We employ a nested system of global and regional climate models, linked to regional and urban air quality chemical transport models utilizing detailed inventories of present and future emissions, to study the relative impact of climate change and changing air pollutant emissions on air quality and population exposure in Stockholm, Sweden. We show that climate change only marginally affects air quality over the 20-year period studied. An exposure assessment reveals that the population of Stockholm can expect considerably lower NO2 exposure in the future, mainly due to reduced local NOx emissions. Ozone exposure will decrease only slightly, due to a combination of increased concentrations in the city centre and decreasing concentrations in the suburban areas. The increase in ozone concentration is a consequence of decreased local NOx emissions, which reduces the titration of the long-range transported ozone. Finally, we evaluate the consequences of a planned road transit project on future air quality in Stockholm. The construction of a very large bypass road (including one of the largest motorway road tunnels in Europe) will only marginally influence total population exposure, this since the improved air quality in the city centre will be complemented by deteriorated air quality in suburban, residential areas.
Worldwide air pollution cause more than 2 million premature deaths annually [
In this work we investigate the role of climate change on future urban air quality and compare it with the effects of changing emissions in Europe and locally in a specific urban area, namely, Stockholm, Sweden. We compare the future year 2030 pollution levels in the urban background air with current levels (ca. year 2010). We also assess the effects of different traffic solutions on future urban air quality. The impact on air quality and population exposure of a road transit scenario, in which a new bypass highway is constructed, is compared to that of a reference scenario where no new roads are created. The comparison is made for year 2030 when the road project should be completed.
Figure
Conceptual overview of the model chain used in the present study. The global climate models (
To assess the present and future climate in Stockholm we make use of European climate data provided by the Rossby Centre [
As an illustration of the expected climate change and intermodel differences, Figure
Three-year-averages of 2 m temperature (a), precipitation (b), and wind speed (c) at Torkel Knutssonsgatan in city center of Stockholm for the period 2009–2011 (striped) and 2029–2031 (grey). Error bars indicate maximum and minimum annual average for the 3 years.
To simulate present and future air quality over Europe, we use a regional chemistry transport model (CTM)—MATCH [
Local (
|
Present 2010 (tons/year) | Reference 2030 (compared to Present) | Road transit project 2030 (compared to Present) |
---|---|---|---|
Local |
15 511 | 58.9% | 59.1% |
Local PM10 | 3 879 | 113% | 114% |
Europe |
|
81% | 81% |
The Stockholm air quality downscaling is performed by operating a high-resolution set-up of MATCH forced with interpolated meteorology from the regional climate model; the methodology follows Gidhagen et al. [
Map showing the air quality downscaling modeling domain (102 × 102 km2, left) and the local domain (36 × 30 km2, right) used for population exposure assessment. The locations of present and planned road transit are indicated with solid and broken lines, respectively. The location of the urban background monitoring station is indicated with a filled circle. The colors indicate population density (number of people in 100 × 100 m2 squares). Thin black lines are roads. White and grey areas indicate water and land.
A detailed local emission database is administered and updated annually. It includes the two counties of Stockholm and Uppsala, covering an area with some 30 municipalities and ca. 2 million inhabitants [
The local air pollution emissions in the Stockholm subregions are described for three different situations: (a) current (2010) situation, (b) a future (2030) scenario with a new transit road, and (c) a future (2030) scenario without the bypass. The transit road is mainly an underground road tunnel (21 km). Emissions are described for all important sectors but the difference in emissions in the three situations is only due to differences in road traffic emissions. Traffic prognoses for the future scenarios are obtained from a national traffic forecast model system called SAMPERS [
The two scenarios with and without the transit road have the same land-use (e.g., with respect to locations of residential areas). With the bypass road the existing congestion tax zone is extended and includes a tax on the highway-ring around the inner city. This additional tax extension is not included in the scenario without the bypass road, the motivation being that there must be a way to bypass Stockholm without having to pay a tax. The decrease in local NOx emissions between the two periods, as estimated from the planned renewal of traffic fleet and stricter vehicle emission limits, is expected to be around 40% (cf. Table
While assessing the environmental consequences of future urban planning scenarios, population exposure can be used as a complement to a pure comparison of air pollution concentrations. In this study we produce exposures using a 100 × 100 m2 resolution population density grid (Figure
The 2008 population data was used to assess exposure both for present and future scenarios (future population projections with a comparable spatial precision do not exist). It is clear that the number of people exposed will be underestimated in future scenarios as population will grow. The merit of population weighted averages is that they take into account both the distribution of pollution levels and the location of densely populated areas. If population will grow without major redistribution between residential areas, then population weighted pollution averages will not change significantly with increasing population. The exposure calculation uses simulated concentrations on a 1 × 1 km2 grid representing outdoor urban background concentrations, that is, no correction is made for indoor concentration levels being different.
