Intercomparison of in situ CRDS and CEAS for measurements of atmospheric N2O5 in Beijing, China

Science of the Total Environment 613–614 (2018) 131–139

Contents lists available at ScienceDirect

Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Intercomparison of in situ CRDS and CEAS for measurements of atmospheric N2O5 in Beijing, China Zhiyan Li a,b, Renzhi Hu a,⁎, Pinhua Xie a,c,⁎, Haichao Wang d, Keding Lu d, Dan Wang e a

Key Lab. of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China Science Island Branch of Graduate School, University of Science and Technology of China, Hefei 230026, China CAS Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361000, China d State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China e School of Mathematics and Physics, Anhui University of Technology, Maanshan 243032, China b c

H I G H L I G H T S

G R A P H I C A L

• A comparison of N2O5 concentrations measured by CRDS and CEAS instruments was carried out in Beijing. • The excellent agreement demonstrated the two instruments can accurately measure N2O5 on different environment. • High relative humidity (N60%) and high concentration of PM2.5 (N200 μg/m3) cause the discrepancies between two systems.

Scatter plots for the entire N2O5 dataset from CRDS and CEAS instruments. The red lines illustrate the linear regression. Time series and correlation of N2O5 mixing ratio between CRDS and CEAS instruments from February 29 to March 1, 2016.

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 4 July 2017 Received in revised form 28 August 2017 Accepted 30 August 2017 Available online xxxx Editor: D. Barcelo Keywords: N2O5 CRDS CEAS RH PM2.5

A B S T R A C T

Dinitrogen pentoxide (N2O5) is one of the basic trace gases which plays a key role in nighttime atmosphere. An intercomparison and validation of different N2O5 measurement methods is important for determining the true accuracy of these methods. Cavity ring down spectroscopy (CRDS) and cavity enhanced absorption spectrometer (CEAS) were used to measure N2O5 at the campus of the University of Chinese Academy of Sciences (UCAS) from February 21, 2016 to March 4, 2016. The detection limits were 1.6 ppt (1σ) at 30 s intervals for the CEAS instrument and 3.9 ppt (1σ) at 10 s time resolution for the CRDS instrument respectively. In this study, a comparison of the 1 min observations from the two instruments was presented. The two data sets showed a good agreement within their uncertainties, with an absolute shift of 15.6 ppt, slope of 0.94 and a correlation coefficient R2 = 0.97. In general, the difference between the CRDS and CEAS instruments for N2O5 measurement can be explained by their combined measurement uncertainties. However, high relative humidity (N 60%) and high PM2.5 concentration (N200 μg/m3) may contribute to the discrepancies. The excellent agreement between the measurement by the CRDS and CEAS instruments demonstrates the capability of the two instruments for accurately measuring N2O5 with high sensitivity. © 2017 Elsevier B.V. All rights reserved.

1. Introduction ⁎ Corresponding authors at: Key Lab. of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China. E-mail addresses: [email protected] (R. Hu), [email protected] (P. Xie).

http://dx.doi.org/10.1016/j.scitotenv.2017.08.302 0048-9697/© 2017 Elsevier B.V. All rights reserved.

Dinitrogen pentoxide (N2O5) plays an important role in nocturnal chemical processes (Wayne et al., 1991; Chang et al., 2011; Brown and

132

Z. Li et al. / Science of the Total Environment 613–614 (2018) 131–139

Stutz, 2012) as the reservoir specie of the nitrate radical which is a major oxidant for pollutants during the night and contributes to the oxidative capacity of the atmosphere. The loss of N2O5 effectively constitutes the loss of NO3 due to the equilibrium between the two species (Brown et al., 2011; Aldener et al., 2006). Meanwhile N2O5 is an intermediate in the conversion of NOx (NOx`NO + NO2) to HNO3 due to its reaction with water in an efficient heterogeneous reaction, indicating the importance of nighttime chemistry in determination of NOx budgets and O3 production (Aldener et al., 2006; Brown et al., 2004; Brown et al., 2006). Furthermore, the presence of N2O5 in the troposphere enables halogen activation and the production of inorganic nitrate by reaction with salt aerosols, forming ClNO2 (Wang et al., 2016). All of these nocturnal processes, such as the removal of nitrogen oxides, production of organic and inorganic nitrate (Atkinson and Arey, 2003), and halogen activation (Phillips et al., 2012; Roberts et al., 2008) can impact the potential ozone formation on the following day (Brown et al., 2006) and the formation of secondary aerosol (Riemer et al., 2003; Fry et al., 2009). Because of the importance of N2O5 in atmospheric chemistry, it has been a high priority that N2O5 is accurately measured. Several techniques have been widely used for tropospheric N2O5 measurement, including chemical ionization mass spectrometry (CIMS), cavity ring-down spectroscopy (CRDS) (Dube et al., 2006; Odame-Ankrah and Osthoff, 2011; Schuster et al., 2009; Ayers et al., 2005) cavity enhanced absorption spectroscopy (CEAS) (Langridge et al., 2008; Kennedy et al., 2011) and laser-induced fluorescence (LIF) (Wood et al., 2003; Matsumoto et al., 2005). CIMS is a mass spectrometry method which uses I− as a reagent ion for N2O5 ionizing. Two kinds of CIMS instruments (Chang et al., 2011) are used for N2O5 ionizing. The first kind of instrument uses an ion–molecule reaction region to directly measure iodide-containing clusters I (N2O5)− (Kercher et al., 2009) and second kind of instrument uses a heated inlet to measure the sum of NO3 and N2O5 (Slusher et al., 2004). The former can avoid additional interference from the reaction of I− with gases which also yield NO− 3 . LIF, CEAS and CRDS instruments generally measure N2O5 by its thermal conversion to NO3 using a heated inlet and subsequent detection of NO3 molecules and make use of deliberate addition of nitric oxide (NO) for instrument ‘zeroing’. The most important contributions to the background signal (i.e., interferences) for the above mentioned techniques are due to scattering processes, such as Mie and Rayleigh scattering. CEAS and CRDS both use high-reflectivity (HR) mirrors to achieve a long effective absorption path with high detection limit and LIF detects NO3 fluorescence within a 700–750 nm spectral window with good selectivity. Several N2O5 intercomparison studies have been conducted in the past decades or so using different platforms, such as photochemical smog chamber, ground sites and aircrafts. During NO3Comp 2007 in SAPHIR (Fuchs et al., 2012), three CRDS instruments and two LIF instruments were involved in the comparison for N2O5 measurements and eleven experiments under a variety of conditions in a smog chamber were carried out. The comparison of N2O5 mixing ratios showed an excellent agreement within the combined accuracy of measurements with slopes of the linear regression range between 0.87 and 1.26. And the comparison measurements indicated that the N2O5 inlet transmission efficiency may decrease in the presence of high aerosol loads, and that frequent filter/inlet changes are necessary to quantitatively sample N2O5 in those environments. For ground-based field observation, the comparisons were mainly performed between CIMS and CRDS. In February 2005 and February 2008, two separate intercomparisons between CIMS and CRDS instruments sampling ambient air in Boulder, Colorado were carried out (Chang et al., 2011). The former comparison was between the TD-CIMS instrument (Slusher et al., 2004) based on NO− 3 and the pulsed CRDS instrument (Dube et al., 2006), exhibiting high correlation between the instruments, though the TD-CIMS consistently reported 30% less N2O5 than CRDS while the latter was between the UWCIMS, based on I (N2O− 5 ), with the same pulsed CRDS instrument showing excellent agreement between the two measurement techniques.

