Aerosol mass concentration

Receptor models are powerful tools for source apportionment of particulates because a vast amount of particulate species characterization data have been collected at many sampling sites worldwide, and because many aerosol species are primary pollutants. Most of the information available is for elemental concentrations, eg, lead, nickel, and alurninum, although more recent measurements have provided data on concentrations of ionic species and carbonaceous compounds. At a sampling (or receptor) site, the aerosol mass concentration of each species i is... 

Charlson, R. J. Atmospheric vuibility related to aerosol mass concentration. A review. Environ. Sci. Technol. 3 913-918, 1%9. 

As a first cut, univariate regression was used to see relationships between pairs of measured variables. The results are summarized in Table 6. It is seen that the particle scattering coefficient is highly correlated with the total fine aerosol mass concentrations, sulfate, and ammonium. To a lesser degree it is correlated with the particle absorption coefficient, potassium, and the unaccounted mass concentration. My. The correlations between the particle scattering coefficient and the nitrate, total carbon (Ct) and crustal species are poor. The poor correlation, r=0.6J, between bgp and the soot concentration is probably an indication of the error in the soot measurement. Although the sulfate and the unaccounted mass are highly correlated with the total mass, they are not well correlated with each other (r=0.49). 

Figure 7. (a) Frequency distribution of St. Louis fine aerosol mass concentration ... 

Most of the studies on size-resolved aerosol mass concentrations in areas with different levels of pollution show that particulate matter typically exhibit a bimodal distribution, with most of their mass being found in the submicron size range (dae < 1pm) and an additional minor mode in the coarse fraction (1 < dae < 10 pm) (Maenhaut et al., 2002 Smolfk et al., 2003 Wang et al., 2003 Gajananda et al., 2005 Samara and Voutsa, 2005). However, with instrumentation allowing more precise measurements, the aerosol mass size distribution was found to be multimodal with the preponderance of a fine mode (dae < 0.2 pm) and an accumulation mode (dae 0.5pm), with a minor coarse mode at d 3-4pm (Raes et al., 2000 Pillai and Moorthy, 2001 Berner et al., 2004). Traditionally, atmospheric researchers classify airborne particles into three size classes coarse (2.5 < c/ ie < 10pm), fine... 

Fig. 3.1 Time series of PMjq aerosol mass concentrations in jig/m for Alta Floresta from 1992 to 1998. Aerosol mass concentration are for particles less than 10pm.

Aerosol mass concentrations are usually determined by filtering a known volume of gas and weighing the collected particulate matter. The mass concentration, p, averaged over the measuring time is found by dividing tJie measured mass by the volume of gas filtered. Atmospheric aerosol mass concentrations range from about 20 for unpolluted air to 200 pg/m for polluted air (1 /ig/m = 10 g/m- ). Federal standards for atmo.spheric air and, in some cases, for industrial emissions are expressed in terms of mass concentrations. There is no simple relationship between the mass and number concentrations unless all particles are the same size. 

Although coagulation (Chapter 7) modifies the size distribution of an aerosol, it causes no change in the mass concentration. The other important internal process within a gas that shape.s the size distribution, gas-to-particle conversion, results in an increase in the aerosol mass concentration. In studying this process we are interested in the mechanisms by which ga.ses are converted to particles, the rates at which conversion takes place, and the distributions of the condensed matter with respect to particle size. 

Ki and K2), closer inspection of the aerosol mass concentration produced for experiments with similarly reacted hydrocarbons showed significantly different total aerosol concentration. The discrepancy between quality of fit for Y vs. Mo and the realization that aerosol mass concentration produced varied significantly for similar hydrocarbon reacted was simply attributed to the fact that the y-axis was not truly independent of the x-axis. (Y is inversely proportional to Mo Mo is the x-axis variable). 

Of critical interest is the large deviation in aerosol produced between the low NOx (conditions more typical of the ambient) and high NOx (conditions often employed in atmospheric reactors) conditions, particularly for aerosol mass concentrations and reacted hydrocarbon concentrations typical of the polluted urban atmosphere (lower left portion of the curve). 

Two commercially available aerosol mass concentration analyzers have been described for the determination of aerosols and particulates [18], The instruments are both portable and relatively rugged and operate on ordinary commercial power. A portable system utilizing an electrostatic precipitator was described for mass concentration measurements in the range of airborne dust particles smaller than 10 /Am [19]. Chuan [20] also described a typical commercial instrument based on the piezoelectric quartz crystal microbalance for the rapid assessment of particular mass concentrations in the atmosphere. The adhesive coating used in the device is non-hygroscopic and non-reactive to the usual concentrations of pollutant gases in the atmosphere, such as CO, SO2, NOjt, and hydrocarbons. 

Table VI summarizes aerosol mass concentrations and composition in different regions of the troposphere. It is interesting to note that average total fine particle mass (that associated with particles of diameter less than about 2 /im) in non-urban continental, i.e., regional, aerosols is only a factor of two lower than urban values. This reflects the relatively long residence time of particles (recall the estimate of a lifetime of fine particles by dry deposition of 10 days). Correspondingly, the average composition of non-urban continental and urban aerosols is roughly the same. The average mass concentration of remote aerosols is a factor of three lower than that of non-urban continental aerosols. The elemental carbon component, a direct indicator of anthropogenic combustion sources, drops to 0.3% in the remote aerosols, but sulfate is still a major compo-...

Terms often used to describe the aerosol mass concentration include total suspended particulate matter (TSP) and PMX (particulate matter with diameter smaller than x pm). TSP refers to the mass concentration of atmospheric particles smaller than 40-50 pm, while PM2.5 and PM10 are routinely monitored. Typical PM2.5 and PM10 concentrations in large cities are shown in Figure 8.14. 

The vertical distribution of aerosol mass concentration typically shows an exponential decrease with altitude up to a height Hp and a rather constant profile above that altitude (Gras 1991). The aerosol mass concentration as a function of height can then be expressed as... 

FIGURE 8.29 Representative vertical distribution of aerosol mass concentration (Jaenicke 1993). 

We can express this timescale in terms of the aerosol mass concentration mp given by... 

The preceding analysis suggests that for polluted airmasses, high temperatures, and small aerosol sizes, the equilibrium timescale is on the order of a few seconds. On the other hand, under conditions of low aerosol mass concentrations, low temperatures, and large particle sizes, the timescale can be on the order of several hours and the aerosol phase may not be in equilibrium with the gas phase. For these conditions, the submicrometer particles are in equilibrium with the gas phase while the coarse particles may not be in equilibrium (Capaldo et al. 2000). 

It is useful to define the aerosol optical depth as the product of coefficients per unit mass of aerosol and the aerosol mass concentration. For sulfate aerosol, for example, we can express the optical depth as (recall that the absorption component for sulfate aerosols is zero)... 

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