Development of a broadband cavity-enhanced absorption spectrometer for simultaneous measurements of ambient NO 3 , NO 2 , and H 2 O

. We describe the characteristics and performances of our newly built broadband cavity-enhanced absorption spectrometer for measurements of nitrate radical (NO 3 ), nitrogen dioxide (NO 2 ), and water vapor (H 2 O). A customized 10 vibration-resistance cavity layout incorporated with N 2 purging on high-reflection mirror surfaces was implemented with a red light-emitting diode (LED) as a light source. In general, this system achieved over 40 km (up to 101.5 km) of effective light path length at 662 nm from a 0.52 m long cavity. For the accurate NO 3 measurement, the measured absorption spectrum of H 2 O was used for simultaneous concentration retrievals with the other species, instead of being treated as interferences to be removed or corrected prior to NO 3 detection. Synthesized N 2 O 5 crystals under atmospheric pressure were used for performance 15 tests of linear response and transmission efficiency. From the standard injection experiments of NO 3 , NO 2 , and H 2 O, high linearities were observed (R 2 ≥ 0.9918). The total NO 3 transmission efficiency through the system was determined to be 81.2 % (±2.9, 1σ) within the residence time of 2.59 seconds. The precisions (1σ) of NO 3 , NO 2 , and H 2 O in 1 Hz measurement from a single pixel on the CCD were 1.41 pptv, 6.92 ppbv, and 35.0 ppmv with uncertainties of 10.8, 5.2, and ≥ 20.5 %, respectively, mainly from the errors al. observed non-negligible amount of N 2 O 5 (up to 35 pptv) during the daytime under high NO x conditions and Foulds et al. (2021) found competitive NO 3 loss by VOC oxidation with photolysis, even in daytime. These findings indicate that the impacts of NO 3 -driven chemistry is not limited to 50 nighttime.


Introduction
The nitrate radical (NO3) has drawn considerable attention due to its significant influence on nocturnal nitrogen oxide chemistry after the first observation in the troposphere (Noxon et al., 1980;Platt et al., 1980;Brown and Stutz, 2012). NO3 is 25 mainly produced from the oxidation of nitrogen dioxide (NO2) by ozone (O3, R1) and is in thermal equilibrium with dinitrogen pentoxide (N2O5) from its further combination reaction with NO2 (R2).

40
As shown in reaction (R6), reservoir species of NO3, N2O5, can undergo heterogeneous reaction to form gaseous nitryl chloride (ClNO2) and/or aqueous nitrate . This uptake on aerosol can act as a dominant nitrate formation path under haze events (Chang et al., 2018;McDuffie et al., 2019;Lin et al., 2020;Liu et al., 2020). Meanwhile, ClNO2 can be photolyzed to NO2 and Cl radical after following sunrise; consequently, it does not only conserve NOx (=NO + NO2) but also accelerate oxidation speed by adding the Cl radical into the atmosphere Le Breton et al., 2018). 45 In addition, the important roles of daytime NO3 and N2O5 have been reported in terms of their contributions to VOC oxidation and aerosol evolution (Geyer, 2003;Brown et al., 2005;Osthoff et al., 2006;Wang et al., 2014;Brown et al., 2016;Brown et al., 2017;Wang et al., 2020;Foulds et al., 2021). Brown et al. (2017) observed non-negligible amount of N2O5 (up to 35 pptv) during the daytime under high NOx conditions and Foulds et al. (2021) found competitive NO3 loss by VOC oxidation with photolysis, even in daytime. These findings indicate that the impacts of NO3-driven chemistry is not limited to 50 nighttime.
One of the main difficulties for accurate NO3 measurement by CEAS is H2O treatment. Due to the strong but narrow absorption by H2O around 660 nm (Ball and Jones, 2003), the contribution of H2O on light extinction needs to be well 70 characterized. Several methods such as look-up table (Langridge et al., 2008;Varma et al., 2009;Suhail et al., 2019), iterative calculation (Kennedy et al., 2011), and frequent NO3 zeroing via NO titration (Wang et al., 2017) were adopted to overcome this issue.
Based on the capability of species-specific absorption cross-section measurements by CEAS (Axson et al., 2011;Chen and Venables, 2011;Young et al., 2011;Kahan et al., 2012;Sheps, 2013;Thalman and Volkamer, 2013;Young et al., 2014;75 Prakash et al., 2018;He et al., 2021;Wang et al., 2022), we suggest a new approach using measured H2O spectrum for simultaneous quantification of NO3, NO2, and H2O, which is simple and efficient enough for atmospheric application. Through this manuscript, we present not only our newly built broadband cavity-enhanced absorption spectrometer (BBCEAS) with detailed descriptions of design and performances but also linearity test results with H2O in atmospherically relevant ranges.
Moreover, we also show the results of the shipborne measurement of NO3 in the Arctic Ocean, indicating successful 80 performance of the instrument in field application.

