A new plant chamber facility, PLUS, coupled to the atmosphere simulation chamber SAPHIR

. A new PLant chamber Unit for Simulation (PLUS) for use with the atmosphere simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) has been built and characterized at the Forschungszentrum Jülich GmbH, Germany. The PLUS chamber is an environmentally controlled ﬂow-through plant chamber. Inside PLUS the natural blend of biogenic emissions of trees is mixed with synthetic air and transferred to the SAPHIR chamber, where the atmospheric chemistry and the impact of biogenic volatile organic compounds (BVOCs) can be studied in detail. In PLUS all important environmental parameters (e.g., temperature, photosynthetically active radiation (PAR), soil relative humidity (RH)) are well controlled. The gas exchange volume of 9.32 m 3 which encloses the stem and the leaves of the plants is constructed such that gases are exposed to only ﬂuorinated ethylene propy-lene (FEP) Teﬂon ﬁlm and other Teﬂon surfaces to minimize any potential losses of BVOCs in the chamber. Solar radiation is simulated using 15 light-emitting diode (LED) panels, which have an emission strength up to 800 µmol m − 2 s − 1 . Results of the initial characterization experiments are presented in detail. Background concentrations, mixing inside the gas exchange volume, and transfer rate of volatile organic compounds (VOCs) through PLUS under different humidity conditions are explored. Typical plant characteristics such as light- and temperature- dependent BVOC emissions are studied using six Quercus ilex trees and compared to previous studies. Results of an initial ozonolysis experiment of BVOC emissions from Quercus ilex at typical atmospheric concentrations inside SAPHIR are presented to demonstrate a typical experimental setup and the utility of the newly added plant chamber.


Introduction
Volatile organic compounds (VOCs) have an impact on atmospheric chemistry and are involved in O 3 , oxygenated VOC (OVOC), and secondary organic aerosol (SOA) formation (Seinfeld and Pandis, 1998).Biogenic VOCs (BVOCs) account for the majority (∼ 90 %) of annually emitted VOCs globally (Guenther, 2002) with isoprene identified as the single most important BVOC (Steiner and Goldstein, 2007).Despite numerous activities exploring the impact of BVOCs on atmospheric chemistry, many uncertainties remain.For instance, isoprene has only recently been found a key species in OH recycling in low-NOx environments (Hofzumahaus et al., 2009;Lelieveld et al., 2008), with exact processes leading to this recycling still being uncertain.Isoprene was found to suppress new particle formation from BVOC photooxidation (Kiendler-Scharr et al., 2009, 2012), while constitutive and stress-induced mono-and sesquiterpene emissions contribute to photochemical particle number and mass formation (Mentel et al., 2009(Mentel et al., , 2013;;Hallquist et al., 2009).Also field studies have repeatedly shown that the total reactivity of air masses towards OH is larger than can be explained, based on measurements of individual compounds reactive to OH (e.g., Edwards et al., 2013;Nölscher et al., 2012;Whalley et Published by Copernicus Publications on behalf of the European Geosciences Union.

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The new plant chamber facility PLUS al., 2011;Di Carlo et al., 2004).This gap in OH reactivity may be explained either by direct emissions or atmospheric oxidation products that are currently not identified as single compounds.It is plausible that large sources of organic compounds have not yet been identified (Hallquist et al., 2009;Goldstein and Galbally, 2007).This lack of understanding of BVOC sources may limit bottom-up estimates of SOA formation and OVOCs such as acetone.
Due to the importance of BVOCs for atmospheric chemistry and the large remaining gaps in process level understanding, BVOCs are a subject of continuous research.One concern with investigations of BVOCs and their impact on atmospheric chemistry arises from the fact that, under natural conditions, BVOCs are emitted as complex blends, whereas many simulation chamber experiments use single BVOCs or simple mixtures to explore atmospheric chemistry processes.The use of direct emissions from plants is a way of progressing towards more realistic experimental simulations.Recently a number of setups implementing the use of direct plant emissions in atmospheric chemistry experiments have emerged, ranging from the investigation of BVOC oxidation within a greenhouse (Pinto et al., 2007;Joutsensaari et al., 2005) to designs implementing two separate chambers for independent variation of emission and oxidation conditions (Timkovsky et al., 2014;Wyche et al., 2014;Hao et al., 2009;Mentel et al., 2009;VanReken et al., 2006).
Here we present details of the design and implementation of a PLant chamber Unit for Simulation (PLUS) for use with our atmosphere simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber).The PLUS chamber setup is described in detail and characterization experiments quantifying VOC transfer efficiencies and plant emission behavior as a function of key physiological parameters are discussed.An example of an ozonolysis experiment in SAPHIR using PLUS as a BVOC source is given to show how such experiments can be used to quantify BVOC impacts on atmospheric composition.

