HONO in Indoor Air Environments: Role of Surfaces and Light

Even a small fraction of indoor HONO undergoing photolysis (generating hydroxyl radicals (OH●) in the near-UV range) will have a large impact on indoor air chemistry (Gómez Alvarez et al., 2013)1. The mechanisms controlling HONO formation are complex - surface nature, light intensity, and humidity levels all play a role in its formation.

The broadband Cavity Ring-Down Spectrometer (CRDS) built in our lab uses a high-finesse optical cavity, a broadband light source (LED), and highly reflective mirrors, providing the capability of cost-effectively measuring nitrous acid (HONO). The HONO generation system (Febo et al., 1995; Wall et al., 2006)2,3 built in our lab will generate highly stable and pure HONO by flowing humidified HCl(g) over a bed of NaNO2(s), allowing calibration of the CRDS system down to low ppbv HONO levels.

For especially organic substrates containing a combination of electron donors (including phenols), and of compounds yielding excited triplet states (i.e. aromatic ketones), NO2 is reduced to HONO, while the aromatic hydrocarbon is oxidized, producing high yields of HONO in the laboratory (up to nearly 100 %) (George et al., 2005)4. Liu et al. (2003)5 characterized paraffinic and polar organic compositions of organic films on an impervious surface (window glass sampled at an indoor site). Considering these compositional findings, it is expected that the photoenhanced reduction of NO2 on indoor organic films will produce significant HONO yields, and may potentially explain the high daytime HONO levels observed indoors, which cannot be explained by known dark reactions. Furthermore, examination of the role of the nature of the surface (e.g. glass versus gypsum), light intensity and humidity levels will give insight into the complex mechanisms controlling HONO formation indoors. Considering the lack of studies employing indoor organic films, a study of the photoenhanced reactions of NO2 on such films is necessary, and is planned soon using the CRDS for HONO detection.

References:

1. 10.1073/pnas.1308310110

2. 10.1021/es00009a035

3. 10.1007/s10874-006-9021-2

4. 10.1039/B417888M

5. 10.1021/es020848i