A method for quantifying trace amounts of gaseous nitrous acid produced by photolysis

Abstract

The application discloses a quantitative method for trace gaseous nitrous acid generated by photolysis, which comprises two modes. Mode one uses a chemical ionization mass spectrometer in positive ion mode to measure the concentration change of volatile organic compounds (VOC) reacted with hydroxyl radicals (·OH) after a switch mercury lamp to quantify the ·OH concentration generated by the mercury lamp irradiation, and further makes nitric oxide (NO) react with ·OH to generate gaseous nitrous acid (HONO), so that the generated concentration of HONO is approximately equal to the change concentration of VOC measured by the mass spectrometer under the condition of NO excess. Mode two uses a NO analyzer to measure the concentration change of NO reacted with ·OH before and after the mercury lamp is turned on to quantify HONO and ·OH, and the ·OH concentration generated by the mercury lamp irradiation and the HONO concentration generated under the condition are both approximately equal to the NO concentration change measured by the NO analyzer.

Description

Technical Field

[0001] This invention relates to the field of environmental quality monitoring technology, specifically to a quantitative method for measuring trace amounts of gaseous nitrous acid produced by photolysis. Background Technology

[0002] Gaseous nitrous acid (HONO) exists in the atmosphere and is an important trace nitrogen-containing gaseous pollutant. In recent years, advancements in atmospheric detection technology have made it possible to detect trace amounts of gaseous nitrous acid in the atmosphere, such as long-path spectrophotometers, differential optical absorption spectroscopy, and mass spectrometry. As a significant source of hydroxyl radicals (·OH) in the atmosphere, gaseous nitrous acid is an important trace gas in the atmospheric chemical cycle. Furthermore, nitrous acid and its reaction products have carcinogenic properties, prompting extensive research. Whether studying the reaction mechanism of gaseous nitrous acid in the atmosphere or calibrating instruments for measuring gaseous nitrous acid, a simple and stable gaseous nitrous acid generation system is needed as a standard source for various studies. Photolysis-generated HONO has many advantages as a standard source for gas generation; however, the challenge lies in how to accurately quantify the HONO generated by this standard source.

[0003] There are three common existing methods for quantitatively measuring gaseous nitrous acid produced by photolysis. The first method is based on the common chemiphotometric method H2O. x The calibration methods have been improved in several ways. One method calculates HONO based on the quantified values ​​of [O3], [H2O], and [O2], as well as the absorption cross-sections of H2O and O2 at 184.9 nm. Another method involves thermally dissociating HONO from a photolysis source and then quantifying the resulting NO2 (Veres et al., 2015). A third method prepares HONO primarily in N2 ([O2] = 0.040%) and quantifies it simply by measuring the NO2 formed from the reaction of NO with HO2 generated by the photolysis of H2O. The third method exhibits lower uncertainty (typically ~10%, 2σ) compared to the first two methods.

[0004] The existing methods for quantitatively generating nitrous acid by photolysis have the following problems: the first method requires characterization of the emission spectrum of a mercury lamp, which is a relatively complex task; the second and third methods indirectly quantify the generated 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] To address the problems existing in the prior art, the present invention provides a quantitative method for trace gaseous nitrous acid generated by photolysis. This method is simpler than existing methods, involves fewer side reactions, and means lower uncertainty.

[0006] To solve the above technical problems, the technical solution of the present invention is as follows:

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

[0008] A VOC gas with a set humidity is introduced into the reaction flow tube. A mercury lamp is used to photolyze water into ·OH. The ·OH reacts with VOC. The VOC flow rate ratio and the degree of light blocking are adjusted to ensure that the generated ·OH reacts completely with VOC. The initial concentration of ·OH is the concentration of VOC consumed. The concentration of substances is detected by a chemical ionization mass spectrometer in positive ion mode during the above process.

[0009] VOC gas is replaced with NO gas, so that the NO gas concentration is greater than the initial concentration of ·OH. ·OH reacts with NO to produce HONO.

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

[0011] The reaction mechanism is as follows:

[0012]

[0013] [OH]0≈△[VOC]=[VOC]0-[VOC].