NO/NO2 and O3 were measured in the urban background site (Torkel Knutssonsgatan) in the city centre of Stockholm (Figure
Simulations of air quality were made for a 102 × 102 km2 domain that includes the Stockholm Metropolitan area as well as Uppsala, the fourth largest city in Sweden. The different combinations of meteorology, European emissions, and local emissions are summarized in Table
Figure
Simulated three-year-average O3 (top) and NO2 (bottom) concentrations in 2009–2011 (case i, left), 2029–2031 (case ii, middle), and 2029–2031 (case iv, right) in the downscaled area over the Stockholm region, based on
Figures The present situation (2009–2011) using European emissions valid for 2010 as well as local Stockholm emissions for 2010. The climate change effect on air quality in Stockholm is assessed through retaining all emissions at their 2010 level, but using the climate around 2030 for both the regional and local air quality simulations. The climate change effect together with the European emission reductions as given by the RCP4.5 scenario, but retaining local Stockholm emissions at the 2010 level. This experiment illustrate the expected evolution of pollution levels in the long-range incoming air. Climate change, time-varying European emissions according to RCP4.5 and also a local Stockholm emission scenario projected for 2030.
Model simulations performed for the large metropolitan area with
Case | Simulation scenario | European emissions | Local Stockholm emissions |
---|---|---|---|
Present meteorology | |||
( |
Present European emissions | RCP4.5 at 2010 | database 2010 |
Present Stockholm emissions | |||
|
|||
( |
Present European emissions | RCP4.5 at 2010 | database 2010 |
Present Stockholm emissions | |||
Future meteorology | |||
( |
Future European emissions | RCP4.5 at 2030 | database 2010 |
Present Stockholm emissions | |||
Future meteorology | |||
( |
Future European emissions | RCP4.5 at 2030 | Reference 2030 |
Future Stockholm emissions |
Observed and simulated levels of O3 diurnal mean (a) and maximum 8-hour levels (b) at Torkel Knutssonsgatan (city centre of Stockholm), for present conditions (i) and simulated projections of future levels with only climate change effect (ii), including also emission changes in Europe (iii), and adding local emission changes in Stockholm (iv), see Table
Observed and simulated levels of NO2 diurnal mean (a) and 98-percentile of hourly levels (b) at Torkel Knutssonsgatan (city centre of Stockholm), for present conditions (i) and simulated projections of future levels with only climate change effect (ii), including also emission changes in Europe (iii) and adding local emission changes in Stockholm (iv), see Table
Figure
The simulation with changed climate but present (2010) European air pollution emissions (ii) demonstrate that the effect of climate change is minor on the average concentrations, this applies both to O3 (Figure
The reduction of European
The last simulation, case iv, (see also Figure
Simulations for the exposure calculations were made with 1 × 1 km2 spatial resolution over the smaller domain covering 36 × 30 km2, which has a population of close to 1.5 million distributed as shown in Figure
The general characteristics of how NO2 and O3 concentrations develop between present and future (Reference scenario without road project) were shown for the larger modeling domain in Figure
Differences in simulated NO2 (a), O3 (b), and PM10 (c) between the 2030 Road project and Reference average concentrations. Unit:
The main difference between the two alternatives is that there will be much less traffic emissions close to the city centre with the new transit road, but this is accompanied with a negative impact on the air quality (NO2 and PM10) west of the city centre. Figure
Table
Population-weighted average exposure given for a total population of 1463780 persons living inside the modeling domain. Model simulations performed for the smaller Stockholm domain with
Simulation scenario | Local Stockholm emissions | Population weighted exposure | |
---|---|---|---|
NO2 | O3 | ||
Present conditions | database 2010 | 6.85 | 54.13 |
Future: Reference | Reference 2030 | 3.62 | 53.05 |
Future: Road project | Road transit project 2030 | 3.60 | 53.06 |
Differences in the number of inhabitants exposed to NO2 (a) and O3 (b) concentration levels “Reference” (2030)—“Present” (2010). The red vertical line indicates zero difference. All simulations made with
Differences in the number of inhabitants exposed to NO2 (a), O3 (b), and PM10, (c) concentration levels between “Road transit project”—“Reference”, both scenarios valid for 2030. The red vertical line indicates zero difference. All simulations made with
As described in Section
Mahmud et al. [
In this study we have used regional downscaling of two different global climate models (
The exposure assessment revealed that all residents can expect considerably lower NO2 exposure in the future. Ozone exposure will change only marginally, partly due to decreased concentrations in the suburban areas and increased concentrations in the city centre.
We have also shown that a very large road transit project (involving the construction of one of the largest motorway road tunnels in Europe) will only marginally influence population exposure, since the improved air quality in the city centre will be complemented by deteriorated air quality in other residential areas.
This work has been cofunded by SUDPLAN: Sustainable Urban Development Planner for Climate Change Adaptation, European Framework Program 7, ICT-2009-6.4 ICT for Environmental Services and Climate Change Adaptation of the Information and Communication Technologies program, Project no. 247708.