All NOx measurements were performed using a modified two-channel chemiluminescence instrument originally purchased from ECO Physics (Dürnten, Switzerland; CLD 790 SR), as described by (Fig. 2 presents the instrument schematic), operated at a total gas flow of 3 SLM, equally divided into the two channels. NO concentrations are measured in the first channel, also referred to as the NO channel, through formation of NO2* via reaction with O3. The resulting excited NO2* emits a photon (>600 nm) detected by a photomultiplier tube and preamplifier setup, and it is recorded as counts per second. The second channel, also referred to as the NOc channel, is structurally identical except for the implementation of a photolytic converter, which converts a known fraction of NO2 to NO prior to the reaction with O3 and is operated at a constant pressure of 110 hPa (105 hPa during the CAFE Africa field experiment). NO2 concentrations are obtained from the difference in counts from each channel and the conversion efficiency Ce as described above (see Eq. ). We use a blue light converter (type 1) purchased from Droplet Measurement Technologies in 2005 (later Air Quality Design, now Teledyne API) equipped with UV LEDs emitting at a wavelength of 397 nm (FWHM=14 nm, as characterized in the laboratory), which is shown in Fig. a. The converter was designed for airborne applications. The inner material is made of porous, optically active PTFE (polytetrafluoroethylene) for providing highly reflective properties. To reduce surface effects the converter was equipped with a quartz cylinder covering approximately half of the PTFE surface (the gas still has contact with the PTFE surface in the ring channel and through the head piece). Please note that this modification was made prior to the CAFE Africa research campaign within a limited time frame and did not have the desired outcome. The sample gas enters the converter sideways into the ring channel and reaches the inner tube via the PTFE head piece, which has four circular recesses, one for each UV LED. The sample gas outlet proceeds analogously. The inner volume of the converter is V=78cm3, which gives a residence time of t=(V×60smin-1)/F×p/pstandard=(78cm3×60smin-1)/(1500cm3min-1)×110hPa/1013hPa=0.34s. The conversion efficiency for this type 1 photolytic converter operated under the conditions described above is approximately 20 % (j=0.66 s-1), which was determined via gas-phase titration (GPT) of NO with ozone. The results obtained with the described type 1 converter were compared to an alternative photolytic converter completely made from quartz glass (type 2), which is shown in Fig. b. For maintaining the reflective properties of the type 1 blue light converter, the type 2 quartz converter was jacketed with optical PTFE, which provides diffuse reflectance of >99 % in the wavelength range 350–1500 nm . The PTFE material was found to provide a higher conversion efficiency in the converter compared to aluminum foil and additionally provides a stable housing for the sensitive quartz tube. The volume of the type 2 converter is 77 cm3, which gives a residence time of t=0.33s and a conversion efficiency of approximately 14 % (j=0.46 s-1) under the operating conditions. The applied LEDs were purchased from LED ENGIN (San Jose, California, USA) (high-efficiency VIOLET LED emitter LZ1-10UB00-01U6, 2–2.2 W, 395–400 nm peak wavelength). Please note that the low conversion efficiencies in both converters result from the operation at low pressures, which we have implemented to pursue aircraft measurements during which altitude changes are accompanied by pressure variations. Operating the converter at lower than minimum ambient pressure levels (max. ∼15 km flight altitude) has the benefit of a constant conversion efficiency. The fractional dissociation of thermally unstable NOx reservoir species increases with increasing pressure in the converter, which can be seen in Fig. S1a in the Supplement. On the other hand, a higher conversion efficiency would be desirable for improved accuracy of the measurement. The main difference between the two converters is that the sample gas flow does not have direct contact with the porous surface of the material for the type 2 quartz converter. Additionally, the sample gas flow in the type 2 quartz converter does not have contact with the LEDs, which likely minimizes the sample gas heating and consequently the thermal interferences when passing through the converter. The reaction chambers (where the reaction of NO and O3 takes places) are operated at a constant temperature of 25 ∘C and a pressure of 9–10 mbar in order to minimize quenching of NO2* by other molecules. The dry ozone flow is humidified with water vapor for maintaining a constant humidity level at all times.

