Source-receptor relationships for surface ozone

A number of published studies have estimated the influence of foreign sources on surface O3

in different parts of the northern hemisphere [TFHTAP, 2007]. These studies have applied a wide variety of techniques (described in Section 4.1.2) and these differences in focus and approach

contribute to the wide range of estimates, which for some S/R pairs disagree in sign and span an order of magnitude. In addition, prior efforts adopted varied regional definitions, metrics and periods of study, making it difficult to draw meaningful, quantitative estimates from a literature survey. By adopting a single approach across all models the HTAP intercomparison limits the factors contributing to differences in individual model estimates to treatment of emissions, chemistry, transport and resolution.

Estimates of intercontinental transport from the HTAP intercomparison for surface O3

The annual and spatial mean surface O3 decreases in each of the receptor regions resulting from 20% reductions of anthropogenic O3 precursor emissions for each source region are given in Table 4.2, along with the range of HTAP model predictions expressed in terms of the standard deviation between the different models. As expected the largest changes occur in the source regions.

For example, a 20% change in the anthropogenic emissions in North America (NA) leads to a mean change in surface O3 of ~1 ppbv, and these source region responses vary from a high of 1.26 ppbv in South Asia to 0.82 ppbv in Europe. The results also show that substantial changes in surface O3 occur far away from the source regions. For example, the annual mean surface O3 mixing ratio in Europe changes by 0.37 ppbv when current emissions in North America are changed by 20%.

To quantify the importance of changes in emissions outside the receptor area on surface O3

within the receptor region, we define the relative annual intercontinental response metric. This represents the ratio of the response in a particular region due to the combined influence of sources in the three other regions to the response from all source regions. This metric varies from 0% (receptor response entirely due to receptor region emissions) to 100% (receptor response entirely due to emissions elsewhere), with 50% representing the point at which the response from emissions outside the region is equal to the response from receptor region emissions. The relative annual

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intercontinental response for annual average surface O3 over each region is shown in Table 4.2, and varies from 43% for Europe, to 32% for NA and SA. This indicates that in all four regions, emission changes in the three other source regions are 50-75% as important as emission changes over the receptor region itself.

Table 4.2. Annual and spatial mean surface O3 response (ppbv) to 20% decreases in anthropogenic precursor emissions (NOx, CO, NMVOC, plus aerosols and their precursors). Values are mean (median) ± one standard deviation across the 15 models that conducted the regional perturbation simulations (SR6). Bold font denotes responses to foreign emission perturbations that are at least 10%

of the response to domestic emission perturbations. Also shown is the relative annual intercontinental response for each receptor region defined as the ratio of the total response in mean surface O3 due to changes in the other three source regions compared to that due to changes in all regions.

Receptor Region

Source Region NA EU EA SA

Annual mean decrease

NA 1.04(1.03)±0.23 0.37(0.37)±0.10 0.22(0.24)±0.05 0.17(0.19)±0.04 EU 0.19(0.18)±0.06 0.82(0.68)±0.29 0.24(0.24)±0.08 0.24(0.25)±0.05 EA 0.22(0.23)±0.06 0.17(0.17)±0.05 0.91(0.86)±0.23 0.17(0.17)±0.05 SA 0.07(0.07)±0.03 0.07(0.07)±0.03 0.14(0.13)±0.03 1.26(1.18)±0.26 Relative annual intercontinental response

