Impact of regional climate change and future emission scenarios on surface O₃ and PM_2.5 over India

Abstract. Eleven of the world's 20 most polluted cities are located in India and poor air quality is already a major public health issue. However, anthropogenic emissions are predicted to increase substantially in the short-term (2030) and medium-term (2050) futures in India, especially if no further policy efforts are made. In this study, the EMEP/MSC-W chemical transport model has been used to predict changes in surface ozone (O₃) and fine particulate matter (PM_2.5) for India in a world of changing emissions and climate. The reference scenario (for present-day) is evaluated against surface-based measurements, mainly at urban stations. The evaluation has also been extended to other data sets which are publicly available on the web but without quality assurance. The evaluation shows high temporal correlation for O₃ (r =  0.9) and high spatial correlation for PM_2.5 (r =  0.5 and r =  0.8 depending on the data set) between the model results and observations. While the overall bias in PM_2.5 is small (lower than 6 %), the model overestimates O₃ by 35 %. The underestimation in NO_x titration is probably the main reason for the O₃ overestimation in the model. However, the level of agreement can be considered satisfactory in this case of a regional model being evaluated against mainly urban measurements, and given the inevitable uncertainties in much of the input data.

For the 2050s, the model predicts that climate change will have distinct effects in India in terms of O₃ pollution, with a region in the north characterized by a statistically significant increase by up to 4 % (2 ppb) and one in the south by a decrease up to −3 % (−1.4 ppb). This variation in O₃ is assumed to be partly related to changes in O₃ deposition velocity caused by changes in soil moisture and, over a few areas, partly also by changes in biogenic non-methane volatile organic compounds.

Our calculations suggest that PM_2.5 will increase by up to 6.5 % over the Indo-Gangetic Plain by the 2050s. The increase over India is driven by increases in dust, particulate organic matter (OM) and secondary inorganic aerosols (SIAs), which are mainly affected by the change in precipitation, biogenic emissions and wind speed.

The large increase in anthropogenic emissions has a larger impact than climate change, causing O₃ and PM_2.5 levels to increase by 13 and 67 % on average in the 2050s over the main part of India, respectively. By the 2030s, secondary inorganic aerosol is predicted to become the second largest contributor to PM_2.5 in India, and the largest in the 2050s, exceeding OM and dust.