Assessment of Particulate Air Pollution by New Sensor. Concepts

VH BERICHTE NR. 1443, 1999 517 Assessment of Particulate Air Pollution by New Sensor. Concepts A. Schmidt-Ott, Th. Kauffeldt, Duisburg (D) ABSTRACT Assessment of particulate air pollution components is only feasible with integrating monitors, such as instruments that determine the total particulate mass (PM), which weight the particle concentration with the particle diameter cubed, Dp^ Currently available monitors are briefly reviewed. In order to obtain a picture about the particulate loading of the air, which contains more information than PM, weightings with other powers of Dp are desirable. This is particularly so, since the importance of the ultrafine particle fi-action (Dp<0.1^m) has been recognized. For routine assessment of particulate pollution in dense networks, inexpensive rehable sensor concepts are required both for total particulate mass and other quantities. Such concepts are shown for a Dp^ (mass) monitor and a Dp^"^ monitor for the ultrafine range. In addition, a simple technique is introduced, distinguishing between the ultrafine solid and toxicologically probably relevant fi-action and the more volatile one, which is mainly of photochemical origin. THE RELEVANCE OF INTEGRATED MEASURES Pollutant components molecularly dispersed in ecosystems (such as SO2 in the atmosphere) are classified according to their chemical composition, and their mass (or mol) concentration is routinely determined. Chemical classification of particulate matter in the atmosphere is presently not feasible on a routine basis. Particles are conventionally assessed via their mass concentration. There is some discussion about whether the mass standard is reasonable or not. Generally speaking, it can be considered as appropriate, if the particulate material is eventually molecularly dispersed, i.e. dissolved. For example, this can be assumed for acid particles in the atmosphere, which are finally precipitated to the ground or waters, where they dissolve. Combustion of organic material produces insoluble solid particles of low volatility. The

518 BERICHTE fraction of such anthropogenic particles is often dominant indoors and outdoors. Inhalation of Diesel exhaust or cigarette particles has been found to be hazardous in various studies. Presently, it is being discussed, whether health risks are rather associated with the chemical nature of adsorbates these particles carry, like polyaromatic hydrocarbons (PAH), or with solid components in the ultrafine size range. Ultrafine solid insoluble particles may be toxic solely because of their small size, and toxic effects have been found to be associated with number rather than mass (Peters and Wichmann, 1997). The particles are deposited in the lung, where they can penetrate the cell membranes. It is interesting to note that ultrafine solid particles are almost exclusively anthropogenic, and must have been essentially absent before man was capable of making fire. Thus natural biological resistence mechanisms with respect to these particles may be weak, since evolution has had little time to develop them. Mass is certainly not a quantity related closely to the toxic effects of insoluble particles, considering that the bulk ofthe particles is "inside" and not accessible. Another fact supporting the statement that the mass concentration is insufficient to estimate health effects is that the efficiency of lung deposition (fig. 1) has a peak in the ultrafine size range between 0.01 and 0.1 ^m (apart from the one between 0.5 and 10 ^m, not shown). The dashed curve in fig. 1 represents the size dependent sensitivity of a mass standard (here PM 2.5). While the main questions conceming toxicological effects remain to be answered, it is clear from this graph that if particles in the ultrafine range have any effect at all on health, a measurement quantity in \ig/m^ will not reflect it. Fig. I Estimated deposition efficiency in the lung and sensitivity of a particle mass monitor (dashed ^ 0,1 1,0 Parficle DiameferlM^l curve) as function of the particle diameter