32% 43% 40% 32%

The spatial distribution of the changes in mean surface O3 levels due to 20% changes in anthropogenic precursors for each of the source regions in springtime are shown in Figure 4.7. As noted above, the largest changes in O3 occur over the source regions, but the influence is shown to extend throughout the northern hemisphere. These results indicate that a 20% decrease in North American anthropogenic emissions decreases mean surface O3 across the northern hemisphere by 0.1-0.5 ppbv. The decrease in European emissions has a marginally smaller impact, reducing mean surface O3 over North America by less than 0.35 ppbv and by 0.1-0.5 ppbv over Asia. The impact from a 20% decrease in East Asian anthropogenic emissions is slightly smaller, generally less than 0.3 ppbv except over western North America. The response to emission reductions over South Asia is more localized, with a decrease of less than 0.1 ppbv across much of the northern mid-latitudes, reflecting greater export into the tropical free troposphere. Over the Arctic, the largest impacts on surface O3 are from European emissions (~0.4 ppbv) and the smallest are from South Asian emissions (<0.1 ppbv). The differences between the contributing models (not shown here) are appreciable, with one standard deviation of 20-50% of the mean value over both source and receptor regions. This variation, a measure of uncertainty in the estimates, is largest at northern mid-latitudes for European emissions, for which the standard deviation reaches ~0.2 ppbv, roughly the same order as the multi-model mean decrease. On an annual basis, the standard deviation is generally less than half of the multi-model mean decrease in surface O3.

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Figure 4.7. Model ensemble mean surface O3 decrease in springtime (in ppbv) for the combined 20% emission reduction over each source region showing the spatial variation over each region and over the Arctic. Each row shows the responses from a particular source region.

The range of estimates across the models participating in the HTAP intercomparison is generally smaller than that obtained from a survey of the literature, see Table 4.3. For a direct comparison with previous studies which have taken differing approaches to estimating

intercontinental contributions, the O3 responses in the HTAP studies derived from 20% emission changes are scaled by a factor of 5 to represent 100% emission changes. The uncertainties associated with this extrapolation are discussed later in Section 4.2.11. The largest intercontinental influence occurs for North American emissions on European surface O3, where the response to foreign emissions is between 1.0 and 2.55 ppbv. The largest S/R relationships are for NA→EU; EU→SA;

EU→EA; and NA→EA. These numbers are significant when compared with the changes in surface O3 due to changes in emissions from domestic sources. The largest values in Table 4.3 from prior studies (e.g., over NA in summer and over EU and EA for annual mean values) were estimated from simulations in which anthropogenic emissions were set to zero, or in which O3 production throughout the tropospheric column over a source region was considered to represent the effect of emissions from the source region. None of the models in the HTAP intercomparison suggest estimates near the upper end of this range.

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Table 4.3. Annual and seasonal mean estimates for the contribution to surface O3 (in ppbv) over the receptor region from anthropogenic O3 precursor emissions in foreign source regions, arranged from largest (first row) to smallest (last row). The top entry in each cell is the full range from the 15 models that participated in the SR6 set of HTAP simulations. The response to the 20% emission perturbation was multiplied by five to estimate the full contribution; the limitations of this approach are discussed in the text. Also shown (italics) is the full range from the published literature included in Table 5.2 in HTAP [2007] and Figure 11 in Fiore et al. [2009], supplemented with estimates from more recent studies [Lin et al., 2010b; West et al., 2009b; Zhang et al., 2009]

