Comprehensive multiphase chlorine chemistry in the box model CAABA/MECCA: Implications to atmospheric oxidative capacity
Abstract. Tropospheric chlorine chemistry can strongly impact the atmospheric oxidation capacity and composition, especially in urban environments. To account for these reactions, the gas- and aqueous-phase Cl chemistry of the community atmospheric chemistry box model CAABA/MECCA has been extended. In particular, an explicit mechanism for ClNO2 formation following N2O5 uptake to aerosols has been developed. The updated model has been applied to two urban environments with different concentrations of NOx (NO and NO2): New Delhi (India) and Leicester (United Kingdom). The model shows a sharp build-up of Cl at sunrise through Cl2 photolysis in both environments. Besides Cl2 photolysis, ClO+NO reaction, and photolysis of ClNO2 and ClONO are prominent sources of Cl in Leicester. High-NOx conditions in Delhi tend to suppress the night-time build-up of N2O5 due to titration of O3 and thus lead to lower ClNO2, in contrast to Leicester. Major loss of ClNO2 is through its uptake on chloride, producing Cl2 , which consequently leads to the formation of Cl through photolysis. The reactivities of Cl and OH are much higher in Delhi, however, the Cl/OH ratio is up to ≈7 times greater in Leicester. The contribution of Cl to the atmospheric oxidation capacity is significant and even exceeds (by ≈2.9 times) that of OH during the morning hours in Leicester. Sensitivity simulations suggest that the additional consumption of VOCs due to active gas and aqueous-phase chlorine chemistry enhances OH, HO2, RO2 near the sunrise. The simulation results of the updated model have important implications for future studies on atmospheric chemistry and urban air quality.
Meghna Soni et al.
Status: open (until 07 Jun 2023)
- RC1: 'Comment on egusphere-2023-652', Anonymous Referee #1, 21 May 2023 reply
- RC2: 'Comment on egusphere-2023-652', Anonymous Referee #2, 22 May 2023 reply
- RC3: 'Comment on egusphere-2023-652', Anonymous Referee #3, 24 May 2023 reply
Meghna Soni et al.
Meghna Soni et al.
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Soni et al. report an updated mechanism for the community atmospheric chemistry box model CAABA/MECCA, adding some 35 gas-phase reactions, 15 aqueous-phase reactions, and 4 reactions governing the partitioning of gases between gas and aerosol phase. The focus is on chemistry of chlorine compounds (Cl2, ClNO2, etc.). Reactive intermediates such as the nitronium ion (NO2+) are treated explicitly. The revised model's was tested using two recent urban data sets collected in New Delhi (India) and Leicester (UK).
The paper is written well. I have a few concerns that the authors will hopefully be able to address in revision.
(1) More explanation is needed to justify the chlorine nitrite chemistry in the mechanism, considering this molecule has not been unambiguously observed in ambient air. The authors are correct that chlorine nitrite may form from Cl+NO2 (e.g., Golden, J. Phys. Chem. A 2007, 111(29), 6772–6780, https://doi.org/10.1021/jp069000x and Niki et al., Chem. Phys. Lett. 1978, 59(1), 78-79, https://doi.org/10.1016/0009-2614(78)85618-8) - these papers should be cited. However, its chemistry is incomplete. ClONO is metastable and converts to ClNO2 (Janowski et al., Berichte der Bunsengesellschaft für physikalische Chemie 1977, 81(12), 1262-1270, https://doi.org/10.1002/bbpc.19770811212; Niki et al. Chem. Phys. Lett. 1978, 59(1), 78-79, https://doi.org/10.1016/0009-2614(78)85618-8) - a reaction that should be added to the mechanism. Note that Niki et al. report a ClONO lifetime of ~150 s.
Be it as it may, the reaction between Cl and NO2 is generally thought to be negligible compared to reaction of N2O5 on chloride containing aerosol, except for unusual environments such as Delhi in winter. This paper thus seems to be tailored towards specific data sets, which should be mentioned in the introduction, and the relevant measurement papers (e.g., Haslett et al., Atmos. Chem. Phys. Disc., https://egusphere.copernicus.org/preprints/2023/egusphere-2023-497/) should be cited.
(2) More discussion as to the applicability of this model is needed.
For example, a limitation of this study is that all species are assumed to be well-mixed. In reality, there will be vertical gradients for most species evaluated here, in particular radical reservoir species such as HONO and ClNO2 (e.g., Young et al., Environm. Sci. Technol. 2012, 46(20), 10965-10973, https://doi.org/10.1021/es302206a). This limitation should be discussed.
Transport phenomena should also be acknowledged (since the model does not include them) and assumptions should be clearly stated. For example, it is well established that ClNO2 formation occurs in the nocturnal residual layer (which contains less NO than the surface layer), and ClNO2 then mixes downward in the morning when the convective mixed layer forms (e.g., Bannan et al., J. Geophys. Res. 2015, 120(11), 5638-5657, https://doi.org/10.1002/2014jd022629; Tham et al. Atmos. Chem. Phys. 2016, 16(23), 14959-14977, https://doi.org/10.5194/acp-16-14959-2016).
Title - specify season of study in title of paper (winter)
line 27 - The authors differentiate between nitryl chloride (ClNO2) and chlorine nitrite (ClONO), citing a the MCM modeling study by Riedel et al (2014). More explanation is needed here since Riedel et al. do not mention chlorine nitrite in their paper.
Chlorine nitrite may form from Cl+NO2 (e.g., Golden, J. Phys. Chem. A 2007, 111, 29, 6772–6780, https://doi.org/10.1021/jp069000x and Niki et al., Chemical Physics Letters 1978, 59(1), 78-79, https://doi.org/10.1016/0009-2614(78)85618-8); however, the reaction between Cl and NO2 is generally thought to be negligible compared to reaction of N2O5 on chloride containing aerosol. It would thus be informative to add the relative contributions of ClONO and ClNO2 to the bottom trace of Figure 2.
Furthermore, ClONO is metastable and converts to ClNO2 (Janowski et al., Berichte der Bunsengesellschaft für physikalische Chemie 1977 81(12), 1262-1270, https://doi.org/10.1002/bbpc.19770811212; Niki et al (1978)) - a reaction that should be added to Table 1.
line 3. Please define the abbreviation CAABA/MECCA.
line 80. Please state here that the full mechanism is shown in the SI.
Page 4 - Table 1. Please state the units for the reaction rate constants.
For the photolysis reactions, please state the maximum (noon) j values.
Page 6, Table 2 - reaction A13 - please subscript the 3 in CH3COO.
Line 100 - The Sander et al. (2014) reference is inappropriate. Cite Ghosh et al., J. Phys. Chem. A 2012, 116, 5796-5805, https://doi.org/10.1021/jp207389y, please.
pg 7 - Figure 1 - The nitronium ion (NO2+) is a potent nitrating agent, and there are many more organic molecules in the aerosol-phase than shown here. Please discuss the limitations of the abridged mechanism.
pg 10 - Figure 2. It would be informative to add the relative contributions of ClONO and ClNO2 to the bottom trace of Figure 2.
The NO3 and N2O5 peaks at noon local time are highly unusual. Consider adding a brief explanatory note to the caption.
pg 10 - Figure 3. The left-hand side graphs suggest that there is no nitryl chloride formation from heterogeneous uptake of N2O5 in Delhi. This seems unlikely considering non-zero mixing ratios of N2O5 are shown in Figure 2 (see also the next comment).
pg 10 line 201. Morning ClNO2 peaks are generally due to vertical transport of ClNO2 produced in the residual layer to the surface. Please cite relevant literature here and discuss.