Particulate nitrate photolysis in the atmosphere

Multiphase and heterogeneous photochemistry is an emerging component of atmospheric and air pollution research. It is primarily driven by reactions of photochemically produced free radicals in the particle phase with dissolved gaseous species. It has signi ﬁ cant implications to promote the oxidation of aerosol particles, one of the most important atmospheric processes for secondary inorganic and organic aerosol formation. Nitrate is an increasingly important component in atmospheric aerosol particles with the trend of dominating over sulfate. Nitrate photolysis has long been known to produce highly reactive oxidants such as hydroxyl radicals in both gas and bulk or cloud phases. Recent studies have found that nitrate photolysis in the particle phase ( i.e. , particulate nitrate photolysis) proceeds faster than bulk solutions or cloud droplets by many orders of magnitude. Factors and mechanisms a ﬀ ecting particulate nitrate photolysis include the formation of solvent cages, pH, and co-existing species, but they remain controversial. Hence, the impact of nitrate photolysis in atmospheric chemistry is still uncertain. This paper reviews the current status of knowledge about the e ﬀ ects of particulate nitrate photolysis, instead of relatively well-known gas-and bulk-phase nitrate photolysis, in the atmosphere. Recommendations for future research directions on the mechanistic understanding of particulate nitrate photolysis and its parameterizations in air quality models are also made.


Introduction
Atmospheric particulate matter (PM) or aerosol particles have signicant impacts on climate, regional air quality, and human health. 1 PM is emitted from many diverse anthropogenic and biogenic sources. 2 During its atmospheric lifetime ($a week), PM is subjected to many processes leading to physical and chemical transformations such as changes in its size, morphology, and chemical composition. 3The Earth's atmosphere is a giant and strongly oxidizing chemical reactor, and hence the atmospheric oxidizing capacity is closely associated with the evolution of PM's composition and properties.Despite the crucial importance of PM in the atmospheric environment, our understanding of the physical and chemical transformation of PM is far from complete.In this paper, we will use the terms PM and aerosol particles interchangeably.
Sunlight, especially in the ultraviolet spectral region, is a source of atmospheric free radicals that drive the chemical changes in the atmosphere. 2 The sunlight reaching low altitudes has a wavelength longer than 290 nm. 4 One of the essential atmospheric photochemical processes is the generation of free radicals through gas-phase photochemistry. 5,6The gas-phase reactions are crucial in ozone depletion in the stratosphere and tropospheric oxidant production and organic oxidation, relevant to the abundance of climate forcing agents. 7owever, the chemistry occurring within or on aerosol particles and cloud droplets is much less known.In this paper, we will focus on the photochemical processes of PM or aerosol particles.Solar radiation gives the energy to initiate photochemical reactions of aerosol particles and gaseous species.Multiphase and heterogenous photochemistry is an emerging eld in atmospheric and air pollution research, and it has the potential to promote atmospheric oxidation greatly. 8he importance of multiphase and heterogeneous photochemistry in the atmosphere has been demonstrated in many laboratory studies.For example, multiphase photolysis of aerosols containing a trace amount of photosensitive compounds (e.g., humic acid) produces strong oxidants (e.g., superoxide, hydroxyl, nitrate, and organic radicals) in the particle phase. 9These in-particle oxidants can lead to fast uptake of non-condensable volatile organic compounds (VOCs) of limonene and isoprene without gas-phase oxidation.That study challenges the traditional view that such noncondensable VOCs need to be oxidized in the gas phase before partitioning into the particle phase to form secondary organic aerosol. 3,10Another example is the reactive uptake of sulfur dioxide (SO 2 ) by irradiated particulate nitrate.Nitrate photolysis can produce in-particle hydroxyl (OH), nitrogen dioxide (NO 2 ) radicals, and nitrite, promoting the oxidation of dissolved SO 2 in the particle phase for sulfate production. 11,12his photochemistry can potentially reconcile the difference between eld measurements and model estimations of sulfate formation during highly polluted episodes in China. 13Thus, multiphase and heterogenous photochemistry has signicant implications in atmospheric chemistry.
Among the photolytic sources of strong oxidants such as irradiated mineral dust, 14,15 iron-organic complexes, 16,17 nitrate/ nitrite, 18,19 hydrogen peroxide, 20 and hypochlorous acid, 21,22 inorganic nitrate anion (NO 3 À ) is an increasingly important component in atmospheric aerosol particles as sulfate concentrations decrease.Sulfate was the dominant inorganic constituent in atmospheric aerosol particles and is mainly formed from SO 2 oxidation.SO 2 emission has reduced globally while there is a modest increase in ammonia emission due to intensied agricultural activity livestock farming following population growth. 23or instance, SO 2 emissions in China have decreased by 75% since 2007, while India is surpassing China as the world's largest emitter of anthropogenic SO 2 . 24Across the United States, SO 2 emissions have decreased at $6% per year from 2001 to 2010. 25 This SO 2 reduction elevates aerosol pH and facilitates nitrate partitioning into the aerosol phase, leading to the growing nitrate dominance over sulfate.][31][32] Inorganic nitrate photolysis 18,33 has an inuence on NO x , OH, and O 3 mass burdens in the atmosphere. 34It can be also used as a photolytic source of OH radicals to remove hazardous organic pollutants in environmental waters in advanced oxidation technologies. 35,36The mechanism of nitrate photolysis has been extensively studied in bulk solutions, 18,33 but that of particulate nitrate photochemistry is not fully assessed despite increasing evidence of the nitrate-dominated aerosols as mentioned above.8][39][40] Several factors/mechanisms affecting particulate nitrate photolysis such as solvent cages, 41 pH, 42 and coexisting species 43 have been suggested in the literature, but they remain controversial.As a result, the impact of nitrate photolysis in atmospheric chemistry is still uncertain.Similarly, organic nitrates play important roles in the atmosphere because their fate including photolysis could affect the NO x recycling and O 3 production. 44,45Photolysis of organic nitrates can produce NO 2 and HO 2 , 46 which may have impacts on subsequent reactions in the particle phase. 47However, there are very few studies on photolysis of particulate organic nitrates, and thus we limit our focus to that of particulate inorganic nitrate.
This paper reviews the current status of knowledge about the effects of particulate inorganic nitrate photolysis, instead of relatively well-known gas-and bulk-phase nitrate photolysis, in the atmosphere.We begin with an overview of nitrate photolysis mechanisms, followed by discussing factors affecting the product yields of particulate nitrate photolysis.We then review the quantum yields and nitrate photolysis rate constants reported in the literature for quantifying the impacts of nitrate photolysis in the atmosphere and used in air quality models.Finally, we summarize chemical reactions related to nitrate photolysis in the particle phase.Particulate nitrate photolysis generates gas-phase oxidants such as NO 2 and HONO, which can tremendously affect ozone and halogen chemistry. 34owever, the gas-phase reactions are not addressed in this review.Critical issues and recommendations for future research directions will be presented at the end of this review paper.

