Thermodynamic influences on size fractionated measurements (PM 2.5, PM 10) of ambient aerosols Astrid C. John,* Thomas A. J. Kuhlbusch and Heinz Fissan Process- and Aerosol Measurement Technology Division, Gerhard Mercator University Duisburg, Bismarckstr. 81, 47057 Duisburg, Germany. E-mail: johnas@uni-duisburg.de; Fax:+49 203 379 3268; Tel.: +49 203 379 3907 Received 23rd April 1999, Accepted 14th June 1999 Mass concentrations of PM 10 and PM 2.5 are planned as new standards for the monitoring of ambient air quality in the European Union. Standard procedure is the removal of particles >10 mm and>2.5 mm aerodynamic diameter, respectively, by impaction in a preseparator.DiVerent samplers work according to diVerent principles of flow control. The influence of ambient temperature, pressure and relative humidity on diVerent devices is calculated to estimate the comparability of various aerosol samplers.Therefore, the eVects of these ambient factors on the volume flow as well as on the cut-oV dp50 are investigated. In a second step, the influence of relative humidity on the flow control device is calculated. The results show that the cut-oV shifts (up to 6.4%) for varying ambient conditions.Therefore, the influence on the impaction process should not be neglected and an ‘ideal sampler’ would measure temperature, pressure and relative humidity and adapt the volume flow to avoid a systematic error in the cut-oV. Introduction Fundamentals of the impaction process Particles >10 mm and >2.5 mm aerodynamic diameter, Until recently, the mass concentration of Total Suspended respectively, are removed from the aerosol by impaction in a Particles (TSP) was the only standard of particulates used for preseparator.The aerosol is accelerated in a jet and particles air quality assessments. According to the ‘Proposal for a of suYcient inertia are deposited on the impaction plate while Council directive relating to limit values for sulphur dioxide, the gas and smaller particles leave the impactor. oxides of nitrogen, particulate matter and lead in ambient The aerodynamic diameter where 50% of the particles impact air’,1 the mass concentrations of particles with aerodynamic is called cut-oV (dp50) and is calculated according to:4 diameters <10 mm (PM 10) and <2.5 mm (PM 2.5) are planned as new standards for ambient air quality.To determine these particle size fractions, the aerosol is sucked through an dp50=S9p Stk50g dj3Ni 4CrpV= (1) inlet and particles >10 mm and >2.5 mm, respectively, are removed. Standard procedure is the separation by an inertial where, particle density, rP=1000 kg m-3; Stokes number, impactor.2,3 Particles smaller than the so called cut-oV diameter Stk50=0.24 (for circular jets); dj, jet diameter; Ni, number of are then collected on a filter.Knowing the sampled particle jets; g, viscosity; C, slip correction factor. mass and volume, the mass concentrations of PM 10 and Rearranging eqn. (1) gives PM 2.5 are calculated. If the cut-oV shifts due to changes of ambient temperature, pressure or relative humidity, too many dp50=S9p 4rP Ódj3 Ni Stk50 S g CVÿ (1a) or not enough particles are collected in the preseparator. Errors result for the determination of particle mass of the size where the first square root combines the invariable parameters, fractions PM 10 or PM 2.5 on the filter.the second one consists of impactor design specific data and Throughout Europe, diVerent samplers with diVerent flow the third square root contains variables that depend on ambient control devices are used which may cause some diYculties in temperature, pressure and relative humidity.the harmonisation of data. For this reason, the eVects of The viscosity g depends on temperature [(eqn. (2)] and ambient temperature (T), pressure ( p) and relative humidity because of the compounds (gi) dry air and water