Besides the photons emitted from relaxation of NO2*, the PMT (photomultiplier tube) signal also includes detected photons from interference reactions, for example the reaction of O3 with alkenes , as well as a dark current signal. Therefore, a pre-chamber measurement is operated for 20 s every 5 min with ozone added to the sample gas flow. The residence time in the pre-chamber allows for the reaction of O3 and NO as well as the relaxation of NO2* before entering the main reaction chamber (pre-chamber efficiency >96 % for the NO channel and ∼100 % for the NOc channel). It is not long enough to convert interfering compounds, which then occurs in the following main chamber. Consequently, during pre-chamber measurements, the PMT signal only includes the interfering signal and the dark current signal . We subtracted the interpolated signal obtained during pre-chamber measurements from the signal detected during main chamber measurements in order to obtain the signal generated from NO. The material of both the pre-chamber and the main chamber is gold-plated stainless steel.

The instrumental background of each channel is determined via zero (synthetic) air measurements from a gas cylinder and can be converted to mixing ratios using calibration measurements with a known NO concentration, which define the sensitivity (countss-1ppbv-1 – parts per billion by volume) of each channel to NO, as shown in Eq. () (after pre-chamber corrections). The signal detected from zero air measurement (counts(zero air)) is subtracted from the signal detected from NO calibration (counts(NOcalibration)) and divided by the absolute concentration of the NO calibration (c(NOcalibration)) to calculate the sensitivity. Dividing the signal detected from zero air measurements by this value gives the instrumental background concentration in mixing ratios, e.g., ppbv. The precision is determined from the reproducibility of the NO calibrations and is  3 % (1σ). The NO concentration is 4.96±0.21 ppmv, which gives a 4 % uncertainty on the secondary standard used. The resulting NO calibration mixing ratio is 15.8±0.7 ppbv. The detection limit is given by the reproducibility of the zero air measurements, which is around 5 pptv for the NO channel and the NOc channel using the type 2 quartz converter. The detection limit is higher when using the type 1 converter, but it is difficult to determine due to the observed memory effects and is estimated at >10 pptv. sensitivity=counts(NOcalibration)-counts(zero air)c(NOcalibration)3c(background)=counts(zero air)sensitivity Please note that the utilized zero air can include a trace concentration of NOx. The manufacturer specifies the maximum concentration of NOx to be 0.1 ppmv (parts per million by volume) .

The CAFE Africa research campaign took place in August and September 2018 and included 14 measurement flights (MF03–MF16), which were performed with the HALO (High Altitude Long Range) research aircraft starting from the campaign base in Sal on Cabo Verde (16.75∘ N, 22.95∘ W). We included data measured during the measurement flights MF10, MF12, MF13, MF14 and MF15 in this analysis (MF11 was a nighttime flight and therefore excluded) for which CLD NO2 measurements were available. An overview of the flight tracks is presented in Fig. . A bypass line provided the instruments with air from the aircraft inlet for which the residence time depended on the ambient pressure level (for high altitudes<0.1 s). The sample line temperature was approximately 25 ∘C. More details on the campaign can be found in .

NO and NO2 were measured via photolysis–chemiluminescence with the instrument described in Sect.  using the type 1 conventional blue light converter equipped with the quartz glass cylinder, operated at a temperature of 313 K and a pressure of 105 hPa (0.32 s residence time). Please note that it was not possible to measure the temperature inside the converter. Instead, the temperature of the gas outflow from the converter in the ring channel was measured, which we equate to the inner temperature. Zero air measurements and NO calibrations using a secondary NO standard (cylinder concentration of 1.187±0.036 ppmv and calibration mixing ratio of 2.97±0.09 ppbv) were performed regularly to determine the variability in the instrumental background and the sensitivity of the channels. The ambient measurement was interrupted every 1–2 h by a 1 min zero air measurement, followed by 1 min NO calibration and another 1 min zero air measurement. These calibration background cycles (CB cycles) were performed four to six times during each measurement flight. We linearly interpolated these instrumental background and calibration measurements to each entire flight. The NO data were processed as described by (5 pptv detection limit at 1 min integration time and 6 % relative uncertainty, 1σ). Please note that the instrumental background for the NO data was determined at 5.0±5.3 pptv during a nighttime measurement on 26 August 2018 of NO as presented by and previously described by . The instrumental background determined via zero air measurement was similar with 4±7 pptv (measured four to six times per MF). For the NO2 data, the NOc channel sensitivity and the instrumental background concentration were calculated after pre-chamber correction according to Eq. (). The ambient NOc signal was divided by the channel sensitivity accordingly. Final NO2 concentrations were determined via Eq. (); this includes subtraction of the calculated (and interpolated) NOc instrumental background concentration and the NO concentration in the NO channel and dividing by the conversion efficiency of the blue light converter, which was 24.2±2.8 % during the campaign. 4c(NO2)=c(NOc)-c(backgroundNOc)-c(NO)Ce

