# Advances in High-Precision NO2 Measurement by Quantum Cascade Laser Absorption Spectroscopy
## 3 Results and Discussion
### Ambient Air Measurements
A closer look at the differences between the CAPS and the QCL-Empa measurements (\(\Delta\)NO\({}_{2,\ CAPS}\), Figure 8b) reveals a diurnal cycle (with the highest contrast during the first period of the campaign, i.e., 23 January-27 January), with maxima corresponding to local noon-time, which correlate well with daily NO maxima. The first hypothesis to explain these observations is an erroneous estimation of the NO\({}_{2}\) formed in the CAPS inlet. As the dominant part of the systematic error comes from the residence time calculation, we tested this hypothesis by calculating the extra (\"missing\") time necessary to compensate for the \(\Delta\)NO\({}_{2,\ CAPS}\). The corresponding values ranged from 0 to 5 s during the first period (23 January-27 January), corresponding to 0-100% increase of the initial estimation, to several several tens of seconds during the second period (27 January-2 February). Thus, the hypothesis can be rejected. Another possibility is the presence of an unaccounted light absorbent in ambient air. The \(\Delta\)NO\({}_{2,\ CAPS}\) day /night time variations are stronger during the first part of the campaign that is impacted by higher ozone mixing ratios, which is a sign for stronger photo-chemistry. Previous studies have shown that dialdehydes absorb light in the wavelength range used by the CAPS instrument to detect NO\({}_{2}\)[48; 49]. The dominant ones in the troposphere, glyoxal and methyl-glyoxal, are produced by photo-chemical oxidation of hydrocarbons (mostly isoprene [50]), which is the same mechanism that drives the formation of ozone during daytime. These compounds are therefore good candidates to explain a positive bias in the CAPS instrument. The day /night cycle is in this case caused by the equilibrium between their sources and sinks, both dominated by photo-oxidation processes [51]. The strongest and most variable disagreement is found between the QCL-Empa and the CLD instrument, with a clear difference between the first and second periods of the campaign. The highest absolute and normalized differences are observed during the first measurement period, characterized by high O\({}_{3}\) and low NO mixing ratios. These conditions are, as mentioned above, usually associated with photo-chemically aged air masses, while the second measurement period is likely dominated by freshly polluted air. This confirms the influence of more oxidized forms of nitrogen oxides on the NO\({}_{2}\) measurement by CLD, which are usually found in higher concentrations in aged air masses. The linear relationship between the NO\({}_{2}\) instruments, after common calibration, on ambient air measurements is summarized in Figure 9. Each point corresponds to the intercept and slope of a linear fit between a given instrument and the QCL-Empa. All combinations of instruments and calibration techniques are presented, using the QCL-Empa as reference. The best agreement is obtained between the two QCL instruments using the calibration factors of the direct calibration methods (CDC and ReGaS), with offsets below 50 pptv and slopes between 0.995 and 1. The best correlation between the CAPS and the QCL-Empa is also obtained using the direct calibration methods, with slopes between 0.990 and 0.995. The fit, however, reveals a non-negligible offset of the CAPS data (between 250 and 350 pptv). The least agreement is obtained for the CLD instrument with slope values from 1.079 to 1.092 and offset values comprised between 200 and 425 pptv.