vapour also (rH ) on diVerent systems were theoretically investigated.First, on relative humidity [eqn. (3)]:4 a constant mass flow was assumed and the influences of changing environmental conditions on the volume flow and cut-oV diameters of a PM 10 and a PM 2.5 sampler, respect- gi(T )=gi,0ST T0 1+Si/T0 1+Si/T (2) ively, were calculated.Next, the volume flow was kept constant and the variations of the cut-oV were determined for diVerent temperatures, pressures and humidity. Finally, the influence g(T)= . i=1 n rigi(T)ÓMiTcrit.,i . i=1 n riÓMiTcrit.,i (3) of relative humidity on the volume flow control in diVerent devices was investigated. Combining the results of these investigations, the ‘ideal sampler’ with regard to the eVects of ambient temperature, where ri, partial volume of compound i; Tcrit., critical pressure and relative humidity for monitoring PM 10 or temperature; Si, Sutherland constant; Mi, constant.The slip correction factor C is a function of temperature, PM 2.5 is proposed. J. Environ. Monit., 1999, 1, 409–412 409pressure and humidity:4 curve (dp84/dp16)0.5 as defined by Peters and Vanderpool5 show changes of less than 0.1%.Hence, only the dp50 is discussed in detail. It should be noted that the following C=1+ 2·l dp50 C1.23+0.41 expA-0.88 dp50 2·l BD (4) discussion is principally valid for rectangular jets (Stk50=0.59) as well. l=2 g pSp RT 8 M (5) Influences on the cut-oV for constant mass flow. Table 1 lists the errors of the cut-oVs 10 mm and 2.5 mm for changes in where l, mean free path;M, molecular weight; R; gas constant. temperature, pressure and relative humidity for constant mass The influence of the ambient conditions on the volume flow flow.The maximum errors are -3.0% and 3.7% for PM 2.5 depends on the flow control device. Assuming constant mass and -2.7% and 3.2% for PM 10. The diVerences for the two flow, the volume flows for diVerent environmental conditions size fractions result from the slip correction factor which can be calculated according to the ideal gas law [eqn. (6)] in depends on the ambient conditions, but also on particle size.relation to a certain volume flow at reference conditions (Vref, These errors in the cut-oVs are much smaller than the changes pref, Tref ).in volume flows due to the adverse influence of the thermodynamic conditions on volume flow and the impaction process. Vÿ = prefVÿ ref Tref T p (6) Influences on the cut-oV for constant volume flow. The above given calculations are based on the assumption that the volume Thermodynamic influences flow varies with changing temperature, pressure and humidity. If these parameters are measured, the volume flow can be kept Generally, reference conditions were set to 15 °C, 1013 mbar constant for varying environmental conditions which is realised (1 bar=105 Pa) and 0% relative humidity.As extreme values for several filtration samplers that collect PM 10 or PM 2.5. can occur during operation of the instruments, a temperature Table 2 shows the errors of the cut-oVs 10 mm and 2.5 mm for range from -30 to 40 °C, pressures from 980 mbar to changes in temperature, pressure and humidity for constant 1040 mbar and dry and saturated air were considered.volume flow for the assumed extreme values of ambient The influences of temperature, pressure and relative conditions. humidity on (a) volume flow and (b) cut-oV were calculated The maximum errors are -6.1 and 3.0% for PM 2.5 and using eqns.