Another comparison between CIMS and NOAA-CRDS performed in Hong Kong for 12 days also showed an excellent agreement, with a correlation coefficient (R2) of 0.93, a slope of 0.99, and an intercept of 30 ppt. During the Role of Nighttime Chemistry in Controlling the Oxidizing Capacity of the Atmosphere (RONOCO) measurement campaigns over the United Kingdom in 2010 and 2011 (Breton et al., 2014), a comparison was conducted between CIMS and BBCEAS. Comparison of the observed N2O5 mixing ratios showed excellent agreement between the CIMS and BBCEAS methods for the whole dataset, with the square of linear correlation coefficient, R2 = 0.89. However, comparison observations between CRDS and CEAS instruments in the field under different environments, such as large-scale RH or PM2.5 concentration, for a long period of time are rare. In this study, we present an intercomparison of N2O5 measurements performed by CRDS and CEAS instruments at the campus of University of Chinese Academy of Sciences (UCAS) from February 21, 2016 to March 4, 2016 to evaluate the performance of two optical techniques under different environmental conditions. Ancillary species were measured during the periods to test the theories of N2O5 formation and destruction and were used to evaluate the difference between the measurements by the CRDS and CEAS instruments. 2. Experiment details 2.1. CRDS instrument 2.1.1. The cavity ring down spectroscopy CRDS is a high sensitivity, direct absorption technique, which has been applied to the measurement of numerous species of atmospheric importance (Ball and Jones, 2003; Berden et al., 2000). Several reviews (Gagliardi and Loock, 2014) have illustrated the detailed principle and application of the technique. In this study, we only provide a cursory sketch to contextualize our contribution to the field. In a CRDS instrument, a laser beam is directed into a high-finesse optical cavity. When the beam is switched off, the light intensity decays exponentially, where a fraction is recorded during its transmission from the back mirror of the optical cavity to a photomultiplier tube (PMT). If some fractions of the beam are absorbed by the gas molecules in the optical cavity, light transmission decreases more rapidly. Gas concentrations are derived from the ring-down time constants when the absorber is in the presence (τ) and absence (τ0) of the cavity. ½A ¼


 RL 1 1 − cσ τ τ0

ð1Þ

Here, c is the speed of light and RL is the ratio of the total cavity length to the length over which the absorber is present in the cavity. [A] is the number density and σ is the absorption cross section for A. 2.1.2. Optical layout S.1 shows the schematic of the homemade CRDS system used for measuring N2O5. This system is based on the system described by Wang et al. (2015) for NO3 detection. The CRDS instrument is mainly composed of a diode laser, a high-finesse optical cavity with HR mirrors, and a PMT. The light source is a red diode laser that produces 100 mW pulses which is modulated by a square-wave with a duty cycle of 8% at a repetition rate of 150 Hz. The laser is temperature-tuned resulting in a wavelength range that includes the absorption peak of the radical nitrate near 661.9 nm. The ring down cavity is formed by two HR mirrors with 2.54 cm diameter, and 1 m radius of curvature dielectric coating separated by 76 cm. The output of the cavity is directed into a PMT (Hamamatsu) after passing through a 660 nm band pass filter placed in front of the PMT to prevent stray light from unwanted wavelengths. The output of the PMT is digitized (at a sampling rate of 1 MHz) using a PCI-6132 oscilloscope card mounted on a personal computer. Air samples first flow through a Teflon filter (25 μm thickness, 4.6 cm diameter, 2.5 μm pore size) to remove particulate matter. New filters are

