We use DOAS to identify trace gases by their unique narrow-band (differential) absorption structures based on Lambert-Beer’s Law. We use active DOAS, which relies on xenon lamps, LEDs, or other manmade light sources, as well as sunlight-dependent passive DOAS. The DOAS method has several advantages:
- Analysis of relative intensity differences does not need absolute calibration (inherent calibration).
- Removal of spectral broad band intensity accounts for Rayleigh and Mie scattering effects, thus enabling the use of solar scattered light as light source.
- Measurement over finite wavelength intervals (typically 100 or more nm) allows for simultaneous detection of several trace gases.
- Detection of selective trace gases with high sensitivity.
Cross sections of selected atmospheric absorbers
Differential reduction in intensity D’ due to differential absorption cross-section σ‘ of an atmospheric trace gas.
I = I0*exp(-σ*c*L)
- σ: cross section
- c: concentration of absorber
- L: light path through absorber
Multi-Axis (MAX-DOAS) and Radiative Transfer Modeling (RTM)
We conduct passive DOAS mostly as MAX-DOAS measurements. This can be accomplished from multiple platforms, based on the ground, ocean vessels, or research aircraft. MAX-DOAS measurements are sensitive to vertical trace gas and aerosol distributions because the instrument observes scattered sun light with different telescope elevation angles.
Vertical trace gas and aerosol profiles can be retrieved through a combination of radiative transfer modeling and inversion techniques.
Schematic of the retrieval algorithm applied to solar stray light measurements carried out in the Volkamer group.
The initial product of solar stray light measurements is Slant Column Density (SCD) which is the integrated concentration of trace gases along all light paths.
This quantity can be inverted in order to quantify aerosol properties (optical and microphysical) and trace gas vertical information.
Cavity-enhanced-DOAS and Chamber studies
The ATMOSpec Lab uses chamber studies to better understand atmospheric chemistry and atmospheric processes without the complicating effects of the entire atmosphere. We have utilized chamber studies to better understand the isoprene oxidation mechanism and identify the temperature dependence of the product distribution . We also use chamber studies to look at aerosol uptake of glyoxal (PSI) and to study glyoxal formation from sea surface microlayer proxies .
EUPHORE Chamber in Valencia, Spain
The LED-CE-DOAS instrument measures concentrations of trace gases in chamber experiments. The volume of the chamber is generally well-mixed, so a portable, broad-band in-situ instrument is ideal.
The instrument is also used for in-situ ambient measurements of diurnal cycles of glyoxal and comparison with the lowest elevation angle of the MAX-DOAS. LED light source options provide coverage over most of the near-UV and visible region.