Wintertime Air Quality
EM—December 2019: This issue of EM highlights several examples of wintertime air pollution issues from around the world.
Air pollution is a major risk factor for premature mortality worldwide. In 2017, ï¬ne particulate pollution is estimated to have contributed to 2.9 million premature deaths globally and ozone pollution to nearly a half million deaths. Air pollution levels vary dramatically throughout the world and by season, and many areas of the world experience especially severe air pollution during winter. For example, the highest concentrations of ï¬ne particulate matter (PM2.5) occur in winter in Delhi, India; Beijing, China; Dhaka, Bangladesh; and San Joaquin Valley, California, USA. The timing of the peak in national average PM2.5 concentrations in the United States recently switched from summertime to wintertime.
Severe wintertime air pollution is driven in part by meteorology. Cold ground temperatures cause the layer of air close to the surface to be cooler than the air above. Layering of cool air under warm air promotes atmospheric stability, which suppresses vertical mixing and dilution of surface emissions. Stagnant air conditions can be intensiï¬ed by high pressure systems that further suppress atmospheric mixing and trap air pollutants near the surface. Wintertime pollution can be especially bad in valleys where horizontal air transport and ventilation is blocked by terrain.
Wintertime air pollution is also driven by pollutant emissions speciï¬c to the season. Cold temperatures lead to enhanced emissions associated with home heating. Use of raw coal for home heating has been identiï¬ed as a major pollution source in Ulaanbaatar, Mongolia, and residential wood combustion is an important pollution source in mountain valleys in the United States. In Northern India, residential biomass burning is the major contributor to PM2.5 concentrations in December–February (see Venkataraman et al., in this issue) and burning of crop stubble from farms in Punjab and Haryana is a major pollution source in October–November. Open burning of municipal solid waste has also been recognized as an important wintertime emission source in Indian cities.
In this issue, several examples of wintertime air pollution are described, from Spain to India to Bangladesh to California.