Effect of the blocked-sites phenomenon on the heterogeneous reaction of pyrene with N₂O₅/NO₃/NO₂

Abstract

To clarify whether the blocking reaction sites problem has a significant impact on heterogeneous reactions, experiments contrasting the order of pyrene (PY) particles' exposure to N₂O₅–O₃ or O₃–N₂O₅ in a heterogeneous process were conducted. Additionally, PY particles were exposed to N₂O₅ (∼8 ppm) in the presence of O₃ (2.5–30 ppm) in a reaction chamber at ambient pressure and room temperature. Our results show that the phenomenon of blocking reaction sites may be ubiquitous on the surfaces of atmospheric aerosol particles, and the N₂O₅-initiated ionic electrophilic nitration may be promoted by NO₃ radical-initiated heterogeneous reactions on the aerosol particle surface. We also found that the operative reaction mechanism strongly depends on the concentrations of the nitric oxides in the atmosphere. Our results provide an explanation as to why 2-nitropyrene (2-NPY), one of the most ubiquitous nitro-polyaromatic hydrocarbon pollutants that exists in both the gas and particle phases, was not observed in previous experiments on the heterogeneous reactions of PY and N₂O₅/NO₃/NO₂.

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Introduction

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous air pollutants resulting from incomplete combustion processes, such as those of diesel and gasoline engines, and biomass or coal burning,¹ and constitute a health risk to the population due to their mutagenic and carcinogenic properties.² Homogeneous and heterogeneous processes, promoted by the interaction of PAHs with atmospheric trace oxidants during their atmospheric transit, are considered important degradation pathways for both gas-phase and particulate PAHs. In particular, the heterogeneous chemistry between particulate PAHs and gas-phase oxidants has been shown to be one of the most important sources of the more toxic and mutagenic PAH derivatives (nitro-PAHs (NPAHs) and oxy-PAHs).^3–6

The mechanism for the heterogeneous formation of NPAHs has been explored. The kinetics and products of the heterogeneous reactions of surface-bound PAHs with NO₂,^7,8 OH radicals,^9–11 N₂O₅,¹² NO₃ radicals,^13,14 O₃,^15,16 and NO⁹ have been investigated using various substances as atmospheric particle models. Because NO₃ radicals play an important role in atmospheric chemistry under dark conditions, particular attention has been paid to the heterogeneous reactions of particulate PAHs upon exposure to N₂O₅/NO₃/NO₂. Zhang et al. reported that mono-nitro-, di-nitro-, and poly-nitro-PAHs and their derivatives were produced in the heterogeneous reactions of suspended PAH particles and NO₃ radicals.¹³ Shiraiwa et al. reported that the rate constants for the surface-layer reactions of PAHs with NO₃ radicals were in the range of 10⁻¹⁵ and 10⁻¹² cm² s⁻¹.¹⁷ Gross et al. reported that the reactive uptake coefficient of the NO₃ radical on the surface of solid PAHs ranged between 0.059 (+0.11/−0.049) at 273 K and 0.79 (−0.21/−0.67) at room temperature.¹⁸ Liu et al. reported that the effective reaction rates of the heterogeneous reaction between suspended four-ring PAH particles and NO₃ radicals were of the order of 10⁻¹² to 10⁻¹¹ cm² s⁻¹, and the uptake coefficient of the NO₃ radical ranges between 0.06 and 0.57.¹⁴ Zimmermann et al. suggested that, for PAHs that exist in both the gas and particle phases, the heterogeneous formation of particle-bound NPAHs represented a minor formation route compared to the gas-phase formation; however, their studies could not disprove that the heterogeneous reaction of a NO₃ radical is a more important sink for PAHs than NO₂, HNO₃, or O₃.¹⁹

Although a mechanism has been recently suggested based on the gas-phase chemistry of PAHs with the NO₃ radical, specific markers for the radical-initiated isomer products (2-nitrofluoranthene and 2-nitropyrene) were not observed during the heterogeneous chemical processes.^18,20,21 Ringuet et al. reported the first observation of the heterogeneous formation of 2-nitropyrene from particulate pyrene oxidation in the presence of O₃/NO₂, questioning its use as an indicator of NPAH formation in the gaseous phase.²² Thus, the reaction mechanism of heterogeneous nitration has not been unequivocally identified.

