Development of a portable cavity ring down spectroscopy instrument for simultaneous, in situ measurement of NO3 and N2O5

Zhiyan Li; Renzhi Hu; Pinhua Xie; Hao Chen; Shengyang Wu; Fengyang Wang; Yihui Wang; Liuyi Ling; Jianguo Liu; Wenqing Liu

doi:10.1364/OE.26.00A433

1. Introduction

Nitrate radical (NO₃) and dinitrogen pentoxide (N₂O₅) are interesting trace gas constituents of the troposphere, which play an important role in nocturnal chemical processes. The NO₃ radical is a major oxidant for pollutants, including a number of volatile organic compounds (VOCs) that are important for photochemical ozone production during the night and for controlling the lifetime of some species [1, 2]. N₂O₅, the heterogeneous loss of which can affect the oxidant ability of NO₃ [3], is the reservoir specie of the NO₃ radical. At night, NO_x is removed from the troposphere mostly via reactions with NO₃ and heterogeneous hydrolysis of N₂O₅ [3–5]. In addition, The heterogeneous reactions of N₂O₅ and NO₃ also play a vital role in the generation of aerosol nitrate, as well as in the aging of secondary organic aerosols [6]. Furthermore, the presence of N₂O₅ in the troposphere enables halogen activation, forming ClNO₂, which can photolyze during the day and yield a highly reactive chlorine radical and NO₂. All of these processes can directly and indirectly have an impact on climate. NO₃ is produced from oxidation of NO₂ by ozone (O₃) [Eq. (1)] and rapidly equilibrates with NO₂ and N₂O₅ [Eq. (2)] in ambient air. The equilibrium partitioning between NO₃ and N₂O₅ is determined by NO₂ concentration and ambient temperature [7]. N₂O₅ is favored by high NO₂ concentrations and low temperatures.

{NO}_{2} {+O}_{3} \to {NO}_{3} {+O}_{2}

{NO}_{3} {+NO}_{2} +M \leftrightarrow N_{2} O_{5} +M

NO₃ is short-lived in sunlit air due to its strong absorption and photolysis by visible light [Eq. (3) and Eq. (4)] as well as its reactivity towards NO [8] (Eq. (5)), a predominantly daytime species. In general, NO₃ is negligible in the daytime; however, under some specific conditions, such as coexistence of high ozone and NO₂, high aerosol scattering coefficient and weak solar irradiation, NO₃ and its reservoir N₂O₅ have been inferred as significant oxidants in the process of the formation of ozone and secondary aerosols [9–11].

{NO}_{3} +hυ \to NO+O(90%)

{NO}_{3} +hυ \to {NO+O}_{2} (10 %)

{NO}_{3} +NO \to {2NO}_{2}

The NO₃ radical reacts rapidly with various VOCs [12] to produce nitric acid and a hydrocarbon radical [Eq. (6)]. The reactions of the NO₃ radical with biogenic hydrocarbons such as isoprene and monoterpenes may be a vital mechanism for the production of secondary organic aerosol [13–15]. N₂O₅ is lost mainly by heterogeneous hydrolysis on aerosol surfaces to produce nitrate [Eq. (7)] or by the reaction between N₂O₅ and H₂O to generate nitric acid [2, 16, 17] [Eq. (8)].

{NO}_{3} +VOC \to products

N_{2} O_{5} +aerosol \to nitrate

N_{2} O_{5} {+H}_{2} O \to {2HNO}_{3}

Given the importance of the NO₃ radical and N₂O₅ in the nocturnal chemical process, in the past dacades, a wide range of surface-based field studies have been conducted to investigate the nighttime chemistry of NO₃ and N₂O₅ in polluted and clean air environments [11, 16–24]. Also, more limited measurements have been made on other platforms, such as aircrafts [14, 25, 26], towers [27, 28] and ships [4]. Moreover, only a small amount of existing research on NO₃ and N₂O₅ can be used to analyzer the vertical profiles of NO₃ and N₂O₅ [29–31]. The simultaneous measurement of NO₃ and N₂O₅ remains an area of considerable interest. Particularly, in densely populated urban areas, the ground-based measurements of NO₃ and N₂O₅ have become more difficult because of local emissions. Thus, it is of great importance to investigate the vertical gradients of NO₃ and N₂O₅ and access the influence of significant sinks of these species in urban area. Particularly, in China, home to several megacities and many large cities owing to its fast-paced urbanizations and industrialization processes, only a few measurements have been reported to investigate the role of NO₃ and N₂O₅ in the atmosphere [11, 32–36],so field measurements should be conducted in these regions to obtain a full picture of the role of the NO₃ oxidation process and N₂O₅ heterogeneous reactions in the formation of aerosol in China.

