Comprehensive multiphase chlorine chemistry in the box model CAABA/MECCA: implications for atmospheric oxidative capacity

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 Chemistry As A Boxmodel Application/Module Efficiently Calculating the Chemistry of the Atmosphere (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 NO_x (NO + NO₂): New Delhi (India) and Leicester (United Kingdom). The model shows a sharp build-up of Cl at sunrise through Cl₂ photolysis in both the urban environments. Besides Cl₂ photolysis, ClO+NO reaction and photolysis of ClNO₂ and ClONO are also prominent sources of Cl in Leicester. High-NO_x conditions in Delhi tend to suppress the nighttime 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 reactivity ratio is up to ≈ 9 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 volatile organic compounds (VOCs) due to active gas- and aqueous-phase chlorine chemistry enhances OH, HO₂, and RO₂ near sunrise. The simulation results of the updated model have important implications for future studies on atmospheric chemistry and urban air quality.

1 Introduction

Chlorine (Cl) radicals are one of the most important players in the tropospheric chemistry (Seinfeld and Pandis, 2016; Ravishankara, 2009). Cl impacts the oxidative capacity of the atmosphere and radical cycling and, therefore, can significantly alter the atmospheric composition (Seinfeld and Pandis, 2016; Faxon and Allen, 2013). In comparison with hydroxyl (OH) radicals, the so-called atmospheric detergent, the much faster reaction rates of Cl with volatile organic compounds (VOCs) enhance the peroxy radical (RO₂) formation and, thereby, the production of ozone (O₃) and secondary organic aerosols (SOAs) (Qiu et al., 2019a; Choi et al., 2020). In addition, Cl radicals can also enhance the oxidation of climate-driving gases (such as methane and dimethyl sulfide) (Saiz-Lopez and von Glasow, 2012). Cl radicals are produced in the atmosphere through photochemistry involving heterogeneous reactions of Cl-containing gases and aerosols (Qiu et al., 2019a; Faxon and Allen, 2013). The major sources of Cl-containing species are anthropogenic activities in continental regions and sea salt aerosols in marine and coastal environments (von Glasow and Crutzen, 2007; Osthoff et al., 2008; Liao et al., 2014; Liu et al., 2017; Thornton et al., 2010; Gunthe et al., 2021; Zhang et al., 2022). The photolysis of reactive Cl-containing species, such as chlorine gas (Cl₂), hypochlorous acid (HOCl), nitryl chloride (ClNO₂), and chlorine nitrite (ClONO), and the reaction of hydrochloric acid (HCl) with OH are known to produce Cl radicals in the lower troposphere (Atkinson et al., 2007; Riedel et al., 2014). With the rise in anthropogenic activities, emissions of Cl-containing species have increased significantly across the globe (Lobert et al., 1999; Zhang et al., 2022), and hence the importance of Cl in local as well as regional atmospheric chemistry has become prominent.

Despite the aforementioned importance, Cl chemistry and the associated mechanisms, especially heterogeneous reactions in the lower troposphere, are not yet fully understood, and the effects of Cl on atmospheric composition, air quality, and oxidation capacity remain uncertain. Field measurements have revealed high concentrations of Cl species over inland regions in addition to coastal and polar regions (von Glasow and Crutzen, 2007; Osthoff et al., 2008; Liao et al., 2014; Liu et al., 2017; Thornton et al., 2010); however, quantitative understanding of continental sources remains poorly understood. This is due to the lack of the relevant heterogeneous and gas-phase chemistry in atmospheric photochemical models despite the range of chemical mechanism complexity used in 3-D chemistry transport models (Xue et al., 2015; Pawar et al., 2023; Pozzer et al., 2022). In addition, the chemistry of Cl compounds has been less studied using the laboratory/chamber experiments. Qiu et al. (2019b) showed that due to inadequate representation of heterogeneous Cl chemistry, the Community Multiscale Air Quality (CMAQ) model underestimated nitrate concentrations during daytime but overestimated during nighttime in Beijing, China. In addition, the uncertainties associated with emission inventories of Cl species can lead to inaccurate estimation of air composition (Zhang et al., 2022; Sharma et al., 2019). For example, Pawar et al. (2023) noticed that even after the inclusion of HCl emissions from trash burning, the levels of nitrate, sulfate, nitrous acid (HONO), etc., still deviated from the observations in Delhi, India, highlighting the need to include emissions from other sectors, such as industries. A few recent studies assessed the impacts of the gas-phase Cl chemistry by including gas-phase ClNO₂ reactions; for example, Xue et al. (2015) reported about a 25 % enhancement in the daytime oxidation of carbon monoxide and VOCs at a coastal site in East Asia. In the same region, the model predicted a 5 %–16 % enhancement in peak ozone with ClNO₂ (≈ 50–200 pmol mol⁻¹) at a mountain top in Hong Kong, China (Wang et al., 2016). The measurements of Cl₂ (up to ≈ 450 pmol mol⁻¹) and ClNO₂ (up to ≈ 3.5 nmol mol⁻¹) were reported from a rural site in the North China Plain, and Cl chemistry was shown to enhance the formation of peroxy radicals (by 15 %) and O₃ production rate (by 19 %) (Liu et al., 2017).

Nevertheless, the heterogeneous chemistry of Cl species remains poorly represented in models and often neglected in large-scale numerical simulations. For example, in several models, the heterogeneous uptake of N₂O₅ on aqueous aerosols yielded nitric acid (HNO₃) via reaction HET1:

$$\begin{array}{}\text{(1)}& {\displaystyle }{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}\left(g\right)+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\left(\text{aq}\right)\to \mathrm{2}{\mathrm{HNO}}_{\mathrm{3}}\left(\text{aq}\right).\end{array}$$

However, N₂O₅ uptake on chloride-containing particles can produce ClNO₂ (Behnke et al., 1997; Thornton et al., 2010), especially in urban environments with strong NO_x emissions (Osthoff et al., 2008; Young et al., 2012). Incorporating heterogeneous mechanism of ClNO₂ into the regional models led to a 3 %–12 % increase in O₃ over northern China (Sarwar et al., 2014; Zhang et al., 2017; Liu et al., 2017). In addition, heterogeneous reactions of Cl-containing species including particulate chloride (pCl⁻), Cl₂, ClNO₂, chlorine nitrate (ClNO₃), and hypochlorous acid (HOCl) are suggested to result in the formation of Cl radicals as well as in recycling of NO_x and HO_x (OH and HO₂) (Ravishankara, 2009; Qiu et al., 2019a; Hossaini et al., 2016; Faxon and Allen, 2013). Very recent measurements suggest a reduction in ClNO₂ formation due to the competition of N₂O₅ uptake among chloride, sulfate, and acetate aerosols (Staudt et al., 2019). These heterogeneous reactions can be of paramount significance in the Cl budget; however, to the best of our knowledge, these have not yet been considered in model simulations.