vn BERICHTE 519 The contribution of ultrafine particles to atmospheric chemistry is uncontested. The impact non-volatile particulate matter mainly consists in chemical or catalytical action of the particle surface. For mass transport of gas molecules to the particle surface, the joint surface or radius are certainly more relevant quantities than total mass, especially if the particles are solid or have an insoluble sohd core. In nucleation processes, the number concentration is the most important measure. The considerations above show that it is difficult to say, which physical or chemical sum parameter(s) give(s) the most relevant representation of the particulate loading of the air. They also show that relying only on the total particulate mass is insufficient. The demand to assess particulate matter via other integral measures in addition to mass is a logical consequence, and corresponding standards are in preparation. Simple inexpensive on-line measuring sensors in large number are required for effective assessment of particulate pollution, in particular to provide a better basis for epidemiological studies investigating health impact. Complete chemical characterization of airborne particles requires sophisticated technology, especially if this is to be done on-line (Ch. A. Noble and K.A. Prather, 1996). On-line methods and even sampling followed by chemical analysis generally have the restriction that they neglect the ultrafine particle fi-action with diameters below 0.1 im. Simple monitors allowing chemical identification of ultrafine particles are presently out of reach. STATE OF THE ART MONITORS Some sensing devices delivering crude information on the nature of the particles or selectively detecting only a certain group have been developed (see Schmidt-Ott, 1991 or Schmidt-Ott, 1998). Examples are the photoacoustic sensor (Petzold et al., 1993) and the Aethalometer (Hansen et al., 1984), detecting carbonaceous particles. Another example is the photoelectric sensor (e.g. Burtscher, Schmidt-Ott, Siegmann, 1984 and Burtscher, Siegmann, 1994), which is sensitive to soot particles covered with polyaromatic hydrocarbons (PAH). The majority of the devices available today for on-line assessment of particulate pollution respond only to physical particle parameters, namely number concentration, joint particle

520 VM BERICHTE surface and/or joint mass (Schmidt-Ott, 1998). Methods of size classification based on impactors or on electrostatic mobility analysis have been used in numerous field studies, but their price is too high and the amount of data they deliver is inadequately large for routine air quality assessment in dense networks. The classical method of monitoring particulate mass concentration is P-absorption. The method is very reliable, but has the disadvantage of requiring a radioactive source. Quartz microbalances have been used with limited success. The adherence of particles to the quartz, oszillating in the MHz regime is a problem, especially if the particles have the typical strongly agglomerated structure of soot particles. Alternative oscillator devices like the TEOM (H. Patashnik, 1975) or a recently reported filter disk resonance method (P. Lilienfeld, R. Steg, 1998) are more promising, since they use lower Irequencies. The Aethalometer (Hansen et al., 1984) and the photoacoustic sensor (Petzold et al., 1993) are based on the absorption of radiation and selectively detect soot. They can also be calibrated in terms of mass, but there is some uncertainty about the quantitative response, if the particles are covered by substances that exhibit smaller absorption than black carbon. The Epiphaniometer (Gaggeler et al., 1989) is a monitoring instrument Requiring little maintenance based on the adsorption of a radioactive isotope to the particle surface. The response is the integrated neutral molecule attachment coefficient of the particles, which represents mass transfer in gas particle interaction for "sticking" molecules. For particles substantially larger than the gas particle mean Iree path (0.066 ^im in air at normal pressure and room temperature), the measured quantity is proportional to the joint particle diameter. For smaller particles the response is proportional to the joint particle surface. The instrument contains a weak radioactive source, and the corresponding legal restrictions apply. Optical particle counters (OPCs) and condensation nucleus counters (CNCs) basically monitor the particle concentration. OPCs additionally give particle size information but are blind in the ultrafine size range. CNCs are presently the only instruments for number concentration measurement in the ultrafine range. Their main disadvantage is their high price, forbidding their use in large monitoring networks.

BERICHTE 521 SIMPLE SENSOR CONCEPTS FOR INTEGRAL MEASURES Present standards (PM 10, PM 2.5) determine the particle mass m* per unit volume of air. This corresponds to a weighting of the particle number concentration by the particle diameter to the third power, Dp^ according to Here N is the particle number concentration, p is the particle mass density and Dpn^ax is the largest particle diameter detected, e.g. 2.5 ^m for PM 2.5. Any particle mass concentration measurement neglects of the small end of the size distribution, and even PMI completely neglects the ultrafine fraction. The considerations above show that future particle monitoring technology must include instruments that weight the particle concentration by other powers of Dp than 3. This should be in addition to the existing particle mass standard, which should be maintained. Non-radioactive inexpensive PM monitors measuring m* are also desirable Finally, a simple technique distinguishing between solid non-volatile (soot or fly ash-like) particles and volatile (liquid) ones that are primarily products of atmospheric photochemical reactions should be helpful in view of the potential health risk (see preceding chapter). Simple sensor concepts corresponding to these three demands are introduced below. a) A SENSOR WEIGHTING CONCENTRATION WITH Dp' ' A simple sensor concept which weights particle number with Dp'-^for Dp<0.1 ^m is shown in fig. 2. Aerosol diffusion aerosol charger ' electrometer pump " Fig. 2 Dp" sensor concept ' ^ 1