With the aim of obtaining a detailed understanding of the chemistry involving NO₃ and N₂O₅, the simultaneous measurement of NO₃ and/or N₂O₅ have been performed by several groups using cavity enhanced absorption spectroscopy (CEAS) [32, 37–39], cavity ring-down spectroscopy (CRDS) [21, 40–45], laser induced fluorescence [46, 47] and chemical ionization mass spectrometry (CIMS) [48–50]. With respect to CIMS which combines the ion-molecule chemistry with mass spectrometry detection, fundamental measurement involves the reaction of I⁻ (the reagent ion) with NO₃ to form the NO₃⁻ ion that can be detected at 62 amu. However, this method may suffer interference, such as N₂O₅, HNO₃ and HO₂NO₂ [48, 49, 51].CIMS method based on iodide cluster ion at m/z 235 has been successfully used in N₂O₅ measurement [50]. With respect to the optical methods, the measurement of NO₃ is based on the strong absorption of NO₃ in the visible spectrum at 662 nm whereas the measurement of N₂O₅ is determined by using a heated channel to decompose it into NO₃ or from the calculation according to the fast equilibrium among N₂O₅, NO₃ and NO₂. The thermal dissociation system of LIF is similar to that of CRDS for N₂O₅ detection, but for the fomer NO₃ is measured by LIF technique. The LIF technique has achieved a detection limit of 8 and 11 pptv for NO₃ and N₂O₅ with an intergration time of 10 min, respectively [47]. However, due to the relatively low fluorescence quantum yield of NO₃, this method has not been widely used. The CEAS and CRDS techniques are two similar optical techniques. The ringdown signal of a light pulse circulating inside a ringdown cavity both depends on the reflectivity of the mirrors and the attenuation by gases and aerosol particles present in the cavity. But CRDS monitors the absorption information at the peak absorbtion wavelength while CEAS requires obtaining the obsorber’s absorption data over a wide wavelength. The reported NO₃ and N₂O₅ detection sensitivities for CEAS and CRDS achieve similar detection performance ranging from 0.5 pptv–8 pptv with a high temporal resolution of a few seconds. The high-speed measurements and universal detection capabilities are advantageous since the instrument can rapidly respond to concentration changes. CEAS and CRDS can achieve background measurement by adding excess NO to titrate NO₃ and N₂O₅. However, the advantage of NO₃ removal for those two methods comes at the expense of potential wall losses, which have to be characterized. In recent years, a spectroscopic instrument based on a mid-infrared external cavity quantum cascade laser (EC-QCL) was developed to measure N₂O₅ directly based on the N₂O₅ strong absorption bands in the mid-infrared region centered approximately at 1,245 cm⁻¹ [52]. The minimum detection limit was 15 ppbv with an integration time of 25 s and it was down to 3 ppbv in 400 s with an optical absorption path-length of Leff = 70 m. This method has just been used in a chamber experiment, not in the field environments.

Based on the current research development on NO₃ and N₂O₅ in China, the following two requirements have been proposed for the instruments. First, instruments should have a high time resolution and a high detection limit to capture the rapid change of NO₃ and N₂O₅, such as in a ground site where many local source emissions exist. Second, instruments should have a compact structure to apply to the vertical profile measurement, such as on a movable carriage. Here, we report a two channel CRDS system for the detection of NO₃ and N₂O₅. Previous CRDS instruments used by our group for the detection of NO₃ have been described [53] and successfully deployed in ground sites. Major modifications to the device include the introduction of a second cavity to enable the simultaneous measurement of NO₃ and N₂O₅ and the optimization of operating parameters i.e., flow rates and residence time as well as the improvement of detection limit and time resolution. The instrument we describe here is able to detect pptv levels of NO₃ and N₂O₅ on timescales of a few seconds in a well defined air-mass (point measurement). In addition, the instrument has a small footprint of 110 × 40 × 35 cm, low power consumption of < 300W and a total weight of < 40 kg, such that it can be widely used in different platforms. The instrument was deployed to detect the mixing ratios of NO₃ and N₂O₅ in the urban city of Beijing in summer and winter campagains.