Z. Li et al. / Science of the Total Environment 613–614 (2018) 131–139

changed every 2 or 3 h to reduce N2O5 losses. A 40 cm long piece of PFA tube with an external diameter of 6.25 mm and an internal diameter of 3.8 mm serving as a thermal dissociation of N2O5 is placed between the filter and cavity inlet. This preheater is thermally stabilized at 140 °C to make N2O5 convert to NO3 completely. Then the air samples flowing through the cavity which are heated to 80 °C to prevent the recombination of NO3 with NO2 and allow detection of N2O5·When the sample flow rate is 3.5 L/min, sample gas residence time in the instrument is 0.52 s. Each mirror is purged with 150 sccm dry filtered nitrogen to prevent the contamination of the optical surface from the aerosol particles. A total of 10 ppm NO is added at a flow rate of 50 sccm through an MFC to the air after the filter for a 60 s period out of every 300 s to obtain a NO mixing 150 ppb. The data obtained within the first 30 s period after adding NO is excluded from the data collection, taking into the residence time account. S.2 shows a typical time series of concentration for the field observation of N2O5, where N2O5 is converted to NO3 by the thermal decomposition described above. Although this method detects the sum of ambient NO3 and N2O5, we take all measurements as N2O5 due to the high N2O5:NO3 ratios expected at low temperatures and the high NO2 concentrations observed. For example, under 0 °C and 10 ppb of NO2 condition, the N2O5:NO3 ratio, calculated from the equilibrium constant between NO3 and N2O5, is 190. NO3 is therefore negligible in the dataset. During the whole campaign, the ratio of NO3 to the sum was b5%. 2.1.3. Instrument properties According to Osthoff et al. (2007), the temperature dependence of the NO3 absorption cross-section at 662 nm is given σ = [(4.582 ± 0.096) − (0.00796 ± 0.00031) × T(K)] × 10−17 cm2 molecule−1. The effective absorption cross-section at standard conditions can be acquired by convolving the NO3 absorption cross-section (Yokelson et al., 1994) with a laser spectrum. After scaling this absorption cross-section to 348 K, the final effective absorption cross-section at the center wavelength of 661.9 nm is determined as the 1.74 × 10−17 cm2 molecule−1. To obtain the concentration, the ratio of the total cavity length to the length over which the absorber is present in the cavity, RL, should be determined, which cannot be accomplished simply by using the ratio of the distance between the two RH mirrors and the distance between the inlet and the outlet due to the turbulence of purge flow. Based on the known O3 absorption cross section, we got the fitted RL value to be 1.196 by flowing known different identified concentrations of O3 into the CRDS cell (Wang et al., 2015). The main uncertainty of N2O5 is its inlet transmission efficiency. Because NO3 radicals and N2O5 are reactive species which are easily lost, the inlet is constructed from Teflon tubing and fittings. According to Dube et al. (2006), the most important contribution to N2O5 sampling efficiency is the wall loss for NO3 in the heated channel, with a value of 0.2 ± 0.05/s. The residence time is 0.52 s when the sampling air arrives at the centre of the cavity from the heated inlet at a flow rate of 3.5 L/min. Thus, the wall loss of NO3 in the preheated channel and the cavity is estimated to be 10 ± 3%. Meanwhile, N2O5 losses on clean filters are negligible (Dube et al., 2006). However, due to filter aging (Fuchs et al., 2012; Fuchs et al., 2008), the transmission efficiency would decrease with an uncertainty of 3%. Thus, N2O5 transmission is estimated to be 90% (−3%, +6%) at the given residence time. The theoretical limit of detection (LOD) of this instrument is given by:

½NO3 min ¼

pffiffiffi 2RL Δτ cστ20

ð2Þ

133

the detection limit of NO3 radical is approximately 3.9 ppt (1σ) for a 10 s integration time. It is important to consider systematic errors, in particular when comparing two different instruments measuring the same parameter. While the CRDS technique is a self-calibrating procedure, the largest contributions to systematic error come from the absolute calibration of the cross sections reported in the literature. The differential cross sections of NO3 differ significantly within the literature. Therefore a systematic error of ±10% for the retrieved N2O5 concentration is accurate. The overall uncertainty of N2O5 measurement is estimated to be ±12% in our system when taking into account the following part: the uncertainty of the NO3 radical effective absorption cross-section measured by Yokelson et al. (1994) and Osthoff et al. (2007) (±10%), the uncertainty of RL (±5%) and the uncertainty of the inlet transmission efficiency (±3%) and the uncertainties of NO3 radical caused by the baseline shift during the period of 3–5 min (±2%), resulting in an overall uncertainty in NO3 radical of ±12% (1σ) for the CRDS system. 2.2. CEAS instrument CEAS is an optical technique to detect atmospheric trace gases that rely on measurements absorption spectrum of a specific wavelength window. CEAS was widely used in detection of targeting species of atmospheric interest both in laboratory experiments and field in the past decades (Washenfelder et al., 2008; Platt et al., 2009; Thalman and Volkamer, 2010). The CEAS instrument used for comparison is from Peking University (Wang et al., 2017). Briefly, light is emitted by a single light emitting diode with peak wavelength of 665 nm, FWHM of 25 nm and then transmits into the cavity. The cavity separated by 50.0 cm consists a pair of high reflective (HR) mirrors with reflectivity of about 0.999936 at 660 nm. The leaked intensity from the exited mirror of the cavity is received by the Ocean Optics QE65000 spectrometer through an optical fibre with 100 μm diameter, and 0.22 numerical aperture. The preheater temperature of the CEAS instrument to make N2O5 decompose is 120 °C, different from that of the CRDS instrument. The cavity temperature is 80 °C to prevent the recombination of NO3 and NO2 to N2O5which is the same as that of the CRDS instrument. The flow rate is 2 slm with the total transmission efficiency of N2O5 82.9%. In order to retrieval the concentration of the target species, several key parameters including: the cross sections of the target species, effective cavity length, mirror reflectivity and reference spectrum have been taken into account. Briefly, the cross section of the NO3 for the heated cavity (353 K), is calculated by scaling the NO3 cross section profile from Yokelson et al. (1994)) to 353 K based on that from Osthoff et al. (2007) and then convolving the scaled cross section with an instrument function. The effective cavity length is determined with standard gas of NO2. Mirror reflectivity is measured by flowing the same amount of pure N2 and He in the cavity and getting the Rayleigh scattering signals of two species in the cavity. The reference spectrum used for data obtained is from the spectrum which was added NO into the ambient sample flow. This method can enhance the fitting precision by avoiding the complicated fitting of water vapour absorption. The typical 1σ detection limits of the CEAS instrument is 1.6 ppt in 30 s. The uncertainty of the measurement is 19%, governed by systematic errors in the absorption cross section of NO3 (±13%), errors in the inlet gas transmission efficiency (±4–11%) and the mirror reflectivity (± 5%) and effective absorption path length (± 13%). A summary of the properties of these two instruments during this campaign is provided in Table 1. 2.3. Ancillary measurements

