the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Sep 2025
High-purity nitrous acid (HONO) generation and quantification using broadband cavity-enhanced absorption spectroscopy (BBCEAS)
Abstract. Nitrous acid (HONO) is a key atmospheric precursor to hydroxyl radicals (OH), which drive oxidation processes in the troposphere. Accurate quantification of HONO is essential for modelling atmospheric chemistry but is often challenging due to low ambient concentrations, significant wall losses from instruments with inlets, and interference from co-produced nitrogen dioxide (NO2). Laboratory generation methods frequently struggle to produce HONO in both high purity and usable quantities. This study introduces a simple, scalable method for generating high-purity HONO via the reaction of hydrochloric acid (HCl) and sodium nitrite (NaNO2) in a multi-bubbler system. The method incorporates a thermal "catch and release" mechanism to selectively trap HONO and separate it from NO2 prior to analysis. While the melting point of NO2 is well-established at -9 °C, the melting point of HONO was monitored and determined to be -4 °C, enabling the thermal separation mechanism to function effectively. Broadband cavity-enhanced absorption spectroscopy (BBCEAS) was used for detection and quantification in the 329–344 nm region. Systematic optimization of bubbler temperatures, carrier gas flow rates, capture durations, and moisture levels yielded HONO purities greater than 96 %. This method enables reliable and flexible production of high-purity HONO, making it well-suited for use in laboratory-based chamber studies and instrument calibration applications.
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Status: closed
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RC1: 'Comment on egusphere-2025-4183', Anonymous Referee #2, 19 Oct 2025
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AC1: 'Reply on RC1', Alexis Harper, 06 Nov 2025
We appreciate the reviewer’s time and participation in the review process for our manuscript. Below are our responses, in bold, to the concerns raised by the reviewer.
In response to the general comment “While the study offers clear advances in calibration techniques for reactive nitrogen species, area require clarification regarding the broader applicability.” the following has been added to the manuscript (additions to existing sections are italicized):
“By thermally separating HONO from NO2, the method achieves high purity while producing sufficient quantities for laboratory use and experimental applications. Beyond controlled environments, its simplicity, reproducibility, and scalability (up or down) make it suitable for integration into diverse analytical setups and for deployment in long-term atmospheric monitoring both in chamber setups and as standards and calibration sources for field monitoring sites.”
“When paired with broadband cavity-enhanced absorption spectroscopy (BBCEAS), this generation method supports sensitive, selective, and high-throughput HONO detection, with broad applicability in both laboratory and field-based research. The HONO source can be easily coupled to other detection platforms, including spectroscopic, chemical ionization, or mass spectrometric techniques, and thus enables comparative and intercalibration studies across multiple measurement frameworks.”
“Future applications may include integration with field-deployable HONO sensors, automated sampling systems, or kinetic studies exploring the influence of HONO on radical-driven oxidation pathways. Ultimately, this method provides a robust, transferable foundation for investigating HONO chemistry across varied environments by enhancing our capacity to isolate, quantify, and mechanistically understand one of the atmosphere’s most influential trace species.”
Technical Corrections
1. Title: Remove "(HONO)" and "(BBCEAS)".
The authors have chosen to retain the chemical formula and acronym in the title following journal precedent and convention.
2. Sec. 2.2: Typo: "350–420 nm region" → "360–410 nm" (Fig. 3 shows data to 410 nm).
Figure 3 shows the mirror curve from 325 nm to 355 nm and the fitting window, shown in Figure 4 shows the fitting window of 329 nm to 344nm. We believe there may have been a misunderstanding, as no data are available above 345 nm.
3. Fig. 2: Label axes of HONO synthesis schematic (flow rates, temperatures).
Figure 2 has been updated as suggested. We also took this opportunity to revise Figure 1 to ensure consistent formatting throughout the manuscript.
4. Eq. 5: Define "I" and "I₀" for the absorption cross-section calculation.
The sentence on page 5, line 131 now reads, “This reflectivity value, along with the intensity of the reference spectrum (I0(λ)) and the spectrum of interest (I(λ)),” and the sentence on page 6, line 141 now reads, “where I0 is the intensity of light in N2, and I is the intensity of light in our target sample.”
5. Table 1: Units missing for "HONO Yield" (±%?)
We appreciate the reviewer’s careful attention to detail. While the current version of the manuscript does not include any tables, we have carefully reviewed the text to ensure that units for all reported % yields are consistently stated with their associated ±% uncertainties.