Instrumental setup
BBCEAS is a sensitive technique to directly measure the abundance of target species and/or its optical extinction properties, introduced by Fiedler et al. (2003). Details on the working principle can be found in Ball and Jones (2003) as well as Gagliardi and Loock (2014). Briefly, this technique is based on the measurement of light extinctions in a relatively broad wavelength 85 range. System based on this method basically consists of a broadband light source, a high-finesse optical cavity formed by a pair of high-reflection (HR) mirrors, and detector(s). The light from the source resonates inside the cavity and rapidly reaches a steady state with the wavelength-specific attenuated intensity through the loss processes of (1) transmission, diffraction, and absorption by HR mirrors, (2) scattering by particles and gases (i.e. Mie and Rayleigh scattering), and (3) absorption by sampled trace gases in the cavity. The transmitted light from the cavity is then detected by the spectrometer and charge-coupled 90 device (CCD) or photodiode array to monitor the light extinction spectrum.
The schematic diagram of our single channel BBCEAS is shown in Figure 1. The material, dimensions, and design of the novel cage system on optical mounts for stable performance with respect to vibration and pressure changes during the field deployment were adopted from the glyoxal (CHOCHO) and nitrous acid (HONO) instrument described in Min et al. (2016).
Detailed descriptions of the optical layout, flow system, and data acquisition system are described in the following section. 95

Optical layout
An LED (LZ1-10R202-0000, LedEngin, Germany) centered at 660 nm is used as a broadband light source and is mounted on a home-built module with copper plate, thermoelectric cooler, heatsink, XYZ translator (LP-1A-XYZ, Newport Corp., USA), and fan to control the position and temperature (20±0.1 °C) precisely. The light from the LED is collimated by an offaxis parabolic mirror (50328AU, Newport Corp., USA) and enters the cavity. The HR mirrors (FiveNine Optics, USA) for 100 high-finesse optical cavity are mounted at a distance of 51.5 cm occupied by a Teflon cell (1 in. outer diameter (o.d.), and 1 mm thickness). The light exiting the cavity is then focused by another parabolic mirror (50331AU, Newport Corp., USA), filtered through a colored band-pass filter (660±5 nm, FB-660-10, Thorlabs Inc., USA), and coupled to a fiber collimator (74-UV, Ocean Optics Inc., USA). The customized optic fiber (Seokwang Optical Co., Korea) with linearly assembled seven 200 μm diameter cores are aligned along the slit axis of the spectrometer (HRS-300MS-NI, Princeton Instruments Inc., USA). 105 Light is then transmitted to a diffraction grating (1200 G/mm, 750 nm blaze, 68 mm × 68 mm) and dispersed with respect to its wavelength. A CCD is used as a detector (PIXIS-2KX, Princeton Instruments Inc., USA) to monitor the spectra of the final transmitted light intensity, and is cooled to -70 °C to minimize the dark current. The wavelength coverage from 632 to 691 nm and the spectral resolution of 0.47 nm as full width at half maximum (FHWM) were calibrated with the narrow Ne emission lines (NE-2, Ocean Optics Inc., USA). The entire optical layout was housed in a temperature-controlled optic box to maintain 110 constant performances regardless of environmental changes.
The overall configuration of the optics is similar to the system described by Min et al. (2016); however, the main difference is that we have an independent cavity for individual channel rather than two channels sharing one parabolic mirror plate. This modification minimizes potential interferences of light leaking from the adjacent cell and provides convenience in operation and maintenance aspects (i.e. HR mirror cleaning and LED swapping). 115