The main goal of the present study is to investigate the role of chlorine chemistry in chemically contrasting urban environments. In this regard, we incorporate comprehensive gas-phase and heterogeneous Cl chemistry into a state-of-the-art box model. Section 2 provides a detailed description of the Cl chemistry mechanism with gas-phase and heterogeneous reactions. Section 3 describes the model setup, and Sect. 4 shows the simulation results which include a detailed investigation on (i) the sensitivity of air composition to chlorine chemistry, (ii) the production and loss of Cl and ClNO₂, (iii) the role of Cl in the atmospheric oxidative capacity (AOC), and (iv) the sensitivity to ${\mathrm{ClNO}}_{\mathrm{2}}+{\mathrm{Cl}}^{-}$ reaction.

2 Mechanism development

The community box model Chemistry As A Boxmodel Application/Module Efficiently Calculating the Chemistry of the Atmosphere (CAABA/MECCA, Sander et al., 2019), has been used in this work. A comprehensive gas- and aqueous-phase mechanism of chlorine chemistry has been added to MECCA, here used within the box model CAABA. The gas-phase and heterogeneous chemistry implemented in MECCA is described in the following subsections, and the full mechanism is shown in the Supplement.

2.1 Gas-phase chlorine chemistry

A total of 36 inorganic, organic, and photolysis reactions which are key contributors of Cl radicals were added to the mechanism (Table 1). The mechanism includes the inorganic reactions of Cl with NO_x, NO₃ (G1–G4), the reactions of Cl-containing species with OH and NO (G5–G7), and the reactions between Cl-containing species (G8–G9) (Qiu et al., 2019a; Burkholder et al., 2015; Atkinson et al., 2007). ClONO is formed through the reaction of Cl with NO₂ (G2) and exists as a metastable intermediate (Janowski et al., 1977; Niki et al., 1978; Golden, 2007). This intermediate subsequently transforms into ClNO₂ (G10), with an average conversion time of ≈ 12 h (ranging from 4 to 20 h), and the corresponding rate constant is $\mathrm{2.3}\times {\mathrm{10}}^{-\mathrm{5}}$ s⁻¹ (Janowski et al., 1977). The Cl-initiated oxidation of organic species, i.e., alkanes (C₃H₈, C₄H₁₀), aromatics (benzene (C₆H₆), toluene (C₇H₈) and xylene (C₈H₁₀)), alcohols (CH₃OH, C₂H₅OH), ketones (CH₃COCH₃, MEK), isoprene (C₅H₈), and other organic compounds (C₂H₅CHO, HOCH₂CHO, BENZAL, GLYOX, MGLYOX), has also been included (G11–G31). The corresponding kinetic data are based on the International Union of Pure and Applied Chemistry and NASA Jet Propulsion Laboratory data evaluations (Atkinson et al., 2006, 2007; Burkholder et al., 2015), as well as from the literature (Niki et al., 1985, 1987; Green et al., 1990; Shi and Bernhard, 1997; Sokolov et al., 1999; Thiault et al., 2002; Wang et al., 2005; Rickard, 2009; Wennberg et al., 2018). In addition, photolysis reactions (G32–G36) resulting in production the of Cl are also added to the module (Atkinson et al., 2007). The abbreviations of species mentioned in Table 1 are kept similar to that in the Master Chemical Mechanism (MCM) nomenclature (Rickard, 2009).

Qiu et al. (2019a)Burkholder et al. (2015)Burkholder et al. (2015)Qiu et al. (2019a)Atkinson et al. (2007)Atkinson et al. (2007)Atkinson et al. (2007)Atkinson et al. (2007)Atkinson et al. (2007)Janowski et al. (1977)Rickard (2009)Rickard (2009)Rickard (2009)Rickard (2009)Atkinson et al. (2006)Rickard (2009)Sokolov et al. (1999)Wang et al. (2005)Wennberg et al. (2018)Wennberg et al. (2018)Shi and Bernhard (1997)Atkinson et al. (2006)Atkinson et al. (2006)Atkinson et al. (2006)Atkinson et al. (2006)Niki et al. (1987)Atkinson et al. (2006)Niki et al. (1987)Niki et al. (1985)Green et al. (1990)Atkinson et al. (2006)Atkinson et al. (2006)Atkinson et al. (2006)Thiault et al. (2002)Atkinson et al. (2007)Atkinson et al. (2007)Atkinson et al. (2007)Atkinson et al. (2007)Atkinson et al. (2007)

Table 1 Gas-phase chlorine reactions and corresponding rate constants added to MECCA. The rate constants are expressed in units of cubic centimeters per molecule per second (cm³ molecule⁻¹ s⁻¹) unless otherwise specified. Model-simulated maximum noontime J values for Delhi are provided.

2.2 Heterogeneous chemistry

The aqueous-phase and heterogeneous chemistry of Cl compounds added to MECCA is described in Table 2. In the present study, we assume that N₂O₅ is in equilibrium between the gas and aqueous phase (H2) according to Henry's law, and the dissociation of N₂O₅(aq) to nitronium ion (${\mathrm{NO}}_{\mathrm{2}}^{+}$) and nitrate (${\mathrm{NO}}_{\mathrm{3}}^{-}$) occurs according to Reaction (A1). The rate constant for the recombination reaction of ${\mathrm{NO}}_{\mathrm{2}}^{+}$ and ${\mathrm{NO}}_{\mathrm{3}}^{-}$ is 2.7×10⁸ mol⁻¹ L s⁻¹, calculated based on Bertram and Thornton (2009) and Staudt et al. (2019). The acid dissociation of nitric acid (HNO₃) in aqueous phase (A3) also results in the formation of ${\mathrm{NO}}_{\mathrm{2}}^{+}$ (Sapoli et al., 1985).