Nitrate photolysis mechanism
Nitrate anion (NO 3 À ) is a crucial chromophore in environmental waters.Nitrate can be photolyzed in both aqueous 48 and crystalline states. 49In this section, we briey review nitrate photolysis mechanisms relevant to the atmosphere.For more detailed mechanisms of nitrate photolysis, we refer readers to the previous reviews. 18,33s a result of the absorption of UV photons, electrons of nitrate anions can move from their ground state to an unoccupied or partially occupied molecular orbital of higher energy.Because the energy level of UV light is of the same order as the enthalpies of the covalent bonds, this additional energy results in a bond cleavage, which splits the excited nitrate, [NO 3 À ]*, into two fragments, a process known as photolysis (R1).Nitrate photolysis produces oxidants such as hydroxyl (OH) and nitrogen dioxide ) ((R2) and (R3); Fig. 1). 50Furthermore, [NO 3 À ]* can isomerize (R4).These photoproducts can have potentially signicant impacts on subsequent reactions in aerosol particles, as will be discussed in Section 5.The absorption spectrum of NO 3 À is dominant by a weak n / p* band around 302 nm (3 ¼ 7.2 M À1 cm À1 ) and a much stronger p / p* band at 200 nm (3 ¼ 9900 M À1 cm À1 ). 19,48Excitation in the n / p* band (l > 280 nm) mainly proceeds through (R2) and (R3), whereas excitation in the p / p* band (l < 280 nm) proceeds via the two primary photo-processes (R2) and (R4). 19 3 À !hv ½NO 3 À * (R1) Under atmospherically relevant irradiation (i.e., >300 nm), (R2) and (R3) are more relevant, and (R4) is negligible at >280 nm.Goldstein et al. reported the quantum yield of ONOO À is lower than 0.2% under 300 nm illumination. 19The pK a (O À / OH) is around 12, and the primary fragment O À is immediately protonated in water to yield OH radicals. 33OH radical is also a precursor for H 2 O 2 , another important oxidant in the atmosphere.However, H 2 O 2 has not been detected as a signicant photoproduct from nitrate photolysis at l > 200 nm, likely due to the very low concentration and short lifetime of OH. 18 (R3) is a potentially important source of nitrite (NO 2

À
).  À in Fig. 1), following subsequent reactions (Table 1).The photolysis quantum yield of (R2) Presence of organic compounds (Org.) for OH production is 1.35 AE 0.3% at 298 K under 302 nm irradiation. 20,36,51The quantum yield of (R3) for NO 2 À production is 1.1 AE 0.2% under 313 nm irradiation at pH > 5, 50 indicating that these two channels ((R2) and (R3)) may be of comparable importance in nitrate photolysis.These photolysis quantum yields are affected by many factors that will be discussed in Section 3.

Factors affecting particulate nitrate photolysis
While nitrate photolysis has been investigated for decades in laboratory experiments, theoretical studies, and eld measurements, understanding how physical and environmental conditions affect particulate nitrate photolysis is far from complete.
Most of the previous experimental works were performed in diluted bulk solutions, which are useful to reveal the chemistry in aqueous solutions such as in cloud droplets.However, reactions in atmospheric aerosol particles with signicantly higher nitrate concentrations and surface area to volume ratios can differ from those in bulk solutions.In this section, we discuss how various factors alter particulate nitrate photochemistry.

Bulk versus interface: solvent cage and surface propensity
The absorption cross section of NO 3 À in aqueous solution at 310 nm is 25 times that of gas phase HNO 3 due to symmetry breaking of NO 3 À by hydration. 57However, the water molecules surrounding NO 3 À form a solvent cage and retard nitrate photolysis.Specically, the fragments generated from nitrate photolysis are initially surrounded by a cage of solvent (water) molecules.Their diffusion out of the cage competes with the regeneration of nitrate anions by recombining the fragments.The recombination accounts for the reduced quantum yields of nitrate photolysis in the aqueous phase compared to the gas phase. 41he solvent cages near the air-water interface are less complete.As a result, the recombination processes are inhibited, increasing the quantum yield of nitrate photolysis.Aerosol particles have a much larger surface area to volume ratio than bulk solutions. 8Hence, the relative contribution of interface reactions to the overall reaction can be higher for smaller particles. 585][66][67] This is still a controversial topic.Therefore, more work on the surface propensity of nitrate in aerosol particles should be warranted in the future.Our experimental results showed that the surface propensity of nitrate can be enhanced by the presence of co-existing species (i.e., halide ions), 37 which will be covered later.