Recent studies have suggested that high concentrations of N₂O₅ and NO₂, either in experiments or the real atmosphere, may prevent the NO₃ radical from being accommodated on particle surfaces and reacting; this may suppress NO₃ radical-initiated heterogeneous reactions.^18,19,21 To explore whether this problem of blocking reaction sites has a significant impact on the heterogeneous reactions that occur on the PAH particle surface, we conducted an in-depth investigation of the heterogeneous oxidation of suspended pyrene (PY) particles by N₂O₅/NO₃/NO₂ in the presence of O₃. Contrasting order-of-exposure experiments between PY and N₂O₅–O₃ and O₃–N₂O₅, and a series of heterogeneous reactions of suspended PY particles by exposure to N₂O₅ (∼8 ppm) in the presence of O₃ (at concentrations ranging from ∼2.5 to ∼30 ppm) were conducted.

Experimental section

Experimental setup

All experiments were conducted in the dark at room temperature (298 ± 3 K) and atmospheric pressure (∼96 kPa). The relative humidity in the chamber ranged between 40% and 50%. The schematic diagram of the experimental setup, shown in Fig. 1, was described in previous studies.¹⁴

Fig. 1 The schematic diagram of the experimental setup.

The experimental setup consists mainly of a 120 L aerosol reaction chamber,²³ online and offline analytical instruments, an aerosol generator, an ozone generator, and a N₂O₅-vapor manipulator. The online and offline analytical instruments include a laboratory-built vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS), a scanning mobility particle size (SMPS), an ozone monitor (Model 202, 2B Technologies Corp.), and a gas chromatograph/mass spectrometer (GC-MS). The VUV-ATOFMS was used to online-monitor the PAH particles and their reaction products in the reaction chamber. A detailed description of the VUV-ATOFMS has been presented elsewhere.²⁴ The SMPS, consisting of a differential mobility analyzer (DMA, TSI 3081) and a condensation particle counter (CPC, TSI 3010), was employed to measure online the size distribution and mass concentration of the PY particles. The measured mean diameters and mass concentration of PY particles were 367 ± 20 nm and 244 μg m⁻³, respectively; the geometric standard deviation of the particles was 1.2. The particle surface-to-volume ratio was ∼6.43 × 10⁹ nm² cm⁻³ in the experiments. The partitioning ratio of PY was estimated to be 4.3 × 10⁻⁴ cm³ μg⁻¹ under our experimental conditions (seeing ESI†). The GC-MS consisted of an HP model series 6890 gas chromatograph coupled with an HP model 5973 mass-selective detector with a 70 eV electron impact ionizer (Agilent Technologies, Massy, France); this was used offline to identify the nitropyrene isomers (NPY).

N₂O₅/NO₃/NO₂ preparation and aerosol generator

The procedures for producing the aerosol particles and N₂O₅ were similar to those recently reported.^14,25,26 NO₃ radicals and NO₂ were generated from the thermal decomposition of N₂O₅ at room temperature (eqn (R₁)). N₂O₅ was synthesized by dehydrating concentrated nitric acid. Fuming nitric acid (∼20 mL) was introduced into a glass bottle placed in a 223 K cooling bath. Then, P₂O₅ powder was gradually added into the nitric acid and thoroughly mixed until the slurry was too thick to stir. In preparation of N₂O₅, a ∼25 mm thick layer of P₂O₅ powder was placed over the slurry (the mixture of P₂O₅ and fuming nitric acid) to eliminate nitric acid moisture. Next, the bottle was heated in a 313 K water bath. The gaseous N₂O₅ from the slurry was extracted by a pump and collected in a 1 L flask placed in a liquid nitrogen-cooled Dewar. The synthesized N₂O₅ powder collected in the liquid nitrogen trap appeared as pure white crystals. The collected N₂O₅ powder could be further purified by vacuum pumping at 263 K to remove minor amounts of NO₂. A digital refrigerated circulator bath (233–373 K, DCW-4006, China) was used to maintain a constant temperature in the cooling bath. A glass trap containing N₂O₅ powder was placed in the cooling bath. Since our laboratory lacks the corresponding detecting instruments, N₂O₅ cannot be measured in an independent way. The concentration of N₂O₅ was controlled by changing its vapor pressure by adjusting the temperature of the bath. O₃ was prepared by passing O₂ through a commercial ozonizer (NBF30/W); its initial concentration in the mixture was measured with an ozone monitor.