Flow and data acquisition system
Air sample is drawn into perfluoroalkoxy alkenes (PFA) inlet tubing with a constant flow rate of 2.5 slpm (standard liter per minute) by a mass flow controller (Alicat Scientific, USA) and a scroll pump (IDP-3, Agilent Technologies Inc., USA). The design of the coaxial inlet following Min et al. (2016) is used to minimize the pressure change during the mirror reflectivity measurements and sampling cycles (see Sect. 3.1.1). On the downstream of the inlet, a 2 µm polytetra-fluoroethylene (PTFE) 120 membrane filter (R2PJ047, Pall Corporation, USA) is used to minimize the light extinction owing to Mie scattering by sampled aerosols in the cavity as well as to prevent reflectivity degradation of HR mirrors. The air then passes through 1/16 in. inner diameter (i.d.) PFA tubing into the cavity to minimize NO3 loss by shortening the residence time with the scheme of reduced pressure operation as in Fuchs et al. (2008). The temperature and pressure inside the cavity are measured at the outflow of the cell. 125 Another difference with the cavity system in Min et al. (2016) is the addition of purging system. As an active strategy to prevent sampled air contact on HR mirror surface, ultra-high purity (UHP) N2 flows (>99.999 %, 20 standard cubic centimeter per minute, sccm) are introduced on each side of the cavity mirrors via custom-designed PFA cell flanges with orifices (50 µm, SS-1/8-Tube-50, Lenox Laser, USA). The effect of purge flow on the volume occupied by the air sample in the cavity was estimated as in Sect. 3.1.2. 130 Our instrument is operated and controlled automatically with the customized software programmed by LabView (National Instruments) for flow rates and temperatures of LED and CCD. The transmitted light spectra and other auxiliary data including temperatures, pressures, and flow rates for the extinction spectrum calculation are acquired via this program as well.

Characterization
From the spectrum of measured light extinction inside the cavity, ( ), the number density ( ) of species can be 135 calculated from the equation (1).
Here, is the transmission efficiency from the inlet to the cavity, corresponds to the absorption cross-section, stands for mirror reflectivity, and is effective cavity length. The ℎ and refer to the optical extinctions due to Rayleigh and Mie scattering, while we neglect owing to the aerosol filter on the inlet airway. The intensities of light transmitted 140 from the cavity with and without the absorbing species are symbolized as and ,0 , respectively. Here, we define ,0 as the light intensity when the cavity is filled with dry zero air (ZA) only. Thus, for the accurate quantification of based on equation (1), not only the reference spectrum but also the instrumental parameters such as , , ,0 , and should be characterized beforehand.

Determination of cavity parameters 145
3.1.1 Mirror reflectivity, ( ) ( ) can be derived from the well-known Rayleigh scattering differences in two species and we selected helium (He) and ZA as shown in equation (2). For our instrument, 2.75 slpm flow of He or ZA (99.999 % each, Daedeok Gas Co. Ltd.) is overflowed into coaxial inlet tubing so that 2.5 slpm flow is introduced into the cavity same as the flow rate of sample, while the rest of the flow is streamed towards the outside of the inlet to minimize the pressure change inside the cavity. 150 In equation (2), refers to the transmitted spectral intensity filled with gas species . ℎ is the theoretically calculated effective light path length under the assumption that light attenuation is solely driven by the mirror and the cavity length, .

