Preparation of hybrid calibrated absorption cross sections for a compact UV-DOAS measurement

Abstract. A low-cost differential optical absorption spectroscopy (DOAS) spectrometer has been calibrated using a hybrid approach in which multiple absorption cross sections (ACSs) are measured and compared with previously obtained ACSs. The ACSs obtained from the measurement in this study are referred to as precise measurement (PM) ACSs, while those obtained from the simulation are referred to as hybrid simulation (HS) ACSs. The compact UV grating spectrometer was used to measure PM ACSs at different pressures and temperatures ranging from 295.75 K to 298.15 K. The spectral range was 230–320 nm with a spectral resolution of 0.28–0.39 nm. Uncertainties were evaluated and traceable to SI units. The relative standard measurement uncertainty of the PM ACS for o-xylene, p-xylene, SO₂, benzene, styrene and toluene were 3.1 %, 3.1 %, 3.1 %, 3.0 %, 3.0 % and 2.9 %. respectively. This combined relative standard uncertainty includes contributions from optical path length (1.4 %), CRM concentration (≤0.74 %), absorbance repeatability (≤1.5 %), pressure (0.78 %) and temperature (2.1 %). ACSs for reactive trace gases without CRMs were derived using the HS method. This involved convolving literature spectra with measured instrumental functions (IFs) and subsequently applying spectral fitting to achieve ACSs that are well-aligned with those that would be obtained if measured with this spectrometer. The uncertainty of the HS ACSs results from the referenced ACSs, the determination of the IFs and the fitting procedures. The uncertainty of the referenced ACSs is taken directly from the literature and ranges from 0.05 % for benzaldehyde and formaldehyde to 5 % for oxygen. The uncertainty of the IFs is 0.67 %, 0.74 % and 0.65 % in regions I, II and III respectively. The total uncertainty for HS ACS is estimated to be 0.25 % for benzaldehyde, 1.01 % for NO₂, 0.95 % for formaldehyde, 0.75 % for p-cresol, 0.74 % for m-xylene, 4.13 % for phenol, 4.19 % for ethylbenzene, 2.83 % for ozone in region I, 3.51 % for ozone in region II, 5.24 % for oxygen in region I and 5.31 % for oxygen in region II, taking into account the above uncertainty components. In a laboratory measurement of a gas mixture (benzene, toluene and ortho-xylene), the difference between the measurement values of the certified reference material (CRM) produced by the gravimetric method and the DOAS measurement is 8.5 % for benzene, 1.6 % for toluene and 4.9 % for ortho-xylene. Therefore, the uncertainty of the DOAS system was estimated to be 15.2 % for benzene, 8.2 % for toluene and 11.6 % for o-xylene, taking into account the uncertainties of the ACS (≤3.1 %), the fitting procedure (≤0.6 %), the difference with the CRM value (≤8.5 %) and the path length (1.4 %). In general, the system is ready for use in field measurements using the long-path DOAS technique.