N₂O₅(g) ⇄ NO₂(g) + NO₃(g) (R1)

An electric tube furnace, equipped with two tandem quartz tubes [50 cm (length) × 3 cm (inner diameter)] wrapped in heating tape so as to create a linear hot-to-cool temperature gradient, was used to produce the PY aerosol. Because of the complexity of natural particles, azelaic acid was used to produce nuclei in this study. It has the advantage of having limited reactivity toward gas-phase oxidants and can form a stabilized aerosol distribution.²⁷ Azelaic acid was placed in the first tube (463 ± 1 K), and the PY sample in the second tube (453 ± 1 K). A volumetric flow of 0.6 L min⁻¹ of N₂ controlled by a mass flow controller (MFC, D08-2F) was used to send the mixed aerosols coated with PY into the reaction chamber through the electric tube furnace.

Product collection and analysis

After exposure to N₂O₅/NO₃/NO₂/O₃ in the reaction chamber, the suspended particles in ∼75% of the chamber volume were collected with a pre-cleaned glass microfiber filter (GMF, 25 mm diameter, 0.7 μm pore size, Whatman). A schematic diagram of the collection procedure is shown in Fig. S1.† The filter was connected to a sample pumping (ACO-016, 450 L min⁻¹). The collected PY was extracted with ∼10 mL dichloromethane. The extraction was carried out for 1 min in a KH-5200B sonicator (Kunshan Hechuang Sonicator Co. Ltd), and concentrated to a volume of ∼2 mL by evaporation under a gentle stream of high purity nitrogen, and then analyzed offline using GC-MS. The relative yields of formed NPY isomers under different experimental conditions were compared through the relative abundances of NPY from the results of GC/MS analyses.

The National Institute of Standards and Technology (NIST) Mass Spectral Library 2005 was employed to identify the reaction products. An aliquot of the samples (5 μL) was introduced into the GC-MS system in the pulsed splitless mode. The column used for the analyses was a 30 m × 0.25 mm i.d. × 0.25 μm film thickness DM-1701ms (Agilent Technologies). The temperature of the vaporizer was kept at 270 °C. The initial oven temperature was set to 40 °C for 2 min; it was then increased step-by-step to 150 °C (by 20 °C min⁻¹), 220 °C (by 10 °C min⁻¹), and 270 °C (by 5 °C min⁻¹); the temperature was kept for 10 min at each of the 3 levels. Helium was used as the carrier gas at a constant flow rate of 1 mL min⁻¹. The interface temperature was kept at 270 °C throughout the GC-MS assay. A mass range between m/z 50 and 500 was used for quantitative determinations. The GC-MS data were obtained via GC-MS selected ion monitoring (SIM) of the molecular ion (m/z 247).

Chemicals

PY (98%, Sigma Aldrich), azelaic acid (98%, Sigma Aldrich), dichloromethane (chromatographic grade, Sinopharm Chemical Reagent Beijing Co., Ltd.), and absolute ethyl alcohol (≥99.7%, Sinopharm) was used in the experiments.

Exposure experiments

Wall loss

Both NO₃ radicals and N₂O₅, as two major oxidants in our study, could not been online monitored due to the limitation of our current experimental condition. Thus, the wall loss characterized for NO₃ radical and N₂O₅ could not been characterized. As for the wall loss of O₃ and NO₂, 30 ppm O₃ and 10 ppm NO₂ were separately introduced into the chamber. The results showed that their concentrations are basically constant after their concentrations stabilization. Thereby, the wall loss of O₃ and NO₂ in the chamber could be negligible.

The wall loss of PY particles monitored with the VUV-ATOFMS in the absence of oxidants are below 5% for 500 s, thus, the wall loss of PY particles have not apparent impact to their heterogeneous degradation.