Effective cavity length, 165
The volume occupied by the sample in the cavity should be accurately evaluated. Due to the reduction in the sample volume by addition of the purge flow, the differs from both the cavity length defined by mirror separation (51.5 cm) and the physical displacement of sample in and out ports (47.5 cm). This parameter has been commonly quantified by the injections of known amounts of standard gas such as H2O (Kennedy et al., 2011), O3 (Dubé et al., 2006;Fuchs et al., 2008), and NO2 (Schuster et al., 2009;Wang et al., 2017). In this study, we filled the cavity with 5 ppmv NO2 standard (in N2, Korea Research 170 Institute of Standards and Science, KRISS) and compared the retrieved NO2 number densities with and without the purge flow.
From this experiment, we concluded that the purge volume takes 5.25 cm 3 in the cylindrical cell (1 in. o.d. and 1 mm thickness) and thus estimated as 50.28 cm with an uncertainty of 5.2 % (1 % from the standard gas, 2.9 % and 2.6 % from retrieval errors, and 3.2 % from 2 reported by Bogumil et al. (2003)).

Absorption cross-section,
Number density is determined from the optimal fit of ( ) on the light extinction spectrum, ( ), within broad spectral ranges. For 3 , convolved spectrum of literature cross-section from Yokelson et al. (1994) was used because of its simple outstanding absorption features as shown in Figure 3. Its uncertainty was reported as 10 %.
Measured spectra were used for H2O and NO2 to compensate for the imperfections in line-shape determination. These could 180 be originated from the astigmatic bias on CCD pixels and from the environmental changes (i.e. pressure and temperature) in instrumental operation conditions. In order to minimize the impacts of these accumulated errors on concentration retrievals, measured spectra were used in previous studies (Min et al., 2016;Liang et al., 2019;Barbero et al., 2020) based on the fact that the CEAS technique is widely used to characterize the wavelength-dependent light extinction properties of chemical species (Thalman and Volkamer, 2010;Axson et al., 2011;Chen and Venables, 2011;Young et al., 2011;Kahan et al., 2012;185 Sheps, 2013;Thalman and Volkamer, 2013;Young et al., 2014;Prakash et al., 2018;Jordan et al., 2019;He et al., 2021;Wang et al., 2022).
As mentioned before, the treatment of H2O absorption features is crucial for accurate NO3 measurement due to its strong but narrow absorption lines around 660 nm. To prevent this issue, the measured H2O spectrum scaled with literature and relative humidity (RH) probe was used as 2 . For that, H2O was produced by flowing injected via the constant flow of ZA 190 through a deionized water bubbler at room temperature of 22.7 °C and pressure of 991.5 hPa with 12.3 % relative humidity as averages, while activated carbon denuder (6 mesh, Ecotech Pty Ltd., Australia) as well as Drierite filter (8 mesh, Thermo Fisher Scientific, USA) were installed on upstream of the bubbler to remove possible contaminants in ZA cylinder. An averaged H2O spectrum from 15-minute injection was scaled with the literature spectrum from HITRAN2020 database (Gordon et al., 2022) in the range of 659.28-671.94 nm and the humidity transmitter (HMT337,Vaisala,Finland,uncertainty: 195 1 %) data.
Due to the weak but complex absorption features of NO2 in the fitting range, we measured the absorption spectrum produced from 3-minute injection of NO2 standard gas (10 ppmv in N2, uncertainty 1 %, KRISS) passing through a trap immersed in dry ice to ensure H2O-free condition. The acquired spectrum was scaled to the literature one from Bogumil et al. (2003) for the range of 649.64-672.79 nm where the most apparent NO2 absorption features exist with minimum fitting error. 200 Although the application of the measured spectra improves the fitting performance in general, the uncertainties of 2 and 2 inevitably increased (≥20.1 % and 3.3 %, respectively) compared to the literature's (≥20 % and 3.2 %) owing to the additional errors caused by fitting procedure (1.3 % and 0.036 %) and number density calculation (0.86 % for both). Note that we only provide the lower limit error in 2 propagated from the absorption line-by-line uncertainties in HITRAN2020 (≥20 %). 205