Facility description
Experiments were conducted in the atmosphere simulation chamber SAPHIR, coupled with a newly developed plant chamber, PLUS, in Jülich, Germany.In brief SAPHIR consist of a double-walled Teflon (FEP) film of cylindrical shape (volume 270 m 3 ) and is used for simulation experiments studying atmospheric chemistry processes under atmospherically relevant conditions.Representative concentration ranges for experiments are ensured by using ultrapure zero air, generated from evaporated liquid nitrogen and oxygen (Linde, purity 99.99990 %).To this clean, dry air, trace gases are added according to experimental purpose.The SAPHIR chamber is described in more detail elsewhere (Schlosser et al., 2009;Bohn et al., 2005;Rohrer et al., 2005).
The PLUS facility consists of three containers, which are installed beneath the SAPHIR chamber.The first container is the gas exchange chamber, which houses the plants.When used for experiments with SAPHIR, this chamber is operated with air from the Linde ultra-high purity zero air supply.A second container houses the process control system including standard instrumentation monitoring plant chamber environmental conditions.The third container houses a system to purify ambient air from NO x , O 3 , VOCs, and particles.This air can be used to supply the gas exchange chamber with cleaned outside air instead of zero air.All three components of PLUS are described in more detail in the following sections.

PLUS plant container design and implementation
A schematic diagram of the container housing the PLUS plant container is shown in Fig. 1.The custom-built container (Ilkazell Isoliertechnik GmbH, Zwickau, Germany) is gastight and temperature-and light-controlled with outer dimensions of 2.39 m × 3.84 m × 2.84 m.Special emphasis of the dynamic plant chamber setup was on contamination-free operation, with some of the contamination prevention strategies being adopted from SAPHIR.The walls of the container have a layered structure to provide good thermal insulation while limiting the risk of outgassing from wall materials (see Fig. 2).From the outside to the inside the wall is built of (i) an outer steel housing, (ii) a spray foam insulation of 120 mm thickness, (iii) a stainless steel wall, (iv) an air buffer volume, and (v) a gastight welded stainless steel containment.The side and back walls of this inner containment are made of double-walled (pre-inflated) stainless steel heat exchange panels.Temperature control inside the container is achieved by adjusting the temperature (0 to 50 • C) of the water-based thermofluid with an externally installed heating/cooling unit (Unistat Chiller, type 055T-H-TP35, Huber, Germany).
The inner volume of the container is divided in three hermetically separated compartments: (i) a light control unit, (ii) the gas exchange unit, and (iii) an interspace also housing the soil.These three compartments are described in more detail in the following.
The light control unit is hermetically separated from the gas exchange unit by means of five glass window panes.Window pane gaskets are made of Viton.The light needed for photosynthesis activity is provided by 15 LED panels (type SL3500, Photon Systems Instruments, Czech Republic).Five rows with three LED panels in each row are installed on a movable frame, ensuring homogenous illumination of the chamber.The spectrum of the LED panels has a bimodal bell-shaped characteristic with the first mode ranging from 400 to 480 nm with a maximum at 440 nm, and the second mode ranging from 480 to 700 nm with a maximum at 570 nm.The intensity of each LED panel can be controlled individually by the process control unit.The intensity of photosynthetically active radiation (PAR) is 0 to 800 µmol m −2 s −1 , as measured at a distance of 1 m.The compartment for the light control unit is air-conditioned by means of an external air cooling system (5.5 kW, Robert Schiessel GmbH, Germany).Recirculating air through the external cooler sustains an (adjustable) continuous underpressure of ∼ 200 Pa with respect to the gas exchange unit, to ensure mass transfer out of the gas exchange unit in case of any leakage on the window panes.An ultrasonic sound leak test proved the tightness of the glass windows.
The gas exchange unit is setup as follows.A rectangular aluminum frame inside the chamber provides the frame for the 1.80 m × 3.52 m × 1.45 m gas exchange volume.Teflon film (FEP, 75 µm thickness, Vector Foiltec, Bremen, Germany) is mounted across each side and the ceiling of the frame separately.The side wall and the ceiling of the frame are assembled such that the inside of the frame is exposed to the FEP film only.The bottom part of the frame is constructed as a table-like structure where the surface of the table, except for the holes where the plant stem and leaves reach into the gas exchange volume, is also completely covered with the FEP film.The seal of this table against the frame is achieved by a silicone-based pneumatic gasket (Flohreus GmbH, Germany), which is installed along a groove in the rim of the table.While inflated (up to 2 bar) the pneumatic gasket presses the FEP film of the table against the FEP film of the frame.When plants are installed in the gas exchange chamber, the stem and leaves protrude through one of six holes in the table into the gas exchange volume, while the soil is kept beneath the table.Each hole is sealed by fusing a rectangular piece of the FEP film around the stem of a plant, separating the soil from the upper parts of the plant.
The piece of rectangular FEP film is then pressed onto the FEP film covering the table using a Teflon ring with a Viton O-ring.
The space around the sealed gas exchange unit is called the interspace.The interspace houses all necessary connections for the installed sensors (e.g.light and temperature sensors), plant pots including soil, tubing for the air supply, and the irrigation system.To ensure a low gaseous contaminant level, all cables and tubing are either made of Teflon or have a Teflon cover.The plant pots are made of stainless steel.The interspace is furthermore flushed with high purity air (up to 1 m 3 h −1 ) and kept at 31 Pa below the gas exchange unit to avoid transfer of gases from this compartment to the gas exchange unit.
The new plant chamber PLUS is run in dynamic mode, which means that it is continuously flushed with purge air to (i) sustain CO 2 for photosynthetic activity of enclosed plants and (ii) allow purging of emitted trace gases and water vapour.The air supply (see Sect. 2.3 for details) is distributed inside the gas exchange unit using six Teflon tubes with an inner diameter (ID) of 0.5 in.Each tube is connected to a Teflon nozzle (0.25 in.ID).Three nozzles are distributed along each side of the plant table, introducing synthetic air into the gas exchange volume.Through the nozzles, flow of air into the gas exchange unit is turbulent, thus ensuring homogeneous mixing.
Air from PLUS can be transferred to SAPHIR via a transfer line tube with an ID of 80 mm and a length of 1.89 m that connects the back plane of the gas exchange unit with the bottom plane of SAPHIR (see Fig. 1).The tube is made of perfluoroalkoxy (PFA) and temperature-controlled (typically 60 • C).Directly prior to the inlet to SAPHIR, two wide-bore valves are situated: one controls the flow towards SAPHIR, the other towards ambient.When only PLUS-related experiments are conducted, the airflow is usually directed to the ambient.To prevent pressure fluctuations within the plant chamber by valve switching, the opening width (resistance) of the exhaust valve can be adjusted by a pneumatic positioner to mimic the internal counter-pressure of SAPHIR.The adjustable positioners implemented in both of the transfer tube valves also allow dynamic operation of SAPHIR, i.e., only a fraction of VOC-loaded PLUS air may be introduced into SAPHIR, while the residual can be vented to ambient.When plants are installed in PLUS, continuous flow through the gas exchange unit ensures that the plants are supplied with CO 2 and that the emissions of the plants are continuously flushed out of the gas exchange unit.
The flow through the gas exchange unit is adjustable between 0 and 42 m 3 h −1 .Normal operation flows are usually between 6 and 30 m 3 h −1 .The pressure inside the gas exchange unit is usually kept at 56 ± 5 Pa over atmospheric pressure.The interspace is also flushed continuously 1 m 3 h −1 from the same air supply.Here, a processcontrolled small-bore proportional valve (type 6024, nominal bore 12 mm, Bürkert GmbH & CO KG, Germany) is