2. Experimental

2.1 Cavity ring-down spectroscopy

CRDS, first demonstrated in 1988 [54] is a scheme for highly sensitive absorption measurements and has been described in several reviews [55–57] and we present it briefly to clarify the notation used in this study. Laser is coupled into a high finesse optical cavity. The leaked intensity from the back of a highly reflective mirror, I(t), exhibits as a single exponential decay with a time constant, τ₀, that is inversely proportional to the mirror transmission. The concertration of the target absorber can be accomplished by comparing the fitted ring-down time constants in the presence (τ) and absence of the absorber (τ₀) using the absorption cross section (σ) at the probing wavelength.

[A]= \frac{R_{L}}{cσ} (\frac{1}{τ} - \frac{1}{τ_{0}})

where, c is the speed of the light, R_L is the ratio of the total cavity length to the length over which the absorber is present in the cavity and τ and τ₀ are ring-down time constants in the presence (τ) and absence (τ₀) of the absorber respectively.

2.2 Optical layout

The optical layout of the two-channel CRDS instrument is described schematically in Fig. 1. Light is provided by an external modulation diode laser (IQm, Power Technology Inc.), which is pulsed through a function generator (DG1022, RIGOL), the laser output is modulated with a square-wave signal at a repetition rate of 1 KHz, which is higher than that used in Dan Wang [53], with an output power of approximately 100 mW. The laser is temperature tuned corresponding to a central wavelength of 662.08 nm. Light emerging from the laser is isolated through an optical isolator (IO-3D-660-VLP, Thorlabs) from the cavities to prevent potentially damaging back reflection from entering the laser. The light is divided into two beams by a 50/50 beam splitter and coupled into two identical 76 cm long optical cavities each constructed from pairs of high reflectivity mirrors, with reflectivities of 0.99998 and radii of curvature of 1 m. The cavity ring-down mirrors are seated in custom-built mirror holders. The sample cells are constructed from PFA Teflon and are held in place by two custom-built aluminum enclosures. NO₃ is detected via its strong ${\tilde{B}}^{2} E^{'} (0000) \leftarrow {\tilde{X}}^{2} A_{2}^{'} (0000)$ electronic transition centered at approximately 662 nm. N₂O₅ is measured in another cavity maintained at 80 °C following its thermal coversion to NO₃ and NO₂ at 140 °C in a heater placed before the cavity entrance. Therefore the sum of ambient NO₃ and thermally dissociated N₂O₅ is measured in this channel. The temperature of the two stage heated channel is close to that of the NOAA-CRDS instrument where ambient air initially flows through a Teflon converter maintained at 140 °C and subsequently into the measurement cell maintained at 80 °C [58]_. The preheater temperature is determined using tape heaters wrapped around a 35 cm-long, 1/4 o.d. and 3/16 i.d. section of Teflon tubing with embedded pt100 thermocouples and temperature controllers (Yudian-Ai509). The time needed for quantitative conversion is mainly limited by the time needed to heat the sampled air. Calculations using a value of k2b = 23.4 s⁻¹ at 80 °C and 1 atm [59] for N₂O₅ indicate that less than 0.1 s N₂O₅ is stoichiometrically converted to NO₃. The specific design of the heater enables the gas to convert to NO₃ completely with the gas residence time of 0.10 s in the tube when the flow rate is 3.5 slm. Each mirror is isolated from the sample flow by a purge volume that is continuously flushed with 100-200 sccm of dry nitrogen to prevent the degradation of reflectivity by aerosols to mirror surfaces during the night. Light exiting the back mirror of the cavity is monitored by a photomultiplier tubes (PMT, Hamamatsu H10721-20) after passing through a 660 nm band pass filters placed in front of the PMTs to prevent stray light from unwanted wavelengths.

Fig. 1 A schematic of the two channel CRDS instrument for the detection of NO₃ and N₂O_5.