Contrast experiments

To establish whether the phenomenon of blocking reaction sites has a significant impact on heterogeneous reaction mechanisms of particulate PY and N₂O₅/NO₃/NO₂, two different types of experiments were carried out. In the N₂O₅–O₃ exposure, the PY aerosol in the chamber was first exposed to ∼1.5 ppm N₂O₅ for ∼3 min, followed by a heterogeneous exposure for 3 min to O₃ (after ∼5.5 ppm O₃ was introduced into the chamber). The sequence of introducing O₃ and N₂O₅ into the chamber in the O₃–N₂O₅ exposure was simply reversed from that of the N₂O₅–O₃ exposure. We assumed that the effect of blocked reaction sites on the heterogeneous nitration mechanism would be negligible, and the observed 2-NPY would be mainly formed in the gas-phase reaction following deposition onto the particles. Based on this hypothesis and considering the same mass concentration of PY aerosol, because N₂O₅ was first introduced in the chamber and can produce the NO₃ radical (R₁), the NO₃ radical-initiated NPY yield in the N₂O₅–O₃ exposure (especially for 2-NPY formed in the gas-phase reaction) should be higher than that in the O₃–N₂O₅ exposure due to the longer exposure time in the chamber.

Effect of different O₃ concentrations

To further clarify whether the high concentrations of N₂O₅ and NO₂ used may block the NO₃ radical from being accommodated on the surface and reacting, a series of heterogeneous reactions of suspended PY particles with N₂O₅ (∼8 ppm) in the presence of different O₃ concentrations (ranging between ∼2.5 ppm and ∼30 ppm) was conducted. The concentrations of O₃ and N₂O₅ in this work are higher than those found in the atmosphere. To investigate whether adsorbed molecules would prevent other reactants from accessing reactive substrate species under real atmospheric conditions, heterogeneous exposures of PY particles to N₂O₅ (∼500 ppb) in the presence of O₃ (∼150 ppb) were also carried out. However, the results showed that no 2-NPY was formed after exposure for 20 min. Since PY particles in the chamber were continuously sedimented as the reaction time was prolonged (>20 min), experiments at lower concentration with much longer reaction times could not be carried out in our reaction chamber. Additionally, to the best of our knowledge, several previous and recent studies have shown that both the effective reaction rate constants of PAHs with NO₃ radicals and the reactive uptake coefficient of the NO₃ radical on the surface of the particulate or adsorbed PY are 5–7 orders of magnitude faster than those of adsorbed PAHs oxidized by O₃, NO₂, and N₂O₅.^{7,14,16,18,28} Thus, it can be concluded that 2-NPY might be formed via the heterogeneous reaction of the NO₃ radical and PY in the real atmosphere if the reaction time is long enough.

Results and discussion

Contrast experiments

Fig. 2A and B show the NPY distributions obtained from the two exposure orders. Similar to the results described by Ringuet et al.,²⁹ the NO₃ radical-initiated NPY isomers (2-NPY and 4-NPY) are clearly observed. However, contrary to our hypothesis, the amounts of 2-NPY and 4-NPY are significantly enhanced in the O₃–N₂O₅ exposure. Previous studies showed that 2-NP was only formed via the gas-phase reaction of PY and the NO₃ radical in the presence of higher NO₂ concentrations in the gaseous phase.³⁰ Additionally, Zimmermann et al. concluded that 1-NPY was unlikely to be formed by NO₃ radical-initiation, but rather, by nitration that occurred after the adsorption of N₂O₅. Therefore, the traditional use of 2-NP as a marker of NPAH formation in the gaseous phase seems questionable.^31,32 Our experimental results reveal that the observed 2-NPY arises mainly from the heterogeneous reaction; the contribution of gas-phase 2-NPY is negligible in the experiments. The ratio of the NO₃ radical-initiated isomers (2-NPY and 4-NPY) to 1-NPY increases from ∼37.8% in the N₂O₅–O₃ exposure to ∼80.1% in the second experiment. Given that O₃ is first introduced into the chamber, a mixture of PY particles and O₃ should be obtained within 3 min, and more reaction sites on the PY particle surfaces should be surrounded by O₃ in the O₃–N₂O₅ exposure. We thus concluded that more reactive NO₃ radicals may be formed on the particle surface via the surface reaction (R₂), and the NO₃ radical-initiated heterogeneous reaction should be promoted on the particle surface.³³

NO₂ + O₃ → NO₃ (R2)

Fig. 2 The NPY isomer distributions observed in the N₂O₅–O₃ (A) and the O₃–N₂O₅ exposure (B).