T. Hohaus et al.: The new plant chamber facility PLUS
installed to adjust the pressure difference between the interspace surrounding the gas exchange unit and ambient atmosphere.The pressure in the interspace is kept at a constant pressure of 25 ± 2 Pa over atmosphere.Differential pressures are monitored by means of two fully metal-sealed differential pressure gauges (type Barathon 226A, 200 Pa, MKS, Germany) to control the PLUS pressure gradients.All three valves (plant chamber exhausts and sheath air exhaust) are process-controlled, i.e., the desired pressure differences can be set and are controlled by a feedback control loop in the automatic process control system.The stepwise increase in overpressure from ambient conditions outside the chamber to 56 Pa in the gas exchange unit prevents diffusion of possible contaminants from the outside into the gas exchange unit.The pressure of the PLUS gas exchange unit is always kept 6 ± 2 Pa above the pressure in the SAPHIR chamber.This pressure difference ensures flow from the PLUS gas exchange volume to SAPHIR while PLUS is coupled to SAPHIR.

Air supply and mixing
Two air supply systems can be used for the gas exchange chamber, the SAPHIR air supply and a system utilizing cleaned outside air.As mentioned above the air supply used for SAPHIR is based on mixing O 2 and N 2 from liquid reservoirs (Linde, purity 99.99990 %).This ultra-clean air can be used for PLUS when CO 2 is added to the gas exchange chamber inlet to supply the plants with CO 2 .This air supply is always used in experiments where both chambers are coupled.
Usually for standby operations of the PLUS plant chamber cleaned outside air is used.Ambient air is compressed into a stainless steel buffer volume (500 L, 7 bar) by means of four oil-free scroll pumps (Mark, OFMS 15).The largebore (100 mm ID) inlet tube of the compressor is made from stainless steel and has a recyclable stainless steel mesh filter cartridge (1 µm pore size) for aerosols, which is built into a stainless steel housing installed outdoors at ca. 2 m above ground.The compressed air is dried to a dew point of typically below −40 • C using two columns of molecular sieve and is subsequently scrubbed from VOC contaminants by means of an additional column filled with activated charcoal (both Zander KA-MT 10-95).Highly volatile organic compounds, which might break through this first purification stage, are removed by a second-stage heated VOC converter (Bekokat type CC-120, Beko, Germany).Aerosols are removed by two pure Teflon particle filter units (FEP/PTFE, Fluoroflow 33-14710-002-5-E, with 0.2 µm cut off size and 33-14710-825-5-E with 0.05 µm cut off size, Parker Hannifin GmbH, USA) installed in stainless steel housings (VBA-111-BE, Parker Hannifin GmbH, USA).
Trace gases, e.g., CO 2 , VOCs, and water vapor can individually be added to the air supply.Both VOC standard compounds and CO 2 are added from gas bottles to the synthetic airflow.Mixing ratios of both additions are controlled by two mass flow controllers (Mass Stream 6311, Bronkhorst, Germany).CO 2 concentration at the PLUS inlet is kept at the upper range of naturally occurring conditions of about 500 ppm to maintain vital photosynthesis.Depending on the mass flow rate through the chamber and the amount of leaf biomass installed, typical CO 2 mixing ratios at the outlet (and representative of inside chamber) are about 450 ppm.Water vapor can be added to the purge air by vaporization of purified (Milli-Q, Millipore Corporation) water in a 25 mm outer diameter stainless steel tube with a length of 30 cm which is heated to 350 • C. Adjusting the liquid flow sustained by a peristaltic pump, the relative humidity of the purge air can be varied between 0 and 100 % .

