HONO Formation from Heterogeneous Photochemistry of Nitrogen Oxides on Urban Grime

Abstract:

Multiple field studies have indicated that there is an important unknown fixed, near-ground, photochemical source of HONO. We will discuss the photochemical production of HONO from urban grime, surface films that form on impervious surfaces in an urban environment, as a possible process contributing to this source. In a field study carried out in Fall 2014, we measured decreased nitrate in urban grime samples that were exposed to direct sunlight, compared to those that were shielded. These measurements support previous laboratory experiments showing a fast photochemical loss of nitrate from collected urban grime upon illumination with simulated sunlight. Using Incoherent Broad-Band Cavity Enhanced Spectroscopy (IBBCEAS) to quantify the gas phase products, we show that HONO(g) is a product of urban grime photochemistry, demonstrating the ability of grime photochemistry to re-activate surface nitrate. This chemistry does not occur on dry samples; we will discuss the relative humidity (RH) dependence in the context of our measurements of water uptake onto grime samples made using a quartz crystal microbalance (QCM).