Concurrent measurements of nitrate at urban and suburban sites identify local nitrate formation as a driver for urban episodic PM pollution.

Nitrate (NO) is often among the leading components of urban particulate matter (PM) during PM pollution episodes. However, the factors controlling its prevalence remain inadequately understood. In this work, we analyzed concurrent hourly monitoring data of NO in PM at a pair of urban and suburban locations (28 km apart) in Hong Kong for a period of two months.