Ambient Aerosol Size Distributions

As a result of particle emission, in situ formation, and the variety of subsequent processes, the atmospheric aerosol distribution is characterized by a number of modes. The volume or

Fine Particles Coarse Particles

Ultrafine Particles

Fine Particles Coarse Particles

Ultrafine Particles

Ultrafine Particles Number Mass

0.01 0.1 1 10 Diameter (micrometers)

FIGURE 8.10 Typical number and volume distributions of atmospheric particles with the different modes.

0.01 0.1 1 10 Diameter (micrometers)

FIGURE 8.10 Typical number and volume distributions of atmospheric particles with the different modes.

mass distribution is dominated in most areas by two modes (Figure 8.10, lower panel): the accumulation (from ~ 0.1 to ~2 pm) and the coarse mode (from ~2 to ~50 pm). Accumulation-mode particles are the result of primary emissions; condensation of secondary sulfates, nitrates, and organics from the gas phase; and coagulation of smaller particles. In a number of cases the accumulation mode consists of two overlapping submodes; the condensation mode and the droplet mode (Figure 8.10, upper panel) (John et al. 1990). The condensation submode is the result of primary particle emissions, and growth of smaller particles by coagulation and vapor condensation. The droplet submode is created during the cloud processing of some of the accumulation-mode particles. Particles in the coarse mode are usually produced by mechanical processes such as wind or erosion (dust, seasalt, pollens, etc.). Most of the material in the coarse mode is primary, but there are some secondary sulfates and nitrates.

A different picture of the ambient aerosol distribution is obtained if one focuses on the number of particles instead of their mass (Figure 8.10, upper panel). The particles with diameters larger than 0.1pm, which contribute practically all the aerosol mass, are negligible in number compared to the particles smaller than 0.1 pm. Two modes usually dominate the aerosol number distribution in urban and rural areas: the nucleation mode (particles smaller than 10 nm or so) and the Aitken nuclei (particles with diameters between 10 and 100 nm or so). The nucleation mode particles are usually fresh aerosols created in situ from the gas phase by nucleation. The nucleation mode may or may not be present depending on the atmospheric conditions. Most of the Aitken nuclei start their atmospheric life as primary particles, and secondary material condenses on them as they are transported through the atmosphere. The nucleation-mode particles have negligible mass (e.g., 100,000 particles cm~3 with a diameter equal to 10 nm have a mass concentration of <0.05 pg m-3) while the larger Aitken nuclei form the accumulation mode in the mass distribution.

Particles with diameters larger than 2.5 pm are identified as coarse particles, while those with diameters less than 2.5 pm are called fine particles. The fine particles include most of the total number of particles and a large fraction of the mass. The fine particles with diameters smaller than 0.1 pm are often called ultrafine particles.

Atmospheric aerosol size distributions are often described as the sum of n lognormal distributions where N¡ is the number concentration, Dpi is the median diameter, and ct, is the standard deviation of the ¡'th lognormal mode. In this case 3n parameters are necessary for the description of the full aerosol distribution. Characteristics of model aerosol distributions are presented in Table 8.3 following the suggestions of Jaenicke (1993).

8.2.1 Urban Aerosols

Urban aerosols are mixtures of primary particulate emissions from industries, transportation, power generation, and natural sources and secondary material formed by gas-to-particle conversion mechanisms. The number distribution is dominated by particles smaller than 0.1 pm, while most of the surface area is in the 0.1-0.5 pm size range. On the contrary, the aerosol mass distribution usually has two distinct modes, one in the submicrometer regime (referred to as the "accumulation mode") and the other in the coarse-particle regime (Figure 8.11).

The aerosol size distribution is quite variable in an urban area. Extremely high concentrations of fine particles (less than 0.1 pm in diameter) are found close to sources (e.g., highways), but their concentration decreases rapidly with distance from the source (Figure 8.12).

There is roughly an order of magnitude more particles close to a major road compared to the average urban concentration. However, the concentration of these particles decays rapidly because of dilution in a characteristic distance of roughly 100 m from the road (Zhu et al. 2002). The increase in mass concentration next to major roads is usually smaller, roughly 10-20% of the urban background. Part of this mass increase is in the Aitken and accumulation modes because of the fresh combustion particles, but a significant part of it can be in the coarse mode due to resuspension of dust particles from traffic (Figure 8.13).