Source/Receptor Annual DJF MAM JJA SON

NA

→

^{EU 1.00-2.55} _2.-15. ^0.70-2.35 _0.4 ^1.05-3.10 _0.2 ^1.05-2.65 _-0.3-5. ^1.00-2.45 _-0.9

EU

→

^{SA 0.70-1.60 0.55-2.00 0.75-1.95 0.75-1.80 0.65-1.45}

EU

→

^{EA 0.60-1.85} _0.-5.4 ^{0.35-2.95 0.75-2.80} _3. ^0.40-1.15 _3. ^0.65-1.70

NA

→

^{EA 0.60-1.55} _0.2-4.5 ^{0.65-2.05 0.50-1.60 0.30-1.35 0.60-1.70}

EA

→

^{NA 0.50-1.55} _1. ^{0.50-1.80 0.65-2.05} _4. ^0.30-1.15 _1.-3. ^0.45-1.20

NA

→

^{SA 0.50-1.15 0.40-1.55 0.60-1.45 0.35-0.85 0.45-1.00}

SA

→

^{EA 0.50-1.10 0.50-1.30 0.55-1.25 0.30-0.90 0.45-1.05}

EU

→

^{NA 0.40-1.55} _0.2-0.9 ^{0.30-1.90 0.55-1.95 0.30-2.05 0.45-1.15}

EA

→

^{SA 0.40-1.45 0.40-2.30 0.30-1.05 0.30-0.95 0.25-1.85}

EA

→

^{EU 0.35-1.30} _0.8-7. ^{0.35-1.45 0.40-1.70 0.30-1.05 0.35-1.05}

SA

→

^{EU 0.20-0.75 0.20-1.00 0.20-0.85 0.15-0.55 0.05-0.70}

SA

→

^{NA 0.20-0.65 0.20-0.90 0.25-0.85 0.10-0.45 0.15-0.60}

Seasonal responses

The annual mean surface O3 responses presented in Table 4.2 mask a large seasonal variability in the response to emission perturbations, as is evident from Table 4.3. Surface O3

responses to 20% emission changes in individual models are shown as a function of month in Figure 4.8. There is significant variability in the calculated responses in both source and receptor regions which varies by season and location. However, there is good agreement between models on the strong seasonal variations. For example, the response of European surface O3 to North American emissions, which averages 0.37 ppbv over the year, varies between 0.47 ppbv in spring, when the effects of intercontinental transport are generally largest in the Northern Hemisphere, and 0.29 ppbv in summer when O3 production from domestic emissions is greatest. The smallest responses are from South Asian emissions over North America, which average 0.07 ppbv over the year but range from 0.10 ppbv in February to 0.03 ppbv in August. At northern mid-latitudes the intercontinental influence is largest in boreal spring, with a secondary peak in fall in some locations, and is smallest during

summer when southerly flow is more prevalent over these regions. For the South Asian region there is less variation and the seasonality is reversed, reflecting the transition between dry and monsoon

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seasons. The combined response of surface O3 over South Asia to foreign emissions varies from 0.4 to 0.9 ppbv, approximately 50% of the response due to domestic emissions reductions, and the

seasonality of domestic and intercontinental influences is similar, in contrast to the other regions considered here. Over the Arctic, the largest surface O3 response from all source regions occurs between April and June, with a secondary maximum in October and November, and this strong seasonality is most greatly influenced by European sources. These results indicate that while the intercontinental influence over many regions is important, decreasing domestic emissions is more effective at decreasing the highest O3 levels (e.g., as occur in July in EA, EU, and NA) [Fiore et al., 2009; Reidmiller et al., 2009].

Figure 4.8. Monthly mean surface O3 decreases (in ppbv) over receptor regions for the combined 20% emission reductions (HTAP SR6 simulations) showing (1) the seasonality of the responses (black line: ensemble mean ±1σ), and (2) the variability between models (grey lines). Each row shows the responses from a particular source region. For clarity, the vertical scale for the source regions (shaded panels, 0-2.5 ppbv) is different from that for the receptor regions (0-1 ppbv). The bottom row summarises the ensemble mean response over each receptor region from other source regions; the black line is the sum of the O3 decreases to emission changes in the three other regions (four for the Arctic).

The role of methane (CH4)

Emissions of chemically-active species may have a long-term effect on receptor regions through indirect effects on chemical composition that are often overlooked. O3 precursors alter the

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abundance of atmospheric OH radicals and thus affect the abundance and distribution of atmospheric CH4; these changes lead to changes in O3 abundance on a longer time scale reflecting the response time of CH4, ~12 years [Prather, 1996; Wild and Prather, 2000]. This effect can be particularly significant for NOx emissions which have a relatively large effect on OH, and lead to long-term reductions in O3, which partly counterbalance the short-term increase [Derwent et al., 2001; Wild et al., 2001]. Most studies of intercontinental transport have focused on periods of no more than a year or two, and therefore neglect these long-term responses; however, more thorough analysis requires that these transient effects are also included. Where the response time of CH4 has been characterized in a model, these long-term O3 responses can be estimated.