However, a rather different scenario should take place in the N₂O₅–O₃ exposure, i.e., the NO₃ radical-initiated reaction mechanism may be suppressed to some extent because the large concentrations of N₂O₅ and NO₂ introduced at first may interfere with the heterogeneous reaction between the NO₃ radical and PY by blocking NO₃ radicals from being accommodated on the surfaces and reacting. Additionally, we note that the amount of the 1-NPY isomer observed in the O₃–N₂O₅ exposure is significantly higher than that in the N₂O₅–O₃ exposure. We speculate that the higher NO₃ concentration in the O₃–N₂O₅ exposure and the larger NO₃ uptake coefficients on the PY particle surface (approximately 4–5 orders of magnitude higher than those of N₂O₅) cause a larger production of N₂O₅ on the surface of the PY particles;¹⁸ to some extent, this indirectly promotes the N₂O₅-initiated ionic electrophilic nitration mechanism. However, additional studies are needed to clarify whether the NO₃ radicals and NO₂ can form N₂O₅ near the PY surface. Based on our experimental results, a higher N₂O₅ concentration (in the real atmosphere and in the absence of O₃) can inhibit NO₃ radical-initiated heterogeneous reactions. In contrast, the higher NO₃ concentration can promote N₂O₅-initiated electrophilic nitration. Thereby, we conclude that the two reaction mechanisms should be competitive and mutually reinforce each other to some extent. The operative reaction mechanisms are highly dependent on the concentrations of the nitric oxides (N₂O₅ and NO₃ radicals) on the surface of atmosphere particles. High concentrations of N₂O₅ and NO₂ under either experimental or real atmospheric conditions can prevent NO₃ radicals from being accommodated on the surfaces and reacting, thereby suppressing the formation of the NO₃ radical-initiated isomers.

Effect of different O₃ concentrations

Increasing NO₃ radical concentrations and decreasing NO₂ concentrations were observed with increasing O₃ concentrations (Table 1). In all different O₃ concentration exposures, the heterogeneous reaction time and the mixing times between the PY particles and oxidants (O₃ and the NO_x species) were consistent (∼8 min). Fig. 3A–E show the NPY isomers distribution obtained from these exposures. The experimental results from Ringuet et al. reported that formation of 2-NPY by the heterogeneous reaction of PY with O₃/NO₂ was clearly observed.²⁹ However, this is not consistent with the results obtained in the real atmosphere, since no 2-NPY is formed in the atmospheric heterogeneous reaction of N₂O₅/NO₃/NO₂/O₃ due to a lower NO₃ concentration. This may result from the lower NO₃/N₂O₅ ratio at 298 K in the real atmosphere compared to that used in this study. The NO₃/N₂O₅ ratio at 298 K in the real atmosphere was estimated to be ∼0.006 using the following equation:where [NO₂] is the NO₂ concentration in the real atmosphere (up to ∼200 ppb in polluted air),^34–38 and K_eq represents the equilibrium constant for N₂O₅(g) ⇄ NO₂(g) + NO₃(g). In the present study, the [NO₃]/[N₂O₅] ratios (ranging between 0.018 and 0.61, Table 1) in the chamber experiments are significantly larger than those in the ambient atmosphere. It should be noted that a remarkable increase in the 2-NPY yield is observed as the initial concentration of O₃ is gradually increased (Fig. 3A–D), which further questions its use as an indicator of NPAH formation in the gaseous phase. This may be caused by the formation of more reactive NO₃ radicals, or a gradual increase in the [NO₃]/[N₂O₅] ratios, which occurs when the O₃ concentration is increased in the chamber (Table 1). In this case, the NO₃-initiated reaction mechanism may be favored in the heterogeneous reaction process over the N₂O₅-initiated ionic electrophilic nitration mechanism, since the equilibrium concentration of N₂O₅ is constant under different O₃ concentrations. We thus conclude that the blocking mechanism occurring on the particle surface in the heterogeneous process should be gradual. Similar to the observations of the discussed contrast experiments (Fig. 2), the 1-NPY isomer yield (Fig. 3A–C) also increased with the initial concentration of O₃ (from 2.5 to 13.5 ppm). This confirms that the N₂O₅-initiated ionic electrophilic nitration mechanism may be reinforced to some extent due to the higher uptake coefficient on the particle surface and the higher gas-phase concentration of NO₃ radicals. Surprisingly, the yields of 2-NPY and 1-NPY (Fig. 3E) in the presence of ∼30 ppm O₃ are significantly smaller than those shown in Fig. 3B–D. This may result from the increase in O₃ concentration and the decrease in NO₂ concentration that occur under equilibrium conditions. We suggest three possible explanations for this phenomenon. First, it should be noted that the NO₂ concentration is clearly lower than that of the NO₃ radical under equilibrium conditions when ∼30 ppm O₃ is added into the chamber (Table 1). The NO₃ radical-initiated mechanism requires both NO₃ and NO₂ to proceed (i.e., the NO₃ initiates the reaction and NO₂ is required at a later step to form the final product). Atkinson et al. also suggested that the gas-phase formation of 2-NPY in the N₂O₅/NO₃/NO₂ exposure was a function of the NO₂ concentration (with an excess of NO₂).⁴ Thereby, the lower NO₂ concentration in the chamber may suppress the corresponding NO₃-initiated reaction mechanism. Similarly, we concluded that this may also occur for the heterogeneous formation of 2-NPY. Second, the TOF mass spectra of some ozonation products of PY located at m/z = 205, 218, and 250 are clearly observed, because the heterogeneous reaction between particulate PY and O₃ may dominate when an excess of O₃ (∼30 ppm) is employed (Fig. 4).³⁹ Finally, although our previous study suggests that only trace amounts of pyrenequinone (m/z 232) are produced during the O₃-only exposure (∼30 ppm),³⁹ according to this work, significant amounts of pyrenequinone and mono-nitropyrenequinone (m/z 277), as well as 2-NPY (Fig. 3), are the main NO₃ radical-initiated products due to the occurrence of NO₃ radicals in the presence of ∼30 ppm O₃. The mono-nitropyrenequinone is believed to form from further reaction of the pyrenequinone with NO₃ radicals.