Spectral fitting
The Levenberg-Marquardt least-squares fitting software, DOASIS (Kraus, 2006), was applied for the spectral fitting of the extinction spectrum between 659.28 and 671.94 nm. Fourth-order polynomial was applied to account for the optical drift and/or unaccounted extinctions such as absorption by ambient ozone. Fit order was selected based on the resulted fitting statistics (i.e. fit coefficient uncertainties, root mean square of residuals, and chi-square of residuals, Figure S1), which needs 210 to be verified for different measurement applications. The reference spectra were allowed to be shifted within ±1.0 nm and squeezed freely for the 3 but the measured ones were set to share the degree of horizontal shift and squeeze together. Figure  4 shows an example of simultaneous retrievals of 5.45 pptv NO3, 5.75 ppbv NO2, and 5620 ppmv H2O with polynomial and fit residual from the ambient measurement during the Arctic shipborne mission (acquired at the open ocean on 26 August 2021, 17:11:41 UTC described in Sect. 4) with 2 seconds integration time. 215

NO3 wall loss evaluation
Even though CEAS is able to measure the target species without concentration calibration using chemical standards in regular operation, indeed, the loss of NO3 along the airway needs to be evaluated. Many previous works carefully characterized transmission efficiency of NO3 for their instruments (Aldener et al., 2006;Dubé et al., 2006;Fuchs et al., 2008;Schuster et al., 2009;Kennedy et al., 2011;Wagner et al., 2011;Hu et al., 2014;Wang et al., 2015;Sobanski et al., 2016;Wang et al., 220 2017;Li et al., 2018). In line with this, we conducted a series of NO3 injection experiments with a custom-built NO3 generation system.

NO3 generation
NO3 was delivered from the synthesized N2O5 crystal under atmospheric pressure with the home-built system ( Figure 5).
To produce gaseous N2O5 via reactions (R1) and (R2), 5 to 10 % of O3 was generated by corona discharge (Nano 15, Absolute 225 Systems Inc., Canada) of O2 (300 sccm of 99.999 % O2 in N2, Daedeok Gas Co. Ltd.). NO2 (2 % in N2, AirKorea Co. Ltd.) was added in two different positions (500 and 200 sccm, respectively) into a quartz reactor (5 cm i.d. and 50 cm length) for efficient production of N2O5 crystals. To minimize HNO3 formation in the reactor, any H2O which can be present in the gas supplies as well as on all surfaces in the generation system was removed by heating the reactor (up to 120 °C) before the injection and by applying dry ice traps in front of the reactor during the synthesis. After the synthesis, additional O3 was 230 introduced on white N2O5 crystals to flush out the remaining HNO3 for at least 30 minutes. The crystals were used immediately or trapped with dry ice and stored at -78 °C for later use.
Sub-ppbv to a few tens ppbv of NO3 was produced by thermal equilibrium with the sublimated N2O5 introduced with a small flow of dry ZA (15-40 sccm) as a carrier gas passing the trap. Unlike previous studies Kennedy et al., 2011;Odame-Ankrah and Osthoff, 2011;Wang et al., 2015;Wu et al., 2020), we did not provide any additional heat to shift 235 the equilibrium towards NO3 because the amount of NO3 through this method was large enough to cover the ranges of typical atmospheric NO3 mixing ratios in urban night conditions.

NO3 transmission efficiency,
NO3 losses ( 3 ) of individual parts along the airway before the detection region (i.e. overflow inlet, filter, and cavity cell) were quantified via continuous injection (at least 5 minutes) of the synthesized NO3 under dark condition. Differences in NO3 240 concentrations with and without each part of the flow system were acquired by periodic switching between two conditions under the same residence time using a three-way solenoid valve. In order to determine the effect of NO3 loss on the aerosolaccumulated filter, filters with total ambient suspended particle loadings of 2480, 4075, and 12308 µg 203, 335, and 1010 µg cm -3 were compared with a clean one.  Wang et al. (2015)). Interestingly, the used filters showed no significant differences compared to the clean one regardless of the ambient aerosol loadings within the experimental range (93.1 (±0.1, 1σ), 92.9 (±0.1), and 92.7 (±2.0) % for 2480, 4075, and 12308 µg 203, 335, and 1010 µg cm -3 , respectively) which agree with Fuchs et al. (2008) and Zhou et al. (2018). From the results, total 3 was quantified to be 81.2 % (±2.9, 1σ) under 2.5 slpm sampling condition. 260