Additional measurements of atmospheric trace gases during CAFE Africa including O3, CO, CH4, HO2, OH, NO2 and water vapor as well as the photolysis frequencies jNO2 and jPNA were used in this study. O3 was measured via UV absorption and chemiluminescence with the FAIRO (Fast AIRborne Ozone) instrument (total measurement uncertainty of 2.5 %; ). CO and CH4 were measured via quantum cascade laser absorption spectroscopy (total measurement uncertainty of 4.3 % and 0.3 %, respectively; ). HO2 and OH were measured with the custom-built HORUS (HydrOxyl Radical measurement Unit based on fluorescence Spectroscopy) instrument via fluorescence spectroscopy . Please note that these data are still preliminary, and the measurement uncertainty is estimated at 50 %. Additional NO2 concentrations for comparison were measured via differential optical absorption spectroscopy (miniDOAS) with a detection limit of about 5 pptv and an uncertainty depending on the altitude and cloud cover of typically 40 pptv . Water vapor was measured via direct absorption by the tunable diode laser system SHARC (Sophisticated Hygrometer for Atmospheric ResearCh) (accuracy of 5 %, detection limit typically in the range of 2–3 ppmv) . The photolysis frequencies were calculated from actinic flux densities measured with a spectral radiometer (Meteorologie Consult GmbH, Metcon, Koenigstein, Germany) (uncertainty<15 %) . Please note that all measurement data were converted to a uniform timescale with a 1 min time resolution as a basis for this analysis.

For calculating the photostationary-state NO2 concentrations during CAFE Africa, we assume that NO2 production occurs through reaction of NO with O3 (Reaction ), HO2 (Reaction ) and RO2 (Reaction ). showed that RO2 is well-represented by CH3O2 during CAFE Africa via model simulations (80 % at 200 hPa of altitude and up to 90 % below), which we therefore use as surrogate for describing all organic peroxy radicals. In analogy to , we describe NO2 loss by photodissociation as shown in Reaction (). Other loss pathways for NO2, for example, via OH can be neglected (<1 %) . R3NO+HO2→NO2+OHR4NO+CH3O2→NO2+CH3O NO2 concentration in photostationary state can therefore be obtained via Eq. (), whereas the concentration of CH3O2 is calculated with the help of Eq. (), which was derived by . For the calculation via Eq. () we assume that CH3O2 and HO2 formation occurs through CH4 and CO oxidation, respectively. We estimate an uncertainty of around 20 % resulting from these assumptions. Propagating the measurement uncertainties of HO2, CH4 and CO suggests a 50 % uncertainty in the calculated CH3O2 data. The NO2 PSS data have an uncertainty of 22 % regarding the trace gas measurements according to Gaussian error propagation (uncertainty of rate coefficients is considered negligible). 5[NO2]PSS=[NO]×(kNO+O3×[O3]+kNO+HO2×[HO2]+kNO+CH3O2×[CH3O2])jNO26[CH3O2]=kCH4+OH×[CH4]kCO+OH×[CO]×[HO2] The temperature-dependent rate coefficients were obtained from the data sheets of the .