Citation: https://doi.org/10.5194/egusphere-2025-4183-AC1
-
AC1: 'Reply on RC1', Alexis Harper, 06 Nov 2025
-
RC2: 'Comment on egusphere-2025-4183 19Oct2025', Anonymous Referee #1, 20 Oct 2025
This reader would appreciate strengthening the BBCEAS detection discussion by adding some statistics about observed repeatability of HONO retrieval especially in cases where NO2 and HONO are both detected. The strength of the method proposed relates to the strength of the HONO retrieval from the BBCEAS spectra. Adding error bars to Figs. 6 - 8 may enhance understanding of the stability of the production method and give others a sense how robust the method may be for their own purposes.
Citation: https://doi.org/10.5194/egusphere-2025-4183-RC2 -
AC2: 'Reply on RC2', Alexis Harper, 06 Nov 2025
We thank the reviewer for their time and valuable feedback on our manuscript. Our responses to the reviewer’s comments are provided below, bolded.
- This reader would appreciate strengthening the BBCEAS detection discussion by adding some statistics about observed repeatability of HONO retrieval especially in cases where NO2 and HONO are both detected. The strength of the method proposed relates to the strength of the HONO retrieval from the BBCEAS spectra.
Statistics for the repeatability of the HONO retrieval and error analysis have been conducted and are now described in the text as follows, “Fit residuals were consistently unstructured, white noise distributed around zero, and an order of magnitude or smaller lower than the fitted extinction of HONO which serves as an indication of a good fit. The 1-minute limit of detection (LOD) calculated from this fit residual, while measuring both NO2 and HONO, is 6.56 × 108 molecules/cm3 for HONO and 4.94 × 108 molecules/cm3 for NO2, which is at least 3 orders of magnitude below the concentrations measured in this experiment thus indicating confidence in the fit to the extinction. The fit is not stretched or shifted at all and, therefore, had negligible bias towards either species when trying to fit the other.”
- Adding error bars to Figs. 6 - 8 may enhance understanding of the stability of the production method and give others a sense how robust the method may be for their own purposes.
Figures 6-8 have been updated with error bars. In addition to this, a paragraph describing how the error analysis for the purity/yield was done has been added, “The total 1σ uncertainty in HONO and NO2 was determined by combining all independent sources of error, including uncertainties in temperature, cavity length, pressure, and the reported absorption cross-section uncertainties under these conditions of 5% for HONO (Bongartz et al., 1991) and 3% for NO2 (Vandaele et al., 1998). The total uncertainty was determined by combining the individual uncertainties in quadrature to be 3.5% for NO2 and 5.3% for HONO.”
Citation: https://doi.org/10.5194/egusphere-2025-4183-AC2
-
AC2: 'Reply on RC2', Alexis Harper, 06 Nov 2025
Status: closed
-
RC1: 'Comment on egusphere-2025-4183', Anonymous Referee #2, 19 Oct 2025
This manuscript introduces an integrated system for generating high-purity HONO and quantifying it via BBCEAS. The work addresses a significant gap in atmospheric chemistry research, providing a critical tool for studying HONO's role in oxidation processes. The validation experiments demonstrate robustness. While the study offers clear advances in calibration techniques for reactive nitrogen species, area require clarification regarding the broader applicability. The manuscript needs enhancements in presentation quality to reach its full potential.
Technical Corrections
- Title: Remove "(HONO)" and "(BBCEAS)" .
- Sec. 2.2: Typo: "350–420 nm region" → "360–410 nm" (Fig. 3 shows data to 410 nm).
- Fig. 2: Label axes of HONO synthesis schematic (flow rates, temperatures).
- Eq. 5: Define "I" and "I₀" for the absorption cross-section calculation.
- Table 1: Units missing for "HONO Yield" (±%?)
Citation: https://doi.org/10.5194/egusphere-2025-4183-RC1 -
AC1: 'Reply on RC1', Alexis Harper, 06 Nov 2025
We appreciate the reviewer’s time and participation in the review process for our manuscript. Below are our responses, in bold, to the concerns raised by the reviewer.
In response to the general comment “While the study offers clear advances in calibration techniques for reactive nitrogen species, area require clarification regarding the broader applicability.” the following has been added to the manuscript (additions to existing sections are italicized):
“By thermally separating HONO from NO2, the method achieves high purity while producing sufficient quantities for laboratory use and experimental applications. Beyond controlled environments, its simplicity, reproducibility, and scalability (up or down) make it suitable for integration into diverse analytical setups and for deployment in long-term atmospheric monitoring both in chamber setups and as standards and calibration sources for field monitoring sites.”
“When paired with broadband cavity-enhanced absorption spectroscopy (BBCEAS), this generation method supports sensitive, selective, and high-throughput HONO detection, with broad applicability in both laboratory and field-based research. The HONO source can be easily coupled to other detection platforms, including spectroscopic, chemical ionization, or mass spectrometric techniques, and thus enables comparative and intercalibration studies across multiple measurement frameworks.”
