Special Topic: Global Environment and Atmospheric Pollution
Future Projection of Global Air Pollution
- Toward Understanding Chemistry-climate Interaction -
Future prediction experiments of global atmospheric pollution are conducted with CHASER, a chemistry coupled climate model. The experiments show that future distributions of tropospheric ozone are largely influenced by climate change (global warming), as well as by increases in emissions of pollutants (precursor gases). Here introduced is a challenge toward climate change prediction in the context of chemistry-climate interaction.
Kengo Sudo
Atmospheric Composition Research Program
Frontier Research System for Global Change (FRSGC)

Chemistry Coupled Climate Model

  Ozone in the troposphere, one of the significant greenhouse gases, controls concentrations of other pollutants and greenhouse gases such as methane and halofluorocarbons through chemical reactions. Tropospheric ozone chemistry also affects formation of sulfate aerosol, which plays a significant role for climate change and causes acid rain. Since the most of tropospheric ozone is produced associated with air pollution (emissions of ozone precursors), we need to assess the impacts of future pollution on climate and atmospheric environment, especially in Eastern Asian regions, where significant economic development is expected.
  For such purpose, we have developed a chemistry coupled climate model CHASER*1to simulate tropospheric ozone chemistry and its impacts on climate. The CHASER model is also used for short-term forecast in the "chemical weather forecasting system". Using this model, we have started predicting future changes in ozone and other pollutants distributions and their impacts on climate.

Significant Roles of Climate Change in Future Prediction of Air Pollution

  We perform future simulations of important pollutants such as ozone and sulfate using CHASER with the IPCC scenarios. In the simulation which considers changes in emissions of ozone precursors (nitrogen oxides, carbon monoxide, etc.) only, significant increases in surface ozone are calculated in eastern Asia in response to enhanced chemical production of ozone with the expected emission increases (e.g., Fig. 1). Our simulations also show that enhanced ozone production in the upper troposphere over eastern Asia has a huge impact on future global distributions of ozone owing to rapid intercontinental transport out of the region.
   However, our recent study suggests that these changes in tropospheric ozone associated with emission changes can be modulated by future climate change (warming). Fig. 2b shows the differences in the simulated zonal mean ozone distributions between with and without climate change for 2100. With climate change, lower tropospheric ozone levels are reduced due to increased chemical loss of ozone associated with water vapor increases. Upper tropospheric ozone in the high latitudes also decreases reflecting the rises in the tropopause height induced by climate change. On the other hand, the model shows increases in upper tropospheric ozone in the low-mid latitudes due to climate change. These ozone increases are attributed to the enhanced ozone input from the stratosphere (Fig. 3) resulting from enhancement in the stratospheric and tropospheric circulation with climate change. These results imply that for prediction of future ozone pollution, it is necessary to take into account the feedbacks from climate change as well as emission changes.

Fig. 1 Surface ozone increases (ppbv) predicted for 2050 and 2100 with the IPCC SRES-A2 emission scenario. Surface ozone levels in eastern Asia are simulated to increase by ~50% in 2050 and by ~100% in 2100 relative to the present-day.
Fig. 2 Annual and zonal mean ozone distribution predicted for 2100 with the A2 scenario in the control experiment (with emission changes only) (a); climate change (warming) induced changes in the ozone distribution (shown as the warming experiment minus the control experiment) for 2100 (b). In (b), the zonal mean tropopause is also shown for 2100 (solid line) and 1990 (dashed line).

Fig. 3 (Upper panel): temporal evolution of net stratospheric ozone influx to the troposphere for 1990-2100 calculated in the climate change (warming) and control experiment.(Lower panel): global and annual mean surface air temperature change relative to 1990 in the warming experiment. The decreases in net stratospheric ozone input in the control experiment are due to increases in tropospheric ozone associated with the emission increases.

Need for the Future Prediction in the Framework of Earth System

Ozone, while it is one of the important factors affecting climate, it also has significant influence on climate by chemically interacting with methane and aerosols. In addition, this interaction is subject to some changes with climate change as discussed above*2. It is therefore necessary to understand air pollution and climate as one system. Using the chemistry coupled climate model, we are working on prediction of future changes in climate and atmospheric environment in the context of chemistry-climate interaction. Also, the CHASER model is expected to play an important role in the integrated Earth system modeling at FRSGC which is part of the "MEXT's (Ministry of Education, Culture, Sports, Science and Technology) Project for Sustainable Coexistence of Humans, Nature, and the Earth".

*1 CHASER simulates production and distributions of pollutants such as ozone and sulfate, considering emissions of precursors (NOx, CO, SO2, etc.), transport, chemical reactions, and deposition in the CCSR/NIES/FRSGC climate model.
*2 Our future simulations as discussed here also show that temporal evolution of methane and aerosols is highly dependent on future changes in ozone and climate.

Sudo K., M. Takahashi, and H. Akimoto., Future changes in stratosphere-troposphere exchange and their impacts on future tropospheric ozone simulations, Geophysical Research Letter, Vol.30, No.24, 2256, Doi:10.1029/2003GL018526, 2003.
Frontier Newsletter/No.25
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