(1)–(6) and are illustrated in Fig. 1. -6.4 and 3.2% for PM 10. Influences on the volume flow Comparison of the influences of constant mass flow and constant volume flow. Comparing the results in Tables 1 and Table 1 lists the resulting volume flows and their variations 2, the following conclusions can be drawn: At reference for diVerent p, T and rH in reference to the flow of 1 m3 h-1 temperature and humidity, varying pressure and constant at reference conditions. For the assumed range of temperature, volume flow, no deviation for the cut-oV was found for pressure and humidity, the maximum deviations are -17.8% PM 10 and only -0.1% and 0.1% for PM 2.5.This is due to and 12.3%, respectively. the pressure dependence of the slip correction factor which is Knowing the exact volume that was sucked through a more important for small particles. For constant mass flow at sampler is necessary for the calculation of the concentration these conditions, the deviations of the cut-oV are in the order of the ambient aerosol.of -1.7% and 1.4%, which is a result of changes in volume flow for these conditions.Influences on the cut-oV Looking at varying temperature and diVerent pressures, the Calculations derived from eqn. (1) of the influence of errors in the cut-oV are generally more pronounced for conthermodynamic conditions on the steepness of the cut-oV stant volume flow (e.g., -6.0% and -6.4% for PM 2.5 and PM 10 at -30 °C, 1013 mbar and dry air) than for constant mass flow (+2.3% and +1.9% for PM 2.5 and PM 10 at these conditions).This surprising result is explained by the temperature dependence of the viscosity. For constant mass flow, deviations in the volume flow due to changing environmental conditions are partly compensated by changes of viscosity. For constant volume flow, changes of viscosity result in more distinct errors of the cut-oVs. Small shifts in the cut-oV occur for saturated air compared to dry air at the same conditions because of changes in viscosity which is not only dependent on temperature, but also on the composition, that means the water content of the air Fig. 1 Influencing factors for volume flow and impaction process. which is expressed as relative humidity. Table 1 Errors of volume flows and cut-oVs 10 mm and 2.5 mm for changes in temperature, pressure and relative humidity assuming constant mass flow and Vÿ ref=1 m3 h-1 T/°C 15 15 15 -30 -30 -30 40 40 40 0 0 15 30 p/mbar 1013 980 1040 1013 980 1040 1013 980 1040 1013 1013 1013 1013 rH (%) 0 0 0 0 0 0 0 0 0 0 100 100 100 Vÿ /m3 h-1 1.00 1.03 0.97 0.84 0.87 0.82 1.09 1.12 1.06 0.95 0.95 1.00 1.05 DVÿ (%) 0.0 3.4 -2.6 -15.6 -12.8 -17.8 8.7 12.3 5.9 -5.2 -5.2 0.0 5.2 dp50;2.5 mm/mm 2.50 2.46 2.54 2.56 2.51 2.59 2.47 2.43 2.50 2.52 2.52 2.50 2.48 Ddp50;2.5 mm (%) 0.0 -1.7 1.4 2.3 0.6 3.7 -1.3 -3.0 0.1 0.8 0.8 0.1 -0.8 dp50;10 mm/mm 10.00 9.83 10.13 10.19 10.02 10.32 9.90 9.73 10.03 10.06 10.07 10.01 9.94 Ddp50;10 mm (%) 0.0 -1.7 1.3 1.9 0.2 3.2 -1.0 -2.7 0.3 0.6 0.7 0.1 -0.6 410 J.Environ. Monit., 1999, 1, 409–412Table 2 Errors of the cut-oVs 10 mm and 2.5 mm for changes in temperature, pressure and relative humidity assuming constant volume flow T/°C 15 15 15 -30 -30 -30 40 40 40 0 0 15 30 p/mbar 1013 980 1040 1013 980 1040 1013 980 1040 1013 1013 1013 1013 rH(%) 0 0 0 0 0 0 0 0 0 0 100 100 100 dp50;2.5 mm/mm 2.50 2.50 2.50 2.35 2.35 2.35 2.57 2.57 2.58 2.45 2.45 2.50 2.54 Ddp50;2.5 mm (%) 0.0 -0.1 0.1 -6.0 -6.1 -6.0 2.9 2.8 3.0 -1.9 -1.8 0.1 1.8 dp50;10 mm/mm 10.00 10.00 10.00 9.36 9.36 9.36 10.32 10.32 10.32 9.80 9.80 10.01 10.19 Ddp50;10 mm (%) 0.0 0.0 0.0 -6.4 -6.4 -6.4 3.2 3.2 3.2 -2.0 -2.0 0.1 1.9 Table 3 Errors of volume flow and cut-oV when neglecting the From these calculations, it can be deduced that neither the influence of relative humidity for the rotameter operation at constant mass flow nor at constant volume flow is appropriate for sampling PM 10 or PM 2.5 without system- T/°C 0 15 30 atic errors for varying ambient temperatures, pressures and p/mbar 1013 1013 1013 relative humidity as they occur e.g.for day-to-night-changes rH (%) 100 100 100 DVÿ (%) 0.12 0.30 0.71 or if the samplers are operated in the south as well as in the Ddp50;10 mm (%) -0.06 -0.15 -0.35 north of Europe.As the volume flow is the