Staudt et al. (2019)Bertram and Thornton (2009)Staudt et al. (2019)Sapoli et al. (1985)Staudt et al. (2019)Behnke et al. (1997)Roberts et al. (2008)Knipping et al. (2000)Grigorev et al. (1987)Grigorev et al. (1987)Staudt et al. (2019)Staudt et al. (2019)Staudt et al. (2019)Staudt et al. (2019)Ryder et al. (2015)Heal et al. (2007)Iraci et al. (2007)Coombes et al. (1979)Fried et al. (1994)Frenzel et al. (1998)Müller and Heal (2001)Sander (2015)

Table 2 Aqueous-phase and heterogeneous chlorine reactions added to MECCA.

Thus, produced nitronium ion (${\mathrm{NO}}_{\mathrm{2}}^{+}$) reacts reversibly with chloride (Cl⁻), yielding ClNO₂ (A4, A5) (Staudt et al., 2019; Behnke et al., 1997). After outgassing according to Henry's law (H3), ClNO₂ is photolyzed in the gas phase, producing Cl and NO₂ (Ghosh et al., 2012; Sander et al., 2014). ClNO₂ uptake on chloride-containing aerosols results in the formation of Cl₂ and nitrite ion (${\mathrm{NO}}_{\mathrm{2}}^{-}$), as shown by Reaction (A6) (Roberts et al., 2008). Chamber experiments suggest the formation of Cl₂ from the self-reaction of OH⋅Cl⁻ (A7), which gets formed via the reaction of OH with Cl⁻ (Knipping et al., 2000). Through another channel of reversible Reactions (A8, A9), $\mathrm{OH}\cdot {\mathrm{Cl}}^{-}$ reacts with aqueous chloride and produces ${\mathrm{Cl}}_{\mathrm{2}}^{-}$, which can yield Cl₂ through subsequent reactions (Grigorev et al., 1987). The ${\mathrm{NO}}_{\mathrm{2}}^{+}$ uptake on aqueous chloride to form ClNO₂ (A4) is ≈ 500 times faster than ${\mathrm{NO}}_{\mathrm{2}}^{+}$ reaction with H₂O (A10) (Staudt et al., 2019). At the same time, experimental studies revealed a strong competition of ${\mathrm{NO}}_{\mathrm{2}}^{+}$ to react with Cl⁻ and with other nucleophiles (e.g., ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$) and aqueous organic compounds, e.g., phenol, methanol, and cresol (A11–A16) (Staudt et al., 2019; Ryder et al., 2015; Heal et al., 2007; Iraci et al., 2007; Coombes et al., 1979). These reactions could suppress the formation of ClNO₂, and also the corresponding rate constants for Reactions (A11)–(A14) are similar to the ${\mathrm{NO}}_{\mathrm{2}}^{+}$ + Cl⁻ reaction yielding ClNO₂, i.e., 7.5×10⁹ mol⁻¹ L s⁻¹ (Staudt et al., 2019; Ryder et al., 2015; Heal et al., 2007). Methanol reacts with ${\mathrm{NO}}_{\mathrm{2}}^{+}$ (A15) and forms aqueous methyl nitrate (CH₃NO₃) (Iraci et al., 2007). Phase exchange for CH₃NO₃ and nitrophenol (HOC₆H₄NO₂) is shown by Reactions (H4) and (H5), respectively. The heterogeneous chemistry just discussed is implemented in MECCA and is summarized in Fig. 1. The rate constant for the ${\mathrm{NO}}_{\mathrm{2}}^{+}$ reaction with methoxyphenol is about ≈ 10 000 times smaller than the ${\mathrm{NO}}_{\mathrm{2}}^{+}$ + phenol reaction (Kroflič et al., 2015), so it is not considered in this study. In addition, nitration reactions of other alcohols (e.g., catechol and polyphenols) could be potentially important; however, due to unavailability of corresponding rate constants, these reactions are not considered in this study; nonetheless, future studies calculating the kinetics of these reactions are recommended.

Figure 1 Aqueous-phase and heterogeneous chemistry added to MECCA.

In this study, Cl reacts with hydrocarbons and acetone via H abstraction and hence does not lead to the formation of any Cl-containing molecules, such as chloroacetone. This means that there are no such reactions in MECCA in which the Cl atom becomes part of the organic molecule. The reaction of Cl atoms with isoprene proceeds mainly via addition, and it produces chlorine-containing organics (Ragains and Finlayson-Pitts, 1997; Fan and Zhang, 2004). However, here we have simplified the mechanism by not considering the fate of organohalogens. Therefore, for future research, it would be valuable to investigate the chemical kinetics of such reaction kinetics and their importance in the formation of organohalogen compounds.

3 Box model setup

The chemistry described in Sect. 2 has been added into community box model CAABA/MECCA v4.4.2 (Sander et al., 2019). A comprehensive gas- and aqueous-phase tropospheric chemistry involving a total of 3330 reactions was utilized for the simulations, and the full set of reactions are presented in the electronic supplement. The gas-phase chemistry of organics like terpenes and aromatics is treated by the Mainz Organic Mechanism (MOM) (Taraborrelli et al., 2012, 2021; Nölscher et al., 2014; Hens et al., 2014). The aqueous-phase chemistry of oxygenated VOCs is treated by the Jülich Atmospheric Mechanism of Organic Chemistry (JAMOC) (Rosanka et al., 2021). The numerical integration of the chemical mechanism is performed by the kinetic preprocessor v2.1 (KPP) (Sandu and Sander, 2006). The photolysis rate constants (J values) are calculated by the submodel JVAL, based on the method by Landgraf and Crutzen (1998). The Cl chemistry is expected to be more prominent during winter conditions due to the higher concentration of Cl-containing species in the boundary layer (Thornton et al., 2010; Gunthe et al., 2021; Sommariva et al., 2021), and, therefore, simulations are performed for the winter season. Hence, the model is set up for typical winter conditions of two different urban environments: Delhi (28.6^∘ N, 77.2^∘ E), India, and Leicester (52.4^∘ N, 01.1^∘ W), United Kingdom. Simulations are performed for a 5 d period (17–21 February 2018), and the output of the fifth day is considered for the analysis; by then, radicals have achieved an almost steady state. The typical environmental conditions used in the simulations for Delhi (Tripathi et al., 2022) and Leicester (Sommariva et al., 2021) are summarized in Tables 3 and S1.

Table 3 Environmental conditions of Delhi and Leicester in the model simulations.

