# Advances in High-Precision NO2 Measurement by Quantum Cascade Laser Absorption Spectroscopy
## 2 Materials and Methods
### Field Inter-Comparison Setup and Instrumentation
#### 2.2.2 Calibration Methods  
To ensure comparability between the NO2 instruments, they were all calibrated simultaneously prior to the ambient air measurements. During the calibration phase, each of the four instruments was sampling in parallel from the common calibration line. The CLD instrument was connected to this line via a 5 m /4 inch PFA tubing. Three different calibration methods were applied: dilution of a 1 ppmv NO2 cylinder, gas phase titration of NO by ozone and permeation from a wafer type NO2 permeation tube.  
The cylinder dilution calibration (CDC) was done using NO2 in N2 from a 10 L cylinder provided by the Dutch Metrology Institute (VSL, Delft, Netherlands), with a nominal concentration of 1.00 \(\pm\) 0.04 ppmv (k = 2). The content of this cylinder was diluted by synthetic air to generate concentrations at the relevant range (from 0 to 100 ppbv). The 1 ppmv NO2 gas flow was controlled by a SilcoNert@2000 coated mass flow controller (MFC; range 0-500 mL/min, Vogtlin Instruments GmbH, Muttenz, Switzerland). The synthetic air flow was controlled by a second MFC (range 0-10 L/min, Vogtlin Instruments GmbH, Switzerland), both calibrated against a traceable Molbox system (Fluke Calibration, Everett, WA, USA). During the whole calibration phase, the dilution flow was kept constant at 5 L/min, and the 1 ppmv NO2 flow was varied step-wise between 0 and 500 mL/min. An overflow of about 1 L/min was located 2 m downstream of the mixing point to ensure constant pressure at the inlet of the NO2 instruments.  
The gas phase titration (GPT) calibration was carried out using a 20 ppmv NO in N2 SI-traceable standard cylinder (50 L) from the National Physical Laboratory (NPL, Teddington, UK). The GPT reaction itself (NO + O3 = NO2 + O2) took place in a commercial GPT unit (MKAL-GPTM, BREITFUS Messetchnik GmbH, Harpstedt, Germany), which includes an O3 lamp, a NO + O3 reactor and a dilution system. The total gas flow was kept constant at 5 L/min, and different NO2 concentrations were obtained by changing the light intensity of the O3 lamp. NO2 concentrations were retrieved from the NO concentrations measured with the CLD instrument, i.e., by assuming that the difference in NO concentration measured without and with the addition of O3 correspond to the NO2 concentration produced in the GPT instrument with a yield of 100%.  
For the permeation calibration (ReGaS), a portable gas generator (ReGaS1) built by the Federal Institute of Metrology (METAS, Bern-Wabern, Switzerland) was used. This generator comprises a dilution system and a SilcoNert@2000 coated oven, with capacity for one permeation tube, similar to the ones described in [PERSON] et al. [47]. The delivered NO2 concentration depends on the permeation unit used, the oven temperature and the set flow. In this work, we used a NO2 permeation tube (VICI AG International, Schenkon, Switzerland) that had a permeation rate of 605 ng/min at 38 \({}^{\circ}\)C and 2.6 bar. This permeation tube was calibrated at the METAS using a magnetic suspension balance (TA instruments, New Castle, DE, USA) 9 months prior to the inter-comparison. The set concentration used in this work ranged from 20 to 65 ppbv with a total flow ranging from 4.5 to 5.3 L/min (with one or two dilution steps), from which at least 0.5 L/min was overflowing.