only parameter in the impaction process which can be influenced in a given system [see eqn. (1a)], the environmental parameters have to be measured and for these conditions, the volume flow through and cut-oV if the calibration of the rotameter has been carried the impactor Vÿ I which is needed to keep the cut-oV constant out with dry air.has to be calculated [eqn. (7)] and adjusted by the device. Orifice. In the orifice, a pressure diVerence ( p1-p2) is measured and for a given density of the air (rD), the mass Vÿ I= dj3·9·p·Stk50·g(T)·Ni dp 2 50·4·C·rp (7) flow at the device Mÿ D can be calculated according to:10 Mÿ D=kÓ( p1-p2)rD (12) Influences of relative humidity on the flow rate where k=constant. If the adaptation of the volume flow is realised and the As the mass flow through the impactor is also the mass flow filtration sampler for PM 10 or PM 2.5 runs as an ‘ideal through the orifice: sampler’, there is one more aspect to point out which is often neglected in commercial flow control devices.The water con- Mÿ I=Mÿ D (13) tent of air, expressed as relative humidity, influences the and density of air (rsat).The latter is needed for calculations in flow control devices.6 Vÿ I= Mÿ I rI (14) rsat,T=raq,0 paq,T p0 +r0 A1- paq,T p0 Bp p0 T0 T (8) the volume flow at the impactor Vÿ I can be calculated. where paq,T=partial pressure of water vapour at T. Vÿ I=k Ó( p1-p2)rD rI (15) Normally, only temperature and pressure are measured to determine the density of air. For diVerent, frequently used This means for the pressure diVerence ( p1-p2) that has to flow control devices, the eVect of neglecting relative humidity be adjusted in the orifice: on volume flow and cut-oVs were calculated.7,8 Rotameter. The rotameter is calibrated for reference ( p1-p2)= Vÿ I2 rI2 k2 rD (16) conditions (Vÿ Sc for rref).The volume flow at the device Vÿ D for diVerent environmental conditions (rD) can be calculated Neglecting relative humidity when calculating the density of according to:9 the air leads to errors (see Table 4) of volume flow and cut-oVs. Mass flow controller. In a mass flow controller, relative Vÿ D=Vÿ ScSrref rD (9) humidity influences the heat capacity of the gas which is needed for the calculation of the mass flow.If the mass flow If the rotameter is operated behind the filter ( pD), the pressure controller is calibrated for dry air, the reading of the instrument drop at the filter has to be taken into account and the volume Mÿ Sc has to be multiplied by a correction factor F which flow through the impactor Vÿ I with the pressure pI is calculated accounts for the diVerent heat capacities of dry air and water using the ideal gas law: vapour.11 F=1/[1.00·(percentage of dry air) Vÿ I=Vÿ D pD pI (10) +1.15·(percentage of water vapour)] (17) To obtain the correct volume flow Vÿ I through the impactor, the volume flow Vÿ Sc, to which the scale (calibrated for reference Table 4 Errors of volume flow and cut-oV when neglecting the conditions) of the device has to be set, is calculated by influence of relative humidity for the orifice combining eqn.(9) and eqn. (10): T/°C 0 15 30 p/mbar 1013 1013 1013 Vÿ Sc=Vÿ I pI pD SrD rref (11) rH (%) 100 100 100 DVÿ (%) 0.12 0.30 0.71 Neglecting relative humidity when calculating the density of Ddp50;10 mm (%) -0.06 -0.15 -0.35 air leads to the following errors (see Table 3) of volume flow J.Environ. Monit., 1999, 1, 409–412 411Table 5 Errors of volume flow and cut-oV when neglecting the ability of results (e.g. summer5winter, or even more extreme influence of relative humidity for the mass flow controller winter in northern Europe5summer in southern Europe). To avoid these systematic errors in the sampling of ambient T/°C 0 15 30 particles, temperature, pressure and humidity have to be p/mbar 1013 1013 1013 measured and the volume flow has to be adjusted