7.3. Results and discussion

### 7.3.1. Continuous monitoring of atmospheric HONO

The developed QCL instrument was employed for monitoring daytime and nighttime variation of HONO in an urban environment near a road with moderate traffic. Continuous monitoring of HONO mixing ratio variation was performed during a campaign of several days. Figure 28 shows time series of the mixing ratios of 15 min averages of HONO and NO2 and the corresponding ratios of HONO/NO2 along with the solar radiation for the field measurements from 14 to 18 January 2013 (16–18 of them are snowy days). NO2 was measured by a NOx analyzer (Environmental SA). The solar radiation was recorded by a weather station (Davis Vantage Pro2, Montanay). The measured HONO mixing ratios ranged from 1.40 ppb to

6.76 ppb, with a mean value of 3.33 1.03 ppb, whereas the mean and maximum mixing ratios

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy

http://dx.doi.org/10.5772/intechopen.72948

233

NO2 is known to be an important precursor for the formation of HONO or to have a common source. As mentioned in the introduction, the mixing ratios of HONO and NO2 were found to be highly correlated in many field observations [77, 86]. The regression analysis (shown in Figure 29) of the combined data sets indicates good correlation between HONO and NO2 mixing ratios, displaying an intercept of 0.83, slope of 0.12, and R<sup>2</sup> of 0.70. This slope can be interpreted as an upper limit for estimate of the HONO exhaust fraction of NO2 emissions. The two parts marked with purple rectangles in Figure 28 implied other sources of HONO formation, because of the increasing fraction of HONO/NO2 with decreasing NO2 mixing ratios. The higher mixing ratios of HONO in the morning are considered as products of heterogeneous reactions of NO2 on wet surfaces during nighttime. The two green rectangles in Figure 28 show a record of HONO mixing ratio variation with solar radiation on snow days. A photochemically enhanced HONO production from snowpack under solar radiation can be seen [73]. Finding the missing sources and the formation mechanism of HONO in the atmosphere is

In conclusion, we overviewed our recent developments of several gas sensors based on TDLAS technology for in situ monitoring of hazard gases, including CH4, CO2, CO, HONO, H2S, and

12CO2. Good understanding of the sources and sinks of these hazard gases requires

of NO2 were 21.32 7.36 ppb and 50.70 ppb, respectively.

Figure 29. Correlation between HONO and NO2 during the measurement period.

still the actual topic for tropospheric HONO chemistry.

7.3.2. Possible sources of HONO

8. Summary and outlook

13CO2/

Figure 28. Time series of HONO, NO2, the solar radiation, and HONO/NO2 during the field measurements from 14 to 18 January 2013.

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy http://dx.doi.org/10.5772/intechopen.72948 233

Figure 29. Correlation between HONO and NO2 during the measurement period.

6.76 ppb, with a mean value of 3.33 1.03 ppb, whereas the mean and maximum mixing ratios of NO2 were 21.32 7.36 ppb and 50.70 ppb, respectively.

#### 7.3.2. Possible sources of HONO

measured with a pressure transducer (Pfeiffer Vacuum, CMR 361). Temperature of the multi-pass cell was maintained at 30C (within 0.1C) in order to avoid deposit of aqueous nitrous acid on the optical cell wall (especially on the cell mirrors) and to avoid any artifact production due to heterogeneous reaction inside the cell. The two detector outputs were sampled with a fast data acquisition digital oscilloscope (LeCroy Wavesurfer 104Xs-A). The data was then transferred to a

The developed QCL instrument was employed for monitoring daytime and nighttime variation of HONO in an urban environment near a road with moderate traffic. Continuous monitoring of HONO mixing ratio variation was performed during a campaign of several days. Figure 28 shows time series of the mixing ratios of 15 min averages of HONO and NO2 and the corresponding ratios of HONO/NO2 along with the solar radiation for the field measurements from 14 to 18 January 2013 (16–18 of them are snowy days). NO2 was measured by a NOx analyzer (Environmental SA). The solar radiation was recorded by a weather station (Davis Vantage Pro2, Montanay). The measured HONO mixing ratios ranged from 1.40 ppb to

Figure 28. Time series of HONO, NO2, the solar radiation, and HONO/NO2 during the field measurements from 14 to 18

personal computer for further data processing.

7.3.1. Continuous monitoring of atmospheric HONO

7.3. Results and discussion

232 Green Electronics

January 2013.

NO2 is known to be an important precursor for the formation of HONO or to have a common source. As mentioned in the introduction, the mixing ratios of HONO and NO2 were found to be highly correlated in many field observations [77, 86]. The regression analysis (shown in Figure 29) of the combined data sets indicates good correlation between HONO and NO2 mixing ratios, displaying an intercept of 0.83, slope of 0.12, and R<sup>2</sup> of 0.70. This slope can be interpreted as an upper limit for estimate of the HONO exhaust fraction of NO2 emissions. The two parts marked with purple rectangles in Figure 28 implied other sources of HONO formation, because of the increasing fraction of HONO/NO2 with decreasing NO2 mixing ratios. The higher mixing ratios of HONO in the morning are considered as products of heterogeneous reactions of NO2 on wet surfaces during nighttime. The two green rectangles in Figure 28 show a record of HONO mixing ratio variation with solar radiation on snow days. A photochemically enhanced HONO production from snowpack under solar radiation can be seen [73]. Finding the missing sources and the formation mechanism of HONO in the atmosphere is still the actual topic for tropospheric HONO chemistry.

