Quantitative method for trace gaseous nitrous acid generated by photolysis

Abstract

The invention discloses a quantitative method for trace gaseous nitrous acid generated by photolysis, which comprises two modes, in the first mode, a chemical ionization mass spectrometer in a positive ion mode is used for measuring the concentration change of a volatile organic compound (VOC) which reacts with a hydroxyl radical (. OH) after a mercury lamp is turned on and off so as to quantify the. OH concentration generated by irradiation of the mercury lamp, and further, nitric oxide (NO) reacts with. OH to generate gaseous nitrous acid (HONO); and the generation concentration of the HONO under the condition of excessive NO is approximately equal to the change concentration of the VOC measured by the mass spectrometer. In the second mode, the concentration change of NO reacting with the. OH before and after the mercury lamp is turned on is measured by the NO analyzer to quantify the HONO and the. OH, and the. OH concentration generated by irradiation of the mercury lamp and the HONO concentration generated under the condition are approximately equal to the NO concentration change measured by the NO analyzer.

Description

Technical Field

[0001] The invention relates to the technical field of environmental quality monitoring, and in particular to a quantitative method for trace gaseous nitrous acid generated by photolysis. Background Art

[0002] Gaseous nitrous acid (HONO) exists in the atmospheric environment and is an important trace nitrogen-containing gaseous pollutant. In recent years, due to the progress of atmospheric detection technology, the detection of trace gaseous nitrous acid in the atmosphere has become possible, such as long-path absorption photometer, differential optical absorption spectroscopy, mass spectrometry detection technology, etc. Gaseous nitrous acid is an important source of hydroxyl radicals (·OH) in the atmospheric environment and an important trace gas in the atmospheric chemical cycle. In addition, nitrous acid and its reaction products have carcinogenic properties. Researchers have conducted a lot of research work on this. Whether it is the study of the reaction mechanism of gaseous nitrous acid in the atmosphere or the calibration of gaseous nitrous acid measuring instruments, a simple and stable gaseous nitrous acid generation system is needed as a standard source for various studies. Photolysis to produce HONO as a standard source of gas generation has many advantages, but the problem faced is how to accurately quantify the HONO produced by the standard source.

[0003] There are three common methods for quantifying gaseous nitrous acid produced by photolysis. The first method is the common chemical photometric method HO x Improvements of the calibration method, in which HONO is calculated based on the quantified values ​​of [O3], [H2O], and [O2] and the absorption cross sections of H2O and O2 at 184.9 nm; a second method is to thermally dissociate the HONO formed by the photolysis source and then quantify the resulting thermally dissociated NO2 (Veres et al., 2015); a third method in which HONO is mainly prepared in N2 ([O2] = 0.040%) and simply quantified by measuring the NO2 formed by the reaction of NO with HO2 produced by the photolysis of H2O. The third method has lower uncertainty (usually ~10%, 2σ) than the first two.

[0004] The problems with the above-mentioned existing quantitative methods for producing nitrous acid by photolysis are as follows: the first method requires the characterization of the emission spectrum of the mercury lamp, which is relatively complicated; the second and third methods indirectly quantify the produced HONO by measuring NO2. Even though the third method has lower uncertainty than the second method, the reactions involved in these two methods are relatively complex due to the presence of O2, and there is still much room for improvement. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a method for quantifying trace gaseous nitrous acid produced by photolysis. The method is simpler than the existing method, involves fewer side reactions, and means lower uncertainty.

[0006] In order to solve the above technical problems, the technical solution of the present invention is:

[0007] In a first aspect, the present invention provides a method for quantifying trace gaseous nitrous acid produced by photolysis, comprising the following steps:

[0008] A VOC gas with a set humidity is introduced into the reaction flow tube, and a mercury lamp is used to photolyze water into ·OH, and ·OH reacts with VOC. The VOC flow ratio and the degree of light shielding are adjusted so that the generated ·OH reacts completely with VOC, and the initial concentration of ·OH is the concentration of VOC consumed. In the above process, a chemical ionization mass spectrometer in a positive ion mode is used to detect the concentration of the substance;

[0009] Replace VOC gas with NO gas, so that the NO gas concentration is greater than the initial OH concentration, and OH reacts with NO to generate HONO;

[0010] At this time, the concentration of the generated gaseous nitrous acid is the initial concentration of ·OH.

