INSTRUMENTATION

Portable CO₂ meters

For the measurement of the CO₂ concentration, we used single beam and dual wavelength CO₂ infrared sensors: Chauvin Arnoux CA1510 and Aranet 4. The CO₂ sensor is a non-dispersive infrared (NDIR) one.

Portable CO₂ meters.

The fundamentals of these devices is presented in the following scheme.

Schematics of a double channel NDIR and an example of this type of sensors.

These sensors consist of a single IR radiation source and two independent IR detectors (photodiodes). It consists of a reference channel that measures the transmitted IR radiation at 3,91 mm, where CO₂ does not absorb and a measurement channel that detects CO₂ at 4,26 mm, where it strongly absorbs. The aging or drift of the lamp (if it is incandescent, mainly) affects both sensors equally.

Fast mobility Particle Sizer (FPMS) spectrometer

The analysis of the size distribution of the ultrafine particles present in the air is carried out using an FMPS spectrometer (TSI model 3091).

FMPS spectrometer mod. TSI 3091 and a photograph of the device attached to a smog chamber.

The classification of particles with a diameter (D_p) between 5.6 nm and 560 nm (0.56 μm) results from the differences in their differential electric mobility according to their size, density and charge, as they travel through the column of a Differential Mobility Analyzer (DMA). In the DMA, the particles are deflected radially outward and collected on a series of electrically insulated electrodes located on the outer wall of the analyzer.

The normalized concentration of the number of particles or number of particles per unit volume (dN/dlogD_p in particles /cm³) is determined by measuring the electric current collected in a series of electrodes (as shown in the figure). The results at each time are therefore expressed as dN/dlogD_p for each particle size or total particle concentration (N). This equipment allows to follow dynamic processes with response times of 1 s, that is, the size distribution is completed in 1 s, so that sudden changes in the level of ambient particles are in real time.

Proton Transfer Ionization Time-of-Flight Mass Spectrometer (PTR-Tof-MS)

A commercial PTR-ToF-MS (IONICON, model PTR-TOF 4000) is available in our group to monitor in real time the secondary pollutants generated in the simulation chambers from the gas-phase reactions of OH or Cl atoms. This equipment uses H₃O⁺ as the primary ion for the ionization process by charge transfer, so the observed ions are MH⁺. This ionization source allows the detection of most of the volatile organic compounds (VOCs), although it does not allow the detection of chlorinated species or certain hydrocarbons without functional groups. The mass spectra of the gas mixture were recorded with a temporal resolution of several seconds and the detected mass range was established between 10 and 390.86 amu. The PTR-MS acquires all the quantification parameters and automatically calculates the absolute concentrations of all measurable VOCs in ppb over time. In addition, the calibration of m/z is made using those of H₃¹⁸O⁺ (m/z=21) from H2O and C₆H₅I⁺ (m/z=203.94) from C₆H₄I₂.

Photograph of the PTR-ToF-MS equipment coupled to an atmospheric simulation chamber.