VOC emissions are taken from the CAMS inventory (Sindelarova et al., 2014; Granier et al., 2019) and are adjusted iteratively in magnitude for better agreement with observations. CAMS-GLOB-ANT v5.3 (0.1^∘ × 0.1^∘) (Granier et al., 2019) provides emissions of anthropogenic VOCs (e.g., benzene and toluene), while emissions of biogenic VOCs (e.g., isoprene) are from CAMS-GLOB-BIO v3.1 (0.25^∘ × 0.25^∘) (Sindelarova et al., 2014). Emissions of HCl and particulate chloride are included from Zhang et al. (2022) and adjusted iteratively towards reported levels of Cl-containing species (Gunthe et al., 2021; Sommariva et al., 2021). The Mainz Organic Mechanism (MOM) dry deposition scenario (Sander et al., 2019) is activated in the model. Ground-based lidar measurements of boundary layer height (BLH) during wintertime, performed as a part of the European Integrated project on Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) project, are utilized for the simulations at Delhi (Nakoudi et al., 2019). The diurnal variation in BLH in Leicester is extracted from the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation reanalysis dataset ERA5 (Hersbach et al., 2020). Air composition in the model has been initialized based on previous studies (Table S1; Zhang et al., 2007; Lanz et al., 2010; Lawler et al., 2011; Sommariva et al., 2018, 2021; Gunthe et al., 2021; Tripathi et al., 2022). The values of aerosol properties (e.g., radius, liquid water content, and chemical composition) incorporated in the simulations for both Delhi and Leicester are provided in Table S1. We constrained the model with the parameterized function best representing the observed diurnal variations of NO_x (Fig. 2) (Tripathi et al. (2022); Sommariva et al. (2018, 2021), https://uk-air.defra.gov.uk/data/, last access: 4 December 2023), which helped in better reproducing the diurnal variations of some VOCs (e.g., isoprene) and ozone. Diurnal observations of HONO from Sommariva et al. (2021) are used for Leicester. For Delhi, however, HONO could not be constrained due to a lack of observations.

4 Results and discussion

The model captures the patterns in O₃ variability at both locations (Sommariva et al., 2018; Nelson et al., 2021; Chen et al., 2021; Sommariva et al., 2021; Nelson et al., 2023) to an extent, as shown in Fig. 2. O₃ is underestimated after ≈ 16:00 LT in Leicester, mainly due to titration by high NO and lack of adequate dynamics/transport of O₃ in the model. Entrainment seems to improve O₃ after midnight towards the observed values (Fig. 2i). Simulated isoprene is in agreement with diurnal observations in Delhi (Tripathi et al., 2022) and in accordance with the observed mean level in Leicester (Sommariva et al., 2021). The nitrate radical (NO₃), which is a nighttime oxidant, is formed through reaction between NO₂ and O₃ (G37). NO₃ can react with NO₂, forming N₂O₅, which can again produce NO₃ and NO₂ through thermal dissociation (G38).

Figure 2 Diurnal variations of $\mathrm{NO},{\mathrm{NO}}_{\mathrm{2}},{\mathrm{O}}_{\mathrm{3}},{\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{8}},{\mathrm{NO}}_{\mathrm{3}}$, and N₂O₅ mixing ratios in Delhi (a–f) and Leicester (g–l). The unusual and negligible nighttime NO₃ in Delhi is attributed to the nearly non-existent O₃, due to titration by higher concentrations of NO. This leads to the negligible nighttime N₂O₅ in this region. Although mixing ratios of NO₃ and N₂O₅ peak during the daytime, their levels remain quite low. The mean value of observed C₅H₈ in Leicester is shown by the red-colored long dashed line.

$$\begin{array}{}\text{(G37)}& {\displaystyle }{\mathrm{NO}}_{\mathrm{2}}+{\mathrm{O}}_{\mathrm{3}}⟶{\mathrm{NO}}_{\mathrm{3}}+{\mathrm{O}}_{\mathrm{2}}\end{array}$$

$$\begin{array}{}\text{(G38)}& {\displaystyle }{\mathrm{NO}}_{\mathrm{3}}+{\mathrm{NO}}_{\mathrm{2}}+\text{M}\rightleftharpoons {\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}+\text{M}\end{array}$$

As seen in Fig. 2e, NO₃ remains negligible during the nighttime (≈ 18:00–07:30 LT) in Delhi due to unavailability of O₃ under high-NO conditions (up to 200 nmol mol⁻¹). Interestingly, despite its very short lifetime (≈ 5 s), about ≈ 0.1 pmol mol⁻¹ of NO₃ is sustained during daytime. This is primarily due to prevailing levels of NO₂ (≈ 30 nmol mol⁻¹) and O₃ (≈ 40 nmol mol⁻¹). Such unusual daytime enhanced NO₃ has been reported in recent studies, for example, 5–31 pmol mol⁻¹ of NO₃ in Texas, USA (Geyer et al., 2003). Aircraft measurements during the New England Air Quality Study showed ≈ 0.5 pmol mol⁻¹ of NO₃ within the boundary layer (≤ 1 km) during noontime (Brown et al., 2005). The calculated NO₃ levels using steady-state approximation showed 0.01–0.06 pmol mol⁻¹ of NO₃ for the 1997–2012 period at urban sites in the UK (Marylebone Road London, London Eltham, and Harwell) (Khan et al., 2015a). Horowitz et al. (2007) suggested that NO₃ in tenths of picomoles per mole (pmol mol⁻¹) during daytime over the eastern United States results in the formation of ≈ 50 % isoprene nitrates through oxidation of isoprene, which could further affect the formation of O₃ and SOA significantly (Horowitz et al., 2007). Following to higher NO₃, up to 8 pmol mol⁻¹ of N₂O₅ is simulated during daytime in Delhi (Fig. 2f). Similar unusual daytime high levels of N₂O₅ (≈ 21.9 ± 29.3 pptv) during wintertime were recently measured at Delhi using a high-resolution iodide adduct chemical ionization mass spectrometer (Haslett et al., 2023).

Enhanced NO₃ ≈ 2.6 pmol mol⁻¹ and N₂O₅ ≈ 330 pmol mol⁻¹ are simulated after midnight in Leicester (Fig. 2k, l). In contrast to Delhi, the daytime simulated levels of NO₃ are negligible as it gets removed rapidly during the daytime by photolysis and through its reactions with NO, HO₂, RO₂, and VOCs (Khan et al., 2015b). In conjunction with high NO from ≈ 16:00 LT to near midnight that titrates O₃, the corresponding NO₃ and N₂O₅ is negligible (following Reactions G37 and G38). Nighttime high and negligible daytime levels of NO₃ and N₂O₅ are their typical features which are generally reported in the literature (Brown et al., 2001; Seinfeld and Pandis, 2016).

