ALBERT

Description: To investigate the energy, matter and reactive and non-reactive trace gas exchange between the atmosphere and a spruce forest in the German mountain region, two intensive measuring periods were conducted at the FLUXNET site DE-Bay (Waldstein-Weidenbrunnen) in September/October 2007 and June/July 2008. They were part of the project "ExchanGE processes in mountainous Regions" (EGER). Beyond a brief description of the experiment, the main focus of the paper concerns the coupling between the trunk space, the canopy and the above-canopy atmosphere. Therefore, relevant coherent structures were analyzed for different in- and above canopy layers, coupling between layers was classified according to already published procedures, and gradients and fluxes of meteorological quantities as well as concentrations of non-reactive and reactive trace compounds have been sorted along the coupling classes. Only in the case of a fully coupled system, it could be shown, that fluxes measured above the canopy are related to gradients between the canopy and the above-canopy atmosphere. Temporal changes of concentration differences between top of canopy and the forest floor, particularly those of reactive trace gases (NO, NO2, O3, and HONO) could only be interpreted on the basis of the coupling stage. Consequently, only concurrent and vertically resolved measurements of micrometeorological (turbulence) quantities and fluxes (gradients) of trace compounds will lead to a better understanding of the forest-atmosphere interaction.

Description: 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.

Description: This paper describes the background, instrumentation, goals, and the regional influences on the HUMPPA-COPEC intensive field measurement campaign, conducted at the Boreal forest research station SMEAR II (Station for Measuring Ecosystem-Atmosphere Relation) in Hyytiälä, Finland from 12 July–12 August 2010. The prevailing meteorological conditions during the campaign are examined and contrasted with those of the past six years. Back trajectory analyses show that meteorological conditions at the site in 2010 were characterized by a higher proportion of southerly flow than in the other years studied. As a result the summer of 2010 was anomalously warm and high in ozone making the campaign relevant for the analysis of possible future climates. A comprehensive land use analysis, provided on both 5 and 50 km scales, shows that the main vegetation types surrounding the site on both the regional and local scales are: coniferous forest (Scots pine and/or Norway spruce); mixed forest (Birch and conifers); and woodland scrub (e.g. Willows, Aspen); indicating that the campaign results can be taken as representative of the Boreal forest ecosystem. In addition to the influence of biogenic emissions, the measurement site was occasionally impacted by sources other than vegetation. Specific tracers have been used here to identify the time periods when such sources have impacted the site namely: biomass burning (acetonitrile and CO), urban anthropogenic pollution (pentane and SO2) and the nearby Korkeakoski sawmill (enantiomeric ratio of chiral monoterpenes). None of these sources dominated the study period, allowing the Boreal forest summertime emissions to be assessed and contrasted with various other source signatures.

Description: To investigate the energy, matter and reactive and non-reactive trace gas exchange between the atmosphere and a spruce forest in the German mountain region, two intensive measuring periods were conducted at the FLUXNET site Waldstein-Weidenbrunnen in September/October 2007 and June/July 2008. They were part of the project "ExchanGE processes in mountainous Regions" (EGER). Beyond a brief description of the experiment and links to the already published results of both experiments, the main focus of the paper is the problem of the coupling of the trunk space, the canopy and the atmosphere. Therefore, the relevant coherent structures were analyzed in different canopy levels and an already published coupling classification was applied to gradients and fluxes. It could be shown that fluxes above the canopy are only related to the gradient between the canopy and the atmosphere in the case of a fully coupled system. Changes in the concentration of especially reactive trace gases (NO-NO2-O3 and HONO) could only be interpreted together with the coupling stage. Finally it was pointed out that the combination of air chemical measurements with micrometeorological turbulence measurements is urgently needed to understand the biosphere-atmosphere interaction.

Description: This paper describes the background, instrumentation, goals, and the regional influences on the HUMPPA-COPEC intensive field measurement campaign, conducted at the Boreal forest research station SMEAR II (Station for Measuring Ecosystem-Atmosphere Relation) in Hyytiälä, Finland from 12 July–12 August 2010. The prevailing meteorological conditions during the campaign are examined and contrasted with those of the past six years. Back trajectory analyses show that meteorological conditions at the site were characterized by a higher proportion of southerly flow. As a result the summer of 2010 was anomalously warm and high in ozone making the campaign relevant for the analysis of possible future climates. A comprehensive land use analysis, provided on both 5 and 50 km scales, shows that the main vegetation types surrounding the site on both the regional and local scales are: coniferous forest (Scots pine and/or Norway spruce); mixed forest (Birch and conifers); and woodland scrub (e.g. Willows, Aspen); indicating that the campaign results can be taken as representative of the Boreal forest ecosystem. In addition to the influence of biogenic emissions, the measurement site was occasionally impacted by sources other than vegetation. Specific tracers have been used here to identify the time periods when such sources have impacted the site namely: biomass burning (acetonitrile and CO), urban anthropogenic pollution (pentane and SO2) and the nearby Korkeakoski sawmill (enantiomeric ratio of chiral monoterpenes). None of these sources dominated the study period, allowing the Boreal forest summertime emissions to be assessed and contrasted with various other source signatures.

Description: 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 good agreement. 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 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. = 1.04 ± 0.16), while HO2 mixing ratios are significantly underpredicted (HO2mod./HO2obs. = 0.3 ± 0.2) and simulated OH reactivity does not match the observed OH reactivity. The simultaneous underprediction of HO2 and OH reactivity in periods in which OH concentrations were simulated well, suggests that the missing OH reactivity is an unaccounted 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. Nevertheless, a major fraction of the OH recycling occurs via the reaction of HO2 with NO and O3 in this terpene dominated environment.