To address the importance of these effects, additional HTAP model studies were undertaken in which the globally-fixed CH4 abundance (1760 ppbv in the base simulations) was decreased by 20%. From these simulations, we estimate the response to 20% decreases in regional anthropogenic CH4 emissions as described in HTAP [2007] and Fiore et al. [2009], and compare the results to the response from 20% reductions in NOx, NMVOC, and CO, the O3 precursors that have traditionally been regulated, in Figure 4.9. While domestic emission controls on NOx, NMVOC and CO combined are clearly most effective for lowering domestic O3, the O3 response to anthropogenic emissions of CH4 from distant source regions is nearly as large as that to emissions of the traditional O3 precursors in these regions. This highlights the importance of controlling CH4 emissions for improving air quality as well as for reducing total greenhouse gas emissions that influence climate. Fiore et al.

[2008]explored the contribution of CH4 emissions to air quality and climate, and estimated that anthropogenic sources of CH4 accounted for about 50 Tg of the annual mean tropospheric O3 burden and contributed about 5 ppbv to global mean surface O3, based on results from earlier model studies.

Figure 4.9. Model ensemble surface O3 decrease (ppbv), annually and spatially averaged over the HTAP regions from 20% decreases in anthropogenic emissions of NOx, CO and NMVOC (red) versus 20% decreases in anthropogenic CH4 (blue). Influence of each source region on surface O3 within the same region (termed “domestic”, left panel), and the sum of the O3

responses to emission changes within the three other source regions(termed “foreign”, right panel). [Adapted from Figure 2 of Fiore, A. M., et al. [2010], Interactions between climate and air quality, (Section 6.1) in Air Pollution Modelling and its Application XX, edited by D. G.

Steyn and S. T. Rao, with kind permission of Springer Science and Business Media B.V.10.]

The long-term response of O3 through the effects of precursor emissions on OH and thus CH4

was found to be negligible (less than 3% for all months and all regions) for the HTAP simulations in which anthropogenic O3 precursor emissions were reduced simultaneously (SR6). However, this long term response has a larger influence when O3 precursor emissions were altered individually. For example, the long-term response reduced the overall response to foreign NOx emission changes by 15-20% when only NOx emissions were perturbed, and increased the overall response to foreign NMVOC emission changes by 10% and to foreign CO emission changes by 30-40% [Fiore et al., 2009].

Considering these findings and other studies quantifying the long-term impact of O3 precursor emissions on surface O3 [West et al., 2007; West et al., 2009b], we conclude that the long-term feedback may be important when individual O3 precursor emissions are perturbed by different amounts. In Table 4.2 and Figures 4.7 and 4.8 we focus on the SR6 set of simulations in which NOx, NMVOC and CO were reduced together by 20% and the long-term feedback through CH4 is negligible.

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FINDING (S/R relationships ozone): The impact of 20% changes in anthropogenic emissions in one region on annual regional-mean surface ozone in the other regions lies between 0.07 and 0.37 ppbv, based on the ensemble mean from the HTAP simulations. However, these values mask large temporal and geographic variability, and responses can vary by more than a factor of two from month to month and from location to location within a region.

FINDING (S/R relationships ozone): The relative annual intercontinental response for annual mean surface O3 is found to vary from 43% for Europe, to 40% for EA, to 32% for NA and SA in the HTAP simulations. These results indicate that in all four regions, emissions changes in the other three source regions are about half as important as the same domestic emissions change.

FINDING (CH4): Anthropogenic sources of CH4 are estimated to contribute about 50 Tg to the annual mean tropospheric O3 burden and about 5 ppbv to global mean surface O3. Controlling CH4 is of major importance in limiting increases in baseline surface ozone, and has additional benefit for climate.

RECOMMENDATION: The temporal and geographic variability in S/R relationships needs to be better characterized so that the influence of intercontinental transport can be more reliably quantified in critical locations (e.g., population centres, sensitive ecosystems) over the annual cycle.

Im Dokument Emission Inventories and Projections (Seite 169-175)