of the manual Will be discussed in a small tutorial ahead of the experiment Why is the particle size important for ambient air Why need the particles to be charged in order to be measured with this technique What happens if particles carry multiple charges Can you think of other techniques to measure particle sizes For which sized may they be best suited and why Please finish the following exercises before the day of the experiment e Consider the formulas and calculate the min max size range from the possible voltages Hint It is simplest to do this graphically Plot the voltage as a function of the particle diameter Get the size range from this linear plot e Divide the size range into size bins e g 10 per decade e Calculate the appropriate voltage for each size Diff
4. ass from the DMA entrance to the CPC This delay time needs to be determined so it can be added to all further measurements To measure the delay time a filter can be connected upstream of the DMA entrance When the concentration at the CPC has dropped to zero remove the filter The time it takes now until incoming particles have reached the CPC and its readout shows a concentration settled at a stable level is called the time lag Page 8 Differential Mobility Particle Sizer 4 Experiments 4 1 Exercises After the flows have been adjusted and the instrument has been brought to a working state the size distributions of aerosol particles can be measured In this experiment the size distribution of the aerosols which a standard tea light candle produces shall be compared to the size distribution of ambient urban aerosols Perform all experiments twice to have two size distributions for comparison 4 2 Candle The candle stands inside a glass volume so that the aerosols it produces are not diluted The ventilator is used to distribute the aerosols equally in the volume Since the candle needs a lot of oxygen add a flow of 10 l min of filtered compressed air to the volume Mount the tubes into the volume and ensure that the sampling tube is fixed in the upper part Wait until the candle burns stable before you start sampling Monitor the CPC concentration at one of your smaller size bins to ensure a stable concentration has been reached Measure
5. climate effects especially with respect to the radiation budget of the atmosphere In addition to light scattering in the atmosphere as in the case of fog or smog they can directly scatter solar radiation back to space and therefore cause radiative cooling direct aerosol effect Furthermore aerosols can act as cloud condensation nuclei leading to cloud formation The radiative properties of the clouds depend on the number and type of aerosol indirect aerosol effect The size range of atmospheric aerosols is from about 10 nm to 100 um Thereby particles in the order of 0 1 lum are a matter of particular interest since they have the highest scattering intensities longest atmospheric lifetimes and are the majority of cloud condensation nuclei The size distribution of polydisperse aerosols can be determined with a DMPS System Differential Mobility Particle Sizer A DMPS consists of a DMA Differential Mobility Analyzer transmitting only particles with a certain size and a CPC Condensation Particle Counter counting these particles In this lab experiment the size distributions of different types of aerosol particles will be determined with a DMPS system The setup is shown in Figure 1 2 Instruments 2 1 DMA and electrical mobility A DMA separates charged particles according to their electrical mobility Electrically charged particles move in the electrical field according to their electrical mobility The DMA is a aerosol ae
6. er DMPS A DMPS consists of a DMA and a CPC Particles are first size selected with the DMA and then counted with the CPC 2 4 Flowmeter A flow meter is an instrument for measuring and calibrating volume flows of gases The Gilibrator instrument Figure 4 we use in this experiment works with a soap bubble rising with the flow It measures air flows in both directions i e in a sucking or blowing mode It is important that incoming air enters the lower hose connection and exits the upper connection At the base of the flow cell is a bubble generator By pushing the button down a ring lowers HE Figure 4 Gilibrator flowmeter Page 7 Atmospheric Physics Lab Work itself into the soap solution creating a film Releasing the button lifts the film to the flow tube and it rises with the air stream Sensors measure the velocity of the bubble and the volume flow rate is calculated Note that the Gilibrator measures volume flows To compare these flows e g to a Mass Flow Controller volume flows have to be temperature and pressure compensated using the ideal gas law 3 Practical advice 3 1 DMA flow checks Disconnect the CPC before calibrating the DMA flows When changing tubing connected to the CPC always switch off the CPC pump before The sheath and sample flows of the DMA used in the experiment need to be controlled prior to the experiment as the selected particle diameter directly depends on the air flow First of all the