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By digital fitting using an oscilloscope card (PCI 6132, 2.5 MHz) at a rate of 1.0 × 10⁶ samples s⁻¹ per channel, the 2,000 ring-down traces are transferred to a computer, co-added and averaged on the occasion when data are acquired on the data acquisition board for a continuous period of 2.5 s. This data acquisition method, which was the same as that used by osthoff [43], can both improve the fitting rate compared with finite sampling and avoid data loss compared with continuous sampling. The LM fitting method is used to obtain the average of the ring-down traces. The effective absorbtion optical path excees 30 km, with the fitting ringdown-time of approximately 100 μs (τ₀), which is sufficient for NO₃ radical and N₂O₅ measurements. An example cavity decay trace is shown in Fig. 2.

Fig. 2 An example of cavity ring-down signal and the fitting result. The lower panel is the fitting residual trace.

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The inlet consists of several parts. First sample air is brought into the instrument enclosure by a short length of 0.4 cm inner diameter tubing. The sample air passes through a 47 mm-diameter, 2µm-pore size, 25 µm-thick Teflon filter to remove all optically active particles from the sample air flow. The filter is usually changed every one or two hours beacause of aging effects, although changes occur more frequently in heavily polluted environment. A T-piece PFA Teflon connector was used to connect the two sample channels. The slow flow system is separated into two air samples immediately below the filter each of which is controlled by electronic mass flow controllers at the exhaust. A pump with a capacity of 11 L is applied to both flows. Flow rates are usually 5 slm and 3.5 slm for the ambient temperature channel and the two-stage heated channel, respectively.

The ring-down time of the absorber in the absence of the cavity is determined by the titration reaction of NO with NO₃ [Eq. (6)] [19, 21, 40]. NO (10 ppm) is added at a flow rate of 50 sccm through an MFC to the air after the filter every 300 s for a period of 60s. The added NO is about 1.4 × 10¹² molecule/cm³ at a total flow of 8.5 slm. The rate constant of Eq. (5) is 2.6 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at 298 K and is weakly dependent on temperature. Based on the evaluated rate coefficient [60], this amount of NO should result in complete titration of NO₃ within 0.1 s. This process disturbs the equilibrium between NO₃ and N₂O₅, and leads to NO₃ formation by N₂O₅ dissociation, such that the NO added titrates all NO₃ and N₂O₅. When NO is added, NO₃ is converted into NO₂ in its reaction with NO before entering the cavity, so that the NO₃ absorption can be selectively switched on and off. The data obtained for the first 30 s period after adding NO are excluded from the data collection, taking the residence time into account. The largest uncertainty of the NO₃ radical caused by the background shift during the 5 min period is approximately ± 0.5 pptv (the variations of O₃, NO₂ and H₂O are estimated at ± 2 ppbv, ± 2 ppbv and ± 2%, respectively).

The data also should be corrected due to the loss of NO₃ and N₂O₅ during transport through the cavities. Because the ambient N₂O₅ concentration is determined from the two measured results in ambient channel and heated channel, the NO₃ transmission efficiency in ambient channel (Te(NO₃)amb) and the NO₃ + N₂O₅ transmission efficiency (Te(N₂O₅)) in heated channel have to be known to accurately retrieve the ambient concentration of NO₃ and N₂O₅.

In the case of NO₃ in ambient channel, the loss should include the wall loss in the sampling tube and the detection cell as well as the loss in the filter and filter holder. The NO₃ wall loss has been measured by flowing a stable NO₃ source into the cavity as described previously [53], the linear fit of NO₃ concentration with the residence time results in a NO₃ first order loss rate coefficient and associated uncertainty of k = 0.19 ± 0.02 s⁻¹,which is in good agreement with that measured by Dube et al. [41] (0.20 ± 0.05 s⁻¹). The residence time through the inlet tube to the midpoint of the ring-down cell is 0.74 s when the flow rate is 5 slm, so that the NO₃ wall loss is 14% ± 1%. The loss in the filter holder and filter has been measured by repeated flowing NO₃ (generated in the reaction of NO₂ with O₃) into the cavities with the insertion and removal of a filter holder and a filter. The results show that the loss in a filter holder and a clean filter is 17 ± 5% while the loss increased by 2% in a used filter and filter holder. During the campaign, the filter is changed every 0.5 - 2 h depending on the aerosol loadings. For example, under heavy polluted conditions (PM2.5 > 200 μg/m³), the filter is changed every 0.5 h to reduce the impact of the filter aging on the measurement. Overall, the NO₃ transmission efficiency (Te(NO₃)_amb) is 69% ± 5%.

