Ability to track long-term trends in hemispheric transport from existing

Due to great year-to-year variability, long-term monitoring (typically more than a decade) at carefully chosen sites is essential to assess trends in intercontinental transport from surface observations. The interannual variability at receptor sites is largely driven by the temporal and spatial variability of upwind emissions and of the weather and climate patterns that determine transport processes. The characterization of trends in hemispheric transport is especially difficult because local effects, with possibly confounding temporal trends, often obscure the influence of that transport.

Measurements intended to characterize intercontinental transport are best carried out at remote sites that are relatively free of local and regional impacts. Island sites are ideal, especially those that are

sparsely inhabited or minimally developed. Coastal and mountain sites may also be suitable so long as attention is given to local and regional sources that might impact on the site and to the vagaries of local circulations, e.g. sea-breeze cycles or mountain-induced circulations induced by diurnal heating and cooling cycles and regional-scale forcing induced by the wind field interacting with the mountain mass. Unfortunately, there are very few data sets that meet these requirements. In the following sections we present some examples.

3.6.1 Characterization of ozone trends

A number of studies and assessments have been made in an effort to elucidate large-scale trends in O3 concentrations using data from continental networks. The EMEP Assessment (EMEP, 2004) is typical. It shows that trends across Europe are dominated by changes in regional emissions, thus making it difficult to elucidate the impact of long-range transport. Nonetheless, at some relatively remote continental sites it is possible to extract information on trends by the careful selection of samples based on air mass trajectories or other criteria. Examples are shown in figures 3.6 and 3.22, which all show positive temporal trends. However, a critical uncertainty exists regarding the validity of the O3 concentration trends presented in these figures. Oltmans et al. (2006) have recently reviewed evidence for temporal trends in tropospheric ozone from surface and ozone sonde sites. They generally find increasing trends in surface O3 over Europe in agreement with the results presented here in figures 3.22 and 3.6a. However, they find no significant trend in western North America, a conclusion in clear disagreement with the result presented here in figure 3.6b. This disagreement, unless it can be resolved by further analysis, strongly suggests that existing surface observations are not adequate to track conclusively the trends in transport of ozone in the lower troposphere.

Figure 3.22 Trends determined by samples based on air mass trajectories. Linear trends in median, normalized ozone concentrations at Kårvatn, mid-Norway, based on data filtered by back trajectory for only the background transport sectors. The boxes mark the annual medians, and the error bars the 25- and 75-percentiles (EMEP, 2005).

3.6.2 Characterization of trends in dust transport

Mineral dust is a major and sometimes dominant aerosol component over many ocean regions (Cakmur et al., 2006; Mahowald et al., 2005). North Africa is the world’s most active dust source (Engelstaedter et al., 2006; Prospero et al., 2002) with strong transport to the Atlantic, the Caribbean, North America and Europe (Perry et al., 1997; Prospero, 1999). Winds also carry large quantities of dust out of Asia and over large areas of the North Pacific (Prospero et al., 1989) and across North America (Husar et al., 2001; VanCuren, 2003). The resulting dust concentrations downwind from

these two regional sources exhibit strong seasonal cycles with the maximum in boreal summer (African source) or spring (Asian source). In addition, there is considerable interannual and long-term variability that is linked to weather and climate processes in the continental source regions.

The intercontinental transport of African dust is perhaps the best-documented example of long-range transport on intercontinental scales because of the long-term surface-level measurements made in a network of island and coastal stations in the Atlantic as well as in Miami, Florida. The longest record is from Barbados, West Indies, where daily measurements began in 1965 and continue to the present (figure 3.17) (Prospero and Lamb, 2003). Most notable in that record is the sharp increase in dust transport in the early 1970s in response to the onset of prolonged drought in the Sahel-Soudano region. Drought, and the corresponding increase in dust transport, appears to be linked to large-scale climate processes. Although concentrations are somewhat lower in Miami than Barbados and the dust transport season is shorter, the longer-term variability is similar at both sites.

Measurements at Bermuda between 1987 and 1998 also show the strong and persistent impact of African dust comparable to that in Miami.

Measurements of Asian dust made on Midway Island from 1981 to 2001 show that there were periods in the mid-1980s and in 1998 when dust transport increased sharply (Prospero et al., 2003).

There had been considerable speculation that the increase in dust activity in the late 1990s was due to increased impacts on land use resulting from the rapid economic expansion in China. The record on Midway argues against this explanation because there was no evidence of a long-term trend, only a sharp increase in the late 1990s that is now attributed to drought that occurred at that time. This conclusion is now generally accepted based on dust studies in China.

Temporal trends in dust transport, at least from these two major global source regions, have been reliably documented from surface observations. However, there is considerable uncertainty regarding our ability to extend these measurement records over the coming decades due to a lack of resources committed to such efforts.