4.1 Sensitivity of air composition to chlorine chemistry

To investigate the effects of Cl chemistry on air composition, other than comprehensive chemistry simulation discussed in the previous section (simulation: NEW, i.e., chemistry already present in the model + newly added gas- and aqueous-phase chlorine chemistry), two additional simulations have been performed, which are (1) OLD (this includes the default chemistry already present in the model) and (2) NOCL (OLD minus chlorine chemistry, i.e., without Cl chemistry). The OLD simulation also encompassed some basic chlorine chemistry that was part of the model prior to its update (full mechanism is also shown in the Supplement). Figure 3 shows the comparison of Cl, ClNO₂, ClONO, OH, HO₂, and RO₂ variations among the three simulations in Delhi and Leicester. Figure S5 shows the differences in diurnal variations of Cl, ClONO+ClNO₂, OH, HO₂, and RO₂ in the NEW simulation with the NOCL and OLD simulations.

Figure 3 Model-simulated diurnal variations of Cl, ClNO₂, ClONO, OH, HO₂, and RO₂ at Delhi (a–f) and Leicester (g–l).

The Delhi environment is mainly characterized by two peaks in Cl, a predominant sharp peak just after sunrise followed by a broad shallow peak during noontime, corresponding to different mechanisms as discussed in the next section. With newly added chemistry (NEW simulation), a sharp peak in Cl is seen near sunrise, with the maximum values attained being ≈ 3.5 fmol mol⁻¹ (8.75×10⁴ molec cm⁻³) in Delhi (Fig. 3a). A broad smaller peak with a magnitude of ≈ 0.8 fmol mol⁻¹ maximizing around noontime is seen, which is ≈ 4 times smaller than the first morning peak. The OLD simulation also shows a sharp peak in Cl near sunrise in Delhi, with a maximum of ≈ 11 fmol mol⁻¹ (2.75×10⁵ molec cm⁻³). Cl get suppressed by up to ≈0.01 pmol mol⁻¹ of maximum value in the OLD simulation, in the presence of added chlorine chemistry (NEW) as shown in Fig. S5. Similar to Cl, a peak is seen in ClONO+ClNO₂ of ≈ 100 pmol mol⁻¹ with sunrise, which gradually decreases and attains ≈ 7 pmol mol⁻¹ from nearly 11:00–16:00 LT. Afterwards it increases to ≈ 20 pmol mol⁻¹ from late evening as shown by Fig. 3b and c. The pathways for the formation of ClNO₂ and ClONO were absent in the earlier version of the model (OLD). Simulated OH, HO₂, and RO₂ show a prominent peak just after sunrise in the presence of Cl chemistry for both the OLD and NEW simulations (Fig. 3d, e, f). As a consequence of greater oxidation of VOCs by Cl, enhanced levels of OH by 0.05 pmol mol⁻¹ (up to a factor of ≈ 1.8), HO₂ by 0.21 pmol mol⁻¹, and RO₂ by 0.1 pmol mol⁻¹ are noted with added Cl chemistry compared to the NOCL case (see Fig. S5). No significant changes are seen in noontime levels of OH and HO₂, whereas ≈ 1.1 times more RO₂ is produced with added Cl chemistry (NEW) compared to the OLD simulation.

The model-predicted Cl peaks at ≈ 2 fmol mol⁻¹ (5.2×10⁴ molec cm⁻³) during sunrise in Leicester (Fig. 3g). In contrast to Delhi, suppressed Cl (up to ≈ 3.2 times) with a narrow peak is simulated by the OLD simulation in comparison with the NEW simulation containing newly added Cl chemistry at Leicester. In contrast to negligible nighttime ClONO+ClNO₂ in Delhi, it shows a strong build-up over Leicester during 00:00–04:00 LT with a maximum of ≈ 40 pmol mol⁻¹, with higher levels (up to 50 pmol mol⁻¹) prevailing until about sunrise. ClONO+ClNO₂ is negligible during midday until midnight, in accordance with N₂O₅ in Leicester as shown in Fig. 2l. Previous studies have demonstrated that the formation of ClNO₂ occurs within the nocturnal residual layer, which contains lower levels of NO compared to the surface layer. Subsequently, ClNO₂ mixes downward during the morning when the convective mixed layer develops (Bannan et al., 2015; Tham et al., 2016). However, the present study does not account for the the effect of transport processes due to the limitations of the box model. The effects of added Cl chemistry on OH, HO₂, and RO₂ are more prominent in Leicester compared to Delhi. The NEW simulation shows strong enhancements in OH (up to ≈ 2 times), HO₂ (up to ≈ 5 times), and RO₂ (up to ≈ 8 times) after sunrise, which is gradually progressive, resulting in higher levels during noontime as well (Figs. 3, S5). Remarkably elevated levels of RO₂ (by a factor of ≈ 2) are prominent during the noon hours. Such elevated levels of RO₂ could favor enhanced levels of secondary organic aerosols in Leicester. The impact of Cl chemistry on aerosols (${\mathrm{NO}}_{\mathrm{2}}^{+}$, ${\mathrm{NO}}_{\mathrm{3}}^{-}$, and oxalic acid) is discussed in Supplement Sect. 2.2 (Fig. S6). Though significant differences in ${\mathrm{NO}}_{\mathrm{2}}^{+}$, ${\mathrm{NO}}_{\mathrm{3}}^{-}$, and oxalic acid are seen due to Cl chemistry, further measurements are required for validation. In the next sections, we have analyzed the observed behavior of Cl and ClNO₂ in the NEW simulation over both the locations in more detail.