pH
Aerosol acidity plays a critical role in atmospheric processes. 68It affects chemical compositions, gas-particle partitioning, and toxicity 68 through various oxidation reactions, either directly or indirectly. 69,70Here we refer acidity to as the activity of hydrogen ions or pH of aerosol particles.The inherent nitrate photolysis rate constant is not pH-dependent, and the molar light absorptivity of nitrate in aqueous solution is not sensitive to pH in the range of 2 to 6. 42 The rate of NO 2 production from nitrate thin-lm photolysis is pH-independent under the atmospherically relevant pH range (pH ¼ 0.5-6). 71OH production from nitrate photolysis requires the protonation of O À , but it is also insensitive at pH < 9, 20 due to its high pK a (O À /OH) of 12. 72 Nonetheless, pH has signicant impacts on the effective quantum yield for production of NO 2 À /HNO 2 , a particular category of products with emerging oxidative potential. 11,50ince the pK a of HNO 2 is around 3, the speciation of NO measured nitrite production and found that its quantum yield increases with pH and remains constant at pH higher than 4.5. 50Thus, aerosol pH is the determining factor in the distribution of NO 2 À /HNO 2 in the gas phase or aqueous phase.

RH
Nitrate is hygroscopic, and the amount of water uptake is sensitive to the counter cation, particle size (Kelvin effect), and relative humidity (RH). 78Particularly, RH is an important parameter to determine the phase of nitrate-containing particles. 79In this section, we cover RH effects on nitrate photolysis in aqueous solutions or droplets followed by those in the solid phase.
In general, lower RH increases nitrate concentration in aqueous particles due to reduced liquid water content in the particles, leading to higher nitrate photolysis rates.However, the quantum yield of nitrate photolysis also depends on nitrate concentration.Concentrated nitrate solutions have lower quantum yields for nitrite production than diluted ones have. 80t 310 nm irradiation and pH ¼ 4, the quantum yield of calcium nitrate solution for nitrite production signicantly decreases from (1.4 AE 0.1) Â 10 À2 to (4.2 AE 0.3) Â 10 À3 as nitrate concentration increases from 0.01 to 15 M. In contrast, a similar decrease was not observed in sodium nitrate solutions.This $30 folds decrease in the nitrite quantum yield of calcium nitrate solution is attributable to the blue shi of the n-p* absorption (i.e., away from actinic wavelengths) with increasing nitrate concentration.Specically, the absorption peak of calcium nitrate solutions blue-shis from 302 to 294 and 289 nm as the concentration increases from 0.01 to 6.0 and 14.9 M, respectively.In contrast, the blue shi for sodium nitrate solution was minimal: from 303 nm at 0.01 M to only 301 nm at 6.2 M. 80,81 In summary, higher nitrate concentration reduces the quantum yield due to the blue shi and therefore the photolysis rate constant at a given photon ux.However, it may also increase the overall nitrate photolysis rate, which is the product of nitrate concentration and nitrate photolysis rate constant.
At low RH, nitrate can exist as crystalline solids.The quantum yields of solid-phase nitrate photolysis are four orders of magnitude lower than those of aqueous phase one. 82Nonetheless, emissions of gaseous NO 2 at >1 ppb min À1 83 and HONO at < 0.06 ppt min À1 84 are possible from solid-phase nitrate photolysis when thin water lms or so-called surface adsorbed water (SAW) are present on the solid surface.While the exact role of SAW in nitrate photolysis remains poorly understood, the presence of SAW likely promotes the dissolution of solid nitrate to form aqueous nitrate in SAW.Because RH regulates the amount of SAW, 85 the formation of nitrate photolysis products is expected to be RH-sensitive.The increased amount of SAW might increase the availability of nitrate in its dissociated form and then enhance the quantum yields.

Ice and snow
Nitrate photolysis on ice and snow has signicant implications. 49,90,91Nitrate can be embedded in snow pack and ice via deposition, heterogeneous dissolution of HNO 3 (g), and freezing the water contained nitrate (e.g.sea and lake).Gaseous products from nitrate photolysis are observed in much higher quantities from snow packs than that from aqueous solutions. 91,92A comprehensive review on nitrate photolysis in ice/snow is available elsewhere. 93Here we briey introduce the main features of nitrate photolysis on ice and snow.Similar to the situation in the aqueous phase, nitrate photolysis on ice and snow proceeds faster at the air-snow interface than that in the bulk. 88,92In addition to the partial solvation at the interface which allows gas phase products to easily escape, intramolecular geometrical distortion of nitrate anions at interface resulting in an increase in the absorption cross section of nitrate enhances nitrate photolysis. 92In snow, nitrate photolysis likely occurs in the liquid like region on the surface of ice grains, or in cracks between ice grains. 94Highly variable quantum yields of nitrate photolysis on ice and snow are reported because they are strongly inuenced by the location of nitrate anions in an ice grain, 95,96 and the co-existing species (e.g., Cl À ). 97Meusinger et al. have proposed two photochemical domains of nitrate photolysis: photolabile nitrate anion and nitrate anion buried within the ice grain. 95Photoproducts produced from photolabile nitrate anion can escape the ice grain and hence the quantum yields are higher than those of nitrate anion buried within the ice grain.In contrast, photoproducts from buried nitrate anions are likely to undergo a recombination reaction to regenerate nitrate anion (Fig. 1).