Process control unit and analytical instrumentation
For ease of user operation the PLUS facility is equipped with an automated control process unit and standard instrumentation used to monitor environmental conditions and trace gas exchange.All process control is based on industrial software and hardware (S7, Siemens, Germany).Parameter settings (flow rate and direction, gas addition, light intensity, temperature, pressure gradients, soil water content etc.) can be set by an automatic process control system, or follow preprogrammed adjustable diel courses.For most of these parameters, two-stage (alarm and shut down, respectively) upper and lower threshold levels are preset for cases of maloperation and/or malfunction.
Pressures inside the PLUS chamber are monitored by pressure gauges (MKS Barathon 226A).As mentioned before, transfer of air from the PLUS gas exchange chamber to SAPHIR is ensured by relative overpressures.These are monitored continuously, and upper and lower threshold values are defined as shutdown criteria.Light intensity in the chamber is varied by regulating the output of the LED panels between 0 and 100 %.Monitoring of the light intensity is provided using a quantum sensor (LI-190SZ-50, LI-COR GmbH, Germany) installed inside the gas exchange volume, situated in the center of the plant table at about canopy height of the installed plants.Distributed over the canopy height of the installed plants, a set of six leaf temperature sensors (type K) is applied to measure leaf temperature.Gas temperature inside the gas exchange volume is also measured by a type K temperature sensor which is located approximately in the center (height-, length-and width-wise) of the gas exchange volume.The soil moisture of each plant is measured individually using a moisture sensor (UMS GmbH Munich, EC-5).The irrigation system is automated such that water drips into the plant pot if the soil moisture falls below a threshold value until the soil moisture rises above a defined threshold.
For gas-phase measurements, a heated (60 • C) sampling line (6 mm Teflon tube) is installed 15 cm away from the transfer line tube.The position of the outlet line is chosen such that the measurements at the outlet are representative of

Environmental conditions Value
Temperature stability without influence of LED lamps Temperature stability with influence of LED-Lamps the gas mixture transferred through the transfer line tube.A second sampling line (6 mm Teflon tube) is connected to the PLUS inlet, where the air mixture can be measured before it enters the gas exchange unit.Gas-phase CO 2 and water vapor concentration are measured at the PLUS inlet and outlet by cavity ring down spectroscopy (CRDS, Picarro, model G2301).Measurements for the inlet and outlet are obtained by switching via a valve system between both measurement ports.This allows for continuous measurement of the plants transpiration and respiration rates.VOC mixing ratios are measured using a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS, Ionicon, Austria; Jordan et al., 2009).A valve system is used to switch the measurement point of the PTR-ToF-MS between the PLUS inlet, PLUS outlet, and SAPHIR chamber.Calibrations were performed with diffusion sources.The sensitivity of the instrument as checked by custom-made compressed gas standard containing about 670 ppb of isoprene, α-pinene, acetone, other carbonyl compounds, and aromatics.The gas standard was diluted to mixing ratios from 2 to 13 ppb by means of an advanced liquid calibration unit (LCUa, Ionicon, Austria).For the accuracy of the VOC measurement see Table 2.For VOC specification two GC/MS/FID systems (7890N GC, 5975 B/C MS, Agilent, USA) are directly coupled to SAPHIR (Kaminski, 2014).Samples were taken every 30 min by an online thermal desorption unit (TDSG, Gerstel, Germany).The GC systems were specifically optimized for the quantification of labile BVOCs using silanized surfaces.Ozone was removed in the enrichment system by chemical titration with NO.The substance identification was done by the comparison of the samples with certified standard gas mixtures (Apel-Riemer Environmental Inc., USA).In cases where no authentic standards were available the substances were identified by using the specific mass spectra or in the case of the monoterpenes, in addition to the mass spectra identification, the identity is verified by using Kovats indices.Due to its excellent signal stability, the FID signal was used for compound quantification.

Temperature and flow stability
As mentioned, the temperature inside the PLUS chamber is adjusted and controlled using heat exchangers at three walls of the container.While this system has relatively slow response to changes in temperature set points (1-3 • C h −1 ; see Table 1), it provides good temperature stability in the range 13-35 • C. The system thus allows for the investigation of temperature dependence of biogenic emissions (see below).
Table 1 provides further details on the temperature and flow stability performance of PLUS.