### 8. Summary and outlook

In conclusion, we overviewed our recent developments of several gas sensors based on TDLAS technology for in situ monitoring of hazard gases, including CH4, CO2, CO, HONO, H2S, and 13CO2/ 12CO2. Good understanding of the sources and sinks of these hazard gases requires instruments capable of performing high sensitivity, high precision, high specificity, high spatial resolution, and fast in situ measurements. TDLAS is an effective method to measure these gases' mixing ratios and multiple parameters with these advantages. The methane detection system based on TDLAS can simultaneously detect CH4, C2H2, and C2H4 rapidly and effectively in open environment, and the response time is less than 2 s. The MDLs of these three gases can meet the requirements for the detection of natural gas leakage to petrochemical industry. The accuracy of making an alarm is 100%, which can be used in natural gas station and valve room gas leakage detection. The detection limit of CO detection system based on TDLAS technology is 0.25 ppm with an integration time of 30 s, which basically realizes the high sensitivity detection of CO in the near infrared and satisfies the requirements for those situation that have a higher measurement requirement of CO such as alarming of coal spontaneous combustion and mine safety production. The experimental results of H2S show that the system based on TDLAS has a good linearity and stability with a quick response time of 24 s and a low detection limit of 240 ppb. This indicates that the system has the feasibility of realtime online monitoring in many applications. The measurement system of CO2 isotopologues has realized the high measurement precision of 0.2‰ for δ13C; the next step is to carry out calibration to get the correct isotope ratios and achieve long-term stability measurements. Good understanding of the important roles of HONO in the key chemical processes of hydroxyl radicals and the sources of HONO requires correct detection of the HONO mixing ratios. A QCL-based instrumental system was designed to measure the atmospheric HONO. The regression analysis indicates good correlation between HONO and NO2. But increasing HONO mixing ratios with decreasing NO2 also indicates other sources of HONO formation. Finding the missing sources and the formation mechanism of HONO in the atmosphere is still a great challenge for tropospheric HONO chemistry.

Author details

Pengshuai Sun<sup>1</sup>

References

07.016

, Bian Wu<sup>1</sup>

\*Address all correspondence to: fzdong@aiofm.ac.cn

Mechanics, Chinese Academy of Sciences, Hefei, China

3335. DOI: 10.1016/j.saa.2004.01.033

284. DOI: 10.1007/s00340-006-2357-0

3 University of Science and Technology of China, Hefei, China

Express. 2016;24(10):A943-A955. DOI: 10.1364/OE.24.00A943

2013;124(124):592-594. DOI: 10.12693/APhysPolA.124.592

2014;14(7):8403-8418. DOI: 10.5194/acpd-14-10429-2014

Xiaojuan Cui1,2, Fengzhong Dong1,2,3\*, Zhirong Zhang1,2, Hua Xia1,2, Tao Pang1

Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei Anhui, China

1 Anhui Provincial Key Laboratory of Photonic Devices and Materials, Anhui Institute of

2 Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine

[1] Zhang Z, Pang T, Yang Y, Xia H, Cui X, Sun P, Wu B, Wang Y, Sigrist MW, Dong F. Development of a tunable diode laser absorption sensor for online monitoring of industrial gas total emissions based on optical scintillation cross-correlation technique. Optics

[2] Dong F, Liu W, Chu Y, Li J, Zhang Z, Wang Y, Pang T, Wu B, Tu G, Xia H, Yang Y, Shen C, Wang Y, Ni Z, Liu J. Real-time in situ measurements of industrial hazardous gas concentrations and their emission gross. InTech Publisher. 2011:66-90. DOI: 10.5772/27161

[3] Nelson DD, McManus B, Urbanski S, Herndon S, Zahniser MS. High precision measurements of atmospheric nitrous oxide and methane using thermoelectrically cooled midinfrared quantum cascade lasers and detectors. Spectrochimica Acta A. 2004;60(14):3325-

[4] Wojtas J, Bielecki Z, Stacewicz T, Mikolajczyk J, Rutecka B, Medrzycki R. Nitrogen oxides optoelectronic sensors operating in infrared range of wavelengths. Acta Physica Polonica.

[5] Vardag SN, Hammer S, O'Doherty S, Spain TG, Wastine B, Jordan A, Levin I. Comparisons of continuous atmospheric CH4, CO2 and N2O measurements-results of InGOS travelling instrument campaign at Mace head. Atmospheric Chemistry and Physics.

[6] Wu T, Zha Q, Chen W, Xu Z, Wang T, He X. Development and deployment of a cavity enhanced UV-LED spectrometer for measurements of atmospheric HONO and NO2 in Hong Kong. Atmospheric Environment. 2014;95(1):544-551. DOI: 10.1016/j.atmosenv.2014.

[7] Lima JP, Vargas H. Mikl'os a, Angelmahr M, Hess P. Photoacoustic detection of NO2 and N2O using quantum cascade lasers. Applied Physics B: Lasers and Optics. 2006;85(2):279-

, Shuo Liu1,3, Luo Han1,3, Zhe Li1,3 and Runqing Yu1,3

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Although parts of these gas analysis experiments are just results under laboratory conditions, we are improving the stability and SNR of these systems with the aim of putting them into practical application. To date we have developed all-fiber gas sensor to detect CH4, O2, C2H2, and C2H4, portable CH4 sensors, CO2 analyzer, CO analyzer, and so on. Some of them have been put into the application. The development of these gas sensors would be beneficial for the implementation of environmental protection policies and expand their application in energy, public safety, and medical science. The TDLAS technology also shows high potential for monitoring all kinds of hazardous gases in the atmosphere from surface layer to troposphere combined with a wide spectral application range from the near infrared to mid-infrared.
