Rapid hydrolysis of NO2 at High Ionic Strengths of Deliquesced Aerosol Particles.

Nitrogen dioxide (NO2) hydrolysis in deliquesced aerosol particles forms nitrous acid and nitrate and thus impacts air quality, climate, and the nitrogen cycle. Traditionally, it is considered to proceed far too slowly in the atmosphere. However, the significance of this process is highly uncertain because kinetic studies have only been made in dilute aqueous solutions but not under high ionic strength conditions of the aerosol particles. Here, we use laboratory experiments, air quality models, and field measurements to examine the effect of the ionic strength on the reaction kinetics of NO2 hydrolysis. We find that high ionic strengths (I) enhance the reaction rate constants (kI) by more than an order of magnitude compared to that at infinite dilution (kI=0), yielding log10(kI/kI=0) = 0.04I or rate enhancement factor = 100.04I. A state-of-the-art air quality model shows that the enhanced NO2 hydrolysis reduces the negative bias in the simulated concentrations of nitrous acid by 28% on average when compared to field observations over the North China Plain. Rapid NO2 hydrolysis also enhances the levels of nitrous acid in other polluted regions such as North India and further promotes atmospheric oxidation capacity. This study highlights the need to evaluate various reaction kinetics of atmospheric aerosols with high ionic strengths.

In Situ Surface Tension Measurement of Deliquesced Aerosol Particles.

The surface tension of aerosol particles can potentially affect cloud droplet activation. Hence, direct measurement of the surface tensions of deliquesced aerosol particles is essential but is challenging. Here, we report in situ surface tension measurements based on a novel method that couples a linear quadrupole electrodynamic balance (EDB) with quasi-elastic light scattering (QELS). The EDB-QELS is validated using surface tension measurements of atmospherically relevant inorganic and organic droplets. The surface tension results reasonably agree with the reference values in the range of ∼50-90 mN m-1. We find a significant size dependence for sodium chloride droplets containing surface-active species (sodium dodecyl sulfate) in the size range of ∼5-18 µm. The surface tension increases from ∼55 to 80 mN m-1 with decreased size. Relative humidity (RH)-dependent surface tensions of mixed ammonium sulfate (AS) and polyethylene glycol droplets reveal the onset of liquid-liquid phase separation. Droplets containing water-soluble matter extracted from ambient aerosol samples and 2.3-2.9 M AS exhibit a ∼30% reduction in surface tension in the presence of ∼50 mmol-C L-1 water-soluble organic carbon, compared to pure water (∼72 mN m-1). The approach can offer size-resolved and RH-dependent surface tension measurements of deliquesced aerosol particles.

Detection of Fibril Nucleation in Micrometer-Sized Protein Condensates and Suppression of Sup35NM Fibril Nucleation by Liquid-Liquid Phase Separation.

Elucidating the link between amyloid fibril formation and liquid-liquid phase separation (LLPS) is crucial in understanding the pathologies of various intractable human diseases. However, the effect of condensed protein droplets generated by LLPS on nucleation (the initial step of amyloid formation) remains unclear because of the lack of available quantitative analysis techniques. This study aimed to develop a measurement method for the amyloid droplet nucleation rate based on image analysis. We developed a method to fix micrometer-sized droplets in gel for long-term observation of protein droplets with known droplet volumes. By combining this method with image analysis, we determined the nucleation dynamics in droplets of a prion disease model protein, Sup35NM, at the single-event level. We found that the nucleation was unexpectedly suppressed by LLPS above the critical concentration (C*) and enhanced below C*. We also revealed that the lag time in the Thioflavin T assay, a semi-quantitative parameter of amyloid nucleation rate, does not necessarily reflect nucleation tendencies in droplets. Our results suggest that LLPS can suppress amyloid nucleation, contrary to the conventional hypothesis that LLPS enhances it. We believe that the proposed quantitative analytical method will provide insights into the role of LLPS from a pathological perspective.

Development of spray-drying-based surface-enhanced Raman spectroscopy.