Here [NO3]min is the smallest measurable concentration of NO3 and Δτ is the smallest measurable difference between time constants. Using RL = 1.196, τ0 = 100 μs, σ = 1.74 × 10− 17 cm2 molecule− 1 and Δτ = 0.3 μs for a 10 s integration time in the field measurement,

Ancillary measurements of the chemical species included NO (chemiluminescence), NO2 (cavity enhanced absorption spectrum), ozone (UV absorption), PM2.5, PM10 as well as meteorological chemical

134

Z. Li et al. / Science of the Total Environment 613–614 (2018) 131–139

species of wind speed and direction, relative humidity and temperature were also measured at the field site. The computer clocks for all measurements were found to deviate b 5 s over the course of a day. 2.4. Site description The campaign was conducted from February 21, 2016 to March 4, 2016, deployed on the campus of UCAS which is located in the northeast of Beijing, China. The observation site is close to Jingjia National Highway (G111), impacted mainly by the emission from vehicles in the vicinity and large amounts of local NO. The CRDS and CEAS instruments measured N2O5 at the same time. The sampling inlet was about 15 m above the ground. However, the inlet directions of the two instruments were slightly different; that of the CEAS instrument faced north and that of the CRDS instrument collecting the air mass through a funnel, as shown in S.3. The distance between the two inlets was about 1 m. 13 days of data from this campaign were presented within this paper for comparison. 3. Results 3.1. Meteorological parameters S.4 shows the time series of key parameters such as relative humidity, temperature, wind direction and wind speed for all experiments from 21 February to 4 March 2016. The measurements suffered from variable concentrations of these parameters. Measurements were averaged to 1 min time intervals for the analysis shown below. During the whole campaign, the temperature showed clean nocturnal trend, with an average value of 1.1 °C. In the last three days, the temperature gradually increased to a high value of 19.1 °C. The relative humidity also exhibited similar variations at nighttime period, changing between 7% and 83%. A wind rose displayed in S.5 reflected that wind direction was mainly from east to west. Thus, the observation was mainly influenced by the air mass of local emissions or transmitted from the city. According to the air pollutants levels, the whole campaign could be divided into six clean nights (the nights of February 20, February 22 to February 25, February 28) with a daily average of PM2.5 b 30 μg/m3 and seven polluted nights (the nights of February 21, February 26 to February 27, February 29 to March 3) with the highest value of PM2.5 above 30 μg/m3. During the last four nights, PM2.5 concentration gradually increased and exceeded 200 μg/m3 on the night of March 3. PM10 concentration changed consistently with PM2.5 concentration and the highest value could reach about 400 μg/m3. 3.2. NO2, NO, O3 and N2O5 mixing ratios Fig. 1 shows the time series of N2O5 mixing ratios from the CEAS and CRDS instruments for all experiments and key parameters that are relevant for formation and loss of N2O5 such as NO, NO2, O3 from February 21 to March 4, 2016. The two instruments were sufficiently sensitive to measure N2O5 concentrations during the entire campaign. The mean N2O5 concentrations were 148 ppt for CRDS and 156 ppt for CEAS, with a median CEAS/CRDS N2O5 ratio of 1.24. NO2 mixing ratios

Table 1 A summary of the properties for CRDS and CEAS instruments. Method Light source Time resolution 1σ detection limit 1σ uncertainty Flow rate Cavity length NO3 titration frequency Mirror reflectivity