4.2 Production and loss of Cl and ClNO₂

The sources and sinks of Cl in Leicester and Delhi are presented in Fig. 4. Panel (a) delineates the sources and sinks of Cl radical on the diurnal scale in Delhi. The morning sharp peak in Cl radical is caused mainly by the photolysis of Cl₂ with a maximum rate of 1.2×10⁷ molec cm⁻³ s⁻¹. The shallow secondary peak is due to the reaction HCl+OH with a noontime rate of $\approx \mathrm{0.4}\times {\mathrm{10}}^{\mathrm{7}}$ molec cm⁻³ s⁻¹. However, there is a smaller contribution from other reactions (photolysis of ClNO₂, ClONO, and reaction of ClO with NO) to the morning peak, which have negligible contributions during the daytime. Interestingly, there is a strong consumption of Cl to oxidize VOCs (peak rate $\approx \mathrm{2.4}\times {\mathrm{10}}^{\mathrm{7}}$ molec cm⁻³ s⁻¹) during sunrise and a lesser consumption during the rest of the day. Cl+NO₂ is also a Cl sink during the morning time in Delhi. The Cl-initiated oxidation of VOCs in the morning hours in Delhi may lead to formation of secondary organic aerosols and new particle formation, which opens up pathways of future research in this direction. In addition to Cl₂ photolysis ($\approx \mathrm{1.0}\times {\mathrm{10}}^{\mathrm{6}}$ molec cm⁻³ s⁻¹), photolysis of ClNO₂ and ClONO as well as ClO+NO reaction (total rate $\approx \mathrm{0.8}\times {\mathrm{10}}^{\mathrm{6}}$ molec cm⁻³ s⁻¹) are other prominent sources of Cl in Leicester. VOCs are the major sink for Cl (rate $\approx \mathrm{1.3}\times {\mathrm{10}}^{\mathrm{6}}$ molec cm⁻³ s⁻¹), followed by NO₂ (rate $\approx \mathrm{0.6}\times {\mathrm{10}}^{\mathrm{6}}$ molec cm⁻³ s⁻¹).

Figure 4 Production and loss rates of (a, b) Cl and (c, d) ClNO₂ in Delhi (a, c) and Leicester  (b, d).

We further analyzed the production and loss pathways of ClNO₂, as shown in Fig. 4c and d. While the major source of ClNO₂ is through the Cl+NO₂ reaction with a reaction rate ≈ 3 × 10⁵ molec cm⁻³ s⁻¹ in Delhi, the aqueous-phase reaction ${\mathrm{Cl}}^{-}+{\mathrm{NO}}_{\mathrm{2}}^{+}$ (≈ 3.4 × 10⁵ molec cm⁻³ s⁻¹) is the prominent source in Leicester corresponding to the peak ClNO₂ (Fig. 2h, p). Though the gas-phase reaction Cl+NO₂ is discussed in the literature (Burkholder et al., 2015; Qiu et al., 2019a), to the best of our knowledge, such an unusually higher contribution of this reaction (seen in Delhi) as compared to the aqueous-phase reaction of ${\mathrm{Cl}}^{-}+{\mathrm{NO}}_{\mathrm{2}}^{+}$ has not been reported in any study. The reaction of Cl with NO₂ (≈ 1.1 × 10⁵ molec cm⁻³ s⁻¹) is the major ClNO₂ source during sunrise in Leicester. In contrast, there is a lower contribution of the Cl⁻ + ${\mathrm{NO}}_{\mathrm{2}}^{+}$ reaction (rate ≈ 1 × 10³ molec cm⁻³ s⁻¹) in ClNO₂ production in Delhi. The prominent sink for ClNO₂ is through its heterogeneous reaction with Cl⁻ (≈ 1.8 × 10⁵ molec cm⁻³ s⁻¹ or 7.2 × 10⁻¹⁵ mol mol⁻¹ s⁻¹) in Delhi almost throughout the day, while its loss through the photolysis (≈ 0.5 × 10⁵ molec cm⁻³ s⁻¹ or 2 × 10⁻¹⁵ mol mol⁻¹ s⁻¹) is also an important sink during the daytime. We are using ClNO₂ uptake coefficient γ = $\mathrm{9}\times {\mathrm{10}}^{-\mathrm{3}}$ from Fickert et al. (1998) in the simulation. The sensitivity simulation with $\mathit{\gamma }=\mathrm{1}\times {\mathrm{10}}^{-\mathrm{5}}$ (Haskins et al., 2019) results in a considerably slower (by a factor of ≈ 270 and ≈ 17, near sunrise and during midday, respectively) loss rate of ClNO₂ with Cl⁻ than in the NEW simulation over Delhi. ClNO₂ loss through the reaction ${\mathrm{ClNO}}_{\mathrm{2}}+{\mathrm{Cl}}^{-}$ (≈ 2.7 × 10⁵ molec cm⁻³ s⁻¹ or 1.0 × 10⁻¹⁴ mol mol⁻¹ s⁻¹) is its major sink in Leicester from midnight to midday, while photolysis (≈ 0.3 × 10⁵ molec cm⁻³ s⁻¹ or 1.1 × 10⁻¹⁵ mol mol⁻¹ s⁻¹) is the smaller sink from sunrise to midday here. The diurnal variation in Cl₂ and its production and loss mechanisms over Delhi and Leicester are shown by Figs. S1 and S2. In conjunction with the major loss of ClNO₂, the ${\mathrm{ClNO}}_{\mathrm{2}}+{\mathrm{Cl}}^{-}$ reaction is the major contributor to Cl₂ formation over Delhi and Leicester.

We also calculated ClNO₂ yield from ${\mathrm{NO}}_{\mathrm{2}}^{+}$ (Fig. S3), which is the ratio of $P_{{\mathrm{ClNO}}_{\mathrm{2}}}/L_{\mathrm{total}}$, where $P_{{\mathrm{ClNO}}_{\mathrm{2}}}$ is the rate of ClNO₂ production through the Cl⁻ + ${\mathrm{NO}}_{\mathrm{2}}^{+}$ reaction and L_total denotes the loss rate of ${\mathrm{NO}}_{\mathrm{2}}^{+}$ through its reactions with Cl⁻, H₂O, ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$, HCOO⁻, CH₃COO⁻, phenol, CH₃OH, and cresol (A4, A10–A16). ClNO₂ yield is ≈ 0.9 over Delhi, indicating the strongest loss of ${\mathrm{NO}}_{\mathrm{2}}^{+}$ is through its reaction with Cl⁻, which is also mimicked in Fig. S4a, showing the same concentrations of ClNO₂ as in the NEW simulation and when other ${\mathrm{NO}}_{\mathrm{2}}^{+}$ reactions (A10–A16) are turned off (simulation: without other NO${}_{\mathrm{2}}^{+}$ reactions). ClNO₂ yield over Leicester is between ≈ 0.40 and 0.55, which is about half the yield in Delhi. Stronger ClNO₂ yield in Delhi could be attributed to ≈ 2 times higher Cl⁻ than Leicester. Lesser ClNO₂ yield in Leicester portrays the importance of ${\mathrm{NO}}_{\mathrm{2}}^{+}$ loss reactions (A10–A15) other than with Cl⁻, which could be seen through Fig. S4b where ClNO₂ is increased by more than 2 times during the early morning hours when A10–A15 reactions are kept inactive in the model. The determination of ClNO₂ yield using cavity ring-down spectroscopy and chemical ionization mass spectrometry shows yield ranging between 0.2 and 0.8 for Cl⁻ concentrations of 0.02 to 0.5 mol L⁻¹ (Roberts et al., 2009). The measurements of ClNO₂ yield for coastal and open-ocean waters were found to be between 0.16 and 0.30, which is suppressed by up to 5 times compared to equivalent salt containing solutions due to the addition of aromatic organic compounds (e.g., phenol and humic acid) to synthetic seawater matrices (Ryder et al., 2015).