Gas purity and background concentrations
After 2 days of flushing the empty gas exchange chamber with synthetic air from the Linde supply, the purity of gas exiting the gas exchange volume was determined by measuring with the PTR-ToF-MS at the inlet and outlet of the gas exchange chamber (see Fig. 1).The PTR-ToF-MS was switched between measuring 15 min at the PLUS inlet and 105 min at the PLUS outlet for a total duration of 12 h.The difference in concentration between the outlet and inlet measurements determines the type and level of contamination inside of the plant gas exchange volume.A relative increase of 48 pptv of acetone between the PLUS inlet and outlet was detected as being the compound with the largest contribution.Isoprene was also identified with a relative contribution of 14 pptv.No other significant contamination could be detected.The most likely source for these compounds inside the plant gas exchange volume is the silicon-based pneumatic seal, which seals the plant table against the frame of the gas exchange volume.Overall, contaminant concentrations in the lower ppt range are about 3 orders of magnitude lower than VOCs emitted biogenically, and can be considered negligible for the purpose of our atmosphere simulation experiments.

Mixing in the PLUS gas exchange volume
To ensure quantitative transfer of plant emitted BVOCs to the SAPHIR simulation chamber, turbulent conditions in the gas exchange unit are sought.Mixing behavior in the gas exchange chamber was studied by the flush out behavior of CO 2 , acetone, α-pinene, and nopinone.CO 2 was used as a conservative, chemically inert tracer not interacting with wall surfaces.For this experiment the chamber was purged with a relatively low airflow rate (100 L min −1 ) to exert a relatively low air speed in the inlet nozzles and long residence time within the chamber of about 90 min.CO 2 and VOCs were added to the flow of synthetic air until constant concentrations were reached inside the gas exchange volume.Concentrations for CO 2 , acetone, α-pinene and nopinone were 455 ppm, 83, 78, and 86 ppb, respectively.Then addition of CO 2 and VOCs to the synthetic air was stopped, while the flow of synthetic air was kept constant at 100 L min −1 .As-   suming perfect mixing the trace gas concentration should depend on the flow of synthetic air in the chamber and the volume of the chamber only and is expected to follow an exponential decrease.
with C t being the trace gas concentration at the time t, C t−1 the concentration at the previous time step, Q PLUS,t the flow through the gas exchange volume at time t, V PLUS the volume of the gas exchange volume.Figure 3 shows the change in concentration, ln(C t ) vs. time for CO 2 , acetone, α-pinene and nopinone in the gas exchange volume as measured and calculated.Inhomogeneous mixing in the gas exchange volume (e.g., by stratification or air pockets/regions inside the chamber, where CO 2 -rich or VOC-rich air remains) would result in deviation from the expected exponential concentration decrease.The ln(C t ) decreased linearly over a period of approximately 8 h, equivalent to exchanging the gas volume in the chamber five times.No significant deviations from the exponential decrease in the trace gas concentration could be observed during this time period.Therefore we can assume that the gas exchange chamber is well mixed by using turbulent mixing from the six injection nozzles in the plant table.Note that no significant difference between CO 2 and VOCs was observed, indicating that, in the time range explored, VOCs are not lost on chamber walls.Additionally the CO 2 flush out was also used to determine the effective volume of the gas exchange chamber.By applying a linear fit to the CO 2 concentration and using an average of the flow into the chamber during the first 5 h of the experiment, the effective volume of the chamber was determined to 9.32 ± 0.5 m 3 , in agreement with the geometric volume of the gas exchange unit of 9.187 m 3 .