3.6.3 Characterization of trends in transport of sulphate and nitrate aerosol

Measurements of NO3 and nss-SO4 concentrations on island sites in the Pacific and the Atlantic can provide temporal trends in the long-range transport of these pollutants from the Asian and North American source regions. The trend in pollution SO4 can be estimated by computing the

“natural” non-sea-salt sulphate based on the MSA concentrations and subtracting it from the total nss-SO4.

On Midway, NO3 and pollution SO4 concentrations show a strong seasonal cycle with maximum concentrations in spring, coincident with the increased dust transport noted previously.

This cycle is attributed to the impact of pollution transport from Asia. Figure 3.23 shows that anthropogenic SO4 concentrations approximately doubled from 1981 to the mid-1990s, a trend that closely matches the increase in SO2 emissions from China over that period (Streets et al., 2000).

Nitrate concentrations (not shown) yield a similar trend over the same time period. The data also suggest that anthropogenic SO4 and NO3 decreased in the late 1990s, as did emissions. However the data become less reliable because of the effects of the very strong El Niño in 1997–1998 and because of operational difficulties that led to the termination of the observations in 2001.

The annual mean concentrations of nss-SO4 and nitrate NO3 measured in Bermuda (figure 3.24) show coherent patterns. Aerosol nss-SO4 decreased steadily from the start of the measurements until the mid-1990s, then stabilized and finally increased slightly thereafter. The emissions of SO2

from United States sources decreased substantially over this time period by an amount that roughly corresponds to that observed at Bermuda, yet the timing of the decreases are markedly different— the United States SO₂ emissions decreased sharply between 1994 and 1995 when more stringent controls were mandated. In contrast, there is no substantial change in nitrate (NO3) over this period, in agreement with the trend in NOx emissions, which show only a very slight increase over the period.

The substantial differences between the aerosol record and the emissions record demonstrate the challenges in attempting to monitor long-term trends at distant receptor sites.

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Figure 3.23 Anthropogenic sulphate concentrations on Midway Island compared to the emissions of SO2 from China (Prospero et al., 2003).

Bermuda BCO: Annual (Daily) Means 1989 - 1998

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

Nitrate Aerosol (ug/m3)

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

nss-Sulfate Aerosol (ug/m3)

Figure 3.24 Annual mean nss-sulphate and nitrate concentrations on Bermuda during onshore winds compared to the eastern United States SO2 (Tg S/yr) and NOx emissions (Tg N/yr). (Based on data discussed in part in Savoie et al. (2002)

Aerosol SO4 and NO3 measurements have also been made continuously on Barbados since 1989. This record suggests that NO3 remained unchanged and that nss-SO4 decreased by about 20 per cent. Approximately half of the NO3 and nss-SO4 at Barbados is attributable to anthropogenic sources (Savoie et al., 2002), predominately from European sources (Hamelin et al., 1989). If we assume that the change in nss-SO4 concentration is due to the transport of pollutants and if we take into consideration that about half of the nss-SO4 is natural (from DMS), then the actual decrease in pollutant SO4 is roughly 40 per cent, a change that is consistent with the sharp drop in European SO2

emissions over this time period (EMEP, 2004). However the absence of a discernable change in Barbados NO3 concentrations is puzzling in the light of the substantial reduction of European emissions of NO_x, roughly 25per cent (EMEP, 2004).

3.6.4 Summary, remaining uncertainties and future needs

There are analyses that derive trends in the long-range transport of O3, dust and anthropogenic NO₃ and SO₄ aerosol. However, there are critical uncertainties. The disagreement over the validity of derived trends in O3 is of great concern, and limits their utility for the testing of models. The available data for O3 and aerosols are limited to a small number of sites, and for O3

generally to only the past two decades. In some cases (especially for dust), large interannual variability obscures more systematic trends. The derived trends in NO3 and pollutant SO4 aerosol transport are usually consistent with estimated trends in the precursor emissions, but there is at least one disagreement (the NO₃trend in Barbados). There is considerable uncertainty regarding our ability to extend these measurement records over the coming decades.

Long-term monitoring data are important for the development and testing the wide variety of models currently being used to address aerosol transport issues (Textor et al., 2005). Such models currently play an important role in addressing environmental issues on regional scales and these models are being extended to larger scales. However, we will only be able to rely on the models when they are thoroughly tested against long-term data sets acquired on continental and global scales. It is notable that in the IPCC 2001 assessment, the only aerosol data available for model evaluation over the oceans was that obtained in the University of Miami aerosol network that was in operation during the 1980s and 1990s. That network no longer exists. Thus there is no data available for future model development.