Measurements of HONO in a Forested Region of Alberta Impacted by Oil Sands Processing Facilities

Abstract:

During the summer of 2013, measurements of NO₂, SO₂ and HONO were made at a ground site in a rural forested region of Alberta known to be impacted by several oil sands processing facilities. Such sources are known to be the main sources of NO_x and SO_x emissions in the region. Nitrous acid (HONO) can be emitted directly from combustion sources in addition to being formed from the heterogeneous hydrolysis of NO₂ on moist surfaces. The measurements were made both day and night by Differential Optical Absorption Spectroscopy (DOAS) along a 2.304 km optical return path through the forest canopy using a 30-corner cube reflector. Nitrous acid (HONO) is of particular relevance to atmospheric chemistry as it can contribute to photochemical production of the OH radical, the main daytime oxidant in the troposphere, which was estimated to exceed 2∙10⁷ molec cm^-3 on several days during the study, giving rise to rapid aerosol formation. Typically in urban areas HONO mixing ratios accumulate during the night, and are lost the next morning due to photolysis (HONO + hv→ OH + NO₂). During this study however, HONO mixing ratios were found to be quite low with a median nightime mixing ratio of ~ 200 ppt. This is attributed to the very low nighttime NO₂ precursor levels, perhaps in combination with a high surface area for deposition on the coniferous canopy. The low nighttime mixing ratios of NO₂ and SO₂ were driven by limited nocturnal mixing of elevated plumes down to the surface, while high daytime levels of NO₂ and SO₂ were episodic and frequently driven by daytime fumigation of the elevated plumes to the surface with additional contribution from surface sources of NO_x. Daytime mixing ratios of HONO persisted longer than expected with median noontime mixing ratios of >50ppt. While this can imply an additional daytime source of HONO as has been observed by many others, it may also result from reduced photolysis rates below the canopy. This issue is being pursued more quantitatively at the current time.