We consider the NO2 reservoir species PAN (peroxyacetyl nitrate), MPN (methyl peroxy nitrate) and PNA (pernitric acid). PAN was measured during CAFE Africa via chemical ionization mass spectrometry (CIMS) . MPN and PNA were not measured and instead estimated via photostationary-state calculations as suggested by . PNA (HO2NO2) production occurs through reaction of HO2 and NO2 (), while PNA loss is described by Reactions ()–() either through thermal decomposition, photolysis or reaction with OH . PSS HO2NO2 concentrations can then be calculated via Eq. (). k is the rate coefficient for each reaction and jPNA is the photolysis frequency for Reaction (). R5HO2+NO2+M→HO2NO2+MR6HO2NO2+M→HO2+NO2+MR7HO2NO2+hν→productsR8HO2NO2+OH→products 7[HO2NO2]PSS=k5×[HO2][NO2]PSSk6+jPNA+k8×[OH] MPN production and loss terms are in analogy to PNA, as shown in Reactions ()–(), except for the reaction with OH, which is negligible . The calculation in PSS is performed via Eq. (). k represents the rate coefficients and jMPN is the photolysis frequency for Reaction (). During CAFE Africa, only the photolysis frequency jPNA was evaluated because reliable molecular data for MPN were missing. As suggested by we assume identical UV cross sections of MPN and PNA and therefore jMPN to be identical to jPNA. R9CH3O2+NO2+M→CH3O2NO2+MR10CH3O2NO2+M→CH3O2+NO2+MR11CH3O2NO2+hν→products 8[CH3O2NO2]PSS=k9×[CH3O2][NO2]PSSk10+jMPN In the photolytic converter, PNA, MPN and PAN can decompose to NO2 depending on the temperature, pressure and residence time t according to first-order kinetics. The resulting NO2 artifact is determined via Eq. (). [NO2]artifact=[HO2NO2]PSS×(1-exp⁡(-k6×t))+[CH3O2NO2]PSS×(1-exp⁡(-k10×t))+[CH3COO2NO2]9×(1-exp⁡(-kCH3COO2NO2+M×t)) Gaussian error propagation gives an uncertainty of 55 % for the calculated PNA and MPN data. We use the residence time according to the volume and the flow rate in the photolytic converter as described earlier. The actual value could deviate from the calculated one due to unknown flow dynamics and temperature gradients. Assuming 30 % uncertainty in the residence time gives an overall uncertainty of around 60 % in the NO2 formed from PNA and MPN in the photolytic converter.

We propose that the memory effect described in the previous section is strongly affected by NO molecules and is dependent on changes in the introduced relative humidity. In order to show this, we have conducted different experiments in the laboratory to investigate the instrumental background produced by the photolytic converter in the NOc channel. Beside NO and H2O, we suggest that one or more additional factors affect the observed instrumental background signal, which are connected to the light of the LEDs and which we discuss at the end of this section.

For the first set of experiments, we exposed the converter (LEDs switched on) to 15.8 ppbv of NO for 2 h followed by 4 h zero air measurements. The first experiment was carried out under dry conditions, sampling NO and zero air directly from the gas cylinder. For the second experiment, we introduced water vapor by passing zero air through a washing bottle with deionized water before entering the instrument. The thus obtained relative humidity was ∼95 % at ambient temperature and decreased over time with decreasing water temperature (through evaporation). For the third and fourth experiment, zero air was humidified only for the zero air measurement and the NO measurement, respectively. We repeated the latter, introducing a lower relative humidity of ∼35 %, and obtained the same result.