to keep the rH (%) 100 100 100 DVÿ (%) 0.14 0.37 0.86 cut-oV constant as illustrated in Fig. 2. Equations accounting Ddp50;10 mm (%) -0.07 -0.18 -0.43 for the dependence of three widely used flow control devices on changing thermodynamic conditions, especially relative humidity, are also given.Combining the equations for the volume flow control for constant cut-oVs and the correspond- For Mÿ I=Vÿ IrI [eqn. (14)] and Mÿ I=Mÿ D [eqn. (13)] and ing equation for the flow control device, an ‘ideal’ sampling Mÿ D=Mÿ Sc·F (18) flow control to derive comparable results under varying environmental conditions all over Europe has been designed.the mass flow to which the instrument has to be set is Mÿ Sc= Mÿ D F = Vÿ I rI F (19) Acknowledgement This work was realised during a project which was sponsored Neglecting relative humidity leads to the errors mentioned by the Ministerium fu� r Umwelt, Raumordnung und in Table 5 of volume flow and cut-oVs. Landwirtschaft des Landes Nordrhein-Westfalen. The errors are more pronounced for mass flow controllers compared to rotameters and orifices because humidity does not only influence air density but also the heat capacity of air.References 1 Proposal for a Council directive relating to limit values for sulphur Conclusions dioxide, oxides of nitrogen, particulate matter and lead in ambient air, European Union, 1998. It was shown that changing environmental conditions 2 DIN EN 12341 Air quality, Determination of the PM 10 fraction (temperature, pressure, and relative humidity) influence the of suspended particulate matter, Reference method and field test procedure to demonstrate reference equivalence of measurement volume flow (errors of -17.8 to +12.3% for the investigated methods; German version EN12341: 1998; Berlin, 03–1999.conditions), and also directly the impaction process (see 3 US EPA 1987, Ambient monitoring reference and equivalent Fig. 1). Neither constant mass nor volume flow lead to methods, Federal Register 40 CFR Part 53, 1 July 1987. PM 2.5 or PM 10 sampling with constant cut-oVs. Errors were 4 VDI 2066 Part 5, Fraktionierende Staubmessung nach dem generally more pronounced for constant volume flow (-6.4 Impaktionsverfahren, Kaskadenimpaktor, November 1994, to +3.2% for PM 10) than for constant mass flow (-2.7 to Berlin. 5 T.M.Peters and R. W. Vanderpool, Modification and Evaluation +3.2% for PM 10). Thus, the shifts in cut-oVs exceed the of the WINS Impactor, R.T.I. report No. 6360–011, Sept. 1996. requirements of the EPA for PM 10 (10 mm±0.5 mm)12 for 6 VDI 2066 Part 1, Staubmessungen in stro�menden Gasen; extreme environmental conditions.We generally conclude that Gravimetrische Bestimmung der Staubbeladung, U� bersicht, neither constant mass nor volume flow should be used to October 1975, Berlin. sample PM 2.5 and PM 10. Even if these errors are looked at 7 P. Profos and T. Pfeifer, Handbuch der industriellen Meßtechnik, as relatively small it has to be noticed that (a) they can be Oldenbourg Verlag, 6. Auflage, 1994. 8 LICOR Dew Point Meter, LI 610 Portable Dew Point Generator, avoided and (b) they are systematic and reduce the compar- Handbook, LICOR, 1991. 9 K. Mennicken, Durchflußmessung aus der Kraft auf angestro�mte Ko�rper (Schwebeko�rper-Durchflußmesser), in Hengstenberg, Messen und Regeln in der chemischen Technik, Berlin, 1964, pp. 366–373. 10 VDI 2463 Part 1, Messen der Massenkonzentration von Partikeln in der Außenluft, Grundlagen, Entwurf, July 1997, Berlin. 11 Installation And Operating Instructions: Brooks Mass flow Controller, Issue 7, Brooks Instrument Division, Hatfield, PA, 08-1989. 12 US EPA 1987, Revisions to the National Ambient Air Quality Standard for Particulate Matter, Federal Register 52 (July 1) 24634–24750. Fig. 2 Ideal flow control for a constant cut-oV. Paper 9/03264I 412 J. Environ. Monit., 1999, 1, 4