Observation and modelling of HO<sub>x</sub> radicals in a boreal forest

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.

Volatile organic compounds (VOCs) in photochemically aged air from the eastern and western Mediterranean

“…For further information please refer to Novelli et al (2014); Hens et al (2014); Martinez et al (2010). The precision for OH measurements is 4.8 × 10 5 molec cm −3 for 4 min time resolution.…”

Comment on "Observation and modelling of HO<sub>x</sub> radicals in a boreal forest" by Hens et al. (2014)

“…Firstly, the reference of Sinha et al (2008) must be an error, since that refers to a publication about the method development of the comparative reactivity method (CRM) together with first field tests conducted in the tropical rainforest of Suriname and the urban atmosphere of Mainz -but not in a boreal forest. We assume that Hens et al (2014) instead are thinking of the paper by Sinha et al (2010). Further, it is correct that Mogensen et al (2011) found a large fraction of unaccounted OH reactivity; however, they did not reach that conclusion using a box model.…”

“…Hens et al (2014) write "Studies on oxidation processes in monoterpene-dominated environments are rare. Direct OH reactivity measurements in a boreal forest, conducted by Sinha et al (2008), and a box model study investigating the OH reactivity budget (Mogensen et al, 2011) revealed a significant fraction of 'unknown OH reactivity'."…”

Oxidation processes in the Eastern Mediterranean atmosphere: Evidence from the Modelling of HO<sub>x</sub> Measurements over Cyprus

“…This scavenger, 3-5 sccm of pure propane in a carrier flow of 8000 sccm synthetic air for CYPHEX, was introduced through an 5 inlet pre-injector (IPI) mounted on top of the inlet nozzle (Novelli et al, 2014a;Hens et al, 2014). The scavenger amount was just sufficient to scavenge off ~95% of atmospheric OH as determined from propane titration experiments on-site.…”

“…To minimize impact of RO2 on OH formation (Fuchs et al, 2011;Hens et al, 2014), only about 30 % of atmospheric HO2 was converted to OH using NO. The required NO flow, about 0.5 sccm pure NO in 10 sccm carrier flow during CYPHEX, was determined from titrations in ambient and calibration air.…”