Transfer efficiency of selected VOCs at different relative humidity
The transfer efficiency of the complete system of SAPHIR-PLUS was studied using a self-prepared gas standard.
The gas standard consisted of acetone, isoprene, α-pinene, nopinone, and methyl salicylate (MeSa) in N 2 (99.999 % purity).The compounds were chosen to represent on the one hand typical BVOC emissions (isoprene, α-Pinene, MeSa) and on the other hand, a significant variety in molecular masses, boiling point, and solubility in water (see Table 3).At the start of the experiment, CO 2 , VOCs from the gas standard and water vapor were added to the synthetic air flowing into the chamber.No plants were installed during this experiment.The airflow into the chamber was 100 L min −1 .The additional flow of VOCs into the synthetic air was chosen such that a constant concentration of about 70 ppbV for each compound was reached inside the PLUS gas exchange volume.A CO 2 concentration of ∼ 455 ppmV was main-  Karl et al. (2008) tained in the PLUS exchange volume throughout the experiment.
The addition of water to the synthetic air was varied during the experiment to achieve different relative humidity levels, while the VOC concentration was kept constant.In total four different relative humidity levels (25 to 100 %) were chosen for this experiment.Once a stable RH was achieved inside the gas exchange unit, PLUS was coupled to SAPHIR and VOCs, CO 2 , and water vapor were transferred from PLUS to SAPHIR.For practical reasons the sequence of experiments was started with RH of 64 %, decreasing stepwise to 50 and 25 % by dilution.In a second experiment the transfer efficiency was examined starting at 100 % RH.Note that the RH in SAPHIR was little affected by the injection of air from PLUS.The RH changed in SAPHIR from 1 to about 7 % RH for the first experiment (for a total coupling duration for all RH steps of 6 h) and 0.5 to 3 % for the second experiment (for a total coupling duration of 3 h).
For evaluation of the transfer efficiency, VOC concentrations were measured by PTR-ToF-MS with a 1 min time resolution switching between the PLUS inlet and PLUS outlet and then subsequently between the PLUS outlet and SAPHIR.After starting VOC addition, the concentration at the PLUS inlet was monitored for about 1 h to verify a stable concentration of VOCs in the inlet flow.Then the PTR-ToF-MS measurements were switched repeatedly to measure between the PLUS outlet and SAPHIR.The VOC concentrations at the PLUS outlet were measured for 20 min before switching for 40 min to the SAPHIR outlet; thus one measurement cycle was 1 h.The coupling of PLUS to SAPHIR was maintained for approximately 1 h, ensuring that concentration changes in SAPHIR due to continuous inflow of VOC-rich air could be measured.
After 1 h PLUS was decoupled from SAPHIR and the RH was changed.This procedure was repeated for all different RH values.Throughout the experiment the CO 2 was used as an inert tracer and measured switching continuously between the PLUS inlet and outlet with one Picarro instrument and separately with a second Pircarro system in SAPHIR.The transfer efficiencies between the PLUS inlet and outlet and between the PLUS outlet and SAPHIR were determined by the ratio of the measured VOC and CO 2 concentration divided by the calculated concentration.The calculated concentration for PLUS was derived by taking the initial concentrations measured at the PLUS inlet and calculating the concentration inside the gas exchange volume, depending only on the flow and the volume.
with C PLUS,t being the calculated concentration in the gas exchange volume at time t, C inlet,t the VOC concentration at the PLUS inlet at time t, Q PLUS,t the flow through the gas exchange volume, V PLUS the volume of the gas exchange volume, and C PLUS,t−1 the previously calculated VOC concentration in the gas exchange volume.Similarly, the calculated concentration in SAPHIR was determined by concentration in PLUS and calculating depending on flow from PLUS into SAPHIR and on the volume of SAPHIR.
with C SAPHIR,t being the calculated VOC concentration in SAPHIR at time t, C PLUS,t the measured VOC concentration in the gas exchange volume, Q PLUS,t the flow from the gas exchange volume into SAPHIR, V SAPHIR the volume of SAPHIR, C SAPHIR,t−1 the calculated VOC concentration in SAPHIR at the previous time step, Q SAPHIR,t the dilution flow of SAPHIR that compensates for sampling and leakage.
Figure 4 shows the result for both transfer efficiency measurements (upper panel between the PLUS inlet and outlet, lower panel between PLUS and SAPHIR) dependent on the RH levels.The inert tracer CO 2 had a transfer efficiency of about 1 for both systems.Since the only loss processes for CO 2 are due to leakage between the PLUS gas exchange volume and the intermediate flushed space or between PLUS and SAPHIR, it can be concluded that losses due to leakages are negligible in this system.The transfer efficiencies for VOCs between the PLUS inlet and PLUS outlet were on average 0.85 with the lowest value being 0.81.For regular operation as a plant chamber, this transfer is of minor importance, as the PLUS inlet is operated with VOC-free air under these conditions.The transfer efficiencies from the PLUS inlet to the PLUS outlet and from the PLUS outlet to SAPHIR are comparable.However the transfer efficiencies from the PLUS outlet to SAPHIR are slightly lower at an average of 0.8, with the lowest value of 0.59 observed at 65% RH.Note that this lowest value was observed for the first stage of the experiment, when transfer lines between PLUS and SAPHIR were not conditioned for VOCs.
It is important to note that there is no significant difference in the transfer efficiencies for different VOCs, indicating that within the range of vapor pressure and polarity investigated here, VOC mixtures emitted from trees enclosed in the gas exchange unit, are transferred to SAPHIR without changes to the relative composition of the VOC mixture.Transfer is furthermore independent of relative humidity in the range of 25 to 100 % for both transfer between the PLUS inlet and outlet and transfer between PLUS and SAPHIR.
Coupling of PLUS to SAPHIR for transfer of BVOCs emitted from trees is achieved by either continuously transferring air from PLUS to SAPHIR or by a short pulsed coupling (timescale of several minutes to hours).Figure 5 displays the temporal evolution of monoterpenes emitted from six Quercus ilex trees.The upper panel shows monoterpene mixing ratios in PLUS together with light intensity (yellow area).The lower panel shows resulting monoterpene concentrations in SAPHIR when a continuous flow of 30 m 3 h −1 was fed from PLUS to SAPHIR.Also shown is the calculated monoterpene concentration applying Eq. (3) to derive monoterpene concentration in SAPHIR.Note that here a transfer efficiency of 1 is implicitly assumed.Apart from the very beginning of the experiment, where calculated monoterpene concentration exceeds observed ones, good agreement is found.At these early times, chamber mixing times may play a role.The overall mixing time of SAPHIR for uniform mixing is 5 min, limiting the ability to observe fast changes in concentration of the inflow from PLUS.