CRDS Diode laser (150 Hz) 10 s 3.9 ppt 12% 3.5 slm 76 cm 5 min 0.999975

CEAS LED 30 s 1.6 ppt 19% 2 slm 45 cm 5 min 0.999936

ranged from the detection limit to N60 ppb, with an average of 13.9 ppb during the night. O3 presented a strong anti-correlation with NO2, with the maximum concentration of 47.8 ppb. High NO level can suppress the production of N2O5, due to the rapid reaction between NO3 radical and NO, which had been observed in several night-time studies at ground level close to local sources of NO. A sudden apparent extinction of N2O5 occurred with a sudden increase in NO on several occasions. 3.3. Comparison results and discussion 3.3.1. Overall comparison The overall time series for N2O5 data detected by CEAS and CRDS instruments show a good agreement in Fig. 2(a). The linear regression exhibits [N2O5]CEAS = 0.94 × [N2O5]CRDS + 15.6 ppt, with a correlation coefficient R2 = 0.97. The deviation from unity is within the combined 1σ accuracies of the two instruments. The agreement between the CEAS and CRDS measurements varies from day to day, thus indicating a day-to-day variability of the sensitivity of two instruments to environmental change. N2O5 concentration measurements of the two instruments below detection limit were excluded from the comparison. A better agreement was obtained when a higher N2O5 mixing ratio appeared with a correlation coefficient R2 = 0.95, a fitted CEAS/CRDS N2O5 ratio of 1.02 and an intercept of 9.29 ppt during the period from 17:00 February 29 to 7:00 March 1 as shown in Fig. 2(c). We note that in this day N2O5 concentrations changed very quickly which may be due to local traffic emission. However, the response of the two instruments to the changes in N2O5 concentrations is rapid and consistent as shown in Fig. 2(b), which demonstrates the capability of two instruments to accurately measure N2O5 with high sensitivity. However, there also exits some period when the response of the two instruments to changes of N2O5 concentration is not consistent. Relatively large discrepancies exist mainly due to low levels of N2O5 and may also due to different environmental conditions such as temperature, relative humidity and PM2.5. The influence of these parameters on two instrument performance will be discussed below. 3.3.2. Comparison at low levels of N2O5 Fig. 3(a) shows that the difference of the N2O5 measurements by the two instruments is larger, with slope = 1.37, if the data are from 6 clean nights (the nights of February 20, February 22 to February 25, February 28) when the highest value of N2O5 is below 140 ppt and PM2.5 is smaller than 30 μg/m3. But the correlation R2 = 0.96, which is close to that of the overall dataset. The data picked above are classified by different ranges of temperature and several differences are found among the different data subsets (shown in Fig. 3[b]), which indicate that changes in ambient temperature may cause the discrepancy. The influence of temperature on the measurement results is mainly from two aspects: the selection of high temperature absorption cross section in the cavity and the thermal dissociation efficiency. For example, when the ambient temperature decreased from 10 °C to 0 °C, the temperature in the cavity decreased by 3 °C at most. Thus, the effective absorption cross section of this temperature decreases by 1% compared that of 348 K for CRDS instruments and the thermal dissociation efficiency decreases to 94.7% from 96.0% with a NO2 concentration of 10 ppb, showing that the measurement results are insensitive to temperature change and that temperature has a small effect on the difference between the measurement of the two instruments. In order to verify the effect of temperature on the discrepancy between the measurements of the two instruments, the correlations between the different temperature data subsets at high N2O5 concentration were investigated. There seems no distinct difference between different data subsets and the fitted slopes are close to unity (Fig. 3[c]). In addition, we notice that most of the points are below 60 ppt. This indicates that the discrepancy between the two instruments at low levels of N2O5 is probably caused

Z. Li et al. / Science of the Total Environment 613–614 (2018) 131–139

135

Fig. 1. Time series of N2O5 mixing ratios (1 min average) by the CEAS and CRDS instruments and compounds which are of importance during the experiment. Data below detection limit were excluded.

by accuracy problems as discuss above over the course of the experiment. 3.3.3. Comparison as a dependence on RH In order to explore the effect of relative humidity on the N2O5 measurement results of the two instruments, we selected data from the nights of two high-pollution days, February 26 and February 27

with PM2.5 concentration b 100 μg/m3, temperature ranged between − 6 °C and 6 °C, RH ranged between 18% and 78% for comparison and classified the data based on RH value. The change in the relative humidity during 5 min periods on the night of February 27 is 2% (Fig. 4[b]) at most. The correlation analysis of the different data subsets show that the difference between the two instruments is increasingly apparent with the increase in RH value (Fig. 4[a]). When the RH value

Fig. 2. (a) Scatter plots for the entire N2O5 dataset from the CRDS and CEAS instruments. The red line illustrates the linear regression. (b). An example of time series and correlation of N2O5 mixing ratios between CRDS and CEAS instruments from February 29 to March 1, 2016 in 1 min average data. (c). Correlation analysis between the measurements of the CRDS and CEAS instruments. The slope and intercept of linear fit (red line) demonstrate a good agreement between the two measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

136

Z. Li et al. / Science of the Total Environment 613–614 (2018) 131–139

Fig. 3. (a) Scatter plots for the N2O5 dataset from CRDS and CEAS instruments in clean days with PM 2.5 concentration below 30 μg/m3. Values below 20 ppt were excluded. The red lines illustrate the linear regression. (b) The CRDS observations of N2O5 plotted against CEAS observations with different temperatures and correlations between these types of N2O5 data from the two instruments. Solid lines and coloured labels give the corresponding results of the linear regression for the different data subsets. Data from clean days with PM 2.5 concentration below 30 μg/m3. Values below 20 ppt were excluded. (c) The figure is similar to panel (b); the data are from the nights of March 1 and March 2 with N2O5 concentrations ranging from the detection limit to about 1400 ppt, PM2.5 concentration b 150 μg/m3 and RH b 60%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

is b60%, the data subsets of the two instruments are consistent and the fitted slopes are close to unity, thereby implying that the influence of water vapour absorption on the measurement of NO3 radicals by the two instruments is identical when obtaining the reference spectrum or background ring down time by titration with NO under low humidity conditions. But when the relative humidity is N60%, the fitting slope is 0.81. It can also be clearly seen that the value of CRDS measurements is significantly higher than that of CEAS measurements (cyan block in Fig. 4[b]) when RH N 60%.