Table 1 The concentrations of NO₂, N₂O₅, O₃ and NO₃ radicals and the [NO₃]/[N₂O₅] ratio in different experiments

Exposures	Initial conc. (molecules cm^−3)		Equilibrium conc. (molecules cm^−3)
	N2O5	O3	N2O5	NO2	NO3	O3	NO3/N2O5
Contrast exposures	6.15 × 10^13	6.2 × 10^13	6.00 × 10^13	4.54 × 10^11	4.23 × 10^11	6.11 × 10^11	0.07
		5.6 × 10^14		1.82 × 10^12	1.67 × 10^11	5.90 × 10^13	0.018
		2.0 × 10^14		1.02 × 10^12	2.99 × 10^12	1.94 × 10^14	0.032
Effect	2.00 × 10^14	3.30 × 10^14	9.50 × 10^13	7.82 × 10^11	3.89 × 10^12	3.28 × 10^14	0.041
		4.90 × 10^14		6.43 × 10^11	4.74 × 10^12	4.87 × 10^14	0.05
		7.40 × 10^14		5.25 × 10^11	5.81 × 10^12	7.32 × 10^14	0.061

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Fig. 3 The NPY isomer distributions observed during the N₂O₅/NO₃/NO₂ exposure in the presence of different initial O₃ concentration ranging from 2.5 ppm to 30 ppm.

Fig. 4 TOF mass spectra of reaction products during the N₂O₅/NO₃/NO₂/O₃ exposure: ∼20 ppm O₃ (black line) and ∼30 ppm O₃ (red line).

Additionally, it has been recognized that O₃ and NO_x (NO₃ radicals, NO₂, and N₂O₅) are ubiquitous and coexistent in the atmosphere. However, most chamber studies of NO₃-derived nitro-PAHs generate NO₃ through the thermal dissociation of N₂O₅ in order to minimize the complexity caused by introducing a second oxidant.^13,18,40,41 Fewer studies have been done using the atmospherically more relevant conditions of introducing both NO_x and O₃ into the chamber to mimic this full range of nighttime oxidation chemistry.²² Thereby, the results obtained also highlight the dependence of the heterogeneous formation of NPAHs on the complicated nature of atmospheric oxidants.

