Black carbon sanitary burden in the Indo-Gangetic plain: exposures, risks and mitigation

11 A large discrepancy between simulated and observed black carbon (BC) surface concentrations over the densely populated Indo-Gangetic plain (IGP) has so far limited our ability to assess the magnitude of BC sanitary impacts in terms of population exposure, morbidity, and mortality. We evaluate these impacts using an integrated modeling framework, including a successfully predicted BC surface concentration in a high-resolution chemical transport model CHIMERE with observationallyconstrained BC emissions, combined with consistent health functions for BC. Population exposure to BC is noteworthy, with more than 60 million people identified living over hotspots of BC concentration (wintertime mean > 20 μg m). A fraction of 62% of the total cardiovascular diseases mortality (CVM) burden for the megacity is found attributable to wintertime BC exposure. The semi-urban area has 50% of the CVM burden attributable to BC exposure in the total population over the IGP. More than 400 thousand lives can potentially be saved from CVM annually, by implementing prioritized emission reduction from the combustion of domestic biofuel in the semi-urban area, and diesel oil in transportation and coal in thermal power plant and brick kiln industries in megacities. 12 BC aerosols are mainly emitted from incomplete combustion processes, whether it is from combustion engines in the 13 automobile sector, residential burning of wood and coal, industrial power stations using heavy oil or coal, field burning of 14 agricultural wastes, or forest and vegetation fires. Recent studies of the health effect of airborne particles provide evidence 15 that BC is more robustly associated with health risks than bulk (non-speciated) particulate matter mass (PM2.5 or PM10), thus 16 suggesting that BC concentration is a better indicator to consider. BC may not be itself a toxic component of fine particulate 17 matter (PM), but it is known to be co-emitted with toxic compounds which are byproducts of incomplete combustion of fuel 18 and can adhere to the BC fractal morphology. BC may thus operate as a universal carrier of a wide variety of chemicals of 19 varying toxicity into the lungs. These chemical can then come in contact with the body’s major defence cells and the systemic 20 blood circulation. Recent studies also indicate health risks (per unit aerosol mass) are as much as 7–8 times larger for BC 21 than for undifferentiated PM 5, . There is thus value in targeting specifically BC emission abatement. 22 Furthermore the spatial and temporal distribution of BC concentration is very heterogeneous in comparison to undifferen23 tiated PM mass. This is attributable to a large number of local combustion sources of BC, while PM also include other species 24 such as dust which comes in part from long-range transport over most part of the Indian subcontinent during summer season 25 and western India during winter season. This heterogeneity may lead to contrast in population exposure to BC compared to 26 that to total PM mass. The World Health Organisation suggested that BC should serve as an additional indicator to PM2.5 27 to quantify human exposure to airborne pollution and assess health effects to such exposure, with consequences on how to 28 evaluate the local or regional mitigation measures aimed to reduce population exposed to combustion aerosols. 29 The IGP region over the Indian subcontinent is known as one of the global hotspots of atmospheric pollutants with very 30 large concentrations of BC aerosols in wintertime. Over India, this region also has the largest population density, with area31 types, e.g., from rural to megacity (refer to Table s2). Epidemiological studies provide evidence of an association between 32 short-term exposure to elevated ambient air pollution (such as that during wintertime) and a higher risk of acute cardiovascular 33 diseases, with systemic oxidative stress induced by air pollution as a potential underlying mechanism 5, . 34 To estimate the magnitude of BC attributable sanitary impacts concerning population exposure, morbidity, and mortality 35 and targeting BC emission abatement for health benefits, a conceptual plan using an integrated modeling framework as im36 plemented in the present study is shown in Figure 1. We obtain a spatially and temporally fine resolved gridded distribution 37 of surface BC concentration for the wintertime over the IGP through BC transport simulation in a chemical transport model 38 (CHIMERE). Simulating atmospheric BC concentration is indeed necessary to provide information on BC aerosols over large 39 areas where atmospheric measurements are sparse, particularly in rural and remote locations comprising of a low population 40 density and experiencing relatively less anthropogenic pollution than urban or megacity sites. 41 Evaluation of BC simulations in CHIMERE over the IGP 42 Meteorological forcing (WRF) BC emission (constrained, baseline) Population exposure to BC (constrsimu, basesimu) (rural, semi-rural, semi-urban, urban, megacity) Simulated BC distribution (0.1° x 0.1°) (constrsimu, basesimu) Exposure response relationship Attributable fraction Cardiovascular diseases mortality (CVM) burden BC attributable CVM burden (CVM BC WIN) (rural, semi-rural, semi-urban, urban, megacity) Mitigation proposed: BC emission reduction through control in fuel consumption from selected combustion activity in a source sector (based on estimates from B2, B5 and B9) (rural, semi-rural, semi-urban, urban, megacity) CHIMERE (BC transport simulations: constrsimu, basesimu ) Source-tagged simulations: FF, BF B1 B2 B3