The ROZE flow system is designed to achieve rapid flushing of the detection cell as required for fast concentration measurements. However, ROZE samples at ambient pressure to maximize sensitivity, necessitating high throughput with a minimal pressure differential. ROZE utilizes a linear diaphragm pump (Thomas 6025SE-150113) that can achieve a flow rate of up to 18 SLM (standard liter per minute) through the system. The pump speed can also be adjusted by varying the supply current and has three pre-set speeds (e.g., 2, 5, and 11 SLM) that can be changed by a switch on the chassis front panel. A flow meter (Honeywell AWM5104) located between the cell exhaust and the pump monitors the sample flow in real time. ROZE uses fluorinated ethylene propylene (FEP) tubing both external and internal to the chassis upstream of the sample cell. External to the chassis, the inlet details depend on the aircraft platform. ROZE has previously used the inlet detailed in Cazorla et al. (2015) when flying on the NASA DC-8 aircraft. The instrument exhaust plumbs directly to an exhaust port near the rear of the aircraft.

ROZE O3 measurements also require knowledge of the reference intensity (I0) as detailed in Eq. (1). A three-way solenoid valve (NResearch TC648T032) switches between the sample line (ambient air from the aircraft inlet) and the zero port, which attaches to an internal Carulite O3 scrubber (2B Technologies) to produce O3-free air. Periodic zeroing during operation captures long-term drift in I0 due to the LED output, PMT response, and changing environmental conditions. Typically, the instrument opens to the O3 scrubber for 10 s every 5 min.

ROZE utilizes a CompactRIO (National Instruments cRIO-9030) that incorporates a real-time operating system and a field-programmable gate array (FPGA). The FPGA is configured for modulation of the LED and subsequent digitization of the PMT signal. To improve measurement precision and remove background due to ambient light scatter, the FPGA modulates the LED at 1 kHz with a 90 % duty cycle (900 µs on and 100 µs off) via an external LED driver (Wavelength Electronics FL591FL). A 16 bit analog-to-digital converter (ADC) digitizes the amplified PMT signal at a digitization rate of 100 kHz. This high rate enables us to average each LED “on” and “off” pulse amplitude. We then take the difference of the on and off signals to remove background noise, both optical (i.e., stray light) and electronic. The 1 kHz differences are further averaged to 10 Hz and recorded. Other diagnostic housekeeping variables (e.g., sample flow, temperatures, and LED power) are recorded at 1 Hz. Additionally, an analog output commands the three-way valve to open to the zero line with a user-defined period and duration.

In practice, the absorbance calculation for ROZE factors in the pressure difference between the sample and zero lines, as derived by Min et al. (2016): 2αO3=IZI-1αcav+αRay,Z+ΔαRay. Analogous to Eq. (1), IZ is the intensity measured when sampling through the zero line (O3-scrubbed air); I is the intensity when sampling ambient air; and ΔαRay=αRay,Z-αRay,S, where αRay,Z and αRay,S give the Rayleigh extinction (αRay=NairσRay) of the zero and the sample, respectively. Using the measured IZ, I, and the known Rayleigh scatter and O3 absorption cross sections, the O3 number density can then be determined as αO3=NO3σO3. The Rayleigh scattering (Bucholtz, 1995) and O3 absorption (Serdyuchenko et al., 2014) cross sections are calculated as the weighted average over the collected spectral range (Fig. 2). Using known cross sections and a calibrated αcav (inverse effective pathlength), the observed change in intensity yields a direct measure of the O3 concentration.

The effective pathlength of the ROZE optical cavity determines the instrument sensitivity to O3 (i.e., the attenuation in intensity per unit O3). The cavity extinction, and thus the effective pathlength, are dictated by the mirror reflectivity as described above but require independent calibration. Calibration can be accomplished via standard addition of O3 or Rayleigh attenuation (in the absence of absorbing species) at varied sample pressures. The former method relies on commercially available O3 generators or sensors for verification, which lack the required accuracy and may drift over time. In contrast, the Rayleigh calibration provides a convenient and straightforward alternative. Both methods are described below.

Figure 4a depicts the ROZE calibration using known concentrations of O3. A commercial O3 source (2B Technologies 306) generated known amounts of O3, with the zero O3 addition serving as the IZ baseline. Per Eq. (2), the slope of the observed attenuation (dI=IZ/I-1) as a function of O3 number density is proportional to the remaining extinction terms (αcav+αRay). Solving for αcav using the O3 cross section and the calculated Rayleigh extinction, the calibration yields an effective pathlength of Leff=108 ± 6 m. The alternate calibration uses the Rayleigh extinction in zero air over a range of cell pressures (Fig. 4b). In the absence of absorbing species, an expression for αcav can be derived following the approach in Washenfelder et al. (2008) as 3αRay=I0I-1αcav. I0 represents the intensity at vacuum, which can be extrapolated from a linear fit of counts as a function of cell pressure. The slope of the observed change in intensity with number density therefore yields a direct measure of the cavity extinction, resulting in an effective pathlength of 104 ± 4 m. The two methods agree to within the 2σ fit uncertainties, and we use Leff as determined by the Rayleigh calibration for subsequent calculations.

