Aarhus Universitets segl

Faciliteter og udstyr

The Aarhus University Research on Aerosol (AURA) atmospheric simulation chamber

Det atomosfæriske simulationskammer, AURA, betsår af en kubisk Teflon pose med et volumen på ∼5 m3. Kammeret hænger fra et metalstativ inden i et temperaturreguleret rum, hvor temperaturen kan justeres fra -16 til 26 °C. Sollys i AURA simuleres ved 24 UV lamper, der er installeret over og under kammeret. Det er muligt at tilslutte instrumenter til AURA gennem rør af rustfrit stål og teflon. I AURA måles der altid temperatur, relativ luftfugtighed, og koncentrationer af ozon og nitrogen oxider (NOx). En mere detaljeret beskrivelse af AURA kan findes i Kristensen et al. (2017).

AURA bruges til simuleringer af dannelse af aerosoler fra mange forskellige kilder fx biogene flygtige organiske stoffer, som oxidation af α-pinene og dimethylsulfid. Når AURA bruges til at danne sekundære organiske aerosoler, fyldes kammeret med en bestemt mængde oxidant (enten ozon eller hydroxyl radikaler), hvorefter der tilsættes en lille mængde af et flygtigt organisk stof. Herefter vil der dannes partikler. Dannelsen og udviklingen af de aerosoler, der dannes fra oxidationsprocesserne, kan følges med flere forskellige instrumenter. Typisk anvendes AURA sammen med en scanning mobility particle sizer (SMPS), som måler størrelsesfordelinger, et aerosol massespektrometer (AMS), som måler den kemiske komposition, og en metode til partikel opsamling til mere detaljeret kemisk analyse gennem fx filteropsamling eller ved brug af en spot sampler. Der er mulighed for at tilkoble andre instrumenter til måling af fx skydannelsesegenskaber og lign.

AURA kan også bruges til forsøg med sea spray, hvor sea spray partiklerne først produceres ved tilkobling af en sea spray tank eller en partikelgenerator, fx en atmomizer, og derefter bliver ældet (dvs. videre oxideret el. andre processer, der ændrer kompositionen af partiklerne). For at partiklerne ældes tilsættes en oxidant (oftes hydroxyl radikaler). Ligesom ved studier af sekundære organiske aerosoler, bruges der mange instrumenter til at følge udviklingen i partiklerne. For eksempel kan der anvendes der en optisk partikel tæller (OPS) og et nephelometer, som måler optiske egenskaber af partiklerne. 

AURA Instrumenter

AURA kan bruges med et væld af instrumenter. Vi har følgende instrumenter til rådighed ved simuleringer i AURA. 

CCN Counter

OPS

Nephelometer

Dew Point Water Activity Meter

Water activity (aw) describes the energy status or escaping tendency of water in a sample. It indicates how tightly water is “bound,” structurally or chemically, in a solution or solid. 

The AquaLab Dew Point Water Activity Meter measures the water activity as the ratio of the vapor pressure of water in a material (p) to the vapor pressure of pure water (po) at the same temperature.

At the group of Atmospheric Physics and Chemistry we investigate how the water activity of atmospheric particles is affected by the chemical composition of the particles. Water activity measurements of solutions often comprising atmospheric particles provide data used to calculate the supersaturation required for particles to activate as cloud condensation nuclei (CCN), thus form clouds in the atmosphere. 

Osmometer

The Osmometer allow for accurate determination of osmolality by measuring the freezing point depression of a liquid sample. 

The osmolality of a solution gives the number of moles of osmotically active species in one kg of water. 

By simple calculations water activity (aw) of a solution can be determined from the osmolality as measured  by the Osmometer. The Osmometer thus enables an alternative approach for determination of the water activity compared to the Dew Point Water Activity Meter.

Low sample volume requirement of the Osmometer (0.15 mL) compared to the Dew Point Water Activity Meter (approx. 7.5 mL) allow for accurate aw measurements to be performed on samples of low volume.  

Atomizers

Atomizers, such as the TSI 3076, enable the production of aerosols by spraying droplets of water or a solution.  

A solution containing the compounds of which particles are to be produced is drawn from a closed bottle connected to the atomizer. Pressurized VOC-and particle-free air is fed to the main unit and expanded through a small nozzle. This creates a jet of high velocity and upon entry liquid is atomized into droplets. Larger droplets impact on the inner back wall and run back to the solution bottle, while smaller droplets continue onwards within the air flow through the output of the atomizer. Subsequently, the aerosol flow is dried by silica diffusion dryers and a Nafion® dryer so that the water evaporates and the solid residual is left as aerosol particles. This procedure produces aerosols with particle in the nanometer regime. 

Scanning Mobility Particle Sizer (SMPS)

The size distribution of aerosol can be measured using a Scanning Mobility Particle Sizer (SMPS). The SMPS system consists of two separate instrumentations: a Differential Mobility Analyser (DMA) followed by a Condensation Particle Counters (CPC)  

The DMA allow for the extraction of a selected size or the size distribution of aerosol particles. The system classifies charged particles according to their aerodynamic mobility using an electrical field.

The CPC allow for online measurement of the total aerosol number concentration. Particles collected through the CPC inlet are grown by condensing a super-saturated vapor (usually 1-butanol) onto the surface of the particles, forming particles large enough to be detected optically.