Effect of co-existing chemical species
Atmospheric aerosol particles consist of a myriad of different components, with diverse spatial differences, temporal variations, and distinctive source dependence. 10,98Most laboratory studies on nitrate photolysis have used only nitrate salts without other atmospherically relevant species.The presence of co-existing chemical species has been reported to inuence aqueous nitrate photolysis through (1) affecting the solvent cage effects, (2) participating in chemical reactions directly or indirectly, and (3) regulating nitrate concentration because of their hygroscopic properties determining the liquid water content.Because the last factor is reasonably well covered in the wealth of literature, 79,99 we will focus on the rst two issues in this review.
3.6.1 Halides.A profound effect of inorganics on nitrate photolysis is the surface propensity of nitrate anions promoted by the coexistence of halide ions. 37,71,100,1013][104] The presence of halide ions can lead to a preferential distribution of nitrate anions at the air-liquid interface due to the formation of a double layer of interfacial halide ions and subsurface cations that further attract nitrate anions. 43,62The surface nitrate anions have incomplete solvent cages or the reduced solvent cage effect (Section 3.1), which gives rise to enhanced production of OH, NO, NO 2 , and NO 2 À /HONO from nitrate photolysis. 43,71,100,1016][107] Nitrate are oen internally mixed with chloride ions in the atmosphere through the chloride depletion reactions of sea spray particles.][115][116][117] Wingen et al. reported that the coexistence of chloride ions results in an enhanced gaseous NO 2 production from deliquesced nitrate aerosol particles under illumination by a factor of 1.6 to 2.4. 43Zhang et al. found that the particulate nitrate photolysis rate constant increases by a factor of 2.0, 1.7, and 2.1 in the presence of Cl À , Br À , and I À , respectively, leading to enhanced sulfate production from heterogenous oxidation of SO 2 by a factor of 1.4, 1.3, and 2.0. 37A linear relation was found between the nitrate photolysis rate constant, j NO 3 À, and the initial molar ratio of Cl À to NO 3 No further enhancement of nitrate photolysis rate constant was observed when [Cl À ] 0 /[NO 3 À ] > 0.2, where j NO 3 À can be considered the same as that at [Cl À ] 0 /[NO 3 À ] 0 ¼ 0.2.Compared with chloride ions, bromide and iodide ions have higher intrinsic surface propensities, 102-104 and therefore their potential impacts on the enhanced nitrate photolysis on a per molecule of halide basis are expected to be comparable or greater.However, the concentrations of bromide and iodide ions are many orders of magnitude lower than that of chloride ([Cl À ] : [Br À ] : [I À ] ¼ 1 000 000 : $1515 : 1), making them insignicant in enhancing nitrate photolysis in typical tropospheric environments. 118-1203.6.2Cations.While cations do not have a pronounced effect on nitrate photolysis in the bulk phase, 50 they can inuence nitrate photolysis in thin lms. 83Richards et al. found that thin lms ($800 nm) of RbNO 3 and KNO 3 produce more gaseous NO 2 than those of Mg(NO 3 ) 2 and NaNO 3 , and Ca(NO 3 ) 2 . 83Molecular dynamics simulations suggested that cations can regulate the surface propensity of nitrate anions. 66,101For instance, the concentration of nitrate anion in the interface region of 2 M KNO 3 thin lm could be ten times higher than that of 2 M NaNO 3 .On the other hand, the formation of contact ion pairings between cation and nitrate anion 121 can reduce the quantum yields. 62For example, Mg(NO 3 ) 2 solution produces NO 2 three times faster than Ca(NO 3 ) 2 solution because it has 50% more free nitrate at the interface, probably due to less contact ion pairings. 83Furthermore, a recent computational study suggests that the ion pairs between cations and nitrate in an aqueous solution can also change the molar absorption coefficient, which would affect the nitrate photolysis rate. 122.6.3Organics.Organic compounds affect the formation of NO 2 À /HONO (N(III)), NO 2 , and OH radicals during nitrate photolysis through three types of chemical reactions: Hdonation, photosensitization, and OH scavenging.H-donation reaction directly transfers hydrogen from organic H-donors, such as organic acids and polyols, to NO 2 to form N(III). 123 On the other hand, photosensitization triggered by light-absorbing organic species, such as aromatic carbonyls and humic-like substances, can indirectly convert NO 2 to HONO.The lightabsorbing organics or photosensitizers absorb light and transfer from their ground state to the singlet excited state.Some molecules (e.g., aromatic carbonyl) at the singlet excited state will be converted to the triplet excited state.The triplet excited state of organic species has a longer lifetime, allowing for interactions/reactions with H donors to form ketyl radicals, which can react with NO 2 to yield HONO. [124][125][126][127] The H-donation reaction and the photosensitization can enhance production rates of photoproducts during nitrate photolysis.For example, Yang et al. reported that the gaseous HONO emission from irradiated thin lms containing nitrate and humic acid reached 16 ppt h À1 , whereas the upper limit without humic acids was just 3.6 ppt h À1 .84 Ye et al. premixed HNO 3 solutions with organic acids, polyols, and aromatic compounds and found that the co-existing organics can enhance the photolysis rate constant of HNO 3 adsorbed on Pyrex glass surface by up to one order of magnitude via H-donation reactions and photosensitization.123 Furthermore, our latest work reported the enhanced nitration (Section 5.3) of vanillin by increased NO 2 formation from nitrate photolysis.The increased NO 2 formation results from the reaction of nitrite with superoxide and OH radicals produced from photosensitizing reactions of vanillin.135 Organic compounds are highly reactive toward OH radicals and increase the effective quantum yields for NO 2 production by suppressing the NO 2 consuming reaction between NO 2 and OH radicals.50 Scavenging of OH radicals can also increase the quantum yields for N(III) production in two ways. Firtly, organic scavengers reduce N(III) oxidation loss by OH radicals by consuming them.Second, some organics such as ethylene glycol and glyoxal react with OH radicals to form O 2 À /HO 2 radicals, which can further lead to secondary formation of N(III) from the NO 2 + O 2 À /HO 2 reaction.11,42,128 It is found that HONO emissions from nitrate photolysis are enhanced by dissolved aliphatic organic matter through enhanced production of superoxide.128 Wang et al. demonstrated the importance of solvated electrons produced from photosensitizing reactions in enhanced nitrite production from nitrate photolysis.158 They suggested that the solvated electrons are mainly scavenged by nitrate, leading to more NO 2 production for further conversion to nitrite.
Highly viscous organic materials could hinder reactions in the particle phase.Liang et al. examined nitrate photolysis in mixed sucrose-nitrate-sulfate particles as a proxy of viscous aerosol particles. 129They found the suppressed nitrate crystallization by the presence of sucrose and the high photolysis rate constants ($10 À5 s À1 ), irrespective of the RH.They observed the formation of enlarged hollow semisolid particles at high sucrose content and low RH, likely due to the release of gaseous species like NO 2 /HONO pushing the viscous materials radially outward.Thus, particulate nitrate photolysis may affect the microphysics of aerosol particles.