Figure  shows the temporal development of the instrumental background signal in the NOc channel after 2 h of NO measurement with a concentration of 15.8 ppbv. The red line shows the experiment under dry conditions, and the blue line presents the experiment under humid conditions. For the yellow line, we performed the NO measurement under dry conditions and the following instrumental background measurement under humid conditions. The green line shows a humid NO measurement and dry instrumental background measurement. Figure a presents the results obtained with the type 1 blue light converter. All experiments showed a decreasing instrumental background signal over time in the NOc channel. For comparison, the instrumental background signal in the NO channel over time is presented in Fig. S5. The lines show no significant trend over time. This indicates that the converter adsorbs NO molecules, for example during NO measurements, and desorbs them during zero air measurements. At t=0 min, the instrumental background signals for all experiments were approximately the same between 90 and 100 pptv but showed a different development over time. The strongest decline by around 25 % over 250 min was observed for a dry instrumental background measurement after a humid NO measurement (green curve). For the opposite case, a humid instrumental background measurement after a dry NO measurement, the signal decreased by around 5 % over the observed time period (yellow curve). Performing the whole experiment under dry or humid conditions (red and blue curve, respectively) had a similar outcome, with an instrumental background signal decrease of approximately 10 %. These observations indicate that the instrumental background measurements in the NOc channel, which were performed during CAFE Africa and used for the data processing according to Eq. (), were consistently too high as they were only run for 2×1 min (per CB cycle) and therefore did not represent the actual instrumental background but an artifact signal. In the laboratory, we observed the strongest effect for a dry instrumental background measurement following a humid NO measurement, which was a likely scenario for the ambient monitoring during CAFE Africa as zero air for an instrumental background measurement was sampled from a gas cylinder (dry conditions) and the measured ambient concentration was subject to ambient meteorological conditions. This would explain the occurrence of negative NO2 concentrations obtained from CLD measurements as mentioned earlier. These experiments also suggest that water molecules might not just promote adsorptive or desorptive processes, but also participate in the surface allocation themselves and compete with NO. Following this hypothesis, the surface spaces would fill with NO during an NO measurement under dry conditions and with both NO and H2O molecules under wet conditions. A subsequent instrumental background measurement under dry conditions would then lead to NO (and possibly H2O) desorption. For a subsequent instrumental background measurement under humid conditions, H2O molecules could actively replace NO molecules because of their higher surface affinity, leading to a temporally longer “leakage” of NO and a slower decrease in the instrumental background signal. The described effect would be highest for a dry NO measurement, which gives the maximum NO surface coverage and a wet zero air measurement (yellow), while it would be lowest for the opposite case for which we observed the lowest instrumental background signal after 250 min (green). This hypothesis fits well with the observations during CAFE Africa presented in Sect. ; we observed a sharp increase in the NO2 signal along with rapid increases in water vapor concentration. H2O molecules could replace adsorbed NO molecules (or other atmospheric compounds), which were detected by the CLD as an artifact signal. Because of the conversion efficiency of 24.4 % of the type 1 blue light converter, the signal difference from the NO and NOc channel was multiplied by a factor of around 4 (compare Eq. ). Therefore, the resulting NO2 signal was distorted by 4 times the actual desorbed NO, explaining the large peaks accompanying altitude descents. As we do not observe these effects in the NO channel (and the two channels are structurally identical), we can exclude that any of the humidity effects are caused by components in the instrument other than the photolytic converter.

We repeated the same laboratory experiments using the type 2 photolytic converter. The resulting temporal development of the instrumental background is presented in Fig. b, which shows significant improvements compared to the type 1 converter. The instrumental background in the NOc channel was many times smaller for the type 2 quartz converter, with mixing ratios of around 10 to 15 pptv, and was, most importantly, constant over time. For stationary long-term experiments it could be possible to measure the instrumental background on the scale of hours. However, for aircraft measurements and the accompanying rapid air mass changes due to the high aircraft velocity, it is vital to obtain a reliable instrumental background measurement within a short time interval, which would be possible with the type 2 converter. Furthermore, changes in humidity did not seem to impact the measurement as all four experiments show the same result. This, too, suggests the suitability of the type 2 converter in aircraft measurements or more generally in field studies impacted by high and changing humidities.

Performing zero air measurements after NO2 measurements had a similar outcome for each of the applied converters. The instrumental background measurement in the type 1 blue light converter showed a decreasing trend over time, while it was constant and significantly smaller in the type 2 quartz converter.

Our assumption that the observed effect is – at least partly – associated with NO molecules is supported by an experiment wherein we heated the type 1 blue light converter with switched-off LEDs with a heat gun and observed a sharp increase in the NOc channel during zero air measurement (following NO calibration measurement). The increase in temperature promoted the desorptive process and had to include NO molecules. Otherwise, the CLD would not have detected any increase in the signal as the converters' LEDs were switched off and NO2 could not form NO via the photolytic reaction. We show the result of the heating experiment in Fig. a. The converter surface was heated for 2 min (under constant movement of the heat gun) at a distance of around 10 cm during zero air measurement. We estimate the surface temperature to not have exceeded 200 ∘C. We observed a peak NO concentration of 2 ppbv (NOc instrumental background was 0 ppbv). In comparison, Fig. b shows the experiment repeated with the type 2 quartz converter, which showed a small increase in the NOc signal, too, but approximately 1 order of magnitude smaller compared to the type 1 blue light converter. The qualitative outcome of this experiment was the same with the LEDs switched on as well as with preceding NO2 (instead of NO) measurement. Please note that a direct comparison of experiments regarding adsorptive and desorptive processes with switched-on and -off LEDs is difficult because the operation of the LEDs increases the temperature within the converter, which, as shown above, strongly impacts the surface allocation.