“Future applications may include integration with field-deployable HONO sensors, automated sampling systems, or kinetic studies exploring the influence of HONO on radical-driven oxidation pathways. Ultimately, this method provides a robust, transferable foundation for investigating HONO chemistry across varied environments by enhancing our capacity to isolate, quantify, and mechanistically understand one of the atmosphere’s most influential trace species.”
Technical Corrections
1. Title: Remove "(HONO)" and "(BBCEAS)".
The authors have chosen to retain the chemical formula and acronym in the title following journal precedent and convention.
2. Sec. 2.2: Typo: "350–420 nm region" → "360–410 nm" (Fig. 3 shows data to 410 nm).
Figure 3 shows the mirror curve from 325 nm to 355 nm and the fitting window, shown in Figure 4 shows the fitting window of 329 nm to 344nm. We believe there may have been a misunderstanding, as no data are available above 345 nm.
3. Fig. 2: Label axes of HONO synthesis schematic (flow rates, temperatures).
Figure 2 has been updated as suggested. We also took this opportunity to revise Figure 1 to ensure consistent formatting throughout the manuscript.
4. Eq. 5: Define "I" and "I₀" for the absorption cross-section calculation.
The sentence on page 5, line 131 now reads, “This reflectivity value, along with the intensity of the reference spectrum (I0(λ)) and the spectrum of interest (I(λ)),” and the sentence on page 6, line 141 now reads, “where I0 is the intensity of light in N2, and I is the intensity of light in our target sample.”
5. Table 1: Units missing for "HONO Yield" (±%?)
We appreciate the reviewer’s careful attention to detail. While the current version of the manuscript does not include any tables, we have carefully reviewed the text to ensure that units for all reported % yields are consistently stated with their associated ±% uncertainties.
Citation: https://doi.org/10.5194/egusphere-2025-4183-AC1
-
RC2: 'Comment on egusphere-2025-4183 19Oct2025', Anonymous Referee #1, 20 Oct 2025
This reader would appreciate strengthening the BBCEAS detection discussion by adding some statistics about observed repeatability of HONO retrieval especially in cases where NO2 and HONO are both detected. The strength of the method proposed relates to the strength of the HONO retrieval from the BBCEAS spectra. Adding error bars to Figs. 6 - 8 may enhance understanding of the stability of the production method and give others a sense how robust the method may be for their own purposes.
Citation: https://doi.org/10.5194/egusphere-2025-4183-RC2 -
AC2: 'Reply on RC2', Alexis Harper, 06 Nov 2025
We thank the reviewer for their time and valuable feedback on our manuscript. Our responses to the reviewer’s comments are provided below, bolded.
- This reader would appreciate strengthening the BBCEAS detection discussion by adding some statistics about observed repeatability of HONO retrieval especially in cases where NO2 and HONO are both detected. The strength of the method proposed relates to the strength of the HONO retrieval from the BBCEAS spectra.
Statistics for the repeatability of the HONO retrieval and error analysis have been conducted and are now described in the text as follows, “Fit residuals were consistently unstructured, white noise distributed around zero, and an order of magnitude or smaller lower than the fitted extinction of HONO which serves as an indication of a good fit. The 1-minute limit of detection (LOD) calculated from this fit residual, while measuring both NO2 and HONO, is 6.56 × 108 molecules/cm3 for HONO and 4.94 × 108 molecules/cm3 for NO2, which is at least 3 orders of magnitude below the concentrations measured in this experiment thus indicating confidence in the fit to the extinction. The fit is not stretched or shifted at all and, therefore, had negligible bias towards either species when trying to fit the other.”
- Adding error bars to Figs. 6 - 8 may enhance understanding of the stability of the production method and give others a sense how robust the method may be for their own purposes.
Figures 6-8 have been updated with error bars. In addition to this, a paragraph describing how the error analysis for the purity/yield was done has been added, “The total 1σ uncertainty in HONO and NO2 was determined by combining all independent sources of error, including uncertainties in temperature, cavity length, pressure, and the reported absorption cross-section uncertainties under these conditions of 5% for HONO (Bongartz et al., 1991) and 3% for NO2 (Vandaele et al., 1998). The total uncertainty was determined by combining the individual uncertainties in quadrature to be 3.5% for NO2 and 5.3% for HONO.”
Citation: https://doi.org/10.5194/egusphere-2025-4183-AC2
-
AC2: 'Reply on RC2', Alexis Harper, 06 Nov 2025
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This manuscript introduces an integrated system for generating high-purity HONO and quantifying it via BBCEAS. The work addresses a significant gap in atmospheric chemistry research, providing a critical tool for studying HONO's role in oxidation processes. The validation experiments demonstrate robustness. While the study offers clear advances in calibration techniques for reactive nitrogen species, area require clarification regarding the broader applicability. The manuscript needs enhancements in presentation quality to reach its full potential.
Technical Corrections