522 BERICHTE The particle carrying air is sucked through a charger and an aerosol electrometer by a pump. In the charger the particles are electrically charged by ion attachment (unipolar diffusion charging). Suitable diffusion chargers are described by Büscher and Schmidt-Ott, 1992 and by Adachi, Kousaka and Okuyama, 1985. The charging probability in this process for a particle of diameter Dp depends on parameters of the charger and is proportional to the so-called ion attachment coefficient for weak charging, if less than about 20% of the particles are charged. Fig. 3 shows the coefficient rjo^ for attachment of positive ions to initially neutral particles, which has been calculated by Adachi et al., 1985 for typical conditions and reasonable assumptions conceming ion mass, mobility and mean free path. For the size range shown, it can easily be deducted from the figure that r (/ is proportional to Dp^'^ within a few percent. Consequently same is tme for the mean charge q : q^const^d/'. (2) The aerosol electrometer consists of an absolute filter in a Faraday cup connected to a sensitive current sensor. The measured electric current I represents the flux of the charged particles in the flowing air stream, and is related to the size dependent mean particle charge and the number concentration via I = ojq{d^)dn, (3) where Q is the volume flux of the aerosol. Taking eq. 2 into account, the response of the sensor of fig. 2 follows max I = const j Dp^-^dN (4) 0 for Dpmax below 0.1 ^m. Thus the particle concentration is weighted in a way that large particles cleariy contribute more than small ones. The weighting with Dp' ' represents a compromise between a mass measurement (Dp^) and a number concentration measurement

ra BERICHTE 523 (Dp^). Regarding the simple sensor concept, cheap and reliable realizations are feasible, and application in air quality assessment in connection with mass sensitive sensors should be discussed. Further investigations and discussion of the needs must show, if means of suppressing the contribution of particles larger than 0.1 ^m to the sensor signal are desirable. 10-12- m E - 5H + O cr Fig. 3 Attachment coefficient r o" for positive ions to initially neutral particles as function 10 50 100 Dp[nm] of the particle diameter Dp b) A SIMPLE MASS CONCENTRATION SENSOR The Dp^ sensor concept illustrated in fig. 4 (Schmidt-Ott, 1998) contains similar elements as the Dp'-' sensor of fig. 2.

524 ra BERICHTE Aerosol field gas conductivity charger sensor - pump - Fig. -I Dp' sensor concept The aerosol enters a so-called field charger, which is a device according to Büscher and Schmidt-Ott, 1992. In field charging the particle charge q is related to Dp via q = const2 Dp (5) The gas conductivity sensor (Schmidt-Ott, 1998) measures the conductivity formed by the charged particles. The sensor output current Ic, which represents conductivity, is proportional to the particle diameter (Dp>0.2^m), the charge and the number concentration. Consequently I, = const^- \ q{dp)dpdn 0.2//w (6) and, together with eq. (5), I, = const^ j Dp^dN i).l/jm {Dp>02 (7) Thus the sensor output is proportional to the particulate mass concentration m, as given by eq.(l). The density of the particle material, p, does not scatter much, and for most purposes the assumption p=l g/cm^ leads to a tolerable error. Due to constraints in connection with the field charging characteristics and the size range limitation for eq. (6), mass proportionality is

