The primary and recycling sources of OH during the NACHTT-2011 campaign: HONO as an important OH primary source in the wintertime

The results suggest that known chemistry cannot constrain HONO and, subsequently, OH during the observational period. When HONO is constrained in the model by observations (< 50 m), the discrepancy between observation and model simulation improves to a factor of 1.3 in the morning and a factor 1.1 in the afternoon, within the 35% estimated instrumental uncertainty. However, the model produces both a morning and afternoon maximum in OH, in contrast to the observations, which show strong evidence for morning OH production but no distinct morning maximum.

Two additional OH sources were also considered, although they do not improve the differences in modeled and measured temporal OH profiles. First, the impact of daytime HONO gradients near the ground surface (< 20 m) was evaluated. Strong HONO gradients were observed between 06:00 and 09:00 MST (mountain standard time), especially within 20 m of the surface.

When constrained to HONO observed below 20 m (rather than 50 m), the model produced an even larger morning OH maximum, in contrast to the observations. Second, Cl atoms from ClNO₂ photolysis producing RO₂ from reaction with alkanes, while significant, produced steady state Cl atom levels (~ 10³ atoms cm⁻³) that were too low to significantly perturb measured OH through reactions of organic peroxy radicals produced from Cl reactions with volatile organic compounds.