Conclusions

This study clearly showed that the N₂O₅-initiated ionic electrophilic nitration mechanism and the NO₃ radical-initiated mechanism may both be operative in atmospheric heterogeneous processes. However, since the concentrations of N₂O₅ (up to ∼10 ppb), O₃ (80–150 ppb), and NO₂ (up to ∼200 ppb) in the real atmosphere are significantly higher than the concentration of NO₃ radicals (which ranges between <10 and 430 ppt),^34–38 these oxidants may block NO₃ radicals from being accommodated on the surfaces and reacting. Thus, NO₃ radical-initiated heterogeneous reactions may be suppressed on the particle surface. The phenomenon of blocking reaction sites may be ubiquitous on the surfaces of atmospheric aerosol particles and have a significant impact on the heterogeneous nitration mechanism. However, the extent to which this affects the heterogeneous reactions of PY and N₂O₅/NO₃/NO₂ in the real atmosphere due to the lower NO_x concentrations will need further investigation. Additionally, this explains why 2-NPY, an indicator for the NO₃ radical-initiated reactions of the parent PAHs, was not observed in the heterogeneous processes investigated in previous studies; our findings provide supplementary knowledge for the heterogeneous reaction mechanism. Furthermore, the blocked-sites phenomenon occurring in the heterogeneous reaction was only investigated in this study from a macroscopic perspective. To further explore the blocked-sites phenomenon in atmospheric heterogeneous processes, computer simulations and experiments, especially with respect to the surface coverage of competing reactants on particle surfaces, are needed.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21277155 and 21207143) and the Creative Research Groups of China (No. 51221892).