Description: Atmospheric concentrations of nitrous acid (HONO), one of the major precursors of the hydroxyl radical (OH) in the troposphere, normally exceed by far the values predicted by the assumption of a photostationary state (PSS) during daytime. Therefore, additional sources of HONO were intensively investigated in the last decades. Here, we present budget calculations of HONO based on simultaneous measurements of all relevant species including HONO and OH at two different measurement heights, i.e. 1 m above ground and about 2 to 3 m above canopy (24 m above ground), conducted in boreal forest environment. We observed mean HONO concentrations during daytime of about 6.5 × 108 molecules cm−3 (26 ppt), more than twenty times higher than expected from the PSS, 0.2 × 108 molecules cm−3 (1 ppt). To close the budgets in both heights a strong additional source term during daytime is required. This unidentified source is maximal at noon (up to 1.1 × 106 molecules cm−3 s−1, 160 ppt h−1) and in general up to 2.3 times stronger above the canopy than close to the ground. The insignificance of known gas phase reactions and also other processes like dry deposition or advection compared to the photolytic decomposition of HONO at this measurement site was an ideal prerequisite to study possible correlations of this unknown term to proposed HONO sources. But neither the proposed emissions from soils nor the proposed photolysis of adsorbed HNO3 contributed substantially to the unknown source. However, the unknown source was found to be perfectly correlated to the unbalanced photolytic loss of HONO.

Description: We present an automated dynamic chamber system which is optimised for continuous unattended flux measurements of multiple non-reactive and reactive trace gases on grassland ecosystems. Main design features of our system are (a) highly transparent chamber walls consisting of chemically inert material, (b) individual purging flow units for each chamber, and (c) a movable lid for automated opening and closing of the chamber. The purging flow rate was chosen high enough to keep the mean residence time of the chamber air below one minute. This guarantees a proven efficient mixing of the chamber volume and a fast equilibration after lid closing. The dynamic chamber system is able to measure emission as well as deposition fluxes of trace gases. For the latter case, the modification of the turbulent transport by the chamber (compared to undisturbed ambient conditions) is quantitatively described by a bulk resistance concept. Beside a detailed description of the design and functioning of the system, results of field applications at two grassland sites are presented. In the first experiment, fluxes of five trace gases (CO2, H2O, NO, NO2, O3) were measured simultaneously on small grassland plots. It showed that the dynamic chamber system is able to detect the characteristic diurnal cycles with a sufficient temporal resolution. The results also demonstrated the importance of considering the chemical source/sink in the chamber due to gas phase reactions for the reactive compounds of the NO-NO2-O3 triad. In a second field experiment, chamber flux measurements of CO2 and methanol were compared to simultaneous independent eddy covariance flux measurements on the field scale. The fluxes obtained with the two methods showed a very good agreement indicating a minimal disturbance of the chambers on the physiological activity of the enclosed vegetation.

Description: Atmospheric measurements of hydroxyl radicals (OH) are challenging due to a high reactivity and consequently low concentration. The importance of OH as an atmospheric oxidant has motivated a sustained effort leading to the development of a number of highly sensitive analytical techniques. Recent work has indicated that the laser-induced fluorescence of the OH molecules method based on the fluorescence assay by gas expansion technique (LIF-FAGE) for the measurement of atmospheric OH in some environments may be influenced by artificial OH generated within the instrument, and a chemical method to remove this interference was implemented in a LIF-FAGE system by Mao et al. (2012). While it is not clear whether other LIF-FAGE instruments suffer from the same interference, we have applied this method to our LIF-FAGE HORUS (Hydroxyl Radical Measurement Unit based on fluorescence Spectroscopy) system, and developed and deployed an inlet pre-injector (IPI) to determine the chemical zero level in the instrument via scavenging the ambient OH radical. We describe and characterise this technique in addition to its application at field sites in forested locations in Finland, Spain and Germany. Ambient measurements show that OH generated within the HORUS instrument is a non-negligible fraction of the total OH signal, which can comprise 30 to 80% during daytime and 60 to 100% during the night. The contribution of the background OH varied greatly between measurement sites and was likely related to the type and concentration of volatile organic compounds (VOCs) present at each particular location. Two inter-comparisons in contrasting environments between the HORUS instrument and two different chemical ionisation mass spectrometers (CIMS) are described to demonstrate the efficacy of IPI and the necessity of the chemical zeroing method for our LIF-FAGE instrument in such environments.

Description: We present an automated dynamic chamber system which is optimised for continuous unattended flux measurements of multiple non-reactive and reactive trace gases on grassland ecosystems. Main design features of our system are (a) highly transparent chamber walls consisting of chemically inert material, (b) individual purging flow units for each chamber, and (c) a movable lid for automated opening and closing of the chamber. The purging flow rate was chosen high enough to keep the mean residence time of the chamber air below one minute. This guarantees a proven efficient mixing of the chamber volume and a fast equilibration after lid closing. The dynamic chamber system is able to measure emission as well as deposition fluxes of trace gases. For the latter case, the modification of the turbulent transport by the chamber (compared to undisturbed ambient conditions) is quantitatively described by a bulk resistance concept. Beside a detailed description of the design and functioning of the system, results of field applications at two grassland sites are presented. In the first experiment, fluxes of five trace gases (CO2, H2O, NO, NO2, O3) were measured simultaneously on small grassland plots. It showed that the dynamic chamber system is able to detect the characteristic diurnal cycles with a sufficient temporal resolution. The results also demonstrated the importance of considering the chemical source/sink in the chamber due to gas phase reactions for the reactive compounds of the NO-NO2-O3 triad. In a second field experiment, chamber flux measurements of CO2 and methanol were compared to simultaneous independent eddy covariance flux measurements on the field scale. The fluxes obtained with the two methods showed a very good agreement indicating a minimal disturbance of the chambers on the physiological activity of the enclosed vegetation.