Gas phase ultraviolet (UV) spectroscopy

The absorption of UV radiation between 10 and 400 nm in certain molecules may cause the following electronic transitions: σσ* (λ <150 nm), nσ* (150 <λ <200 nm) and nπ* and ππ* (200 < λ < 700 nm). The latter transitions also lie in the visible region. The absorbance at a given wavelength, A(λ), is given by the Lambert-Beer law:

A(λ) = σ(λ) ι [R]

where σ(λ) is the absolute absorption cross section at a certain wavelength (cm2 molecule-1), l is the optical path length (cm) and [R] is the concentration of the pollutant (molecules cm-3). From the experimental measurements of A(λ) as a function of [R], σ(λ) is determined from the slope of the A (λ) versus [R] plots.


Gas Phase Fourier Transform IR Spectroscopy (FTIR)

IR absorption spectroscopy is based on the absorption of IR radiation (12000 – 10 cm-1). Like UV-visible absorption spectroscopy, this technique consists of detecting the transmitted IR radiation through a gaseous sample of stable species (reactant and / or product) at each wavenumber (ν ̃) between 4000 and 500 cm-1, in our spectrometers. The absorbance, A(ν ̃), corresponding to each ν ̃ is related to the concentration of the absorbent R inside the cell by means of the Lambert-Beer Law:

A( ν ̃ ) = σ( ν ̃ ) ι [R]

where σ(ν ̃) is the absolute absorption cross sections at the wavenumber ν ̃ (cm2 molecule-1). es el paso óptico (cm) y [R] la concentración del absorbente (molécula cm-3). De esta forma, las secciones eficaces de absorción en el infrarrojo, σ(ν ̃ ), de una especie R se determinan a partir de la pendiente de la representación de A(ν ̃) frente a [R].

Thus, σ(ν ̃) is determined from the slope of A(ν ̃) vs [R]. The recording of IR spectra has a double objective:

1) The determination of the radiative efficiency and global warming potential of a primary pollutants.

2) To monitor the temporal evolution of both the reactant (primary pollutant) or its reaction products (secondary pollutants) in the smog chambers.


Pulsed laser photolysis (PLP, pulsed laser photolysis) coupled to laser induced fluorescence (LIF, laser induced fluorescence)

The PLP technique allows the generation of radicals from the photolytic breakdown of molecules by means of UV radiation. In our studies, a KrF excimer laser (Coherent, model ExciStar 200) provides pulsed radiation at 248 nm, which crosses the reaction cell through quartz windows. The energy of this radiation is on the order of 10 mJ/pulse at 10 Hz. The absorption of this radiation by H2O2 or HNO3 generates OH radicals in their fundamental electronic state, OH (X2П).

H2O2 + hν248nm 2 OH(X2П)

HNO3 + hν248nm OH(X2П) + NO2

The photochemical precursor of OH radicals is introduced into the reaction cell by flowing the bath gas through a bubbler containing an aqueous solution of the precursor.


Glass bubbler with the aqueous solution of the OH-precursor.


The OH radicals generated by the PLP technique are excited to the first electronic excited state, OH(A2 S+, v’=1), by absorbing radiation at ~282 nm, achieved by the doubled frequency of a rhodamine 6G dye laser (Continuum, ND60; LiopTec, LiopStar). The dye laser is pumped by the second harmonic (532 nm) of a Nd-YAG laser (1064 nm). From this excited electronic state OH(A2 S+, v’=1) the spontaneous emission occurs around 309-310 nm.

OH(X2П, v”=0) + hν~282nm OH(A2 S+, v’=1)

OH(A2 Σ+, v’=0) OH(X2П, v”=0) + hν~309nm

The laser induced fluorescence at ~309 nm (ILIF(t)) is focused and detected in a photomultiplier tube, which transforms it into an electrical signal. ILIF(t)) depends exponentially on the reaction time, corresponding to a pseudo-first order kinetics:

ILIF(t) = ILIF(t=0) exp (-k’t)

where k‘ is the pseudo-first order rate coefficient, which depends linearly on the pollutant concentration. To vary the reaction time t, a pulse and delay generators are used to change the delay between the trigger of the photolysis and the excitation lasers. The entire PLP-LIF system is therefore perfectly synchronized.

General view of the PLP/LIF experimental system: 1. Reaction cell; 2. Photomultiplier tube; 3. Power supply of the photomultiplier tube; 4. Bubbler of OH-precursor; 5. Thermostatic bath; 6. Excimer laser; 7. Nd-YAG+dye lasers; 8. Vacuum line and reagent mixing bottles; 9. Pulse/delay generators and data acquisition system.


Relative kinetics coupled to different detection methods

A relative kinetic method consists in determining the rate coefficient of the process under study (k) relative to that of a reference compound (kref) that is well known under the same pressure and temperature conditions. Therefore, in the relative kinetic method, the stable species, that is, the pollutant under study and the reference compound, are monitored as a function of the reaction time. To carry out the kinetic studies and the formation of gaseous products and particulate matter formed in the reactions with OH, Cl, and ozone, under conditions of the terrestrial troposphere near the surface (T = 298 K and P = 1 atm) two smog chambers are used. They consist in a reactor surrounded by several UV lamps where Cl atoms and OH radicals are generated in situ by continuous UV photolysis of Cl2 and H2O2, respectively.

Cl2 + hν (λmax = 360 nm) 2 Cl

H2O2 + hν (λmax = 254 nm) 2 OH

Ozone is generated by electrical discharge in a commercial ozone generator:

O2 + discharge 2 O

O2 + O + M O3 + M M = N2, O2


Therefore, the reactions that take place in the reactor are:

R + Ox → Products k Ox = OH, O3 ó Cl

R → Other losses kP

Ref + Ox → Product kref

Ref → Other losses kref,P

Although the disappearance of the pollutant (R) and the reference compound (Ref) in the reactor is mainly due to the reaction with the oxidant, in any kinetic study it is necessary to consider all possible loss processes of R and Ref (e.g., heterogeneous reaction with the reactor walls, UV photolysis and/or reaction with the precursor of the oxidant). Thus, taking into account the previous reaction scheme, the rate of disappearance of the contaminant and the reference compound is giving by the following equations:

Rearranging both expressions and integrating between time 0 and time t, the resulting integrated rate equation is:

Plotting ln{[R]0/[R]t}-kpt against ln{[Ref]0/[Ref]t}-kref,pt, the relative rate coefficient k/kref is obtained from de slope. Finally, knowing kref, we determine k. The temporal evolution of stable species is followed by mass spectrometry coupled with gas chromatography separation technique (GC-MS), FTIR spectroscopy and/or proton transfer ionization time-of-flight mass spectrometry (PTR- ToF-MS). The GC/MS technique is commonly used in the detection of reaction products.


Experimental system used in the relative kinetic studies.


View of the interior of the 264-L smog chamber and 16-L reactor used in the relative kinetic experiments.