References

1. K. Ravindra, R. Sokhi and R. Van Grieken, Atmos. Environ., 2008, 42, 2895–2921.
2. I. W. G. o. t. E. o. C. R. t. Humans, Some non-heterocyclic polycyclic aromatic hydrocarbons and some related exposures, IARC monographs on the evaluation of carcinogenic risks to humans/World Health Organization, International Agency for Research on Cancer. 2010, vol. 92, p. 1.
3. J. Sasaki, S. M. Aschmann, E. S. Kwok, R. Atkinson and J. Arey, Environ. Sci. Technol., 1997, 31, 3173–3179.
4. R. Atkinson, J. Arey, B. Zielinska and S. M. Aschmann, Int. J. Chem. Kinet., 1990, 22, 999–1014.
5. J. Arey, B. Zielinska, R. Atkinson, A. M. Winer, T. Ramdahl and J. N. Pitts Jr, Atmos. Environ., 1986, 20, 2339–2345.
6. R. Atkinson, J. Arey, B. Zielinska and S. M. Aschmann, Environ. Sci. Technol., 1987, 21, 1014–1022.
7. M. L. Nguyen, Y. Bedjanian and A. Guilloteau, J. Atmos. Chem., 2009, 62, 139–150.
8. J. Ma, Y. Liu and H. He, Atmos. Environ., 2011, 45, 917–924.
9. W. Esteve, H. Budzinski and E. Villenave, Atmos. Environ., 2004, 38, 6063–6072.
10. W. Esteve, H. Budzinski and E. Villenave, Atmos. Environ., 2006, 40, 201–211.
11. K. Miet, H. Budzinski and E. Villenave, Polycyclic Aromat. Compd., 2009, 29, 267–281.
12. R. M. Kamens, J. Guo, Z. Guo and S. R. McDow, Atmos. Environ., Part A, 1990, 24, 1161–1173.
13. Y. Zhang, B. Yang, J. Gan, C. Liu, X. Shu and J. Shu, Atmos. Environ., 2011, 45, 2515–2521.
14. C. Liu, P. Zhang, B. Yang, Y. Wang and J. Shu, Environ. Sci. Technol., 2012, 46, 7575–7580.
15. E. Perraudin, H. Budzinski and E. Villenave, J. Atmos. Chem., 2007, 56, 57–82.
16. K. Miet, K. Le Menach, P. Flaud, H. Budzinski and E. Villenave, Atmos. Environ., 2009, 43, 3699–3707.
17. M. Shiraiwa, R. M. Garland and U. Pöschl, Atmos. Chem. Phys., 2009, 9, 9571–9586.
18. S. Gross and A. K. Bertram, J. Phys. Chem. A, 2008, 112, 3104–3113.
19. K. Zimmermann, N. Jariyasopit, S. L. Massey Simonich, S. Tao, R. Atkinson and J. Arey, Environ. Sci. Technol., 2013, 47, 8434–8442.
20. N. O. A. Kwamena and J. P. D. Abbatt, Atmos. Environ., 2008, 42, 8309–8314.
21. J. Mak, S. Gross and A. K. Bertram, Geophys. Res. Lett., 2007, 34, L10804.
22. J. Ringuet, A. Albinet, E. Leoz-Garziandia, H. Budzinski and E. Villenave, Atmos. Environ., 2012, 61, 15–22.
23. B. Yang, J. Meng, Y. Zhang, C. Liu, J. Gan and J. Shu, Atmos. Environ., 2011, 45, 2074–2079.
24. J. Shu, S. Gao and Y. Li, Aerosol Sci. Technol., 2008, 42, 110–113.
25. C. Liu, J. Gan, Y. Zhang, M. Liang, X. Shu, J. Shu and B. Yang, J. Phys. Chem. A, 2011, 115, 10744–10748.
26. P. Zhang, Y. Wang, B. Yang, C. Liu and J. Shu, Chemosphere, 2014, 99, 34–40.
27. P. Zhang, W. Sun, N. Li, Y. Wang, J. Shu, B. Yang and L. Dong, Environ. Sci. Technol., 2014, 48, 13130–13137.
28. R. M. Kamens, J. Guo, Z. Guo and S. R. McDow, Atmos. Environ., Part A, 1990, 24, 1161–1173.
29. J. Ringuet, A. Albinet, E. Leoz-Garziandia, H. Budzinski and E. Villenave, Atmos. Environ., 2012, 61, 15–22.
30. J. N. Pitts Jr, J. A. Sweetman, B. Zielinska, A. M. Winer and R. Atkinson, Atmos. Environ., 1985, 19, 1601–1608.
31. A. Albinet, E. Leoz-Garziandia, H. Budzinski and E. ViIlenave, Sci. Total Environ., 2007, 384, 280–292.
32. A. Albinet, E. Leoz-Garziandia, H. Budzinski, E. Villenave and J.-L. Jaffrezo, Atmos. Environ., 2008, 42, 43–54.
33. U. Pöschl, Y. Rudich and M. Ammann, Atmos. Chem. Phys., 2007, 7, 5989–6023.
34. M. Aldener, S. S. Brown, H. Stark, E. J. Williams, B. M. Lerner, W. C. Kuster, P. D. Goldan, P. K. Quinn, T. S. Bates and F. C. Fehsenfeld, J. Geophys. Res.: Atmos., 2006, 111.
35. U. Platt, D. Perner, A. M. Winer, G. W. Harris and J. N. Pitts, Geophys. Res. Lett., 1980, 7, 89–92.
36. J. Stutz, B. Alicke, R. Ackermann, A. Geyer, A. White and E. Williams, J. Geophys. Res., 2004, 109, D12306.
37. S. Brown, Science, 2006, 1120120, 311.
38. D. A. Knopf, S. M. Forrester and J. H. Slade, Phys. Chem. Chem. Phys., 2011, 13, 21050–21062.
39. S. Gao, Y. Zhang, J. Meng and J. Shu, Atmos. Environ., 2009, 43, 3319–3325.
40. Y. Zhang, B. Yang, J. Meng, S. Gao, X. Dong and J. Shu, Atmos. Environ., 2010, 44, 697–702.
41. N. Jariyasopit, M. McIntosh, K. Zimmermann, J. Arey, R. Atkinson, P. H.-Y. Cheong, R. G. Carter, T.-W. Yu, R. H. Dashwood and S. L. Massey Simonich, Environ. Sci. Technol., 2013, 48, 412–419.

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Footnote

† Electronic supplementary information (ESI) available: Experimental section. Estimation of oxidants concentrations in the new equilibrium system. See DOI: 10.1039/c5ra24368h

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