[0011] The reaction mechanism is:

[0012]

[0013] [OH]0≈△[VOC]=[VOC]0-[VOC].

[0014] In some embodiments, the VOC is an alkane, an aromatic hydrocarbon, an olefin, a halogenated hydrocarbon, an ester, an aldehyde, or a ketone.

[0015] Among them, alkanes such as methane, ethane, etc.; aromatic hydrocarbons such as benzene, toluene, xylene, etc.; olefins such as ethylene, propylene, etc.; halogenated hydrocarbons such as vinyl chloride, vinyl bromide, etc.

[0016] The VOC can react quickly with hydroxyl radicals and can be quantitatively detected by chemical ionization mass spectrometry. Hydroxyl radicals can be generated by irradiating water vapor with a mercury lamp.

[0017] In some embodiments, the water vapor concentration in the VOC gas is ≥50 mg / m 3 .

[0018] In some embodiments, the VOC flow rate ratio and the degree of shading are adjusted so that the initial concentration of ·OH [·OH]0 is less than the initial concentration of the introduced VOC [VOC]0 and can fully react. In this case, the generated ·OH can almost completely react with the VOC, and the initial concentration of ·OH is approximately equal to the concentration of VOC consumed △ [VOC].

[0019] [OH]0≈△[VOC]=[VOC]0-[VOC].

[0020] In some embodiments, the VOC gas and NO gas are provided by standard gas cylinders or standard gas generation methods.

[0021] In some embodiments, the detection limit of the chemical ionization mass spectrometer is ≤ 0.5 ppbv.

[0022] In a second aspect, the present invention provides a method for quantifying trace gaseous nitrous acid produced by photolysis, comprising the following steps:

[0023] NO gas with a set humidity is introduced into the flow reaction tube, and the mercury lamp is turned on to react;

[0024] The concentration of HONO generated was quantitatively detected by detecting the reduction of NO measured by the NO analyzer before and after the mercury lamp was turned on.

[0025]

[0026] NO+OH→HONO.

[0027] In some embodiments, [HONO]=Δ[NO]=[NO]0=Δ[·OH]≈[·OH]0.

[0028] In some embodiments, the concentration of water vapor in the NO gas is ≥50 mg / m 3 .

[0029] In some embodiments, the detection limit of the nitric oxide analyzer is ≤ 0.5 ppbv.

[0030] One or more technical solutions of the present invention have the following beneficial effects:

[0031] The present invention provides a quantitative method for gaseous nitrous acid generated by photolysis. The quantitative method includes two modes. Mode 1 uses a chemical ionization mass spectrometer in a positive ion mode to measure the concentration change of volatile organic compounds that react with hydroxyl radicals after turning on and off the mercury lamp to quantify the concentration of hydroxyl radicals generated by mercury lamp irradiation, and further allows nitric oxide to react with hydroxyl radicals to generate gaseous nitrous acid. Under the condition of excess nitric oxide, the generated concentration of gaseous nitrous acid is approximately equal to the concentration change of volatile organic compounds measured by the mass spectrometer. Mode 2 uses a nitric oxide analyzer to measure the concentration change of nitric oxide that reacts with hydroxyl radicals before and after the mercury lamp is turned on to quantify gaseous nitrous acid and hydroxyl radicals. The concentration of hydroxyl radicals generated by mercury lamp irradiation and the concentration of gaseous nitrous acid generated under this condition are both approximately equal to the concentration change of nitric oxide measured by the nitric oxide analyzer.

