5.3 Regional-scale chemistry-climate simulations over South Asia show air pollution will continue to be a risk

Rajesh Kumar (ACOM/RAL) and Mary Barth (ACOM/MMM)

High-resolution air quality simulations with the nested regional climate model coupled with chemistry (NRCM-Chem) have been conducted to quantify future changes in air quality over South Asia, which is a region where elevated air pollution levels are already adversely affecting human health and food security. By performing regional-scale simulations, several atmospheric processes (e.g. urban emissions, topographic effects and convection) relevant to air quality can be better represented than global chemistry-climate simulations. Thus, these simulations give much better information as to where air quality exceedances may occur. Three 10-year simulations, one present day conditions and two future scenarios bounding estimates of future emissions, have been performed for two domains. The outer domain covers a broad region of South Asia (see accompanying Figure) at 60-km grid spacing, and the inner domain covers the Gangetic Plain (where over 500 million people live) at 12-km grid spacing.

Sample template image

Figure. 1: Dry- and wet-season daily 8-hour average (MDA8) ozone and 24-h average (DA24) PM2.5 over the parent model domain for present day and future (RCP8.5 and RCP6.0) scenarios. Green colors show areas where MDA8 ozone and DA24 PM2.5 are below the WHO limits of 50 ppbv and 25 µgm-3, respectively, while yellow-red colors depict the areas of exceedance. Black dots in future scenarios indicate that changes are robust (statistically significant at 99% confidence level) relative to present-day conditions.

Based on NRCM-Chem projections, we find that ozone pollution occurs primarily during dry season, while PM2.5 pollution persists throughout the year in South Asia (see Figure). Despite projections of improved air quality elsewhere, our model projections show that air quality in South Asia will either worsen by mid-century (RCP8.5) or remain similar to the present-day conditions (RCP6.0). For present-day conditions, ozone and PM2.5 levels exceed their corresponding World Health Organization (WHO) limits for up to ~ 6 and ~ 10 months respectively. Such exceedances are projected to increase by 70-120 days and 20-40 days in most parts of South Asia by 2050 under RCP8.5. The Indian states located in the Indo-Gangetic Plain and Bangladesh experience the largest number of exceedance days. We estimate that the number of South Asian people living in exceedance areas will increase from ~1.2 billion in 2000 to ~1.9-2.5 billion by mid-century.

These simulations have been supported by the NCAR Strategic Capability Project granted by NCAR/CISL.