For the determination of the N₂O₅ + NO₃ transmission efficiency in the heated channel, the N₂O₅ loss in the filter and the inlet tube before the sampled air flows into the pre-heater as well as the wall loss of NO₃ in the pre-heater and the detection cavity should be taken into account while the loss of NO₃ in the filter can be neglected due to the large ratio of N₂O₅/NO₃. Such as under the conditions of 10 °C and NO₂ of 20 ppbv, the ratio of N₂O₅/NO₃ is larger than 100. Since N₂O₅ itself has a little filter and inlet loss of 2% before flowing into the preheating tube which is minor compared to NO₃ as described previously [23, 41], the only significant contribution to the N₂O₅ sampling efficiency is the wall loss occurring subsequent to its conversion to NO₃ downstream of the heater. The surface materials of the preheating tube and the detection cavity in the heated channel are the same as that in the ambient channel. Therefore, the wall loss reactivity of NO₃ in the ambient will be applicable for the converted NO₃ in the heated channel. The residence time through the heated sections of the flow system to the midpoint of the ring-down cell is 0.63 s, so that the NO₃ in the heated portion of the inlet is 12 ± 2%. The correction transmission factor for the measurement of N₂O₅ + NO₃ was thus determined as 86% ± 2%.

3. Results

3.1 Selection of laser diode and effective absorption cross-section

The diode output is centered at 662.08 nm (as shown in Fig. 3 red lines), lightly to the red of the NO₃ absorption maximum. In selecting the centered wavelength, the interference of other trace gases such as NO₂ [61], O₃ [62] and H₂O [63], whose absorption cross sections are shown in Fig. 3, and the full width at half maximum of the diode laser, which should be significantly smaller than that of the NO₃ radical absorption width, have been considered to ensure the single exponential decay of the measured ring-down signal of the NO₃ radical [64]. To retrieve molecule number densities from the Eq. (7) and Eq. (8), the effective absorption cross section of the ambient absorbers in ambient and hot temperature is necessary in this wavelength. The NO₃ absorption cross section is known to be temperature dependent [65, 66]. By convolution of the laser spectrum and the absorption cross section of the NO₃ radical [67], the NO₃ radical effective absorption cross section is 2.24 × 10⁻¹⁷ cm² molecule⁻¹ for our instrument in ambient temperature (the pink line in Fig. 3). The temperature dependence of the NO₃ cross section was taken from Osthoff et al. [65] and the absolute values were taken from Yokelson et al. [66] and plotted in Fig. 3 as the blue line. The value of effective absorption cross section of NO₃ at 662.08 nm is scaled to 1.80 × 10⁻¹⁷ cm² molecule⁻¹ at 80 °C from 2.24 × 10⁻¹⁷ cm² molecule⁻¹ at 25 °C.

Fig. 3 Effective absorption cross section of NO₃ radical at 298K and 353K, cross section of NO₂, O₃, H₂O and diode laser spectrum.

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3.2 Determination of R_L

Due to the influence of the purge volumes, the ratio of the absorption path length and the mirror distance cannot be simply derived from the actual distance between the inlet and outlet of the sampled gas and the distance between the mirrors. R_L is calibrated regularly by filling the cavity with several different known O₃ concentrations and calculating the slope of the measured optical extinction vs. O₃ as described by Washenfelder [68]. O₃ is generated by passing a stream of O₂ past a 185 nm Hg lamp and the concentration is detected by an O₃ analyzer. The R_L value is 1.20. The fitted slopes were similar for all channels with an uncertainty of 5% which can be attribute to the O₃ analyzer (3%) and the absorption cross section (4%).