[0032] The method is simpler than existing methods and involves fewer side reactions, which means less uncertainty. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a schematic diagram of the device required for the method for quantifying gaseous nitrous acid produced by photolysis according to the present invention.

[0034] Appendix Description

[0035] Table 1 is the theoretical calculation results of the reaction of toluene with hydroxyl radicals at different concentrations under the experimental conditions described in Example 1.

[0036] Table 2 is the theoretical calculation results of the reaction of different concentrations of nitric oxide with hydroxyl radicals under the experimental conditions described in Example 2. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0038] Example 1

[0039] A quantitative method for gaseous nitrous acid produced by photolysis, using mode 1, using a chemical ionization mass spectrometer in positive ion mode to measure the concentration change of volatile organic compounds (VOC) that react with hydroxyl radicals (·OH) after turning on and off the mercury lamp to quantify the concentration of OH produced by mercury lamp irradiation, and further allowing nitric oxide (NO) to react with ·OH to generate gaseous nitrous acid (HONO). Under the condition of excess NO, the generated concentration of HONO is approximately equal to the change in the concentration of VOC measured by the mass spectrometer. Toluene is used as an example of VOC that reacts with OH to explain.

[0040] In this example, the instruments, equipment and materials used are as follows: Figure 1 Assembled as shown.

[0041] Material preparation

[0042] 1. High purity nitrogen;

[0043] 2. Nitric oxide standard gas cylinder (NO / N2, 3ppmv);

[0044] 3. Toluene gas cylinder (C7H8 / N2, 5ppmv).

[0045] Instruments and equipment used

[0046] 1. Quartz flow reaction tube (making the reaction time ≥10s);

[0047] 2. Photolysis chamber (with a built-in low-pressure mercury lamp and an adjustable light shield, which is used to adjust the light intensity to adjust the OH concentration produced by irradiating the flow reaction tube);

[0048] 3. Chemical ionization mass spectrometer with detection limit ≤ 0.5ppbv;

[0049] Toluene gas with a relative humidity of 30% is introduced into the reaction flow tube. After the mercury lamp is turned on, H2O will be photolyzed into ·OH, and then ·OH reacts with toluene:

[0050]

[0051]

[0052] Assuming the operating temperature is 300K, k = 5.59 × 10 -12 cm 3 / molecule·s, reaction time t is 10s, initial toluene concentration [C7H8]0 is 3×10 11 molecules / cm 3 (about 10 ppbv), adjust the opening and closing size of the light shield so that the initial concentration of ·OH [·OH]0 is less than the initial concentration of toluene [C7H8]0. Assume that the initial concentration of ·OH [·OH]0 is 6×10 10 molecules / cm 3 (about 2ppbv).

[0053] Table 1

[0054] OH <![CDATA[C7H8]]> P Before reaction t = 0 a b 0 After reaction t = t ax bx x

[0055] The reaction rate equation for this second-order reaction is:

[0056]

[0057] Substitute the above conditions and we get

[0058]

[0059] Concentration range of [C7H8]:

[0060] 3×10 11 molecules / cm 3 >[C7H8]>3×10 11 -6×10 10 molecules / cm 3 ;

[0061] That is, the concentration range of [·OH] is:

[0062] 8.96×10 4 molecules / cm 3 >[·OH]>7.16×10 4 molecules / cm 3 ;

[0063] It can be seen that when the ratio of the initial concentration of ·OH [·OH]0 to the initial concentration of toluene [C7H8]0 is less than 1:5, [C7H8]:[·OH]>3.35×10 6 , the residual concentration of ·OH [·OH] can be ignored.

[0064] Assuming different concentrations of ·OH, the calculation results are shown in Table 2. When the ratio of [C7H8]0:[·OH]0 is greater than 1.5,

[0065] △[·OH] min ≈△[·OH] max ≈[·OH]0≈△[·OH]=△[C7H8];

[0066] △[C7H8] can be measured by chemical ionization mass spectrometry, that is, the concentration of ·OH produced by mercury lamp irradiation [·OH]0.