The mass concentrations of the accumulation and coarse particle modes are comparable for most urban areas. The Aitken and nucleation modes, with the exception of areas close to combustion sources, contain negligible volume (Figures 8.11 and 8.13). Most of the aerosol surface area is in particles of diameters 0.1-0.5 pm in the accumulation mode (Figure 8.11). Because of this availability of area, transfer of material from the gas phase during gas-to-particle conversion occurs preferentially on them.

TABLE 8.3 Parameters for Model Aerosol Distributions Expressed as the Sum of Three Lognormal Modes

Mode I

Mode II

Mode III

N

Dp

N

DP

N

Dp

Type

(cm"3)

(pm)

log a

(cm"3)

(pm)

log a

(cm"3)

(pm)

log a

Urban

9.93 x 104

0.013

0.245

1.11 x 103

0.014

0.666

3.64 x 104

0.05

0.337

Marine

133

0.008

0.657

66.6

0.266

0.210

3.1

0.58

0.396

Rural

6650

0.015

0.225

147

0.054

0.557

1990

0.084

0.266

Remote continental

3200

0.02

0.161

2900

0.116

0.217

0.3

1.8

0.380

Free troposphere

129

0.007

0.645

59.7

0.250

0.253

63.5

0.52

0.425

Polar

21.7

0.138

0.245

0.186

0.75

0.300

3 x 10-4

8.6

0.291

Desert

726

0.002

0.247

114

0.038

0.770

0.178

21.6

0.438

Source: Jaenicke (1993).

Source: Jaenicke (1993).

Aitken Accumulation And Coarse Mode

10

L \t \

= 65.3 nm

r v \

fi

/ \

=1.47

5

1 \

Particle Diameter, nm

Particle Diameter, nm

Particle Diameter, nm

(c)

Hs=38.8nm

d=1.28 "4*

'* 1 * 1

tP&r >

Particle Diameter, nm

Aerosol Size Distribution
Particle Diameter, nm

FIGURE 8.12 Measured and fitted multimodal number distributions at different distances downwind from a major road in Los Angeles (a) 30 m downwind, (b) 60 m downwind, (c) 90 m downwind, and (d) 150 m downwind. Please note the different scale for the y axis. Modal parameters given are the geometric mean diameter and geometric standard deviation (Zhu et al. 2002). 372

Urban Aerosols
FIGURE 8.13 Aerosol volume distributions next to a source (freeway) and for average urban conditions.

The sources and chemical compositions of the fine and coarse urban particles are different. Coarse particles are generated by mechanical processes and consist of soil dust, seasalt, fly ash, tire wear particles, and so on. Aitken and accumulation mode particles contain primary particles from combustion sources and secondary aerosol material (sulfate, nitrate, ammonium, secondary organics) formed by chemical reactions resulting in gas-to-particle conversion (see Chapters 10 and 14).

The main mechanisms of transfer of particles from the Aitken to accumulation mode is coagulation (Chapter 13) and growth by condensation of vapors formed by chemical reactions (Chapter 12) onto existing particles. Coagulation among accumulation mode particles is a slow process and does not transfer particles to the coarse mode.

Processing of accumulation and coarse mode aerosols by clouds (Chapter 17) can also modify the concentration and composition of these modes. Aqueous-phase chemical reactions take place in cloud and fog droplets, and in aerosol particles at relative humidities approaching 100%. These reactions can lead to production of sulfate (Chapter 7) and after evaporation of water, a larger aerosol particle is left in the atmosphere. This transformation can lead to the formation of the condensation mode and the droplet mode (Hering and Friedlander 1982; John et al. 1990; Meng and Seinfeld 1994).

Terms often used to describe the aerosol mass concentration include total suspended particulate matter (TSP) and PMA (particulate matter with diameter smaller than x |im). 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.

Boston

Boston

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Responses

  • karys
    Which modes usually dominate the aerosol number distribution in urban and rural areas?
    8 years ago

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