Linearity tests
To test the linearity in signal response against the concentrations of species, standard injection experiments were performed.
Multiple mixing ratios were achieved by regulating the degree of dilutions in synthesized N2O5 crystals, NO2 standard (5 ppmv in N2, KRISS, uncertainty: 1 %), and deionized water from the bubbler. As described in section 3.3.2, a slow and steady increase in NO3 was observed, varying from 746 to 1045 pptv under the constant dilution ratio of 1:150 from beginning to end 265 of throughout the experiment ( Figure S2). For tracking this drift in NO3 standard, we alternated various dilution conditions with the base one (dilution ratio set as 1:150, black markers in Figure 7 (a, b)) and applied the linear interpolation of retrieved NO3 concentrations in those conditions (gray dotted line in Figure S2 (a)). This baseline which depicts the changes in NO3 source drift was subtracted from the data for both constant (black) and different (red) dilution conditions. Figure 7 shows the results of standard additions with respect to the elapsed time (left) and other independent concentration 270 evaluation parameters (right; i.e. dilution ratio for NO3, nominal concentration for NO2, and independent RH measurement for H2O). Dashed lines in Figure 7 (b, d, and f) represent the correlations considering their errors (uncertainty of the parameter on x-axis and 1σ variabilities in measurements on y-axis). Dilution ratio of NO3 is calculated from the flow rate of ZA passed over the source divided by the total flow rate and NO2 standard concentration is from nominal concentration on manufacturer's specification with dilution ratio. RH was measured at the inlet tip by the humidity transmitter (HMT337, Vaisala, Finland) 275 with the measurement uncertainty of 1 %.
All species show good linearities (R 2 of 0.9918, 0.9985, and 0.9980 for NO3, NO2, and H2O, respectively) indicating the feasibility of atmospheric applications on those species. For NO2, the intercept of 0.12 ppbv is insignificant considering the limit of detection (will be discussed in Sect. 3.5). However, the intercept of -143 pptv in NO3 is larger than the observed precision of the instrument (Sect. 3.5). It is likely due to the variabilities in our NO3 source and/or the variations in offsets of 280 slow drift correction since the source has the minimum flow rate requirements to operate. In addition, the negligible retrieved NO3 (0.077±1.46 pptv, average and 1σ for 1s integration data) during the N2 injection experiment, described in Sect. 3.5, can be used as an alternative to evaluate the zero offsets.
For H2O, 84 ppmv of the intercept was found which is in a similar order of magnitude as with its precision (35 ppmv for 1 s integration time). This may be attributable to not only the random noise in detection but also the zero offset in the humidity 285 transmitter (uncertainty: 1 %). From this highly linear response of our instrument in H2O measurements in varying atmospheric relevant ranges, we would like to emphasize that the difficulties in retrieval in NO3 measurement due to H2O can be alleviated by simultaneous measurement with instrument-specific absorption spectrum of H2O without any pre-treatment to remove it.