Mie resonances of droplets
Light intensity is a crucial parameter in determining nitrate photolysis rates.In the photochemistry of micrometer-sized spherical droplets, the actinic ux in the droplet can be enhanced due to (i) the Mie resonances, also known as the whispering gallery mode resonances, or the morphologydependent resonances (MDRs), and (ii) the increased light pathlengths in the droplets. 41,130,131MDRs, characterized in terms of the size parameter (i.e., particle diameter Â p/wavelength), have been studied in the physical and chemical characterization of aerosols, especially in laboratory studies including elastic scattering, uorescence, and Raman spectroscopy. 132Although MDRs can yield the orders of magnitudes increase in the internal actinic ux, their contribution to the actinic ux enhancement is not profound when averaged over typical droplet size distributions. 130,133Under broadband solar irradiation (290-600 nm), MDRs and the increased light pathlengths in $2 mm droplet can produce a $2-fold intensity enhancement (2.06 in 1-decene; 1.76 in pure water) in spherical aqueous droplets relative to bulk-liquid solutions. 130,131owever, the role of Mie resonances in enhancing nitrate photolysis has not been experimentally ascertained.

Mineral dusts
Mineral dusts are one of the most signicant contributors to aerosol mass, with an estimated annual emission of 1000-3000 Tg. 134 Recent work reported a synergistic effect of iron-organic complexes and nitrate photolysis in nitrite/nitrous acid generation. 159Previous studies explored nitrate photolysis on the surface of mineral oxides: (1) non-photoactive oxides (NPO; e.g., Al 2 O 3 , SiO 2 ) and (2) photoactive semiconductive oxides (PSO; e.g., TiO 2 ).7][138] Spectroscopic analysis revealed that the interactions between HNO 3 /nitrate and reactive surface sites could distort the molecular structure of HNO 3 /nitrate, which results in a red shi in n / p* absorption and an increase of light absorption cross section relative to gas-phase HNO 3 . 136,139,140dditionally, PSO have excellent photocatalytic capacity via an electron-hole conductive mechanism. 8,14In ambient environment, the adsorbed oxygen (O 2 ) accepts an electron to produce highly reactive O 2 À , facilitating nitrate adsorption and subsequent photoreactions. 8hile nitrate photochemistry on oxide surfaces has been widely investigated, aluminosilicates, which can account for >70% of dust mass, are rarely explored. 141Using NaY zeolite as a model system of aluminosilicates, Gankanda and Grassian found that its photoactivity may be signicantly different from the non-photoactive and photoactive oxides. 142,143N(III) produced from nitrate photolysis can stably exist as the primary product inside the zeolite cage during nitrate photolysis, whereas nitrate photolysis on oxide surfaces mainly produces gaseous NO 2 .Hence, porous materials in mineral dust can potentially act as a platform for producing daytime gaseous HONO.

Quantum yields and photolysis rate constants
The impacts of particulate nitrate photolysis in atmospheric chemistry rely highly on its quantum yields or photolysis rate constants.They are the parameters required for implementing particulate nitrate photolysis mechanisms in air quality modeling. 13They are relatively well constrained for the gas phase and aqueous (bulk) phase photolysis, 6,144 but not for particulate nitrate.This section summarizes the reported quantum yields and photolysis rate constants of nitrate (Tables 2 and 3, respectively) to discuss the current understanding of nitrate photolysis rate constants.

