Observation and modelling of HO<sub>x</sub> 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, 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 x 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.

Estimating the atmospheric concentration of Criegee intermediates and their possible interference in a FAGE-LIF instrument

“…(Junninen et al, 2009). Further details and a more complete description of the site, the instrumentation, and the meteorological conditions during the campaign can be found in Williams et al (2011) and Hens et al (2014). A brief description of the instruments used in this study is given here.…”

“…The OH calibration of the HORUS instrument is obtained via the production of a known amount of OH and hydroperoxyl radicals (HO 2 ) from the photolysis of water at 185 nm using a mercury lamp. A more detailed description of the instrument calibration is reported by Martinez et al (2010) and Hens et al (2014). A calibration factor for the background OH signal observed by the HORUS instrument is currently not available.…”

“…CRM uses an in situ kinetics experiment to measure the OH reactivity based on the competitive scavenging of OH by a reference gas (pyrrole) and atmospheric OH reactants. The overall uncertainty of the method during deployment was 16 %, with a limit of detection of 3.0 s −1 (Hens et al, 2014). Sulfur dioxide (SO 2 ) concentration was measured with a fluorescence analyser (model 43S, Thermo 20 Environmental Instruments Inc.).…”

“…The budget of SCIs is analysed using four different approaches: (1) based on an unexplained H 2 SO 4 production rate (Mauldin III et al, 2012); (2) from the measured concentrations of unsaturated volatile organic compounds (VOCs); (3) from the observed OH reactivity (Nölscher et al, 2012); and (4) from an unexplained production rate of OH (Hens et al, 2014). Secondly, we present measurements obtained using our inlet pre-injector laser-induced fluorescence assay by gas expansion technique (IPI-LIF-FAGE) (Novelli et al, 2014a) during the HUMPPA-COPEC 2010 and the HOPE 2012 campaigns.…”

Investigation of the oxidation of methyl vinyl ketone (MVK) by OH radicals in the atmospheric simulation chamber SAPHIR

“…Field studies, in which OH reactivity was dominated by monoterpenes, showed that models underestimate HO 2 and OH concentrations (Kim et al, 2013;Hens et al, 2014). A chamber study investigating the photochemistry of β−pinene found that an additional HO 2 source is required to explain observed HO 2 and OH and suggested a rearrangement of radical intermediates as explanation (Kaminski et al, 2017).…”