4.3 Role of Cl in atmospheric oxidative capacity (AOC)

In order to understand the role of Cl as an oxidizing agent with respect to the OH radical, we calculated the reactivity of Cl and OH as ${\mathrm{\Sigma }}_{X_i}$ ($k_{\mathrm{radical}+X_i}$ × [X_i]), where the radical is Cl or OH, and [X_i] is the concentration of species X_i (here X_i includes CO, CH₄, primary VOCs, and NMHCs which are initialized in the model) (Fig. 5). The corresponding rate constants for Cl + X_i and OH + X_i reactions are taken from MECCA. The reactivity of both Cl and OH decreases rapidly nearly from sunrise to noontime and afterwards increases gradually at both locations. In comparison to Leicester, the magnitudes of Cl and OH reactivity in Delhi are higher by up to ≈ 1.4 and ≈ 12 times, respectively. However, the $\mathrm{Cl}/\mathrm{OH}$ reactivity ratio in Leicester is up to ≈ 9 times higher than that in Delhi. Cl reactivity is lower (Delhi: ≈ 685 s⁻¹; Leicester: ≈ 553 s⁻¹) during noontime and higher (Delhi: ≈ 750 s⁻¹; Leicester: ≈ 554 s⁻¹) during nighttime and the early morning hours at both locations. The OH reactivity follows a similar pattern to that of Cl in Delhi and Leicester. The ratio of Cl to OH reactivity starts increasing after sunrise, reaching a maximum value of ≈ 42 at nearly 16:00 LT, and then decreases further in Delhi. As mentioned above, the $\mathrm{Cl}/\mathrm{OH}$ reactivity ratio in Leicester shows a double peak pattern, with one peak (≈ 270) during the early morning ≈ 04:00 LT and the other peak (≈ 276) at about 16:00 LT.

Figure 5 Reactivity of Cl and OH with CO,CH₄, and VOCs, as well as the $\mathrm{Cl}/\mathrm{OH}$ reactivity ratio during the simulation period in (a) Delhi and (b) Leicester.

Figure 6 Atmospheric oxidative capacity (AOC) of radicals during (a, b) early morning mean (07:00–09:00 LT) and (c, d) daytime mean (06:00–16:00 LT) in Delhi (a, c) and Leicester (b, d).

We quantified the relative contribution of Cl in atmospheric oxidative capacity (AOC) using the model. AOC represents the sum of oxidation rates of species X_i by oxidants Y (OH, Cl, and other radicals, NO₃ and O₃) (Elshorbany et al., 2009):

$$\begin{array}{}\text{(1)}& {\displaystyle }\text{AOC}=\sum k_{X_i}\left[X_i\right]\left[Y\right]\end{array}$$

where $k_{X_i}$ is the corresponding rate constant for the X_i+Y reaction. Accordingly, the magnitude of AOC depends upon the concentration and reactivity of Cl. Figure 6 shows the contribution of individual oxidants in AOC at both locations. Besides OH, Cl is the second most important oxidant in Delhi, with a significant contribution of 23.4 % during the morning (averaged over 07:00–09:00 LT) and 8.2 % throughout the day (06:00–16:00 LT). In Leicester, Cl is the highest contributor (74.0 %) towards AOC during the morning. In fact, with 34.1 % contribution, Cl is a major oxidant after OH during the daytime. Besides the abundance of Cl, higher reactivity enhances the contribution of Cl in AOC, which is further substantiated by the ratio of Cl reactivity to OH reactivity (Fig. 5b). This ratio indicates that Cl reactivity exceeds OH reactivity by a significant margin, ranging from 265 to 276 times greater throughout the day in Leicester. Such a substantial contribution of Cl in AOC leads to enhancements of RO₂ as seen in Fig. 3(f, l). A prominent morning (07:00–09:00 LT) peak in RO₂ is attributed to the atmospheric oxidation enhanced by Cl during that time. Notably the strongest contribution of Cl in AOC during the early morning in Leicester strengthens RO₂ peak by up to a factor of 8 (Fig. 3l). The role of Cl is predominant in Leicester as well as in Delhi during the early morning, compared to a polluted environment of Hong Kong, China, where Cl contribution was estimated to be 21.5 % (Xue et al., 2015). NO₃ and O₃ were found to play a relatively minor role in AOC at both urban environments.

4.4 Sensitivity to the ClNO₂ + Cl⁻ reaction

In a study conducted by Haskins et al. (2019) using the reacto-diffusive length-scale framework, it was demonstrated that field and laboratory observations could be reconciled by considering an aqueous-phase reaction rate constant for the ClNO₂ + Cl⁻ reaction on the order of ≈ 10⁴ s⁻¹. This reaction rate constant is considerably lower (by ≈ 179 times) than reported in Roberts et al. (2008). In this context, the sensitivity simulation (NEWrate) is performed using a reaction rate coefficient of 5.6 × 10⁴ mol⁻¹ L s⁻¹ (Haskins et al., 2019) for the ClNO₂ + Cl⁻ reaction, for both Delhi and Leicester. As depicted in Fig. S7a, the concentration of Cl remains nearly the same in the NEWrate simulation compared to the NEW simulation over Delhi. However, there are significant changes in the concentration of ClNO₂, as shown in Fig. S7b. The simulated ClNO₂ exhibits a broader peak and is approximately 30 pmol mol⁻¹ higher near sunrise in the NEWrate simulation when compared to the NEW simulation. During the nighttime, approximately 20 pmol mol⁻¹ of ClNO₂ is simulated in the NEWrate simulation, whereas it is negligible in the NEW simulation (see Fig. 3b). Since the Cl concentration is almost similar in both the NEW and NEWrate simulations, the differences in the simulated concentrations of OH, HO₂, and RO₂ remain consistent between the NEWrate or NEW simulations and the OLD and NOCL simulations (refer to Fig. S7d, e, f and Fig. 3d, e, f). The production and loss mechanisms of Cl are similar in both the NEW and NEWrate simulations (see Fig. S8a and Fig. 4a). The contributions from ClNO₂ formation reactions are also similar. However, in contrast to the NEW simulation, the loss of ClNO₂ through photolysis becomes dominant and is ≈ 6 times greater than its loss through the ClNO₂ + Cl⁻ reaction in the NEWrate simulation. The contribution of radicals to AOC is also similar between the NEW and NEWrate simulation, as depicted in Fig. 6a and c and Fig. S9a and c, respectively, over Delhi.