Nitrate photolysis rate constants of ambient aerosol particles
Most works reported that nitrate photolysis rate constants of ambient particles ranged from 10 À5 to 10 À4 s À1 , 38,145,146 which are 100-1000 times that of aqueous solution and gaseous HNO 3 . 18 America.They reported the mean values of 6.1 (AE4.2) Â 10 À5 s À1 for samples collected in Albany, NY (urban area), 1.5 (AE1.2) Â 10 À4 s À1 in Delmar, NY (rural area), 2.3 (AE2.4)Â 10 À4 s À1 from Whiteface Mountain summit (remote area), and 1.9 (AE1.2) Â 10 À4 s À1 from ight sampling. 38Bao et al. reported the nitrate photolysis rate constants of ambient particles sampled in Beijing from 1.2 Â 10 À5 to 4.8 Â 10 À4 s À1 . 145In contrast, Romer suggested the particulate nitrate photolysis rate constants of 7 Â 10 À6 to 2.1 Â 10 À5 s À1 , 10-30 times higher than that of gasphase HNO 3 , based on the aircra observations over South Korea. 146On the other hand, Shi et al. found a limited role in the photolysis of particulate nitrate for gaseous NO x and HONO production. 39Nitrate photolysis rate constants on building material surfaces, plant leaf surfaces, and urban grime have been reported to be 6.0 (AE5.3)Â 10 À5 s À1 , 6.0 (AE8.7)Â 10 À5 s À1 and 1.2 Â 10 À3 s À1 , respectively. 144,147,148Furthermore, Laufs and Kleffmann reported a very low HNO 3 photolysis rate constant on quartz surfaces for HONO formation, implying the negligible contribution of nitrate photolysis to the daytime HONO sources. 149The contradictory results related to HNO 3 surface photolysis were also reported, highlighting the importance of HNO 3 coverage on solid surfaces in the absence versus in the presence of water vapor. 150Thus, the rate constants in the atmosphere are highly variable and uncertain.

Estimation of particulate nitrate photolysis rate constant
There is a growing body of research on the enhancement of particulate nitrate photolysis.Accurate estimation of nitrate photolysis rate constant is key to quantifying atmospheric relevance of enhanced particulate nitrate photolysis.The particulate nitrate photolysis rate constant, j pNO 3 À, is a rst order decay rate constant: One way to measure j pNO 3 À is to quantify the decay of nitrate. 148However, it is challenging due to its small value of reported j pNO 3 À: 10 À6 $ 10 À4 s À1 .For instance, it takes about 12 days to see the nitrate decrease by 1 M at j pNO 3 À of 10 À6 s À1 .Hence, studies on the direct measurements of nitrate decay are scarce (Table 3).In addition, this estimation may be complicated by regeneration reactions of nitrate during nitrate photolysis (Table 1), but the effect of the regeneration reaction (OH + NO 2 ) on the quantication of j pNO 3 À would be minimized in the presence of OH scavengers.Another estimation method measures the gas phase photoproducts of nitrate photolysis such as NO 2 , and HONO, 39 assuming low concentrations in the particle phase given their low Henry's law constants: 151 where P i,gas and N nitrate are the production rate of a given gaseous photoproduct generated from nitrate photolysis and the amount of particulate nitrate exposed to light, respectively.As shown in Table 3, the measurements of gaseous photoproducts have been used to estimate j pNO 3 À in most studies.Gaseous NO 2 and HONO are the main target photoproducts.However, estimations based on the gaseous photoproduct measurements may underestimate j pNO 3 À because they do not include production rates of in-particle NO 2 and NO 2 À /HNO 2 .
Gaseous photoproducts are generated only when in-particle photoproducts partition into the gas-phase.The in-particle photoproducts are subjected to secondary reactions in the particle phase due to the presence of many reactive species, as will be described in Section 5.If the secondary reactions in the particle phase are fast, the photoproducts can be almost entirely consumed before leaving the particle phase into the gas phase, leading to no or low production of gaseous photoproducts and underestimation of j pNO 3 À .To the best of our knowledge, there are no simultaneous measurements of gas and particle-phase photoproducts to better constrain j pNO 3 À .UV irradiance uctuates daily and seasonally as a function of latitude, solar zenith angle, cloud cover, and stratospheric ozone and particle concentrations. 144j pNO 3 À is related to the wavelength-dependent photon uxes received by particulate nitrate, I pNO 3 À(l), the molar absorptivity, 3 NO 3 À(l), and the quantum yield, f NO 3 À (l): where N A is the Avogadro's number.Values of 3 NO 3 À(l) are well known. 86In contrast, the photon uxes and quantum yields for particulate nitrate can be signicantly different from those for bulk nitrate solutions, as discussed in Section 3. Table 2 lists the reported quantum yields, and they are less variable (0.25-1.7) than j pNO 3 À , because only studies of quantum yield from bulk solutions are available.The values of j pNO 3 À are highly variable (Table 3), partly because of various light sources with different wavelengths and intensities used in earlier work in addition to the complicated processes of particulate nitrate photolysis (Section 3).For better comparison of the experimentally determined j pNO 3 À among studies, it can be normalized to that under the typical tropical summer conditions on the ground using the following equation: 40 where j N pNO3 À is the j pNO 3 À normalized to the typical tropical summer condition; j nitrate,0 , and j nitrate are the photolysis rate constants of an aqueous solution under the typical tropical summer conditions and that exposed to the experimental photon uxes, respectively.A value of 3.0 Â 10 À7 s À1 can be used for j nitrate,0 . 6Reporting j N pNO3 À based on j pNO 3 À is recommended for a quantitative comparison of experimental results obtained under different conditions.
Besides nitrate, many other light-absorbing species such as black carbon 152 and brown carbon 153 exist, and they can be internally mixed with nitrate in atmospheric particles. 154,155The incident photon ux, I(l), is then absorbed by nitrate as well as those light-absorbing species in the particle phase.The spectral photon uxes absorbed by a component (i), I ai (l), in lightabsorbing multicomponent mixtures is written by where 3 i (l) and c i are the wavelength-dependent molar absorptivity and the concentration of species i, respectively; l and T(l) are the light path length and the wavelength-dependent transmission of a species i, respectively.Eqn (5) illustrates that the fraction of photon uxes absorbed by nitrate in the particle phase, I pNO 3 À(l), decreases with increasing concentrations of other light-absorbing species.Hence, quantifying the total adsorbed photon uxes, X i 3 i ðlÞc i l; is crucial to constrain j pNO 3 À .Given that the sources and chemical compositions of brown carbon remain to be understood, 153,156,157 the total adsorbed photon uxes, X i 3 i ðlÞc i l; are highly uncertain.
Studies of brown carbon in association with particulate nitrate photolysis are warranted. 135,179Note that eqn (5) assumes homogeneous mixing.If a nitrate particle was covered by lightabsorbing species (e.g., through liquid-liquid phase separation 129 ), such a screening effect would be intensied.