3.3 Calculation of the mixing ratios

Once the transmission efficiency of NO₃ in the ambient channel and the transmission efficiency of NO₃ and N₂O₅ in the heated channel are known, the number densities of NO₃ and N₂O₅ can be calculated using the following equations:

{{[NO}_{3}]}_{amb} = \frac{R_{L}}{{Te(NO}_{3})_{amb} {cσ}_{ambient}} (\frac{1}{τ_{ambient}} - \frac{1}{τ_{0,ambient}})

{{[N}_{2} O_{5}]}_{amb} = \frac{\frac{R_{L}}{{cσ}_{hot}} (\frac{1}{τ_{hot}} - \frac{1}{τ_{0,hot}})}{{Te(N}_{2} O_{5})} {-[NO}_{3}]_{amb}

where τ_ambient and τ_hot are the ring-down time constants observed in the ambient and heated channels, respectively; σ_ambient and σ_hot are the temperature-dependent absorption cross sections of NO₃ at the absorption maximum at 662 nm; τ_{0, ambient} and τ_{0, hot} are the ring-down time constants when excess NO was added to the inlet; Te(N₂O₅) is the transmission efficiency of N₂O₅ through the heated channels; and Te(NO₃)_amb is the transmission efficiency of NO₃ through the ambient channel.

The calculated N₂O₅ mixing ratio above is based on the fact that N₂O₅ has been completely converted to NO₃. In fact, under the conditions when NO₂ concentration is less than 30 ppbv and the cavity temperature is higher than 75 °C, the conversion of N₂O₅ to NO₃ goes fully to its equilibrium value. However, under high NO₂ concentration, the fraction of N₂O₅ that remains undissociated at equilibrium at temperature of 75 °C is large (Fig. 4). The undissociated N₂O₅ is dependent on NO₂ and temperature in the cavity. The NO₂-dependent correction value ranges from 99% to 86% for NO₂ from 0 ppbv to 40 ppbv for the measured N₂O₅. Correction is particularly important in winter because of the high NO₂ concentration and low temperature. For example, in wintertime, when cavity temperature is 75°C and NO₂ is 40 ppbv, the N₂O₅ decomposition rate in the cavity is approximately 86% and the actual N₂O₅ concentration should be obtained by dividing the derived N₂O₅ data based on the equations mentioned above by this decomposition rate. Thus, a higher cavity temperature is used under such conditions, e.g., 85 °C to improve the N₂O₅ dissociation rate. Under the same conditions previously described, the decomposition rate is 93%.

Fig. 4 The undissociated N₂O₅ ratio depend on different temperature, the different color represent different NO₂ concentration.

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4. Discussion

4.1 Interference analysis

Impure NO, which contains 1 × 10¹⁰ molecule cm⁻³ level of NO₂, generates a small negative offset equivalent to 0.04 pptv NO₃ based on the fact that the NO₂ cross section in the wavelength of 662 nm is approximately 2 × 10⁻²¹ cm² molecule⁻¹, which is four orders of magnitude smaller than that of the NO₃ radical. Thus, this amount of NO₂ is too low to warrant correction. In addition, we note that adding NO to air samples containing O₃ (>20 ppbv), which also have a small absorption cross section of 1 × 10⁻²¹ cm² molecule⁻¹ at 662 nm in a typical tropospheric environment would result in the removal of O₃ and formation of NO₂ duo to its reaction with ambient O₃. The effect on the total absorption at 662 nm can be accurately applied. Taking O₃ = 100 ppbv and k (NO + O₃) = 1.9 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ at 298 K and an average reaction time of 0.54 s at the ambient channel, only 1.5 ppbv of O₃ is removed and 1.5 ppbv of NO₂ is generated. As a result, the overall NO₃ equivalent is 0.08 pptv. The reaction of NO with ambient O₃ to produce NO₂ is relatively slow to produce a measurable signal on the ambient channel. However, on the heated channel, taking k (NO + O₃) = 7.8 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ at 413 K with a reaction time of 0.10 s in the pre-heated heater and k (NO + O₃) = 4.3 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ at 353 K with a reaction time of 0.27 s at the center of the heated cavity, a small negative offset equivalent to 0.3 pptv NO₃ is generated. We subtract it from the ambient data set.

4.2 Optimization of signal averaging time, detection limit, and noise sources

For the red diode CRDS system, signal averaging can improve the signal-to-noise ratio. The number of average traces selected is the key issue to achieve a rapid and high sensitivity measurement. Figure 5(a) and Fig. 5(c) show a typical time series of the NO₃ and N₂O₅ instrument baselines when zero air was sampled. The Allan variance plot gives a detection limit of 2.3 pptv (1σ) for NO₃ [Fig. 5(b)] and 3.2 pptv (1σ) for N₂O₅ [Fig. 5(d)] at an interval of 2.5 s respectively. The reactor is heated to above room-temperature is carried out to stabilize the dissociation of N₂O₅, which introduces strong turbulent noise because of variations in temperature in the cavity at the optimal flow rate. Thus, the detection limit of the heated channel for the detection of the sum of NO₃ and N₂O₅ is larger than that of the ambient channel for the detection of NO₃. The optimum signal averaging time is 40 s for NO₃ and 30 s for N₂O₅.