3.3.4. Comparison as a dependence on PM2.5 In order to explore the effect of PM2.5 on N2O5 measurements from the CRDS and CEAS instruments, data gathered up by the two instruments on the last four nights of the campaign were classified by the concentrations of PM2.5 and compared at different concentration of PM2.5 subset under low humidity (RH b 40%) (Fig. 5[a]) and high humidity (40% b RH b 60%) conditions respectively (Fig. 5[b]). Under low humidity conditions, the difference between the results from the two instruments is small and no obvious difference is observed at different PM2.5 concentration subsets with PM2.5 concentration below 200 μg/m3 (Fig. 5[a]). Under relative high humidity conditions (40% b RH b 60%), when the particle concentration is b200 μg/m3, the fitted slopes are close to unity at different ranges of PM2.5 concentration. When the concentration of particles is higher than 200 μg/m3, the fitted slope is 0.74, which is different from the results under low PM2.5 concentration conditions (Fig. 5[b]). To further illustrate the difference between the two measurements as a function of PM2.5 concentration, the normalized difference (ND) of CRDS and CEAS data is plotted versus PM2.5 concentration in Fig. 6. Data is filtered during period from 19:00 February 29 to 22:00

March 3 with RH b 60%. Different kinds of colours represent data subsets from four different days. The ND is defined as follows:    0:5 ND ¼ ½N2 O5 CEAS −½N2 O5 CRDS = ½N2 O5 CEAS  ½N2 O5 CRDS Fig. 6 shows that the difference is within 15% with PM2.5 below 150 μg/m3, which attributes to combined system uncertainties. However, ND is up to 30% when PM2.5 is N200 μg/m3. A detailed plot of continuous mixing ratios of N2O5 (CRDS in red, CEAS in black), PM2.5 concentration (blue) and RH from 17:00 on 3 March to 08:00 on March 4, 2016 shown in Fig. 7(a) clearly reveals the diversity of the two N2O5 measurement results. However, the correlation coefficient (R2) of 0.99 indicates that measurements by the two instruments are consistent (Fig. 7[b]). This discrepancy may be due to the different inlet transmission efficiencies for N2O5 from the laboratory calibration for the two instruments in the presence of high PM2.5 on which N2O5 is taken up. In addition, we assume that the influence of scattering of particles on two measurements is different. The results indicate that the cavity contamination on heavily polluted days is large. Therefore, the filters should be removed frequently and the cavity should be replaced regularly during these days to reduce loss. Moreover, the flow rate can be increased to decrease the residence time of gas in the cavity and reduce loss. 4. Summary and conclusions A comparison of N2O5 concentrations between two different systems, CRDS and CEAS was carried out in winter in a suburban area in Beijing. A total of 13 days of data were obtained and a good agreement between the measurements of the two instruments was received within

Z. Li et al. / Science of the Total Environment 613–614 (2018) 131–139

Fig. 4. (a) The CRDS observations of N2O5 plotted against CEAS observations with different relative humidities and correlations between these types of N2O5 data from the two instruments. Data are from the nights of February 26 and February 27. Solid lines and coloured labels give the corresponding results of the linear regression for the different data subsets. (b) Time series of N2O5 mixing ratios (1 min average) by the CEAS and CRDS instruments and relative humidity (RH) on the night of February 27, 2016. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

their uncertainties. Linear regression showed that [N2O5]CEAS = 0.94 × [N2O5]CRDS + 15.6 ppt with an average correlation coefficient R2 = 0.97. The agreement between the CRDS and CEAS measurements was best (R2 = 0.95, slope = 1.02 and intercept = 9.3 ppt) on the night of February 29 during which period a rapid variation in N2O5 was observed. The excellent agreement between the CRDS and CEAS measurements demonstrates their capabilities for accurately measuring N2O5 with a high time resolution of a few seconds in different environment. However, accuracy problems may cause relatively larger discrepancies at low levels of N2O5 and under several special environmental conditions, such as high RH (RH N 60%) and high PM2.5 concentrations (PM2.5 N 200 μg/m3), large discrepancies also existed. The discrepancy between the measurements of the two instruments indicates that filters should be removed frequently during the heavily polluted days and that increasing the flow rate in the cavity is an alternative approach to reduce the loss. We conclude that the concerted applications of in-situ CRDS and CEAS observations will provide a more complete picture of night-time nitric oxide chemistry.

137

Fig. 5. (a) The CRDS observations of N2O5 plotted against CEAS observations with different amounts of PM2.5 and correlations between these types of N2O5 data from the two instruments. Solid lines and coloured labels give the corresponding results of the linear regression for the different data subsets. Data are picked up at the last four nights of the campaign with RH below 40%. (b) The figure is similar panel (a); but the data are picked up at the last four nights of the campaign with RH between 40% and 60%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Acknowledgements This work was funded by National Natural Science Foundation of China (61575206, 91644107 and 41571130023), the National Key Research and Development Program of China (2017YFC0209401) and

Fig. 6. The normalized difference (ND) of CRDS and CEAS measurements versus PM2.5 (filtered 1 min data during period from 19:00 February 29 to 22:00 March 3) with RH b 60%. Individual ND values are shown as black dots.

138

Z. Li et al. / Science of the Total Environment 613–614 (2018) 131–139

Fig. 7. (a) A detailed plot of the continuous mixing ratios of N2O5 (red in CRDS, black in CEAS), PM2.5 concentration (blue), RH from 17:00 on 3 March to 08:00 on March 4, 2016. (b) Correlation analysis between the measurements by CRDS and CEAS instruments. The correlation of N2O5 mixing ratio by CRDS and CEAS instruments show the similar trend of both instruments. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Open Fund of Key Laboratory of Environmental Optics and Technology, Chinese Academy of Sciences (2005DP173065-2016-03). We also thanks for the effective support of University of Chinese Academy of Sciences and Peking University. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scitotenv.2017.08.302. References Aldener, M., Brown, S.S., Stark, H., Williams, E.J., Lerner, B.M., Kuster, W.C., Goldan, P.D., Quinn, P.K., Bates, T.S., Fehsenfeld, F.C., Ravishankara, A.R., 2006. Reactivity and loss mechanisms of NO3 and N2O5 in a polluted marine environment: results from in situ measurements during New England Air Quality Study 2002. J. Geophys. Res.Atmos. 111 (Article D23S73). Atkinson, R., Arey, J., 2003. Atmospheric degradation of volatile organic compounds. Chem. Rev. 103, 4605–4638. Ayers, J.D., Apodaca, R.L., Simpson, W.R., Baer, D.S., 2005. Off-axis cavity ringdown spectroscopy: application to atmospheric nitrate radical detection. Appl. Opt. 44 (33), 7239–7242. Ball, S.M., Jones, R.L., 2003. Broad-band cavity ring-down spectroscopy. Chem. Rev. 103 (12), 5239–5262. Berden, G., Peeters, R., Meijer, G., 2000. Cavity ring-down spectroscopy: experimental schemes and applications. Int. Rev. Phys. Chem. 19 (4), 565–607.

