Nitrous acid (HONO) during polar spring in Barrow, Alaska: A net source of OH radicals?

HONO was measured by a LOPAP instrument (LOng Path Absorption Photometer) for one month during the OASIS spring 2009 campaign in Barrow, Alaska. HONO concentrations between ≤ 0.4 pptv (DL) and ∼500 pptv were measured. The very high concentrations observed on several days were caused by local direct emissions and were highly correlated with the NOx and CO data. When only "clean days" were considered, average HONO concentrations varied between ≤ 0.4 - 10 pptv. Average HONO/NOx and HONO/NOy ratios of ∼6% and ∼1% were observed, respectively, in good agreement with other remote LOPAP measurement data, but lower than measured in most other polar regions by other methods. The strong correlation between sharp peaks of OH and HONO during daytime, which was not observed for any other measured radical precursor, suggested that HONO photolysis was a major source of OH radicals in Barrow. This was supported by calculated net OH radical production by HONO and O₃ photolysis for which the contribution of O₃ (2%) could be neglected compared to that of HONO (98%). A net extra HONO/OH source necessary to explain elevated HONO levels during daytime of up to 90 pptv/h was determined, which was highly correlated with the actinic flux. Accordingly, a photochemical HONO source is proposed here, in good agreement with recent studies. From the higher correlation of the net HONO source with JNO₂ and [NO₂] compared to Jα(₁D) and [NO³-], photosensitized conversion of NO2 on humic acid containing snow surfaces may be a more likely source of HONO in the polar atmosphere of Barrow than nitrate photolysis.