Abstract
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, OHLIF / OHCIMS = (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 (OHmod. / OHobs. = 1.00 ± 0.16), while HO2 mixing ratios are significantly under-predicted (HO2mod. / HO2obs. = 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 HOx 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 HOx 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.
Highlights
The removal of pollutants and trace gases from the atmosphere is mostly initiated by oxidation processes
The HORUS instrument was operated for the first time using the IPI-laser-induced fluorescence (LIF)-FAGE technique to measure OH during HUMPPA-COPEC-2010
To identify possible measurement interferences, an instrument comparison between the chemical ionization mass spectrometry (CIMS) and HORUS-LIF instruments was conducted at the beginning of the campaign on the ground
Summary
The removal of pollutants and trace gases from the atmosphere is mostly initiated by oxidation processes. The main primary source of OH on a global scale is the photolysis of ozone (O3) and the subsequent reaction of the excited oxygen atom with water vapour (Levy II, 1971): O3 + hν → O2 + O(1D), λ < 320 nm, (R1). Once formed, OH reacts rapidly – with a lifetime of typically less than 1 s. Many atmospheric compounds (e.g. CO or O3) subsequently produce hydroperoxyl radicals (HO2). The oxidation of hydrocarbons by OH leads to the formation of peroxy radicals (ROx = RO2 + HO2). In the presence of nitric oxide (NO), RO2 is converted to HO2, which reacts with O3 or NO forming OH (e.g. Martinez et al, 2003). The main HOx (= OH + HO2) sinks are self-reactions with radicals and the formation of acids and peroxides
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