Breton, M.L., Bacak, A., Muller, J.B.A., Bannan, T.J., Kennedy, O., Ouyang, B., Xiao, P., Bauguitte, S.J.-B., Shallcross, D.E., Jones, R.L., Daniels, M.J.S., Ball, S.M., Percival, C.J., 2014. The first airborne comparison of N2O5 measurements over the UK using a CIMS and BBCEAS during the RONOCO campaign. Anal. Methods 6 (24), 9731–9743. Brown, S.S., Stutz, J., 2012. Nighttime radical observations and chemistry. Chem. Soc. Rev. 41, 6405–6447. Brown, S.S., Dibb, J.E., Stark, H., Aldener, M., Vozella, M., Whitlow, S., Williams, E.J., Lerner, B.M., Jakoubek, R., Middlebrook, A.M., DeGouw, J.A., Warneke, C., Goldan, P.D., Kuster, W.C., Angevine, W.M., Sueper, D.T., Quinn, P.K., Bates, T.S., Meagher, J.F., Fehsenfeld, F.C., Ravishankara, A.R., 2004. Nighttime removal of NOx in the summer marine boundary layer. Geophys. Res. Lett. 31 (Article L07108). Brown, S.S., Ryerson, T.B., Wollny, A.G., Brock, C.A., Peltier, R., Sullivan, A.P., Weber, R.J., Dube', W.P., Trainer, M., Meagher, J.F., Fehsenfeld, F.C., Ravishankara, A.R., 2006. Variability in nocturnal nitrogen oxide processing and its role in regional air quality. Science 311, 67–70. Brown, S.S., Dubé, W.P., Peischl, J., Ryerson, T.B., Atlas, E., Warneke, C., de Gouw, J.A., Hekkert, S.L., Brock, C.A., Flocke, F., Trainer, M., Parrish, D.D., Feshenfeld, F.C., Ravishankara, A.R., 2011. Budgets for nocturnal VOC oxidation by nitrate radicals aloft during the 2006 Texas Air Quality Study. J. Geophys. Res.-Atmos. 116 (Article D24305). Chang, W.L., Bhave, P.V., Brown, S.S., Riemer, N., Stutz, J., Dabdub, D., 2011. Heterogeneous atmospheric chemistry, ambient measurements, and model calculations of N2O5. Aerosol Sci. Technol. 45 (6), 665–695. Dube, W.P., Brown, S.S., Osthoff, H.D., Nunley, M.R., Ciciora, S.J., Paris, M.W., McLaughlin, R.J., Ravishankara, A.R., 2006. Aircraft instrument for simultaneous, in situ measurement of NO3 and N2O5 via pulsed cavity ring-down spectroscopy. Rev. Sci. Instrum. 77 (3), 034101–034111. Fry, J.L., Kiendler-Scharr, A., Rollins, A.W., Wooldridge, P.J., Brown, S.S., Fuchs, H., Dube, W., Mensah, A., dal Maso, M., Tillmann, R., Dorn, H.-P., Brauers, T., Cohen, R.C., 2009. Organic nitrate and secondary organic aerosol yield from NO3 oxidation of beta-pinene