We report a spray-drying method to fabricate silver nanoparticle (AgNP) aggregates for application in surface-enhanced Raman spectroscopy (SERS). A custom-built system was used to fabricate AgNP aggregates of four sizes, 48, 86, 151, and 218 nm, from drying droplets containing AgNPs atomized from an AgNP suspension. Sample solutions of Rhodamine B (RhB) at 10-6, 10-8, and 10-10 M concentrations were dropped onto the AgNP aggregates as probe molecules to examine the enhancement of the Raman signals of the RhB. The ordering of the analytical enhancement factors (AEFs) by aggregate size at a 10-6 M RhB was 86 nm > 218 nm > 151 nm > 48 nm. When RhB concentrations are below 10-8 M, the 86 and 151 nm AgNP aggregates show clear RhB peaks. The AEFs of the 86 nm AgNP aggregates were the highest in all four aggregates and higher than those of the 218-nm aggregates, although the 218-nm aggregates had more hot spots where Raman enhancement occurred. This finding was attributable to the deformation and damping of the electron cloud in the highly aggregated AgNPs, reducing the sensitivity for Raman enhancement. When RhB was premixed with the AgNP suspension prior to atomization, the AEFs at 10-8 M RhB rose ~ 100-fold compared to those in the earlier experiments (the post-dropping route). This significant enhancement was probably caused by the increased opportunity for the trapping of the probe molecules in the hot spots.

Photochemical Reactions of Glyoxal during Particulate Ammonium Nitrate Photolysis: Brown Carbon Formation, Enhanced Glyoxal Decay, and Organic Phase Formation.

Glyoxal is an important precursor of aqueous secondary organic aerosol (aqSOA). Its photooxidation to form organic acids and oligomers and reactions with reduced nitrogen compounds to form brown carbon (BrC) have been extensively investigated separately, although these two types of reactions can occur simultaneously during the daytime. Here, we examine the reactions of glyoxal during photooxidation and BrC formation in premixed NH4NO3 + Glyoxal droplets. We find that nitrate photolysis and photosensitization can enhance the decay rates of glyoxal by a factor of ∼5 and ∼6 compared to those under dark, respectively. A significantly enhanced glyoxal decay rate by a factor of ∼12 was observed in the presence of both nitrate photolysis and photosensitization. Furthermore, a new organic phase was formed in irradiated NH4NO3 + Glyoxal droplets, which had no noticeable degradation under prolonged photooxidation. It was attributed to the imidazole oxidation mediated by nitrate photolysis and/or photosensitization. The persistent organic phase suggests the potential to contribute to SOA formation in ambient fine particles. This study highlights that glyoxal photooxidation mediated by nitrate photolysis and photosensitization can significantly enhance the atmospheric sink of glyoxal, which may partially narrow the gap between model predictions and field measurements of ambient glyoxal concentrations.

Nitrite/Nitrous Acid Generation from the Reaction of Nitrate and Fe(II) Promoted by Photolysis of Iron-Organic Complexes.

Gaseous nitrous acid (HONO) has the potential to greatly contribute to the atmospheric oxidation capacity. Increased attention has been paid to in-particle nitrite or nitrous acid, N(III), as one of the HONO sources. However, sources and formation mechanisms of N(III) remain uncertain. Here, we study a much less examined reaction of Fe(II) and nitrate as a source of N(III). The N(III) production was indirectly probed by its multiphase reaction with SO2 for sulfate production. Particles containing nitrate and Fe(III) were irradiated for generating Fe(II). Sulfate production was enhanced by the presence of UV and organic compounds likely because of the enhanced redox cycle between Fe(II) and Fe(III). Sulfate production rate increases with the concentration of iron-organic complexes in nitrate particles. Similarly, higher concentrations of iron-organic complexes yield higher nitrate decay rates. The estimated production rates of N(III) under simulated conditions in our study vary from 0.1 to 3.0 µg m-3 of air h-1. These values are comparable to HONO production rates of 0.2-1.6 ppbv h-1, which fall in the values reported in laboratory and field studies. The present study highlights a synergistic effect of the coexistence of iron-organic complexes and nitrate under irradiation as a source of N(III).

Enhanced Nitrite Production from the Aqueous Photolysis of Nitrate in the Presence of Vanillic Acid and Implications for the Roles of Light-Absorbing Organics.

A prominent source of hydroxyl radicals (•OH), nitrous acid (HONO) plays a key role in tropospheric chemistry. Apart from direct emission, HONO (or its conjugate base nitrite, NO2-) can be formed secondarily in the atmosphere. Yet, how secondary HONO forms requires elucidation, especially for heterogeneous processes involving numerous organic compounds in atmospheric aerosols. We investigated nitrite production from aqueous photolysis of nitrate for a range of conditions (pH, organic compound, nitrate concentration, and cation). Upon adding small oxygenates such as ethanol, n-butanol, or formate as •OH scavengers, the average intrinsic quantum yield of nitrite [Φ(NO2-)] was 0.75 ± 0.15%. With near-UV-light-absorbing vanillic acid (VA), however, the effective Φ(NO2-) was strongly pH-dependent, reaching 8.0 ± 2.1% at a pH of 8 and 1.5 ± 0.39% at a more atmospherically relevant pH of 5. Our results suggest that brown carbon (BrC) may greatly enhance the nitrite production from the aqueous nitrate photolysis through photosensitizing reactions, where the triplet excited state of BrC may generate solvated electrons, which reduce nitrate to NO2 for further conversion to nitrite. This photosensitization process by BrC chromophores during nitrate photolysis under mildly acidic conditions may partly explain the missing HONO in urban environments.