Nitrate-photolysis-initiated reactions
3][164] Note that reactions induced by OH radicals are not only specic to nitrate photolysis, but also the other OH sources such as phase transfer from gas phase and H 2 O 2 photolysis. 165his section reviews studies of the following reactions promoted by particulate nitrate photolysis: multiphase oxidation of (1) SO 2 and (2) organic compounds, and (3) the formation of nitrated products in the aqueous phase of deliquesced aerosols and cloud droplets.

Multiphase oxidation of SO 2
Particulate nitrate photolysis has recently been found to promote multiphase SO 2 oxidation by generating OH, NO 2 , and NO 2 À /HONO (Fig. 2). 11,12,37SO 2 is dissolved in aerosol liquid water and is present as bisulte or/and sulte depending on the pH of the particle.Under typical acidic conditions (pH < 6), dissolved SO 2 mainly exists as bisulte. 2Bisulte can react with all OH, NO 2 , and NO 2 À /HONO for sulfate production.O 3 is also possible to oxidize bisulte, 2 but this oxidation mechanism was not efficient in our previous study. 11The reaction of bisulte and OH radical forms the sulte radical anion, which initiates the chain reactions involving SO 5 À , HSO 5 À , and SO 4 À in the presence of dissolved O 2 to produce multiple sulfate ions from each attack of OH on dissolved SO 2 . 2 The oxidation by dissolved NO 2 , one of the most feasible mechanisms during the haze events, 166 is a one-step process.During particulate nitrate photolysis at 300 nm, the highest sulfate production is found from the oxidation by NO 2 À /HONO, compared to the other oxidation mechanisms such as OH and NO 2 radicals.A simple parameterization of sulfate production results using the reactive uptake coefficient of SO 2 , g SO 2 , and nitrate photolysis rate, P NO 3 À, gives the following relation: g SO 2 ¼ 1.64 Â P NO 3 À. 11Given that nitrate concentration is as high as 10 M under highly polluted episodes and much faster particulate nitrate photolysis than that in aqueous solution, 8,38,167 g SO 2 can become >10 À5 , which is comparable to the values necessary for explaining the observations in the haze events in China. 168

Multiphase oxidation of organics in aqueous phase secondary organic aerosol formation
Organic aerosol accounts for about 20-90% of the total particulate matter on a global scale. 10A signicant fraction of this organic matter is secondary, i.e., formed in the atmosphere by converting gases into the condensed matter. 169The in-particle OH radicals produced from nitrate photolysis can promote the formation of aqueous-phase secondary organic aerosol (SOA).1][172][173][174] Aqueous SOA yields from the photo-oxidation of phenolic carbonyls in nitrate solution are twice as high as those in sulfate solution due to the efficient generation of OH through nitrate photolysis. 175Our recent work examined the role of particulate nitrate photolysis in the formation of SOA from particle-phase oxidation of glyoxal by OH radicals. 176Interestingly, we did not observe typical oxidation products such as oxalic acid, glyoxylic acid, and higher-molecular-weight products previously reported in the literature.Instead, formic acid/formate was the main oxidation product.In the presence of ammonium as a source of dissolved ammonia, light-absorbing species are formed 177,178 and trigger the photosensitization reactions to promote glyoxal oxidation. 179Particulate nitrate photolysis can alter major reaction pathways of glyoxal oxidation.

Nitration for browning atmospheric aerosol
Nitration is a chemical process that introduces a nitro group into an organic compound.2][193][194][195] Nitrate photolysis is a potential contributor to the nitration process for browning atmospheric aerosol 175,181,182,189   Nitration by NO 2 has been reported for aromatic compounds such as phenols, [198][199][200] methoxyphenols, 181,182,189 benzene, 201 toluene, 202 and catechols. 203Phenols are important precursors for SOA formation, including BrC. 204,205 Among the major nitrated aromatic compounds (NACs) found in the atmosphere are nitrophenols, nitrocatechols, nitrosalicylic acids, and nitroguaiacols. 180,184,1857][208] NACs can be directly emitted into the atmosphere such as by traffic exhaust, 209 and biomass burning, 210 and secondarily formed by the nitration of aromatic precursors in both the aqueous and the gas phases. 189,201,211itrophenol is one of the most abundant nitrated organic species in the atmosphere. 206The most prominent atmospheric process for nitrophenol formation is the nitration of phenol. 212ig. 3 summarizes the proposed reaction mechanisms for the nitration of phenol. 213,214Nitration is initiated by the reaction between phenol and two NO 2 (or N 2 O 4 ) (Fig. 3) via H-atom abstraction or electrophilic addition to the ring, resulting in a radical intermediate which is either a phenoxyl (I in Fig. 3) 215 or hydroxynitrocyclohexadienyl (II). 216The phenoxyl (I) can react with either OH radicals to form hydroxyderivatives (e.g., catechol and hydroquinone, resorcinol to a lesser extent) or with NO 2 to yield nitrophenols. 213Contrastingly, hydroxynitrocyclohexadienyl (II) can undergo H-atom abstraction by O 2 or another NO 2 to form nitrophenols. 200,216 While other pathways of nitrophenol formation are possible, 199,200,215,217 the nitration of phenol via nitrate photolysis in the atmosphere (the presence of oxygen) primarily proceeds through the hydroxynitrocyclohexadienyl (II in Fig. 3).
Nitration of phenols by NO 2 is enhanced in the presence of OH scavengers such as 2-propanol. 213Scavengers inhibit the recombination of OH with NO 2 to regenerate NO 3 À + H + , 218 allowing more NO 2 available for the nitration of phenol.The formation of nitrated phenols via nitrate photolysis was observed to decrease with increasing pH. 213At high pH, N 2 O 4 can react with OH À to form NO 3 À and NO 2