7. erential Mobility Particle Sizer Table of Contents 1 sU Ie I ERREUR 4 Ei VERS Lu oec ici rcx M EPIAR RS rISAU QU dra A MEER US EDI EM E DEN NI VADE 4 21 DMA and electrical mobality c seseermeressersairisuirsscesorseso P 4 SABE DRA de A ESA ERE 6 2 2 Condensation Particle Counter CPC ccssssssssssssssssssossssscssscsssscssscsccsccscscsssscccccccesesesssscsscsssssssscscccecsces 6 2 3 Differential Mobility Particle Sizer C DMPS ccccccccccccscsccssssscsssssssscsssssssssssssccscccesesesesssssscescscceseoesessees 6 EE MEE DUI TIENDE I RIDER 7 3 CIE O ssa sis 8 KEEN ENDE liri EE EDIT 8 RENE Ir Ri ceret 8 4 S dnd mENCEECNTNER mn 9 LN MEE C s I REDDE 9 LRUNMEE SCELTE IEEEL 9 ASS ENADE s sicsscssessssvcsasecoesecesessssesacess easass EE E E E E A 9 AA A a E E E E E E E E E E PR 9 S Pe Me aea UMEN MEME aa aE DNE ME 10 Page 3 Atmospheric Physics Lab Work 1 Introduction Aerosols are liquid or solid particles suspended in a gas Aerosols in the atmosphere can have both natural and anthropogenic sources Besides air pollution and health effects atmospheric aerosols are an important factor in
8. rosol modu ae flow in radioactive source source 1 l min controller 3 way valves sheath air out 5 min aerosol flow out 1 l min membrane pump particle filter blower Figure 1 Experimental setup Page 4 Differential Mobility Particle Sizer Sheath Air In xm Polydisperse Aerosol In Flow Straightener _ Outer Electrode High voltage Rod Inner Electrode Excess lt lt Monodisperse Air Out Aerosol Out Figure 2 Differential Mobility Analyzer Figure from TSI cylindrical capacitor consisting of an inner electrode HV Rod and an outer electrode Figure 2 The incoming sample flow containing the polydisperse aerosol Polydisperse Aerosol In is directed together with laminar particle free sheath air parallel to the HV Rod The horizontal particle velocity is yes E 1 where v is the particle velocity and Z the electrical mobility and E is the field strength The electrical mobility of a particle is defined as the ratio of the constant limiting velocity a charged particle will reach in a uniform electric field to the magnitude of this field Willeke and Baron The electrical mobility depends mainly on the particle size and electrical charge The smaller the particle and or the higher the electrical charge the higher is the electrical mobility The electrical mobility is in general given in dependence of the particle diameter dp by C m Q 31nd where C is the C
9. the concentration step by step for all of your size bins to obtain the full size distribution Note that your measurements should start only after the delay time 4 3 Urban air Do exactly as before for ambient air Use appropriate tubing conductive i e black silicone or metal tubing to sample from outside the window 4 4 Report Your report should have the following structure e Summary of the theory calculation and table of voltage vs diameter Complete measurement report including graphics Derivations and interpretations Results and discussion Calculation of errors Page 9 Atmospheric Physics Lab Work 5 References e Instruction Manual Model 3010 Condensation Particle Counter 2002 TSI Incorporated St Paul MN USA e Operation and Service Manual Series 3080 Electrostatic Classifiers 2006 TSI Incorporated St Paul MN USA e E O Knutson and K T Whitby 1975 Aerosol classification by electric mobility Apparatus theory and applications J Aerosol Sci 6 443 451 e K Willeke and P A Baron Aerosol Measurement Van Nostrand 1993 Page 10
10. unningham slip correction which is a correction to the friction for particles between the continuum and free molecular regime 0 999 d 2N 1 1424 0 sstexn need 3 Curs d p 2N The electric mobility in a DMA for a certain deposition location can be described by the following equation Page 5 Atmospheric Physics Lab Work IH Im P An AV where q is the flow of sheath air at the DMA entry and q the sheath air flow at the DMA exit Knutson and Whitby V is the voltage between inner and outer rod and A is an instrument constant given by 4 E In 5 where L is the DMA length r the radius of the inner electrode from the DMA capacitor and Fouer the radius of the outer electrode from the DMA capacitor i e the inner radius of the DMA cover Depending on their polarity the particles are accelerated either to the outer or the inner electrode The particles whose deposition place at a certain voltage i e electrical mobility matches the position of the gap at the outlet of the DMA Figure 2 pass the DMA i e are size classified The transmitted particles satisfy the following mobility voltage relationship which can be derived from equations 2 and 4 ML M 6 2ne C d By ramping the DMA voltage the selected particle diameter can be changed The DMA used in this experiment has the following dimensions L 44 37 cm Fimer 9 37 mm and Foner 19 61 mm Further it can be assumed that for air at
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