Impacts of increased anthropogenic emissions in Asia on tropospheric ozone and climate

Abstract

A global 3-dimensional chemical transport model (CTM) has been used to study the impact of enhanced emission of pollutants in the Asian region on key chemical species like O₃, OH and nitrogen oxides. An extended set of chemical compounds are included in the model, of which 25 are transported. Diurnal variations are calculated, including the diurnal cycle of photodissociation rates. The Asian region has been selected for this perturbation study since emissions are rapidly increasing in this region, and because there is a large potential for future increases. A 15-month reference run and 2 perturbation studies, both starting in May and running for 4 months are performed, one with increases in NO_x emission of a factor of 2 and one with a factor 4. Increases in the anthropogenic emissions of hydrocarbons and CO are also included. Results for the last month (August) are then compared with the results from the reference run for the same month. In the 3-D model calculations, the adopted perturbations lead to significant increases in surface ozone in the Asian region. It is interesting to notice that enhanced short term ozone values during episodes are becoming more pronounced in the cases with high emissions. Significant ozone increases are also obtained in the upper troposphere due to efficient convective transport. The impact is noticible over a larger region in the free troposphere than in the surface layer due to longer chemical lifetime of ozone, and an efficient transport. A doubling of NO_x emission, gives significant ozone increases up to 30% in the upper troposphere. Calculations of the regionally positive radiative forcing from the increases in tropospheric ozone show values of about 0.5 W/m₂, which is 30–50% of the estimated negative radiative forcing due to the direct effect of sulphate aerosols in the region.