À
. At pH < 3, the formation of nitrophenols can be enhanced by thermal reactions (i.e., in the dark) involving HNO 2 . 219,220An example is the HNO 2 -catalyzed phenol nitration in which phenol directly reacts with N(III) (HNO 2 or N 2 O 3 ) in the dark. 220Although the reaction between phenol and HNO 2 or N 2 O 3 is a thermal process, nitrate irradiation is required to generate nitrite.
HOONO, an isomer of nitric acid (HNO 3 ), is formed upon nitrate photoisomerization (R4).It is a powerful nitrating agent for both phenolic and non-phenolic aromatic substrates such as phenol, benzene, and naphthalene. 201,221,222Although the direct formation of HOONO upon nitrate irradiation requires wavelength that is not atmospherically relevant (<290 nm), it can also be generated from HNO 2 and NO produced from nitrate photolysis via the reaction between NO and O 2 À . 221The irradiation of solid nitrate salts (NH 4 NO 3 , NaNO 3 ) with benzene can also yield phenol and nitrobenzene, possibly due to the generation of OH and NO 2 . 212,223In the presence of hematite (a-Fe 2 O 3 ), 224 signicant enhancement in nitrobenzene formation occurs likely due to the protonation of peroxynitrite (formed upon nitrate photoisomerization) to HOONO. 225

Future directions
We have discussed the potential impacts of particulate nitrate photolysis in the atmosphere.Yet, many issues remain unresolved.Here, we propose the following questions to be addressed to better constrain the impacts of particulate nitrate photolysis.
(1) How much can particulate nitrate photolysis promote multiphase oxidation for the secondary formation of inorganic and organic compounds in the particles, respectively?Earlier works have mainly studied the production of gaseous photoproducts of NO 2 and HONO from nitrate photolysis (Table 3).Particular attention needs to be paid to quantifying the oxidation capacity of particulate nitrate photolysis in the particle phase.While gas phase chemistry is not the focus of this paper, particulate nitrate photolysis can affect gas phase chemistry by producing NO x .The NO x recycling from particulate nitrate photolysis can lead to enhancements in NO x , OH, and O 3 concentrations in the atmosphere. 34It is also possible that photolysis of organic nitrates could inuence the nitrogen cycle and O 3 production, but there are very few studies on this topic. 472) What roles does particulate nitrate photolysis play in the formation and aging of brown carbon aerosols?Nitrate has Fig. 3 Proposed reaction mechanisms for phenol nitration. 213,214een recognized as a nitrating agent in brown carbon formation. 226On the other hand, nitrate photolysis has the potential to accelerate the aging of brown carbon. 175Nonetheless, studies on both the formation and aging of brown carbon during particulate nitrate photolysis are scarce.
(3) What is the role of the surface/interfacial effects (Sections 3.1 and 3.6.1) in promoting nitrate photolysis in the particle phase?When nitrate anions are localized at the air/particle interface, they are not fully solvated.Nitrate photolysis in the incomplete solvent cage can proceed faster than in the complete solvent cage, which is one of the plausible reasons to differentiate nitrate photolysis in the particle phase from bulk solutions.However, whether nitrate anions are so surface-active to affect the rate constant in particles is still controversial.What makes nitrate photolysis in particles so different from in bulk solutions needs to be elucidated.
(4) What parameters best describe particulate nitrate photolysis in air quality models?The photolysis rate constants are one of the most practical parameters that can be used in air quality models to implement particulate nitrate photolysis.The majority of studies have measured photoproducts of gaseous species such as NO 2 and HONO to estimate the rate constants.However, this method might potentially underestimate the constants (Section 4).Measurements of both gas and particle phase photoproducts generated from particulate nitrate photolysis are recommended to better constrain the rate constants.In addition to the photolysis rate constants, the branching ratio of photoproducts N(III) to NO 2 is also an important parameter that affects the product yields during nitrate photolysis.A N(III) : NO 2 molar ratio of 0.33-0.67 was assumed in earlier modeling works. 40,227However, the product yields can be affected by many factors as discussed in Section 3. Systematic studies under more realistic complex aerosol systems such as nitrate particles internally mixed with black carbon, BrC, radical scavengers, surface-active species (e.g., halide ions), and heterogeneously mixed (e.g., liquid-liquid phase separated) particles are needed.

Fig. 2
Fig. 2 Proposed multiphase oxidation of SO 2 promoted by particulate nitrate photolysis.Reprinted with permission from ref. 11 Copyright 2019 American Chemical Society.

Table 1
Reactions involved in nitrate photolysis

Table 2
Quantum yields, F, of nitrate photolysis
and increasing the allergenicity 196 by producing nitrating agents of NO 2 /N 2 O 4 , nitrite (NO 2