Concentrations seen at downwind receptor locations and implications for

From an air quality management perspective, we wish to know: What is the effect of long-range transport of air pollutants on the local concentrations of ozone and aerosols? The answer to this question has two aspects. First, what are the maximum concentrations transported to the receptor location during a transport event? Such maximum impacts can potentially push a local area over its air quality standard. Second, what is the effect of the transport on the average background levels? It is these background levels that determine how much local pollution can be tolerated before exceeding the local standard.

3.4.1 Ozone concentrations

Figure 3.6 illustrates the increase of O3 in marine boundary layer air coming ashore at the west coasts of Europe and North America. The average trends observed imply an increase of approximately 10 ppbv over the preceding two decades. This increase represents a substantial fraction of the air quality standards on the respective continents (120 µg m^-3 or about 60 ppbv, 8-hour average in Europe and 80 ppbv, 8-hour average in the United States). Clearly, less local O3 production in continental regions receiving inflow from the upwind marine area can be tolerated before the ambient standard is exceeded. Figure 3.5 indicates that flow from the European continent also significantly impacts the background O₃ levels over the downwind Asian continent.

Episodic enhancement of surface level O3 associated with Northern Hemisphere transport of O3 and precursors has been observed at high-elevation locations in the western United States (Jaffe et al., 2005). Figure 3.2 shows O₃ concentrations exceeding 70 ppbv. The enhancement of the O₃ concentration above the background of about 40 ppbv was linked to transport of Asian anthropogenic emissions through back trajectory analysis and coincident chemical markers.

Model calculations are required to apportion the observed O₃ concentrations among

“background,” i.e. a long-range transport contribution, and local pollution. Figure 3.13 illustrates results from one such calculation for a relatively high-altitude site in California. The model results indicate that the 17 May plume of anthropogenic emissions (included in figure 3.3) impacted the site during this period. The 7–10 ppbv attributed to long-range transport is only about 10 per cent of the observed concentrations on that particular day, but that enhancement pushed the observed concentrations over the standard. Figure 3.14 illustrates an episode when O₃ concentrations exceeded the air quality standard at a lower altitude surface site (Jaffe et al., 2004). The long-range transport in this episode from Siberian forest fires increased surface level ozone values by about 15 ppbv. The

local and long-range contributions are shown stacked in different orders to demonstrate that transport and local pollution both bear responsibility for exceeding the standard.

Models also suggest that North American emissions have substantial effects on Europe. Li et al. (2002) find in their model that 20 per cent of the exceedance of the European Community ozone standard in the summer of 1997 over Europe would not have occurred in the absence of anthropogenic emissions from North America. These North American effects are relatively small and not easily detected in surface observations, but they are large enough to push the observed concentrations over the standard.

3.4.2 Aerosol concentrations

The discussion of the impact of aerosols on air quality in downwind regions has been generally limited to the episodic transport of dust. One particularly severe example of the transport of Asian dust is the April 2001 episode, which led to elevated surface levels of PM2.5 in Salt Lake City, Utah, in the U.S. Rocky Mountain region and Atlanta, Georgia, in the eastern United States (figure 3.15). The dust was transported initially in the free troposphere, and then brought to the surface through a combination of processes, which differed from region to region. Subsequent mixing with local pollutants led to PM2.5 levels exceeding the new daily standard of 35 µg/m³. During this specific case, the dust passed above the Pacific Northwest (e.g. Seattle, Washington) and had greater impact at the surface in the Rocky Mountains and eastern United States. This event was diagnosed through a combination of air mass trajectory calculations, satellite imagery (episode illustrated in figure 3.11) and use of chemical markers (figure 3.16), proving a clear Asian dust signature crossing the United States. Dust transport from North Africa impacts a wide region of the Western Atlantic, the Caribbean, and the southern and eastern United States (Perry et al., 1997, ; Prospero, 1999). Figure 3.17 shows the monthly mean mineral dust concentrations measured on Barbados and at Miami, Florida, indicating levels approaching or exceeding the current United States daily standard for PM2.5. Strong low pressure systems can sweep African dust in a northerly direction also, across the Mediterranean and into Europe as far north as England, Scandinavia (Engelstaedter et al., 2006) and Iceland (Sodermann et al., 2006).

Figure 3.13 Ozone concentrations observed at Sequoia National Park, California (36 N, 118 W, 1890 m altitude) in May 2002 versus the corresponding Asian pollution enhancements simulated by the GEOS-CHEM model (Hudman et al., 2004). Red symbols indicate the period 17-20 May when observed O3 exceeded the standard.

Figure 3.14 Contributions from three sources to surface O3 for Enumclaw, Washington, on 6 June 2003 (Jaffe et al., 2004).

The impact of the long-range transport of anthropogenic aerosols on local and regional air quality is less well characterized by observations. However, this transport likely has significant effects in particular episodes. For example, the aerosol levels illustrated in figure 3.2 correspond to about one half of the United States daily PM2.5 standard.

Figure 3.15 Contributions from three sources to surface PM2.5 in three United States cities (adapted from Jaffe et al. (2003b)).

3.4.3 Summary, remaining uncertainties and future needs

Long-range transport affects the concentrations of ozone and aerosols at downwind receptor locations, but there exists a clear distinction between the effects of this transport on these two species.

In situ observations document episodic elevated concentrations due to transport of both ozone and aerosols, but the latter are much more pronounced. Analyses of long-term observations also generally find that the background O3 concentration in marine air at the west coasts of Europe and North America has increased over the past two decades. It is clear that O3 is, in effect, primarily a hemispheric pollutant with a rising, hemisphere-wide background; only infrequent significant elevated surface levels result from long-range transport. For aerosols, spectacular enhancement events have been observed at the surface, but very clean background concentrations can still be observed throughout the globe. The episodic enhancement events do yield an average elevation of the total aerosol column over the long term.

Despite observations of the effects of long-range transport, the implications for surface air quality can only be determined from models, and these models are still rather uncertain. The future needs here lie largely in the improvement of models, both through improvement in the model parameterizations and in testing the model’s ability to quantitatively reproduce both the observed episodic enhancements and the long-term increases in background concentrations.

Figure 3.16 Eastern movement of Asian dust plume across the United States based on measurements from the IMPROVE (Interagency Monitoring of Protected Visual Environments) monitoring network. Silicon served as a dust tracer linking elevated PM concentrations to Asian-derived dust air mass (Jaffe et al., 2003b).

Figure 3.17 Monthly mean mineral dust concentrations measured at Barbados (red) and Miami, Florida (blue). (Prospero (1999) updated; Prospero and Lamb (2003) updated)