Light-and temperature-dependent plant emissions
To evaluate BVOC emission characteristics inside PLUS, six potted Mediterranean holm oak (Quercus ilex) trees were installed in the plant chamber.Quercus ilex is a strong monoterpene emitter with emissions being light dependent (Kesselmeier and Staudt, 1999, and references therein).The main compounds emitted from the plants during our experiments were α-pinene, β-pinene, Z-β-ocimene, and limonene as determined by GC-MS/FID (see Fig. 6) in agreement with observations from laboratory plant enclosure and field studies (e.g., Blanch et al., 2007;Llusià and Peñuelas, 1999).
To determine the dependence of emissions on light and temperature, diel courses of light and temperature were preset with 12 h day/night periods (Fig. 7).To simulate field conditions the light intensity was gradually increased over a period of 6 h, followed by a 6 h gradual decrease.VOC emissions were analyzed as a function of temperature (13-31 • C) for periods of up to 3 consecutive days at one temperature.
Figure 7 shows the dependence of the normalized (C/C I,max ) emissions on light intensity at different air temperatures, confirming the well-known strong light response of isoprenoid emissions by Quercus ilex.Total emissions are zero while lights are off in PLUS and increase with increasing light intensity.The maximum concentration of isoprene and monoterpenes in PLUS was reached shortly after the light maximum.Normalized isoprene and monoterpene emissions revealed similar responses to the diel course of light intensity in all temperature regimes.Methanol, however, showed emission pulses after the onset of illumination, as has been observed previously for different plant species both in field and laboratory experiments under controlled conditions (Folkers et al., 2008;Warneke et al., 2002;Nemecek-Marshall et al., 1995).Morning pulses of methanol emissions were explained as a consequence of accumulation in extracellular water of the plant's apoplast when stomata are closed during nighttime (Niinemets and Reichstein, 2003).Methanol is continuously produced and high aqueousphase concentrations built up in the aqueous-phase during nighttime when stomata are closed.In the morning, stomata open and methanol is released until equilibrium is reached between the gas-phase of the substomatal cavity and the atmosphere. 13C-labeling experiments confirmed that the PAR dependence of methanol emissions is not directly linked (< 10 %) to photosynthetic de novo production, but rather originates from precursor compounds with long turnover times for carbon, such as cell wall components (Folkers et al., 2008).Methanol emissions are generally associated with plant growth (Hueve et al., 2007;Fall and Benson, 1996;Nemecek-Marshall et al., 1995;Obendorf et al., 1990).Methanol emissions have been shown to be temperaturedependent in general (Folkers et al., 2008).Note that due to the increase of methanol emissions with increasing temperature (see also below), the relative importance of the morning emissions gradually vanished in PLUS with increasing temperature (Fig. 7).
The same set of experiments was used to study the temperature dependence of the Quercus ilex emissions.Figure 8 shows the concentrations of isoprene, the sum of the monoterpenes, and methanol normalized to the concentrations at 30 • C vs. temperature in PLUS.The 4 h averaged data of peak emissions of isoprene and the monoterpene fol- lowed a well-described exponential increase with increasing temperature (e.g., Guenther et al., 1993).For monoterpenes the emissions increased with a slope (β in Eq. ( 5) in Guenther et al. (1993)) of 0.11 ± 0.02 which is in good agreement with the literature (Staudt and Bertin, 1998).Methanol increased linearly with temperature, with a 10 % increase in peak emissions every ∼ 4 • C between 13 and 31 • C. Note that for these experiments the plants were kept in the plant chamber undisturbed for 109 consecutive days.During that time period the plants showed no signs of stress effects.

BVOC ozonolysis of real plant emissions
To illustrate the applicability of SAPHIR-PLUS as a facility for simulation of atmospheric chemistry processes and impacts of BVOC emissions, the following experiment was conducted.After pulsed injection of Quercus ilex emissions from PLUS to SAPHIR for 3.5 h, PLUS was decoupled from SAPHIR and an ozonolysis experiment in SAPHIR was performed (Fig. 9).After the transfer of BVOCs from PLUS was completed, 30 ppbv of ozone was injected into SAPHIR.The  initial conditions inside the chamber prior to injection were 47 % relative humidity at a temperature of 17 • C. Ozonolysis of BVOCs is known to be a source of OVOCs and SOAs in the atmosphere.One OVOC of interest in this respect is acetone, as it is relatively long lived (global mean lifetime several weeks), and serves as a source of HO x and PAN in the upper troposphere (Jacob et al., 2002;Gierczak et al., 1998).
As can be seen in Fig. 9 (lower panel) the ozonolysis of the monoterpene mixture leads to production of acetone with a maximum concentration gain of 0.68 ppb.Note that the background concentration of acetone in SAPHIR prior to the injection of ozone could be considered constant and was on average 0.22 ppbv.The acetone time series shown in Fig. 9 are corrected for the background concentration.Considering dilution in SAPHIR, this observed acetone production is equivalent to an overall yield of 18.8 % from the mixed monoterpene emissions form Quercus ilex.This yield can be evaluated taking known acetone yields of individual monoterpenes and their relative contributions to Quercus ilex emis- sions (Fig. 6) into account.As no OH scavenger was used in these experiments, the monoterpene are oxidized by OH and ozone.From the decay curve of the individual monoterpenes (determined by the GC-MS/FID), it can be estimated that between 17 to 64 % of the monoterpenes were oxidized by OH, depending on the substance.Based on upper limits in the literature (Atkinson and Arey, 2003) a weighted average molar yield of 17 % is expected from the mixture used in this experiment (see Table 4).This takes relative contributions from ozone and OH oxidation into account.Considering that some acetone could have been produced from compounds which were not measured, such as plant-emitted sesquiterpenes or intermediate oxidation products, the observed 18.8 % acetone yield is in good agreement with upper estimates from literature data.With overall concentrations in the range of atmospheric conditions, this combined contribution of ozone and OH to monoterpene oxidation and product formation may still be considered representative of BVOC oxidation in low-NO x environments.This experiment thus exemplarily shows that, using direct emissions of plants as a BVOC source, a simulation of atmospheric chemistry can be used to derive representative product yields for use in bottom-up estimates of global sources.