Fig. 5 Allan variance plot for the NO₃ radical measurements in ambient channel and heated channel when sampling ambient air. The instrument has 1σ precision about 2.3 pptv and 3.2 pptv in ambient channel and heated channel for a 2.5 s integration time.

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The minimum detection limit of the instrument can be expressed as follows [40, 43]:

{[X]}_{min} = \frac{\sqrt{2} R_{L}}{cσ} \frac{Δ τ}{τ_{0}^{2}}

where [X]_min is the smallest measurable concentration of NO₃ and

Δ τ

is the smallest measurable difference among ring-down times τ, Using RL = 1.2, τ₀ = 89.3 μs, σ = 2.02 × 10⁻¹⁷ cm² molecule⁻¹ and Δτ = 0.16 μs for an integration time of 2.5 s in the field measurement of NO₃ in the normal channel and τ₀ = 117.4 μs, σ = 1.81 × 10⁻¹⁷ cm² molecule⁻¹, and Δτ = 0.35 μs for the sum of NO₃ and N₂O₅ measurement in the heated channel. The detection limits in the normal and heated channels are about 2.0 pptv (1σ) and 3.1 pptv (1σ), respectively which are consistent with the 1σ Allan variance determined from the continuous zero NO₃ and N₂O₅ measurements for the normal and heated channels.

The overall 1σ accuracy of the ambient channel is about ± 8%, as determined by the uncertainties in the absorption cross section ( ± 4%), path length ratio (R_L, ± 5%) and transmission efficiency ( ± 5%) [51]. The estimated fractional uncertainty in the quantity of the sum of ambient and dissociation NO₃ is 15%, taken as the linear sum of a 13% uncertainty in the high temperature NO₃ cross section [65], the transmission efficiency of N₂O₅ + NO₃ in the heat channel (2%) and the uncertainties of ambient NO₃ radical measurement described above ( ± 8%). The concentration of N₂O₅ is determined by subtracting the ambient NO₃ measured in the ambient channel from the concentration of the sum of ambient and dissociated NO₃ measured in the heated channel. When N₂O₅ is approximate to the value of NO₃, the accuracy of NO₃ in the heated channel can significantly affect the accuracy of N₂O₅. However, when the concentration of N₂O₅ is larger than that of NO₃, the uncertainty of N₂O₅ mainly depends on the uncertainty of N₂O₅ conversion.

5. Ambient air measurements

The CRDS instrument for NO₃ and N₂O₅ measurement was deployed at the campus of Institute of Atmospheric Physics, located in urban Beijing, China (39° 58ʹ 28″ N, 116° 22ʹ 16″ E) to test the instrument performance. The site is surrounded by many residential and commercial areas. At 300 m east of the measurement site, there is the Jing Zang Expressway, one of the busiest main roads in the city. Influenced by mixed pollutants emitted from traffic, residential, and industrial sources, the measurement site is a good representative of urban area in Beijing.

In the winter campaign, The CRDS instrument was deployed on a movable carriage and the carriage can ascend or descend the tower at a rate of about 8 m/min. The carriage can also be positioned indefinitely at an arbitrary height. During the nighttime measurement, the carriage was positioned at a height of 240 m. The carriage takes about one hour to ascend and descend once. An example of the vertical profile of N₂O₅ when the carriage was ascending and descending during 00:12 to 01:06 in 11 December is shown in Fig. 6. Data obtained at less than 80m are below the detection limit which are not shown in the figure. This finding can be attributed to local emissions such as NO. Given the cold weather and high NO₂ concentration, the thermal decomposition of N₂O₅ is relatively slow such that the N₂O₅ concentration is higher than the NO₃ concentration during the experiment. In fact, the NO₃ concentration is negligible. Results shows that the nocturnal mixing ratios of these compounds vary widely over short vertical distance scales (10 m or less). However, the N₂O₅ trend during the ascending and the descending processes within one hour is different. The N₂O₅ trend during the ascending process is complicated [Fig. 6(a)], which may be related to the different air flow disturbances. However, during the descending process [Fig. 6(b)], the N₂O₅ concentration is reduced with the carriage going down, which may be dominated by its precursors NO₂ and O₃. Overall, the results show that the instrument can be used to measure the N₂O₅ vertical profile in densely populated areas and solve the difficulties in performing NO₃ and N₂O₅ measurements aloft in urban areas.

