Significance of semivolatile diesel exhaust organics for secondary HONO formation

The atmospheric origin of nitrous acid (HONO) is largely unknown despite its estimated importance as an OH source during daytime due to its rapid photolysis. Recently, primary HONO contained in automobile exhaust as well as secondary HONO formation on soot particles have been invoked as possible HONO sources, but none of them is able to account for the observed HONO to NOx ratios of up to 0.04 in the atmosphere. In this paper, we show that semivolatile and/or water-soluble species contained in diesel exhaust are significantly involved in secondary HONO formation. These species are not associated with soot when the exhaust exits the tailpipe. To quantify these species and to assess the reaction kinetics leading to HONO, experiments were performed in which filtered but hot diesel exhaust gas interacted with a glass surface as well as a water film mimicking dry and wet surfaces to which exhaust might be exposed. A fraction of 0.023 of the NOx emitted was heterogeneously converted to HONO, which is at least three times more than the primary HONO emissions by diesel engines and a fraction of 50 larger than HONO formed on diesel soot particles that do not contain the semivolatile organics.