The major contributions to instrument noise include PMT electrical noise and differential scatter or absorption due to non-uniform flow within the sample cell at high flow rates. The flow diffuser (see Sect. 3.1.2) effectively reduces the flow noise, while decreasing the gain on the PMT amplifier circuit minimizes the PMT electrical noise. The ROZE precision can be determined from the continuous sampling of zero air at a constant pressure. Figure 5 depicts the Allan–Werle deviation plot (1σ) for ROZE (in pptv – parts per trillion by volume – O3 equivalents) as calculated from optical extinction measurements of zero air acquired over 1.5 h at 944 mbar. For short integration times (<10 s), a fit of the data gives a τ-0.47 decay, indicating the Allan deviation closely follows the square root of the averaging time (τ-1/2) as expected for white noise. At the native 0.1 s sampling rate, the 1σ precision for O3 is 80 pptv and reduces to 31 pptv with 1 s averaging. For the given cell pressure and a temperature of 35 ∘C, this translates to a 1σ precision of 6.7×108 molecules cm-3 (1 s average) of O3.

The absolute accuracy of the ROZE measurement depends on uncertainties in the literature-reported values of the O3 and Rayleigh cross sections, the measured cell temperature and pressure, and the calibrated cavity extinction. The reported O3 absorption cross section has an uncertainty of 2 % (Gorshelev et al., 2014), and we estimate a conservative uncertainty of 3 % for the Rayleigh scattering cross section (Bucholtz, 1995). The cell pressure and temperature are accurate to within 0.2 % and 0.5 %, respectively, and the calibrated cavity extinction has an additional 4 % slope uncertainty from the linear fit. These errors propagate through Eq. (2) to yield a total measurement uncertainty of 6.2 % in the O3 number density.

The flush time of the sample cell limits the true instrument response time despite the 10 Hz data acquisition rate. A rapid flush rate is critical for high-spatial-resolution measurements from a fast-moving platform. Additionally, fast concentration measurements are required for sampling turbulent eddies for airborne EC, and the necessary time response scales with aircraft speed. Response times of 10 Hz are typically considered sufficient for ground-based EC (Aubinet et al., 2012), while for airborne EC, a response time of 1–5 Hz is typically sufficient due to larger eddy scales at altitude (Wolfe et al., 2018). Figure 6a shows the instantaneous instrument response to a 10 ms pulse of O3 injected into a zero-air carrier flow using a fast switching valve (The Lee Company IEP series). During this experiment, the pump maintained a sample flow rate of 18 SLM. A series of exponential decay fits for several O3 pulses yields an e-folding time constant of τr=50±4 ms (Fig. 6b), which corresponds to a 3e-fold cell flush rate of 9.5 Hz.

FIREX-AQ flights targeted forest wildfires and agricultural burns. In fresh, concentrated smoke plumes, UV-active species such as SO2, aromatic hydrocarbons, and other volatile organic compounds (VOCs) can give rise to positive artifacts in the O3 signal (Long et al., 2020), as the UV absorption technique lacks selectivity (see Birks, 2015). The potential for overestimating O3 due to interfering absorbers can also be of concern in highly polluted urban environments (e.g., Spicer et al., 2010). In general, these studies demonstrate that UV-absorption-based O3 analyzers are not always ideally suited to such applications. Nonetheless, modifications such using an O3-selective scrubber material (e.g., heated graphite) to preserve VOCs and thus account for interferences in the background (IZ) signal have been shown to reduce positive artifacts (Turnipseed et al., 2017). As we did not substitute the ROZE scrubber for the FIREX-AQ deployment, an onboard, independent measurement of formaldehyde (HCHO) was used as a plume indicator. ROZE O3 data are therefore quality-filtered to remove points sampled within dense smoke plumes using HCHO mixing ratios above 5 ppbv.

The DC-8 FIREX-AQ payload included the NOAA Nitrogen Oxides and Ozone (NOyO3) instrument, a well-established O3 measurement using the chemiluminescence technique (Ryerson et al., 2000; Bourgeois et al., 2020). ROZE operated simultaneously with the NOyO3 instrument during several flights. Figure 7 shows a comparison of ROZE and NOyO3 data for the 30 July 2019 flight over the northwestern United States. During this flight, no fresh smoke plumes were sampled, and no filtering of the ROZE data was necessary. Figure 7a depicts a ∼ 25 min subset of the full time series to illustrate the ROZE instrument precision. Both measurements (averaged to 1 s) track the dynamic features in O3 mixing ratios well. The correlation plot for the full flight (Fig. 7b) demonstrates strong agreement between the two measurements, with a slope of 0.98 ± 0.01 and an intercept of 0.17 ± 0.02 ppbv O3 (r2=0.99). Note the intercept is less than 1 % of the minimum observed O3 mixing ratios for this flight. Comparisons for 15 flights from the campaign indicate a range of 0.96–1.04 in slope and -1.6–1.4 ppbv O3 in intercept (in all cases, this offset is <4 % of the minimum measured O3), consistent with the measurement uncertainty.