[0067] Table 2

[0068] <![CDATA[[C7H8]0:[·OH]0]]> <![CDATA[[C7H8]0]]> <![CDATA[[·OH]0]]> <![CDATA[[C7H8] / [·OH]]]> <![CDATA[[·OH] max ]]> <![CDATA[[·OH] min ]]> <![CDATA[[C7H8] max ]]> <![CDATA[[C7H8] min ]]> <![CDATA[△[·OH] min ]]> <![CDATA[△[·OH] max ]]> 1 3.00E+11 3.00E+11 1.00E+00 3.00E+11 0.00E+00 3.00E+11 0.00E+00 0.00E+00 3.00E+11 1.1 3.00E+11 2.73E+11 5.05E+00 5.94E+10 5.40E+09 2.95E+11 2.73E+10 2.13E+11 2.67E+11 1.2 3.00E+11 2.50E+11 1.96E+01 1.53E+10 2.55E+09 2.97E+11 5.00E+10 2.35E+11 2.47E+11 1.5 3.00E+11 2.00E+11 4.02E+02 7.47E+08 2.49E+08 3.00E+11 1.00E+11 1.99E+11 2.00E+11 1.8 3.00E+11 1.67E+11 3.11E+03 9.66E+07 4.29E+07 3.00E+11 1.33E+11 1.67E+11 1.67E+11 2 3.00E+11 1.50E+11 8.76E+03 3.42E+07 1.71E+07 3.00E+11 1.50E+11 1.50E+11 1.50E+11 5 3.00E+11 6.00E+10 3.35E+06 8.95E+04 7.16E+04 3.00E+11 2.40E+11 6.00E+10 6.00E+10 10 3.00E+11 3.00E+10 3.59E+07 8.36E+03 7.53E+03 3.00E+11 2.70E+11 3.00E+10 3.00E+10 50 3.00E+11 6.00E+09 6.86E+08 4.37E+02 4.28E+02 3.00E+11 2.94E+11 6.00E+09 6.00E+09 100 3.00E+11 3.00E+09 1.62E+09 1.85E+02 1.83E+02 3.00E+11 2.97E+11 3.00E+09 3.00E+09

[0069] Furthermore, the toluene standard gas is replaced with NO standard gas, and the other experimental conditions remain unchanged. The reaction is as follows:

[0070] NO+·OH→HONO;

[0071]

[0072] At this time, k = 9.78 × 10 -11 cm 3 / molecule·s, it can be seen that NO and ·OH have a faster reaction rate than toluene and ·OH, so as long as the NO concentration is greater than the ·OH concentration (see Table 3), the HONO concentration generated at this time [HONO] ≈ [·OH] 0, that is:

[0073] [HONO]=△[·OH]≈[·OH]0=△[C7H8].

[0074] Table 3

[0075]

[0076]

[0077] Example 2

[0078] A quantitative method for gaseous nitrous acid produced by photolysis is proposed. Mode 2 is adopted, and a NO analyzer is used to measure the concentration change of NO that reacts with ·OH before and after the mercury lamp is turned on to quantify HONO and ·OH. The concentration of ·OH produced by mercury lamp irradiation and the concentration of HONO produced under this condition are approximately equal to the concentration change of NO measured by the NO analyzer.

[0079] In this example, the instruments, equipment and materials used are as follows: Figure 1 Assembled as shown.

[0080] Material preparation

[0081] High purity nitrogen;

[0082] Nitric oxide standard gas cylinder (NO / N2, 3ppmv).

[0083] Instruments and equipment used

[0084] 1. Quartz flow reaction tube (making the reaction time ≥10s);

[0085] 2. Photolysis chamber (with a built-in low-pressure mercury lamp and an adjustable light shield, which is used to adjust the light intensity to adjust the OH concentration produced by irradiating the flow reaction tube);

[0086] 3. NO analyzer with detection limit ≤ 0.5ppbv.