The SMPS system is widely used for the measurement of the concentration of fine particles (aerodynamic diameter < 1μm) in smog chamber experiment and the ambient atmosphere.  

Optical Particle Sizer

The TSI Optical Particle Sizer (OPS) is a light, portable unit that provides fast and accurate measurement of particle concentration and particle size distribution using single particle counting technology. 

The OPS enables measurement of particles in the size range of 0.3 - 10 μm, thus stretching beyond the limitations of the SMPS system (<1μm particle size)

At the group of Atmospheric Physics and Chemistry the OPS is employed in conjunction with the SMPS systems, thus enabling accurate measure of particle number in the size range of 10 nm to 10 μm.   

 

Cloud Condensation Nuclei (CCN) Counter

The Cloud Condensation Nuclei (CCN) counter is an instrument used for the measurement of the number concentration of cloud droplets in an aerosol at a given supersaturation. By exposing an aerosol to a variable water vapor supersaturation the CCN counter determines the activated fraction as a function of supersaturation for various aerosols.

The group of Atmospheric Physics and Chemistry at Aarhus University currently possess two separate CCN counters:The Wyoming CCNC-100B CCN Counter and the Droplet Measurement Technologies (DMT) CCN Counter. 

The Wyoming CCNC-100B CCN Counter build at the Department of Atmospheric Science at the University of Wyoming.

The Wyoming CCN counter creates a water saturation vapor pressure gradient within a closed chamber. This is done by establishing a temperature gradient through the chamber, and thereby a region of supersaturation is created. By introducing an aerosol into the chamber the particles are subjected to supersaturation and a laser beam measures the concentration of CCN activated particles (NCCN).

By simultaneously measuring total particle concentration with a Condensation Particle Counter (CPC) the fraction of activated particles (NCCN/NTOTAL) is found.

The Droplet Measurement Technologies (DMT) CCN Counter 

The DMT CCN counter is a continuous-flow thermal-gradient diffusion chamber consisting of 50 cm long columns. Inside the columns, a thermodynamically unstable, supersaturated water vapor condition is created. The supersaturated water vapor condenses on the cloud condensation nuclei in the sample air to form droplets, just as cloud drops form in the atmosphere. As the introduced aerosols in the sample air is transformed into cloud droplets Optical Particle Counter (OPC) using side-scattering technology counts and sizes the activated particles.

In the group of Atmospheric Physics and Chemistry, the CCN counters are used to examine the cloud forming abilities of particles from different sources, such as sea spray particles and natural and anthropogenic particles formed from for the gas-phase oxidation of volatile organic compounds. 

Gas Chromatography with Flame Ionization Detector (GC-FID)

The Agilent 7820 GC system with gas valve inlet system allow for close to online measurement of volatile organic compounds (VOC) in the gas phase. The system utilizes the highly sensitive Flame Ionization Detector enabling the detection of atmospheric relevant concentrations of VOCs, such as isoprene, pinene and toluene. 

The GC-FID is a part of the instrumental array connected to the newly developed environmental chamber at the group of Atmospheric Chemistry and Physics at Aarhus University.  

ARAGORN (AaRhus Atmospheric Gasphase OR Nanoparticle) - flow tube setup

Saturation vapour pressure and the associated temperature dependence (enthalpy ΔH), are key parameters for improving predictive atmospheric models. Generally, the aerosol community lack experimentally determined values of these properties for atmospheric relevant organic aerosol compounds.

One technique to determine saturation vapour pressures is from measured evaporation rates of aerosol particles.

ARAGORN (AaRhus Atmospheric Gasphase OR Nanoparticle) - flow tube setup enables the experimental determination of the evaporation rates of organic aerosol particles.  

Sub-micron particles are generated by nebulization from aqueous solution, dried, and a mono disperse fraction of the aerosol is selected using a differential mobility analyser (DMA). The particles are then allowed to evaporate in the ARAGORN-flow tube that is the central part of the system. It is a 3.5 m long stainless steel pipe with an internal diameter of 0.026 m. Along its length, the flow tube has four ports that can be used for aerosol particle sampling, and measurements of temperature, relative humidity and pressure. Changes in particle size as function of evaporation time are determined using a scanning mobility particle sizer system (SMPS). Physical properties like air flows, temperatures, humidity and pressure are controlled and monitored on several places in the setup.

The saturation vapour pressures are then inferred from the experimental results in the MATLAB® program AU_VaPCaP (Aarhus University_Vapour Pressure Calculation Program). 

Sea Spray Tank

Sea spray particles are formed under windy conditions when air-bubbles get entrained in the ocean by breaking waves. At the surface these bubbles burst resulting in release of droplets containing salt and other components to the atmosphere.

The Sea Spray Tank at the group of Atmospheric Physics and Chemistry is designed to imitate the formation and bursting of bubbles in the marine environment and the subsequently formation of sea spray particles.

The Sea Spray Tank consist of a 40 cm high 16 L stainless steal tank. A total of 10 L of sea water or solutions of known chemical composition can be added to the tank. 

Bubble-busting, and thus particle formation, is implemented in one of two ways:

(1) Water is recirculated through a centrifugal pump and ejected through a nozzle 15 cm above the water surface. 

(2) Clean air is pushed through a diffuser located at the bottom of the tank.

As particles are formed in the head space of Sea Spray Tank, these are transferred to adjacent instrumentation for the investigation of size, number, CCN capability, and chemical composition.