Conclusion
A new plant chamber for use as a source of direct emissions from plants in atmospheric chemistry simulation experiments was setup at SAPHIR.Technical features and implementation of PLUS are described in detail.PLUS is shown to provide an environment in which parameters of importance for plant physiology can be chosen at environmental impor-tance.The emissions from Quercus ilex as a function of both light and temperature observed in the gas exchange chamber are representative of emissions observed in ambient studies.Therefore PLUS can be considered a representative source of BVOC emission mixtures.VOC mixtures were shown to be quantitatively transferred to SAPHIR, ensuring the emission pattern remains unchanged.In an ozonolysis experiment the total yield of acetone from oxidation of Quercus ilex emissions was determined.Future applications of SAPHIR-PLUS can therefore target a wide spectrum of atmospheric chemistry questions ranging from product yield determination (OVOC and SOA), oxidation cycles, and oxidant (OH) recycling to the evaluation of total OH reactivity of both direct emissions and their oxidation products under a variety of oxidation regimes.

Figure 1 .
Figure 1.Schematic diagram of the PLUS plant container.The green lines show the inflow of synthetic air and other gases into PLUS (PLUS inlet) and the red lines are the outflows of PLUS to either instrumentation (PLUS outlet), SAPHIR or the atmosphere.

Figure 2 .
Figure 2. Horizontal cross-section through the multi-layered walls of the triple-cage plant chamber system, including the doublewalled wall opening and flanges for the transfer tube to SAPHIR.

Figure 3 .
Figure 3. CO 2 and VOC flush out characteristic inside the PLUS gas exchange volume.

Figure 4 .
Figure 4. Measured/calculated averaged mixing ratios of VOC gas standard compounds.Upper panel shows the transfer efficiency between the PLUS inlet and PLUS outlet.Lower panel shows the transfer efficiency between the PLUS outlet and the SAPHIR chamber.Error bars shown are the standard deviation (N PLUS = 18, N SAPHIR = 37).

Figure 5 .
Figure 5. Illustration of monoterpene transfer from PLUS to SAPHIR when continuously coupling the chambers with a flow of 30 m 3 h −1 .Yellow shaded areas indicate time periods and intensity of light in the plant chamber; green shaded area indicates coupling of PLUS to SAPHIR.

Figure 6 .
Figure 6.BVOC composition of six Quercus ilex trees.BVOC pattern was measured with GC-MS/FID in SAPHIR during the ozonolysis experiment described in Sect.3.6.The relative contribution of each compound concentration (ppbv) is shown.

Figure 7 .
Figure 7. Light-dependent emissions of Quercus ilex under different air temperature conditions, with C I,max referring to observed concentrations at maximum light intensity (I ) (upward branch).

Figure 8 .
Figure 8. Temperature-dependent emission of Quercus ilex. 1 h averaged data of peak emissions during the photoperiod are used.Error bars are calculated using error progagation based on the standard deviation for the concentration at 30 • C and the measured temperatures.Result of the fit shows that monoterpene emissions increase with a slope of β = 0.11 ± 0.02.

Figure 9 .
Figure 9. PLUS (upper panel) and SAPHIR (lower panel) observed concentrations of monoterpenes (red symbols) in an experiment performing a pulsed transfer of BVOC emissions.After ozone addition to the SAPHIR chamber, formation of oxidation products is observed.Acetone, as one representative OVOC, is shown as measured (black symbols) together with calculated acetone (blue line).

Table 1 .
Temperature and flow stability for typical operational conditions with T PLUS

Table 2 .
LOD and accuracy (3 δ error of the sensitivity) of calibration-based PTR-TOF-MS measurements for VOCs of interest.

Table 3 .
Summary of physical and chemical properties of VOCs (acetone, isoprene, α-pinene, nopinone, and methyl salicylate) used in gas standard for transfer efficiency characterization.VOCMolecular formula Molar mass Boiling point (K) k • H at 298.15 K (mol kg −1 bar)

Table 4 .
(Atkinson and Arey, 2003)ndividual BVOCs by ozone (F O 3 ) and OH (F OH ) together with maximum literature-reported acetone yields during ozonolysis (Y O 3 ) and OH oxidation (Y OH ) taken to calculate maximum weighted average acetone yield(Atkinson and Arey, 2003).