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

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

[0016] This VOC can react rapidly with hydroxyl radicals and can be quantitatively detected by chemical ionization mass spectrometry. Water vapor irradiated by a mercury lamp can generate hydroxyl radicals.

[0017] In some embodiments, the water vapor concentration in the VOC gas is ≥50 mg / m³. 3 .

[0018] In some embodiments, 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 that the reaction is sufficient. In this case, the generated ·OH can react almost completely with the VOC, and the initial concentration of ·OH is approximately equal to the concentration of VOC consumed Δ[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 generating methods.

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

[0022] Secondly, the present invention provides a method for quantifying trace amounts of gaseous nitrous acid generated 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 carry out the reaction;

[0024] The concentration of HONO generated is quantitatively determined by measuring the decrease in NO before and after the mercury lamp is turned on using a NO analyzer.

[0025]

[0026] NO + OH → HONO.

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

[0028] In some embodiments, the concentration of water vapor in the NO gas is ≥50 mg / m³. 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] This invention provides a quantitative method for gaseous nitrous acid generated by photolysis. The method includes two modes. Mode 1 uses a chemical ionization mass spectrometer in positive ion mode to measure the concentration change of volatile organic compounds reacting with hydroxyl radicals after the mercury lamp is switched on and off, thereby quantifying the concentration of hydroxyl radicals generated by mercury lamp irradiation. Further, nitric oxide reacts with hydroxyl radicals to generate gaseous nitrous acid. Under conditions of excess nitric oxide, the concentration of gaseous nitrous acid generated is approximately equal to the change in concentration of volatile organic compounds measured by the mass spectrometer. Mode 2 uses a nitric oxide analyzer to measure the concentration change of nitric oxide reacting with hydroxyl radicals before and after the mercury lamp is switched on, thereby quantifying the 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 these conditions are both approximately equal to the change in nitric oxide concentration measured by the nitric oxide analyzer.

[0032] This method is simpler than existing methods, involves fewer side reactions, and means lower uncertainty. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the apparatus required for the quantitative method of photolysis-generated gaseous nitrous acid as described in this invention.

[0034] Appendix Explanation

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

[0036] Table 2 shows the theoretical calculation results of the reaction between nitric oxide and hydroxyl radicals at different concentrations under the experimental conditions described in Example 2. Detailed Implementation

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

[0038] Example 1

[0039] A quantitative method for gaseous nitrous acid produced by photolysis is presented. Using Mode 1, a chemical ionization mass spectrometer in positive ion mode is employed to quantify the concentration of OH generated by mercury lamp irradiation by measuring the concentration change of volatile organic compounds (VOCs) reacting with hydroxyl radicals (·OH) after the mercury lamp is switched on and off. Furthermore, nitric oxide (NO) reacts with ·OH to generate gaseous nitrous acid (HONO). Under conditions of NO excess, the concentration of HONO generated is approximately equal to the change in VOC concentration measured by the mass spectrometer. Toluene is used as an example of a VOC that reacts with OH for further explanation.

[0040] In this embodiment, the instruments, equipment, and materials used are shown below and arranged in accordance with... Figure 1 The assembly is 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 material flow reaction tube (ensuring reaction time ≥ 10s);

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

[0048] 3. Detection limit ≤ 0.5 ppbv chemical ionization mass spectrometer;

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

[0050]

[0051]

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

[0053] Table 1

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

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

[0056]

[0057] Substituting the above conditions, we get the solution.

[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]:

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

[0063] Therefore, when the ratio of the initial concentration of ·OH [·OH]0 to the initial concentration of the introduced toluene [C7H8]0 is less than 1:5, [C7H8]:[·OH]>3.35×10 6 The remaining concentration of ·OH [·OH] is negligible.