Precision and accuracy
Allan deviation method is often used to determine the instrumental precision and the optimal integration time (Allan, 1966;290 Werle et al., 1993). Minimum detectable extinction for each pixel of the CCD was extracted from 1 hour injection of UHP N2 with 1 second integration time. Figure 8 (a) shows the time series of the light extinction ( 2 ) at 662 nm (corresponding to 1024th pixel) where NO3 absorbance peaks. 2 shows no significant time-dependent changes and it deviates around zero, which is likely dominated by white noise. Figure 8 (b) shows the Allan deviation for the single pixel corresponding to 662 nm.
Up to around 900 s, 1σ precision generally follows the statistical limit which implies that there are no significant integration 295 time-dependent systematic errors up to 900 s but it starts to gradually diverge after that, which is likely due to the instrumental drifts such as changes in conditions of the light source and/or CCD.
By only using the corresponding absorption cross-section on that single pixel, the detection limits for NO3, NO2, and H2O are determined to be 1.41 (0.15) pptv, 6.92 (0.73) ppbv, and 35.0 (3.69) ppmv, respectively, for 1 (60) second(s) averaging under 1 atm and 25 °C condition, by following Fouqueau et al. (2020). However, one should note that the spectral retrievals 300 through the optimized fitting algorithm are likely to produce even lower detection limits than those from the single pixel because this method relies on the absorption features on broad wavelength ranges among hundreds of pixels.
Measurement uncertainties (1σ) for NO3, NO2, and H2O are calculated to be 10.8, 5.2, and ≥20.5 %, respectively, by Gaussian propagation of the errors in absorption cross-section (NO3: 10 %, NO2: 3.3 %, and H2O: ≥20.1 %), effective cavity length (3.4 %), and HR mirror reflectivity (2.2 %). Note that the fitting errors are not included here because the mathematical 305 error varies with target species abundance. If there are strong signatures of target species in the measurement, fitting errors are negligible. However, for the extremely low abundance condition, the absorption features of target species weaken and thus the fit result likely end up to produce physically meaningless number with large error because of the limitations in numerical fitting algorithms. Hence, when the fitting error outweighs the abundance of target species, interpretations should be limited. Table 1 summarized the cavity characteristics and performances of the existing BBCEASs for NO3 measurement. Inferred 310 detection limits with the same integration time are available in Table S1. Our instrument has the longest effective light path length even though the mirror displacement is relatively short. Since this system is able to observe the optical extinctions in the order of 10 -10 cm -1 within 1 second time resolution, we can conclude that our instrument is adequate for measuring ambient NO3 abundances in terms of sensitivity aspect. The capabilities of operation and utilization for actual application are described in the following section. 315

Field deployment
In order to demonstrate the feasibility of the instrument in field measurements, we deployed our system on the Korean ice breaker R/V Araon and operated from late July to early September in 2021 for the expedition in Chukchi Sea and East Siberian Sea of Arctic Ocean (Figure 9 (a)). The instrument was housed in a temperature-controlled seatainer placed on the compass deck (29 m above sea level). Inlet was installed on the window and covered by the weatherproof-designed stainless steel pipe 320 (7.5 cm o.d.). To minimize the loss of NO3 along the sampling line, air was subsampled from the center of main flow (1 in. o.d., 1 mm thickness, PFA tubing, 20 slpm). The profile of the main flow was maintained to be steady and laminar (Reynolds number ≅ 1230) by the blower Manseung Electric Co.,Korea). Total length and residence time inside the main flow was kept to be as short as possible to minimize the loss. However, due to the physical limitation of the instrument placement in the seatainer, the length of subsampled PFA tubes was elongated (length: <1 m, residence time: <1.5 s) and the 325 total transmission efficiency of NO3 for this deployment was decreased by 65.1 % (±2.14, 1σ), quantified by post-campaign experiments through the same method as described in Sect. 3.3. Aerosol filters were replaced by an integrated auto-filter changer (Dubé et al., 2006) only during the early and later stage of the mission near the coastal region of Northeast Asia for every three hours and manually changed with intervals of four to six hours in remote region since the demands for changing the filter were scarce due to the low aerosol loading in the Arctic 330 region. The ( ) and ,0 ( ) were checked every two hours and instantaneously interpolated for real-time ( ) calculation. During the campaign, ( = 662 nm) were varying in the range from 99.9985 to 99.9989 %, which are lower than the best performances of the instrument in the laboratory, mainly due to the difference in proficiency and environment for cleaning optics but still high enough for ambient monitoring of NO3. The negligible change in is a direct evidence that our vibrationresistant design is robust despite the strong vibrations in the platform due to the sea ice breaking activities. During the mission, 335 the averages (±1σ) of temperature and pressure of the sample in the cavity were 22.02 (±0.90) °C and 997.6 (±25.9) hPa. And the changes in absorption cross-sections due to these variations were small to be detected by our instrument. when the NO3 signals were continuously observed well above the detection limit. H2O mixing ratios measured by our instrument were ranged from 4160 to 6510 ppmv (mean of 5580 ppmv) and average (maximum) value(s) of NO2 and NO3 were 3.21 (23.9) ppbv and 2.53 (9.51) pptv, respectively. H2O concentrations measured by both instruments were in good 345 agreement considering the uncertainty of H2O for our instrument (≥20.5 %).
During the campaign, several fresh emissions from R/V Araon were observed, represented by sharp changes in O3 and NO2 as shown in Figure 9 (c) indicating that O3 was titrated by NO and formed NO2. These exhaust emissions lasted from a few minutes to several hours depending on the atmospheric conditions such as wind direction and atmospheric stability as well as 350 movement of the vessel (i.e. sailing or anchoring in one place for marine survey). P(NO3) was generally small but rose up to 0.66 ppbv hr -1 (mean of 0.21 ppbv hr -1 ) depending on whether the sampled air masses were directly influenced by the ship plumes or not, but still lower than those observed in previous works during summertime; 0.31 ppbv hr -1 in rural areas (Geyer et al., 2001a) and 1.10-3.2 ppbv hr -1 in urban areas (Wang et al., 2013;Brown et al., 2017;Zhou et al., 2018) . Within the plume condition, maximum NO3 increased up to 1.85 pptv even though 355 there was sunlight at that time. The trend of NO3 concentration was well-matched with P(NO3) for most of the period indicating the suitability of our instrument for ambient NO3 measurements. Further analysis related with regional impacts of ship plume chemistry together with NO3 oxidation assessment with observed VOCs and DMS would be interesting topics for future studies.