Beyond that, we performed an experiment to investigate how NO calibration measurements affect subsequent zero air measurements in response to different NO concentrations. Figure  shows the influence of the preceding NO concentration level on the following first 5 min of zero air measurement. We have performed 30 min NO calibrations with NO concentrations between 0.25 and 10 ppbv. Red data points represent the instrumental background of the NO channel, and green data points show the instrumental background of the NOc channel. Instrumental background concentrations in the NO channel were independent of preceding NO concentrations. In contrast, for the type 1 blue light converter, instrumental background concentrations in the NOc channel increased with increasing NO concentrations and leveled off for high values, as shown in Fig. a. Measured NO concentrations during CAFE Africa were between 0 and 1 ppbv and were therefore situated in the rising part of the curve. That shows that instrumental background measurements during CAFE Africa were not only too high, but also depended on the preceding NO concentration. We tried to retrospectively correct the NO2 data with a lower instrumental background as obtained from laboratory investigations after several hours of zero air sampling. However, it was not possible to quantify the effect of varying the preceding NO levels. Additionally, the impact of humidity had the exact opposite effect on the NO2 measurements. While the higher-than-actual instrumental background led to lower-than-actual NO2 concentrations, increases in humidity triggered higher-than-actual NO2 concentrations. Figure b shows that the development of the instrumental background in the NOc channel did not depend on the preceding NO concentrations for the type 2 quartz converter. The instrumental background was 12±1 pptv and constant over the whole experiment. The detected instrumental background signal for the NOc channel disappeared when switching off the light of the UV LEDs. A possible explanation can be a trace concentration of NO2 in the utilized synthetic air as mentioned earlier.

While we have shown above that NO and humidity strongly affect the instrumental background measurements in the type 1 blue light converter, it is likely that there are other factors contributing to the observed effects, too. When switching off the LEDs in the type 1 converter, the observed instrumental background decreased rapidly (too rapidly for a sole temperature effect), which we present in Fig. S6a. This suggests that the light of the LEDs impacts the instrumental background in the NOc channel. Many other compounds can be photolyzed to form NO, such as PAN, ClNO2 or BrONO2. However, their absorption cross sections suggest no interference at 397 nm, which is the spectral output maximum of the LEDs . Only small interference could occur with HONO and NO3 at the edge of the spectral output, and this would require the presence of these compounds in the converter, which should not be the case for the described laboratory investigations but is conceivable given the memory effect observations. For comparison, Fig. S6b shows that the effect of switching the LEDs on and off during zero air measurement is marginal when using the type 2 quartz converter. We have performed an uptake experiment for HNO3 (nitric acid) to investigate the adsorptive capacity of the converters. HNO3 in zero air (2500 sccm) was first routed through a bypass, and after reaching a constant signal, the gas flow was changed to include the converter. The HNO3 concentration behind the converter was monitored via chemical ionization mass spectrometry (CIMS). Figure S7a and b show the resulting adsorption behavior for the type 1 and the type 2 converter, respectively. When directing the gas flow through the type 1 converter, the detected HNO3 flux decreased rapidly by around a factor of 4, and we did not observe the signal return to its initial value within 40 min (which is when we terminated the experiment). This indicates a high absorptive capacity or a decay of HNO3 in the converter (or both). Integration of the HNO3 flux shows that the converter took up approximately 1.7×1016 HNO3 molecules in the considered time frame. In contrast, for the type 2 converter, the HNO3 flux decreased, too, but returned to its initial value within 10 min, while it adsorbed only ∼1.7×1015 HNO3 molecules. The observed adsorption capacity can be minimized by coating the quartz surface with FEP (fluorinated ethylene propylene), which provides a highly hydrophobic surface . The number of adsorbed HNO3 molecules was a factor of 4–5 smaller compared to the non-coated quartz converter (Fig. S7c). We did not observe any differences between the coated and non-coated quartz converter regarding the experiments investigating the role of NO and humidity presented above. This uptake experiment shows the high adsorptive capacity of the type 1 blue light converter in comparison to the type 2 quartz converter. In the case of HNO3, which could have been adsorbed by the type 1 converter, e.g., during stratospheric measurements, we hypothesize a potential source of NO or NO2 through, for example, surface-catalyzed chemistry, possibly involving the light of the LEDs or elevated temperature. Again, this experiment underlines the superiority of the type 2 quartz converter over the type 1 blue light converter and suggests applicability for ambient and airborne measurements.