Z. Li et al. / Science of the Total Environment 613–614 (2018) 131–139 evaluated using a gas-phase kinetics/aerosol partitioning model. Atmos. Chem. Phys. 9 (4), 1431–1449. Fuchs, H., Dube, W.P., Cicioira, S.J., Brown, S.S., 2008. Determination of inlet transmission and conversion efficiencies for in situ measurements of the nocturnal nitrogen oxides, NO3, N2O5 and NO2, via pulsed cavity ring-down spectroscopy. Anal. Chem. 80 (15), 6010–6017. Fuchs, H., Simpson, W.R., Apodaca, R.L., Brauers, T., Cohen, R.C., Crowley, J.N., Dorn, H.P., Dube, W.P., Fry, J.L., Haseler, R., Kajii, Y., Kiendler-Scharr, A., Labazan, I., Matsumoto, J., Mentel, T.F., Nakashima, Y., Rohrer, F., Rollins, A.W., Schuster, G., Tillmann, R., Wahner, A., Wooldridge, P.J., Brown, S.S., 2012. Comparison of N2O5 mixing ratios during NO3Comp 2007 in SAPHIR. Atmos. Meas. Tech. 5 (11), 2763–2777. Gagliardi, G., Loock, H.-P., 2014. Cavity-enhanced Spectroscopy and Sensing. Springer, Berlin. Kennedy, O.J., Ouyang, B., Langridge, J.M., Daniels, M.J.S., Bauguitte, S., Freshwater, R., McLeod, M.W., Ironmonger, C., Sendall, J., Norris, O., Nightingale, R., Ball, S.M., Jones, R.L., 2011. An aircraft based three channel broadband cavity enhanced absorption spectrometer for simultaneous measurements of NO3, N2O5 and NO2. Atmos. Meas. Tech. 4, 1759–1776. Kercher, J.P., Riedel, T.P., Thornton, J.A., 2009. Chlorine activation by N2O5: simultaneous, in situ detection of ClNO2 and N2O5 by chemical ionization mass spectrometry. Atmos. Meas. Tech. 2 (1), 193–204. Langridge, J.M., Ball, S.M., Shillings, A.J.L., Jones, R.L., 2008. A broadband absorption spectrometer using light emitting diodes for ultrasensitive, in situ trace gas detection. Rev. Sci. Instrum. 79 (12) (Article 123110). Matsumoto, J., Imai, H., Kosugi, N., Kajii, Y., 2005. In situ measurement of N2O5 in the urban atmosphere by thermal decomposition/laser-induced fluorescence technique. Atmos. Environ. 39 (36), 6802–6811. Odame-Ankrah, Osthoff, H.D., 2011. A compact diode laser cavity ring-down spectrometer for atmospheric measurements of NO3 and N2O5 with automated zeroing and calibration. Appl. Spectrosc. 65 (11), 1260–1268. Osthoff, H.D., Pilling, M.J., Ravishankara, A.R., Brown, S.S., 2007. Temperature dependence of the NO3 absorption cross-section above 298 K and determination of the equilibrium constant for NO3 +NO2 b−N N2O5 at atmospherically relevant conditions. Phys. Chem. Chem. Phys. 9 (43), 5785–5793. Phillips, G.J., Tang, M.J., Thieser, J., Brickwedde, B., Schuster, G., Bohn, B., Lelieveld, J., Crowley, J.N., 2012. Significant concentrations of nitryl chloride observed in rural continental Europe associated with the influence of sea salt chloride and anthropogenic emissions. Geophys. Res. Lett. 39 (Article L10811). Platt, U., Meinen, J., Pohler, D., Leisner, T., 2009. Broadband Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) - applicability and corrections. Atmos. Meas. Tech. 2 (2), 713–723.

139

Riemer, N., Vogel, H., Vogel, B., Schell, B., Ackermann, I., Kessler, C., Hass, H., 2003. Impact of the heterogeneous hydrolysis of N2O5 on chemistry and nitrate aerosol formation in the lower troposphere under photosmog conditions. J. Geophys. Res.-Atmos. 108 (D4) (Article ACH 1–20). Roberts, J.M., Osthoff, H.D., Brown, S.S., Ravishankara, A.R., 2008. N2O5 oxidizes chloride to Cl2 in acidic atmospheric aerosol. Science 321 (5892), 1059. Schuster, G., Labazan, I., Crowley, J.N., 2009. A cavity ring down/cavity enhanced absorption device for measurement of ambient NO3 and N2O5. Atmos. Meas. Tech. 2 (1), 1–13. Slusher, D.L., Huey, L.G., Tanner, D.J., 2004. A thermal dissociation–chemical ionization mass spectrometry (TD-CIMS) technique for the simultaneous measurement of peroxyacyl nitrates and dinitrogen pentoxide. J. Geophys. Res. 109 (Article D19315). Thalman, R., Volkamer, R., 2010. Inherent calibration of a blue LED-CE-DOAS instrument to measure iodine oxide, glyoxal, methyl glyoxal, nitrogen dioxide, water vapour and aerosol extinction in open cavity mode. Atmos. Meas. Tech. 3 (6), 1797–1814. Wang, D., Hu, R.Z., Xie, P.H., Liu, J.G., Liu, W.Q., Qin, M., Ling, L.Y., Zeng, Y., Chen, H., Xing, X.B., Zhu, G.L., Wu, J., Duan, J., Lu, X., Shen, L.L., 2015. Diode laser cavity ring-down spectroscopy for in situ measurement of NO3 radical in ambient air. J. Quant. Spectrosc. Radiat. Transf. 166, 23–29. Wang, T., Tham, Y.J., Xue, L.K., Li, Q.Y., Zha, Q.Z., Wang, Z., Poon, S.C.N., Dubé, W.P., Blake, D.R., Louie, P.K.K., Luk, C.W.Y., Tsui, W., Brown, S.S., 2016. Observations of nitryl chloride and modeling its source and effect on ozone in the planetary boundary layer of southern China. J. Geophys. Res.-Atmos. 121 (5), 2476–2489. Wang, H.C., Chen, J., Lu, K.D., 2017. Development of a portable cavity-enhanced absorption spectrometer for the measurement of ambient NO3 and N2O5: experimental setup, lab characterizations, and field applications in a polluted urban environment. Atmos. Meas. Tech. 10 (4), 1465–1479. Washenfelder, R.A., Langford, A.O., Fuchs, H., Brown, S.S., 2008. Measurement of glyoxal using an incoherent broadband cavity enhanced absorption spectrometer. Atmos. Chem. Phys. 8 (24), 7779–7793. Wayne, R.P., Barnes, I., Biggs, P., Burrows, J.P., Canosamas, C.E., Hjorth, J., Lebras, G., Moortgat, G.K., Perner, D., Poulet, G., Restelli, G., Sidebottom, H., 1991. The nitrate radical-physics, chemistry and the atmosphere. Atmos. Environ., Part A 25 (1), 1–203. Wood, E.C., Wooldridge, P.J., Freese, J.H., Albrecht, T., Cohen, R.C., 2003. Prototype for in situ detection of atmospheric NO3 and N2O5 via laser-induced fluorescence. Environ. Sci. Technol. 37 (24), 5732–5738. Yokelson, R.J., Burkholder, J.B., Fox, R.W., Talukdar, R.K., Ravishankara, A.R., 1994. Temprature dependence of the NO3 absorption-spectrum. J. Phys. Chem. 98 (50), 13144–13150.