Spatial And Temporal Variation

The concentration of atmospheric aerosols varies considerably in space and time. This variability of the aerosol concentration field is determined by meteorology and the emissions of aerosols and their precursors. For example, the annual average concentration of PM2 5 in North America varies by more than an order of magnitude as one moves from the clean remote to the polluted urban areas of Mexico City and southern California (Figure 8.24). Sulfate dominates the fine aerosol composition in the eastern United States, while organics are major contributors to the aerosol mass everywhere. Nitrates are major components of the PM2 5 in the western United States. The EC makes a relatively small contribution to the particle mass in many areas, but because of its ability to absorb light and its toxicity, it is an important component of atmospheric particulate matter.

The composition of the particles in a given area often changes significantly during the year. Species that are produced photochemically usually have higher concentrations during the summer. The higher summertime concentrations of sulfate in the northeastern United States (Figure 8.25) lead to high fine aerosol concentrations and low visibility. On the other hand, aerosol nitrate concentrations often peak in the winter, even if nitric acid is a secondary species produced photochemically. Indeed, there is usually more nitric acid available during the summer in most locations. However, owing to the higher summertime temperatures it remains mostly in the gas phase as nitric acid vapor. During the winter, almost all the nitric acid that is available is transferred to the particulate phase after reaction with ammonia to form ammonium nitrate (see Chapter 10), leading to the higher aerosol nitrate concentrations (Figure 8.25). The seasonal behavior of the organic

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Sulfate

Nitrate

Ammonium

Black carbon

Organic carbon

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Other

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Washington DC (14.5)

Los Angeles (30.3)

Atlanta (19.8)

Mexico City-Ped regal (24.6)

FIGURE 8.24 PM2 5 concentration and chemical composition at various sites in North America (NARSTO 2003). Chemica! spccics for cach site arc presented clockwise in the pie charts in the same order as in the legend.

Los Angeles (30.3)

Washington DC (14.5)

Atlanta (19.8)

Mexico City-Ped regal (24.6)

FIGURE 8.24 PM2 5 concentration and chemical composition at various sites in North America (NARSTO 2003). Chemica! spccics for cach site arc presented clockwise in the pie charts in the same order as in the legend.

particulate matter varies for each location. During the winter there are usually additional sources of organic particles because of the additional heating needs. The mixing heights are also lower, causing an increase in aerosol concentrations. On the other hand, the photochemical production of secondary organic aerosol contributes significant organic aerosol matter during the summer. The result of these competing effects is often a relatively constant organic PM concentration (Figure 8.25). Ammonia is transferred to the particulate phase in an effort to neutralize the acidic components forming ammonium sulfates and ammonium nitrate. The ammonium concentration pattern usually follows that of the sum of sulfate and nitrate.

Variation in atmospheric aerosol concentrations over shorter timescales (e.g., hours) is often quite significant as well. Figure 8.26 shows a rather extreme example of the sulfate concentration in an area increasing from 5 to 60 pg m ' over a period of hours because of the transport of pollution to the area. Then the concentration decreases rapidly with rain

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Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun

2001 2002

FIGURE 8.25 Annual variation of the PM2ii concentration and composition in Pittsburgh, PA (Wittig et a!, 2004a).

and returns to around 20 pg m~ , The concentration of aerosol nitrate is quite sensitive to temperature and often reaches a maximum when the temperature is at its minimum, shortly before sunrise (Figure 8.27).

For species with strong local sources the daily concentration pattern is often determined by the strength of atmospheric mixing. The atmospheric mixing height tends to he lower during the nighttime, so these species often reach high concentrations very early in the morning. The OC and EC concentrations in urban areas sometimes exhibit this behavior (Figure 8.28), peaking during the early stages of rush hour traffic at around 6 am. During the afternoon rush-hour, even if the traffic emissions are similar to those in the morning, the concentrations are considerably lower because they are diluted over a much larger boundary-layer volume. These diurnal variations can be used to determine the importance of local emissions. Areas where most of the organic aerosol is a result of local primary

Hour

FIGURE 8.26 PM25 sulfate concentrations measured on July 21, 2001 in Pittsburgh, PA (Wittig et al. 2004b).

Hour

FIGURE 8.26 PM25 sulfate concentrations measured on July 21, 2001 in Pittsburgh, PA (Wittig et al. 2004b).

Hour

FIGURE 8.27 Measured average PM2.5 nitrate concentration as a function of time during the four seasons in Pittsburgh, PA (Wittig et al. 2004b). The concentration pattern is dominated by the temperature variation in this area with the highest concentrations observed during the periods with the lowest temperature (nighttime, winter).

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OC

r-0~L p-T

EC

Day 1 Day 2 Day 3

Day 1 Day 2 Day 3

FIGURE 8.28 Measured PM10 OC and EC concentrations in Bakersfield, CA during 1995 Integrated Monitoring Study.

traffic emissions exhibit the behavior seen in Figure 8.28. Cities where local traffic is a much smaller source of particles are characterized by a relatively small difference between early morning and afternoon organic aerosol concentrations.

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