In early April, city dwellers of India’s northern provinces experienced once-in-a-lifetime views of the snow-capped Himalayas, thanks to a rare absence of smog. The foggy mixture of particles and gases will certainly reappear in the region once coronavirus restrictions are lifted and the air pollution returns. Particulate matter in the atmosphere is a leading cause of lung disease and may contribute to neurological diseases such as Alzheimer’s. A better understanding of the pollutants that lead to smog formation is necessary for any mitigation effort to succeed.
Some particles in the atmosphere, called primary particles, form directly from combustion or mechanical generation. Other particles, called secondary particles, form from trace gases that condense and stick to the surfaces of existing particles, and sometimes undergo complex chemical evolution. Winter smog, shown in figure 1, is a mixture of both types of particles. Rapid growth is essential to the survival of secondary...
