In-situ Observations of HONO and Other Reactive Nitrogen from On-Road Vehicles

Nitrous acid (HONO) is an important nocturnal radical reservoir and upon photolysis is a source of hydroxyl radicals (OH) and nitric oxide (NO), both central to tropospheric chemistry. Nitrous acid has a large impact on the oxidative capacity in urban environments, often as a significant radical source. A major source of HONO is from the direct emissions from internal combustion engines, both gasoline and diesel. Most determinations of HONO emissions ratios have been conducted using laboratory chassis dynamometer measurements or in-situ roadside or tunnel experiments, however these approaches are susceptible to sampling biases related to actual vehicle operation on road. Additionally, HONO is a trace constituent of vehicle exhaust making it difficult to characterized its emission relative to NOx from on-road measurements.

We present real-world driving condition HONO emission ratios sampled from the NOAA CSL Mobile Lab measured with iodide chemical ionization mass spectrometer and laser induced fluorescence NOx instrument. In situ measurements were made in Salt Lake City, Utah and Denver, Colorado as a part of the Utah Summer Ozone Study (USOS) in July-August 2024. We will use high precision HONO measurements in junction with other co-sampled reactive nitrogen species (e.g. HCN, HNCO, NO, and NO₂) to evaluate the current emission factors from the total vehicle fleet sampled during USOS, as well as by different types of on-road vehicles and engines.