[0087] Furthermore, NO gas with a relative humidity of 30% is introduced into the reaction flow tube. After the mercury lamp is turned on, H2O will be photolyzed into ·OH, and then ·OH reacts with NO. The experimental conditions are consistent with those when NO standard gas is used in Example 1. From Table 2 deduced from Example 1, it can be concluded that NO and ·OH have an extremely fast reaction rate, so when the NO concentration is greater than the ·OH concentration, the following results can be obtained:

[0088] Table 4

[0089] OH NO HONO Before reaction t = 0 <![CDATA[[·OH]0]]> <![CDATA[[NO]0]]> 0 change △[·OH] △[NO] [HONO] After reaction t = 10s <![CDATA[[·OH]0-△[·OH]≈0]]> <![CDATA[[NO]0-△[NO]]]> [HONO]

[0090] Right now:

[0091] [HONO]=△[NO]=△[·OH]≈[·OH]0;

[0092] △[NO] can be obtained by using a NO analyzer, which can determine the concentration of HONO produced and the initial concentration of ·OH produced by mercury lamp irradiation.

[0093] Although the above describes the specific implementation methods of the present invention in combination with the diagrams, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A method for quantifying trace gaseous nitrous acid produced by photolysis, characterized in that: The steps include: A VOC gas with a set humidity is introduced into the reaction flow tube, and a mercury lamp is used to photolyze water into ·OH, and ·OH reacts with VOC. The VOC flow ratio and the degree of light shielding are adjusted so that the generated ·OH reacts completely with VOC, and the initial concentration of ·OH is the concentration of VOC consumed. In the above process, a chemical ionization mass spectrometer in a positive ion mode is used to detect the concentration of the substance; Replace VOC gas with NO gas, so that the NO gas concentration is greater than the initial OH concentration, and OH reacts with NO to generate HONO; At this time, the concentration of the generated gaseous nitrous acid is the initial concentration of ·OH.

2. The method for quantifying trace gaseous nitrous acid produced by photolysis according to claim 1, characterized in that: The VOCs are alkanes, aromatic hydrocarbons, olefins, halogenated hydrocarbons, esters, aldehydes or ketones.

3. The method for quantifying trace gaseous nitrous acid produced by photolysis according to claim 1, characterized in that: The water vapor concentration in the VOC gas is ≥50mg / m 3 .

4. The method for quantifying trace gaseous nitrous acid produced by photolysis according to claim 1, characterized in that: The VOC flow ratio and the degree of shading are adjusted so that the initial concentration of ·OH [·OH]0 is less than the initial concentration of the introduced VOC [VOC]0 and can fully react.

5. The method for quantifying trace gaseous nitrous acid produced by photolysis according to claim 1, characterized in that: The VOC gas and NO gas are provided by standard gas cylinders or a standard gas generation method.

6. The method for quantifying trace gaseous nitrous acid produced by photolysis according to claim 1, characterized in that: The detection limit of the chemical ionization mass spectrometer is ≤0.5 ppbv.

7. A method for quantifying trace gaseous nitrous acid produced by photolysis, characterized in that: The steps include: NO gas with a set humidity is introduced into the flow reaction tube, and the mercury lamp is turned on to react; The concentration of HONO generated was quantitatively detected by detecting the reduction of NO measured by the NO analyzer before and after the mercury lamp was turned on.

8. The method for quantifying trace gaseous nitrous acid produced by photolysis according to claim 7, characterized in that: [HONO]=△[NO]=[NO]0=△[·OH]≈[·OH]0.

9. The method for quantifying trace gaseous nitrous acid produced by photolysis according to claim 7, characterized in that: The concentration of water vapor in the NO gas is ≥50mg / m 3 .

10. The method for quantifying trace gaseous nitrous acid produced by photolysis according to claim 7, characterized in that: The detection limit of the nitric oxide analyzer is ≤0.5ppbv.

Images