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

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

[0066] △[C7H8] can be measured by chemical ionization mass spectrometry, which yields 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, replacing toluene standard gas with NO standard gas, while keeping all other experimental conditions unchanged, the reaction is as follows:

[0070] NO + ·OH → HONO;

[0071]

[0072] At this point, k = 9.78 × 10 -11 cm 3From the molecule-s, we know that NO reacts with ·OH at a faster rate than toluene reacts with ·OH. Therefore, as long as the NO concentration is greater than the ·OH concentration (see Table 3), the concentration of HONO produced is approximately [HONO] ≈ [·OH] 0.

[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. Using mode 2, the concentration change of NO reacting with ·OH before and after the mercury lamp is turned on is measured using an NO analyzer to quantify HONO and ·OH. The concentration of ·OH produced by mercury lamp irradiation and the concentration of HONO produced under the same conditions are approximately equal to the change in NO concentration measured by the NO analyzer.

[0079] In this embodiment, the instruments, equipment, and materials used are shown below and arranged in accordance with... Figure 1 The assembly is 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 material flow reaction tube (ensuring reaction time ≥ 10s);

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

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

[0087] Furthermore, NO gas with a relative humidity of 30% was introduced into the reaction flow tube. After turning on the mercury lamp, H2O was photolyzed into ·OH. Then, ·OH reacted with NO. The experimental conditions were the same as when NO standard gas was used in Example 1. From Table 2 derived from Example 1, it can be seen that NO and ·OH have an extremely fast reaction rate. Therefore, when the NO concentration is greater than the ·OH concentration, the following results can be obtained:

[0088] Table 4

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

[0090] Right now:

[0091] [HONO]=△[NO]=△[·OH]≈[·OH]0;

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

[0093] While the specific embodiments of the present invention have been described above in conjunction with figures and tables, this 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 without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for quantitatively analyzing trace amounts of gaseous nitrous acid produced by photolysis, characterized in that: Includes the following steps: A VOC gas with a set humidity is introduced into the reaction flow tube. A mercury lamp is used to photolyze water into ·OH. The ·OH reacts with VOC. The VOC flow rate ratio and the degree of light blocking are adjusted to ensure that the generated ·OH reacts completely with VOC. The initial concentration of ·OH is the concentration of VOC consumed. The concentration of substances is detected by a chemical ionization mass spectrometer in positive ion mode during the above process. VOC gas is replaced with NO gas, so that the NO gas concentration is greater than the initial concentration of ·OH. ·OH reacts with NO to produce HONO. At this point, the concentration of the gaseous nitrous acid produced is the initial concentration of ·OH; The ratio of the initial concentration of VOC to the initial concentration of ·OH is greater than 1.5; The water vapor concentration in the VOC gas is ≥50 mg / m³ 3 ; The detection limit of the chemical ionization mass spectrometer is ≤0.5 ppbv.

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

3. The method for quantitatively analyzing trace amounts of gaseous nitrous acid generated by photolysis according to claim 1, characterized in that: Adjust the VOC flow ratio and the degree of shading to achieve the initial concentration of ·OH. Less than the initial concentration of the introduced VOC And it can react fully.

4. The method for quantitatively analyzing trace amounts of gaseous nitrous acid generated by photolysis according to claim 1, characterized in that: The VOC and NO gases are provided by standard gas cylinders or standard gas generation methods.

5. The method for quantitatively analyzing trace amounts of gaseous nitrous acid generated by photolysis according to claim 1, characterized in that: Includes the following steps: NO gas with a set humidity is introduced into the flow reaction tube, and the mercury lamp is turned on to carry out the reaction; The concentration of HONO generated is quantitatively determined by measuring the decrease in NO before and after the mercury lamp is turned on using a NO analyzer.

6. The method for quantitatively analyzing trace amounts of gaseous nitrous acid generated by photolysis according to claim 5, characterized in that: 。 7. The method for quantitatively analyzing trace amounts of gaseous nitrous acid generated by photolysis according to claim 5, characterized in that: The concentration of water vapor in the NO gas is ≥50 mg / m³. 3 .

8. The method for quantitatively analyzing trace amounts of gaseous nitrous acid produced by photolysis according to claim 5, characterized in that: The detection limit of the nitric oxide analyzer is ≤0.5 ppbv.

Images