Conclusions 360
This paper describes our newly built cavity-enhanced absorption spectrometer for simultaneous measurements of ambient NO3, NO2, and H2O from their absorption features in 659.28-671.94 nm. High performances in measurement capabilities and simplicity in maintaining and processing schemes were achieved by applying high-reflection mirrors (up to 99.9995 % at 662 nm), by integrating the mirror purge and cage system as well as by simultaneous quantification of H2O using its measured spectrum. Generally, the light at 662 nm travels more than 40 km (up to 101.5 km) within the compact cavity cell (51.5 cm), 365 which enables sensitive measurements of the target species. To overcome the difficulties in H2O treatment for accurate NO3 measurement, the measured absorption spectrum of H2O was used and our instrument showed high linearity for varying atmospheric relevant ranges of H2O. The transmission efficiency of NO3 from the inlet tip to the detection region was evaluated as 81.2 (±2.9, 1σ) % within the residence time of 2.59 seconds from the prepared NO3 addition experiments. Consequently, for NO3, NO2, and H2O, the measurement accuracies and detection limit (1σ, 1 second, estimated from a single pixel CCD) 370 were determined as 10.8 %, 4.7 %, and ≥20.5 % with 1.41 pptv, 6.92 ppbv, and 35.0 ppmv, respectively, which are sufficiently low for ambient applications.
The instrument was successfully deployed aboard the Korean ice breaker R/V Araon and captured not only the background condition of the atmosphere over the open ocean in the Arctic but also the highly structured features of the plumes which originated from the vessel exhaust during the campaign. In addition, the trend of NO3 concentration was well-matched with 375 the calculated P(NO3) which serves as a proof of the potential for active applications of this instrument in further studies not only in urban regions but also in pristine regions with any mobile platforms including aircraft and research vessel.

Data availability
The datasets used in this study are available upon request to the corresponding author, Kyung-Eun Min (kemin@gist.ac.kr).

Author contributions 380
WN and KEM contributed to designing and performing this study. TSR provided the data for O3 and helped for the field deployment. WN and KEM wrote the manuscript with contributions from CC and BP. All co-authors revised the content of the original manuscript and approved the final version of the paper.

Competing interests
The authors declare that they have no conflict of interest. 385