BERICHTE 525 restricted to the size range above 0.2 ^m. This restriction is not of much importance, however, since for the size distributions encountered in practice smaller particles have no significant contribution to the total particulate mass anyway. c) A MONITORING CONCEPT SELECTIVE FOR NON-VOLATILE PARTICLES As mentioned above, those particles in the ultrafine fraction that are sohd and of low volatility are particularly suspicious with respect to health effects. In particular, it is desirable to distinguish between soot-like particles as emitted e.g. from automobiles and more volatile droplets of sulphuric acid or organic species that may also be among the automobile emissions or that are photochemically produced in the atmosphere. More and less volatile aerosol components are conventionally extracted in a continuous flow arrangement by applying a denuder stage before the particle detection. A denuder is basically a heated tube through which the aerosol passes, where the more volatile fraction is evaporated. The vapor to be extracted is chemically bound at the denuder wall, which carries a selective coating. This avoids particle formation by recondensation of the vapor behind the denuder. Recent studies by Kauffeldt and Schmidt-Ott have shown that retention of the vapor is not necessary, and a simple heated tube without chemically active coating does the job, if the detector is sensitive only to charged particles, as in the Dp"" sensors described above. The non-volatile particles retain their charge and contribute to the sensor signal, while recondensed ones are electrically neutral and do not contribute. While denuders are saturated after a certain time and have to be reactivated, the method combining a heated tube with a charge sensitive detector has the advantage of not requiring this maintenance. The concept clearly has the potential of being applied to ambient monitoring but has until now only been used in vehicle emission measurements (Kauffeldt and Schmidt-Ott, 1998). The particle size distribution was determined using a differential mobility particle sizer (DMPS). The experimental arrangement is described in detail the pubhcation cited above. The DMPS system ignores particles without charge, and thus the heated tube concept was applicable to distinguish the potentially toxic ultrafine soot particles from more volatile (Uquid) droplets. Fig. 5 shows the particle size distribution measured for a truck Diesel engine with a particle filter in the exhaust channel and 1:12 dilution.

526 BERICHTE =0 1,00E+09 1,00E+08 1,00E+07 1,00E+06 1,00E+05 1,00E+04 1,00E4=03 1,00E+02 1,00E+01.-1 200 Fig. 5 Partiele size distribution from truck Diesel engine with partiele filter and 1:12 dilution (Kauffeldt and Schmidt-Ott, 1998). The occurrence of a peak of ultrafine particles around 10 nm in size is typical for such an arrangement. Fig. 6 shows this peak as fimction of the heated tube temperature. The peak vanished completely at 175 C, revealing that these particles are not soot. Similarly it was proven that the larger particles are soot. These results show that a heated tube in combination with a particle detector sensitive to charged particles is selective for soot and other non-volatile components. A tube temperature of 350 C is adequate to suppress the contribution of all particulate non-soot vehicle emissions as well as typical smog particles which are formed by atmospheric photochemistry from pollutant gases. In the Dp' ' sensor of fig. 2, the heated tube is inserted between the charger and the aerosol electrometer, as shown in fig. 7. Such a sensor selectively detects ultrafine sohd particles from cars and combustion-like processes in an urban atmosphere.

BERICHTE 527 3,50E+07 3,00E+07 E 2,50E-f07 ^ 2,00E-s-07 -ambient temperature 'loo^c -150 =175 C ^ 1,50E-f07 a I 1,00E+07 5,00E+06 0,00E+00 i^jg. d Ultrafine particle peak fi-om fig 5 as fimction ofthe heated tube temperature. Aerosol diffusion heated aerosol charger tube electrometer Fig. 7 particles Dp'' sensor for selective detection of the sohd fraction of ultrafine CONCLUSION Three simple particle monitor concepts have been explained that cpuld be used in addition to the technologies presently apphed or partially replace them. Monitors based on principles according to these examples or similar ones would deliver more complete and more relevant information on the particulate loading of the air, in particular conceming the ultrafine fi-action. Inexpensive realizations of the concepts are feasible, which allows particulate pollution control in dense networks. In particular, this would provide an important basis for epidemiological studies revealing the health impact of ultrafine particles.

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