the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Aug 2025
Real-Time Measurement of NO and NH3 Concentration Variations Using Direct Absorption Spectroscopy in the Smog Chamber to Analyze NH4NO3 Photochemical Formation Characteristics
Abstract. In urban atmospheric chemistry, NOx and NH3 in the atmosphere are major species participating in the secondary aerosol formation process, causing severe environmental problems such as decreased visibility and acid rain. In order to respond effectively to particulate matter problems, the correlation of precursors should be identified in detail. This study used UV-C light to convert gaseous substances into particulate substances in the small-scale smog chamber to simulate the photochemical reaction. The effects of several operating variables, such as UV-C light intensity, relative humidity, and initial concentrations of O2, NO, and NH3, on the NH4NO3 formation were investigated. Since atmospheric gas species are short-lived, they require a measurement technique with an ultra-fast response and high sensitivity. Therefore, the concentrations of NO and NH3 were measured using Direct Absorption Spectroscopy techniques with the wavenumber regions of 1926 and 6568 cm-1, respectively. NO and NH3 were precisely measured with an error rate of less than 3 % with the reference gas. The results show that NO and NH3 were converted over 98 % when UV-C light intensity was 24 W and relative humidity was about 30 % at 1 atm, 296 K. It also showed that higher UV-C light intensity, O3 concentration, and relative humidity induced higher conversion rates and secondary aerosol generation. In particular, it was experimentally confirmed that the secondary aerosol generation and growth process was greatly influenced by relative humidity.
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Status: open (until 28 Oct 2025)
- RC1: 'Comment on egusphere-2025-3602', Anonymous Referee #1, 30 Sep 2025 reply
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RC2: 'Comment on egusphere-2025-3602', Anonymous Referee #2, 08 Oct 2025
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Review of Jeong et al.: Real-Time Measurement of NO and NH3 Concentration Variations Using Direct Absorption Spectroscopy in the Smog Chamber to Analyze NH4NO3 Photochemical Formation Characteristics. egusphere-2025-3602
This manuscript presents results from chamber experiments where nitric oxide (NO) and ammonia (NH3) are exposed to varying levels of UV-C radiation, relative humidity and oxygen. The work also presents a direct absorption analytical method for the quantitative detection of NO and NH3 in the parts per million (ppm) range. The authors present observed decays in both NO and NH3 under varying conditions and then attempt to explain these and their relevance for atmospheric secondary aerosol formation. Unfortunately, the experimental set-up means the findings have very little relevance to the troposphere where these processes are of interest and I therefore do not recommend publication in AMT. A few of my major concerns with this work are outlined below:
- The use of UV-C radiation is not relevant to the lower atmosphere, and thus the reactions being studied cannot be used to infer important atmospheric processes.
- There is no mention of the impact of wall losses in the small chamber used. NH3 in particular is highly susceptible to wall losses, and these could potentially explain a significant portion of the NH3 signal decrease seen during experiments. Especially in the presence of water vapour.
- The concentration ranges studied (10-100 ppm) are not relevant to tropospheric conditions.
- The choice of AMT for this work I had assumed was due to a novel instrument development, but little information is provided on what appears to be a fairly standard cavity-based absorption method. I also assume that the limit of detection of this system is high, hence the ppm reactant concentrations used.
Citation: https://doi.org/10.5194/egusphere-2025-3602-RC2
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Using a small cylindrical chamber, the manuscript by Jeong et al. aims to study ammonium nitrate (NH4NO3) aerosol formation by measuring gas-phase NO and NH3 concentrations in real-time with direct absorption spectroscopy. Several environmental variables are adjusted (e.g., UV-C light intensity, relative humidity, and initial concentrations of O2, NO, and NH3) in order to see their respective impacts on the loss of NO and NH3. At no point in this manuscript is NH4NO3 itself quantified.
This manuscript is a revised version of a previously submitted manuscript to AMT (egusphere-2024-2762). While I appreciate the time spent by the authors on their revisions, I still have major concerns about this manuscript as noted below. As such, as it is currently formulated, I cannot recommend the manuscript for inclusion in AMT.
MAJOR CONCERNS:
My first major concern is that the authors imply their use of direct absorption spectroscopy (DAS) is somehow novel when employed in chamber studies (Lines 52-65 and Lines 334-340). Direct Absorption Spectroscopy (DAS) is a collective term used to describe various spectroscopic techniques where a molecule absorbs electromagnetic radiation. FTIR and NDIR would both fall under this category, so it makes no sense to say that DAS is better than FTIR or NDIR. Moreover, this paper appears to be using TDLAS, which also is not a novel technique; TDLAS has been well-known for decades in the atmospheric sciences.
I also have concerns about the design of the chamber itself. First, it is a small chamber (3.85 L volume) with a lot of surface area for possible wall loss and side reactions. Additionally, the authors use high precursor concentrations and UV-C lights in order to "enhance photolysis efficiency and accelerate reaction kinetics" (Line 104). No mention is made whatsoever in the paper about how their choice of chamber design improves upon other atmospheric chambers already existing. Moreover, the use of UV-C lights is concerning since it makes me question whether any experimental results from this chamber are atmospherically relevant. Photolysis reactions are wavelength dependent, so studying these photochemical reactions at wavelengths of 180-280 nm is inappropriate if then wanting to relate back to the troposphere in a quantitative way. Additionally, how can the authors ensure that other side reactions are not taking place inside the chamber since UV-C light is being used?
In addition to the technological advances of this manuscript being limited, the insights about NH4NO3 aerosol formation are also very qualitative and limited in this manuscript. The main experimental results of the paper (Figs 4-8) simply report the temporal loss of NO and NH3 for a variety of conditions, but no modeling is done to relate this back to NH4NO3 aerosol formation (even though that is the reported aim of the paper). Rather, the authors simply provide a qualitative conclusion that relative humidity and UV-C light intensity have the greatest impact on NH4NO3 aerosol formation (Line 328). Also, some simple modeling would tell which chemical reactions are dominant; simply listing reactions (see Lines 278-282 as an example) is not helpful for the reader.
OTHER COMMENTS:
- Section 4.1: Simply providing the R-squared does not confirm the high accuracy or precision of the sensor (Line 186). The R-squared shows a high correlation, but what is the sensor's limit of detection at various integration times? Also, the accuracy on the concentration could be calculated knowing the uncertainties on the path length, cross-sections for your selected spectral lines, and the absorbance itself.