Observation and modelling of HO_x radicals in a boreal forest

Abstract. Measurements of OH and HO₂ radicals were conducted in a pine-dominated forest in southern Finland during the HUMPPA-COPEC-2010 (Hyytiälä United Measurements of Photochemistry and Particles in Air – Comprehensive Organic Precursor Emission and Concentration study) field campaign in summer 2010. Simultaneous side-by-side measurements of hydroxyl radicals were conducted with two instruments using chemical ionization mass spectrometry (CIMS) and laser-induced fluorescence (LIF), indicating small systematic disagreement, OH_LIF / OH_CIMS = (1.31 ± 0.14). Subsequently, the LIF instrument was moved to the top of a 20 m tower, just above the canopy, to investigate the radical chemistry at the ecosystem–atmosphere interface. Comprehensive measurements including observations of many volatile organic compounds (VOCs) and the total OH reactivity were conducted and analysed using steady-state calculations as well as an observationally constrained box model.

Production rates of OH calculated from measured OH precursors are consistent with those derived from the steady-state assumption and measured total OH loss under conditions of moderate OH reactivity. The primary photolytic sources of OH contribute up to one-third to the total OH production. OH recycling, which occurs mainly by HO₂ reacting with NO and O₃, dominates the total hydroxyl radical production in this boreal forest. Box model simulations agree with measurements for hydroxyl radicals (OH_mod. / OH_obs. = 1.00 ± 0.16), while HO₂ mixing ratios are significantly under-predicted (HO₂^mod. / HO₂^obs. = 0.3 ± 0.2), and simulated OH reactivity does not match the observed OH reactivity. The simultaneous under-prediction of HO₂ and OH reactivity in periods in which OH concentrations were simulated realistically suggests that the missing OH reactivity is an unaccounted-for source of HO₂.

Detailed analysis of the HO_x production, loss, and recycling pathways suggests that in periods of high total OH reactivity there are additional recycling processes forming OH directly, not via reaction of HO₂ with NO or O₃, or unaccounted-for primary HO_x sources. Under conditions of moderate observed OH reactivity and high actinic flux, an additional RO₂ source of approximately 1 × 10⁶ molec cm⁻³ s⁻¹ would be required to close the radical budget. Nevertheless, a major fraction of the OH recycling occurs via the reaction of HO₂ with NO and O₃ in this terpene-dominated environment.