Fig. 6 N₂O₅ vertical profile at the night of 11 December, 2016. Data below 80m are not shown in the figure.

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In the summer campaign, the instrument was placed in a container and the sample inlet was approximately 3 m above the ground. NO₃/N₂O₅ measurements typically started just before local sunset and stopped after the signals had returned to baseline levels following sunrise. The NO₃ and N₂O₅ time series from 2 June to 22 June, 2017 are shown in Fig. 7. In summer, due to high temperature, the chemical process involved in nighttime NO₃ radical and N₂O₅ reaction is fast. Meanwhile, influenced by vertical transmission, sudden injection of high N₂O₅ was observed, e.g. at the night of 9 June and 12 June, 2017. Thus, our instruments with high sensitivity detection have advantages in capturing these rapid chemical processes. During the entire campaign, the observed mean NO₃ and N₂O₅ mixing ratios were 36.2 pptv and 2.5 pptv, respectively. Data along with temperature and NO₂ concentration obtained by a CEAS instrument observed at nighttime of 12 June, 2017 are shown in Fig. 8. Based on Eq. (2) and taking the equilibrium constant to be Keq = (5.1 ± 0.8) × 10⁻²⁷ × Exp((10871 ± 46)/T) cm³/ molecule, a point-by-point comparison of the measured NO₃ mixing ratio with that calculated (green line in Fig. 9) from the NO₂ and N₂O₅ observations and the temperature was conducted. The results show that NO₃ (calculated) = NO₃ (measured) × 0.9-0.25 pptv, with a correlation coefficient R = 0.94 (Fig. 9), which can provide a confirmation of the measurement accuracy of the instruments.

Fig. 7 NO₃ and N₂O₅ time series from 2 June to 22 June, 2017.

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Fig. 8 O₃、NO₂、NO₃、N₂O₅ and temperature time series from 19:00 on 12 June, 2017 to 03:00 13 June, 2017. NO₃ and N₂O₅ mixing ratios are observed by the CRDS instrument. NO₂ mixing ratios are measured by a CEAS instrument. O₃ mixing ratios are measured by an ozone analyzer (Thermo Fisher 49i). All data are averaged to 1min.

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Fig. 9 Scatter plots for the calculated NO₃ from the equilibrium between NO₂ and N₂O₅ and measured NO₃. The red line illustrates the linear regression.

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6. Conclusions and future work

The operating principles, instrumentation and field deployment of a CRDS system have been described. The instrument is able to make sensitive measurements of the concentrations of short-lived trace gases NO₃ and N₂O₅ via their absorption at 662 nm. The overall uncertainty of the instrument is 8% and 15% for NO₃ and N₂O₅ measurements, respectively. The first field deployment of the instrument was in the urban city of Beijing in December of 2016. The field observation results showed that N₂O₅ had distinct vertical concentration profiles at night in winter. To our knowledge, this study is the first time that a CRDS instrument is applied to detect N₂O₅ vertical profiles in the urban area of Beijing, China. This measurement is meaningful because it can avoid local characteristics and give an insight into turbulence processes throughout the depth of the boundary layer to some extent. Another measurement of NO₃ radical and N₂O₅ performed at nighttime of 12 June, 2017 clearly revealed the equilibrium among NO₂, NO₃ and N₂O₅. Therefore, the capability of the CRDS instrument to make rapid measurements of atmospheric trace gases and capture their spatial and temporal variability can help us understand the nighttime NO₃ chemistry on a large scale.

Funding

National Natural Science Foundation of China (41530644, 41571130023, 61575206 and 91644107); the National Key Research and Development Program of China (2017YFC0209403 and 2017YFC0209401).

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Development of a portable cavity ring down spectroscopy instrument for simultaneous, in situ measurement of NO₃ and N₂O₅

Abstract

1. Introduction