In contrast to Delhi, significant differences are seen in atmospheric composition in Leicester when the rate coefficient of the ClNO₂ + Cl⁻ reaction is altered (as shown in Fig. S7). The peak concentration of Cl becomes ≈ 0.6 fmol mol⁻¹ during the morning hours of NEWrate simulation (Fig. S7g), which is about 4 times lower than the concentration of Cl in the NEW simulation (Fig. 3g). However, due to the slower rate of ClNO₂ consumption with Cl⁻, the simulated ClNO₂ using the NEWrate is significantly enhanced (by ≈ 5 times) compared to NEW simulation, reaching a maximum of about 210 pmol mol⁻¹ around sunrise (see Fig. S7h). Due to lower Cl concentrations, the levels of ClONO also decrease by 3.5 times in the NEWrate simulation (as shown in Fig. S7i) compared to the NEW simulation (Fig. 3i). The dominant peak seen at sunrise in the NEW simulation for OH, HO₂, and RO₂ is significantly reduced with the lower rate of the ClNO₂ + Cl⁻ reaction, as illustrated in Fig. S7j, k, l. Significant changes in the production and loss mechanisms of Cl and ClNO₂ are seen in Leicester when the reaction rate of A6 is changed, as shown in Figs. S8 and 4b. For example, in the NEWrate simulation, other reactions, including the photolysis of ClNO₂ and ClONO, as well as the ClO+NO reaction become prominent sources of Cl (with a rate of approximately 6.0×10⁵ molec cm⁻³ s⁻¹), whereas in the NEW simulation, the major source for Cl is photolysis of Cl₂. The primary source for ClNO₂ production remains the Cl⁻ + ${\mathrm{NO}}_{\mathrm{2}}^{+}$ reaction in both the NEW and NEWrate simulations. However, in the NEWrate simulation, ClNO₂ loss from photolysis becomes the major sink, whereas in the NEW simulation, loss from the ClNO₂ + Cl⁻ reaction is prominent. In addition, remarkable changes in AOC are seen between the NEWrate (Fig. S9b, d) and the NEW simulation (Fig. 6b, d). In the NEWrate simulation, even though Cl remains the major oxidant, its contribution is notably reduced from 74 % (in NEW simulation) to 58.1 % during the early morning hours.

5 Summary and conclusions

Extended gas- and aqueous-phase chemistry of chlorine compounds has been added to the MECCA mechanism. It consists of 36 gas-phase reactions (inorganic, organic, and photolysis reactions). A total of 24 aqueous-phase and heterogeneous reactions have been added, containing detailed chemistry of N₂O₅ uptake on aerosols to yield ClNO₂ and various other competing reactions. The updated model is applied to two different urban environments: Delhi (India) and Leicester (United Kingdom) during wintertime. The major conclusions are the following.

1. The model predicts up to 0.1 pmol mol⁻¹ of NO₃ and up to 8 pmol mol⁻¹ of N₂O₅ during daytime in Delhi. However, nighttime production of NO₃ and N₂O₅ is seen to be negligible primarily due of the unavailability of O₃. In contrast to Delhi, NO₃ and N₂O₅ after midnight in Leicester are ≈ 2.6 pmol mol⁻¹ and ≈ 330 pmol mol⁻¹, respectively. N₂O₅ uptake on aerosols yields ClNO₂, which produces Cl via photolysis.

2. A sharp build-up of Cl with sunrise is mainly through Cl₂ photolysis in Delhi. Besides Cl₂, photolysis of ClNO₂ and ClONO and the reaction of ClO with NO are prominent Cl sources in Leicester. VOCs are the main sink for Cl at both locations, whereas NO₂ is also an important sink for Cl in Leicester. The latter results in the formation of ClNO₂ with a major contribution in Delhi, while ${\mathrm{Cl}}^{-}+{\mathrm{NO}}_{\mathrm{2}}^{+}$ is a stronger source in Leicester. Photolysis is the major sink for ClNO₂ in Delhi; however, its uptake on chloride aerosols is a prominent sink in Leicester.

3. The magnitude of Cl (≈ 750 s⁻¹) and OH (≈ 25 s⁻¹) reactivities is significantly greater in Delhi, particularly during the morning hours, when compared to Leicester. However, the Cl-to-OH reactivity ratio (≈ 270) is pronounced in Leicester, coinciding with higher contribution of Cl in AOC.

4. Sensitivity simulations reveal substantial post-sunrise enhancements in OH, HO₂, and RO₂ radicals, with a prominent secondary peak due to Cl chemistry. Up to 8 times higher RO₂ is simulated in Leicester primarily because of leading role of Cl in AOC potential.

It is important to note that box models, despite their general limitation of neglecting transport phenomena and assuming species to be well mixed, do include highly detailed chemical mechanisms. Furthermore, because the model is initialized with measurements of chemical species at both locations and the modeled levels align with observed data, significant discrepancies in model estimates would be unexpected. Future studies focusing on modeling vertical gradients, in particular for radical reservoir species such as HONO and ClNO₂ (Young et al., 2012), are recommended.

This study highlights the vital role of Cl chemistry in governing the oxidation capacity of the atmosphere and air quality, and therefore it is important to account for it in detailed photochemical as well as in 3-D chemical transport models. This will lead to better quantification of the importance of radicals in atmospheric oxidation and, hence, the formation of ozone as well as secondary aerosols over the regional to global scale. Future studies focusing on secondary aerosol formation and new particle formation from heterogeneous reactions are needed to deepen the understanding of transformation of trace gases to aerosols.