Production of Formate via Oxidation of Glyoxal Promoted by Particulate Nitrate Photolysis.

Particulate nitrate photolysis can produce oxidants (i.e., OH, NO2, and NO2-/HNO2) in aqueous droplets and may play a potential role in increased atmospheric oxidative capacity. Our earlier works have reported on the SO2 oxidation promoted by nitrate photolysis to produce sulfate. Here, we used glyoxal as a model precursor to examine the role of particulate nitrate photolysis in the formation of secondary organic aerosol (SOA) from particle-phase oxidation of glyoxal by OH radicals. Particles containing sodium nitrate and glyoxal were irradiated at 300 nm. Interestingly, typical oxidation products of oxalic acid, glyoxylic acid, and higher-molecular-weight products reported in the literature were not found in the photooxidation process of glyoxal during nitrate photolysis in the particle phase. Instead, formic acid/formate production was found as the main oxidation product. At glyoxal concentration higher than 3 M, we found that the formic acid/formate production rate increases significantly with increasing glyoxal concentration. Such results suggest that oxidation of glyoxal at high concentrations by OH radicals produced from nitrate photolysis in aqueous particles may not contribute significantly to SOA formation since formic acid is a volatile species. Furthermore, recent predictions of formic acid/formate concentration from the most advanced chemical models are lower than ambient observations at both the ground level and high altitude. The present study reveals a new insight into the production of formic acid/formate as well as a sink of glyoxal in the atmosphere, which may partially narrow the gap between model predictions and field measurements in both species.

Nitrate Photolysis in Mixed Sucrose-Nitrate-Sulfate Particles at Different Relative Humidities.

Atmospheric particles can be viscous. The limitation in diffusion impedes the mass transfer of oxidants from the gas phase to the particle phase and hinders multiphase oxidation processes. On the other hand, nitrate photolysis has been found to be effective in producing oxidants such as OH radicals within the particles. Whether nitrate photolysis can effectively proceed in viscous particles and how it may affect the physicochemical properties of the particle have not been much explored. In this study, we investigated particulate nitrate photolysis in mixed sucrose-nitrate-sulfate particles as surrogates of atmospheric viscous particles containing organic and inorganic components as a function of relative humidity (RH) and the molar fraction of sucrose to the total solute (FSU) with an in situ micro-Raman system. Sucrose suppressed nitrate crystallization, and high photolysis rate constants (∼10-5 s-1) were found, irrespective of the RH. For FSU = 0.5 and 0.33 particles under irradiation at 30% RH, we observed morphological changes from droplets to the formation of inclusions and then likely "hollow" semisolid particles, which did not show Raman signal at central locations. Together with the phase states of inorganics indicated by the full width at half-maxima (FWHM), images with bulged surfaces, and size increase of the particles in optical microscopic imaging, we inferred that the hindered diffusion of gaseous products (i.e., NOx, NOy) from nitrate photolysis is a likely reason for the morphological changes. Atmospheric implications of these results are also presented.

Concluding remarks: Faraday Discussion on air quality in megacities.

Megacities are metropolitan areas with populations over 10 million, and many of them are facing significant global environmental challenges such as air pollution. Intense economic and human activities in megacities result in air pollution emissions, inducing high levels of air pollutants in the atmosphere that harm human health, cause regional haze and acid deposition, damage crops, influence regional air quality, and contribute to climate change. Since the Great London Smog and the first recognized episode of Los Angeles photochemical smog seventy years ago, substantial progress has been achieved in improving the scientific understanding of air pollution and in developing emissions reduction technologies and control measures. However, much remains to be understood about the complex processes of atmospheric transport and reaction mechanisms; the formation and evolution of secondary particles, especially those containing organic species; and the influence of emerging emissions sources and changing climate on air quality and health. Molina (DOI: ) has provided an excellent overview of the sources of emissions in megacities, atmospheric physicochemical processes, air quality trends and management, and the impacts on health and climate for the introductory lecture of this Faraday Discussion.

Single particle diversity and mixing state of carbonaceous aerosols in Guangzhou, China.

Many field studies have investigated the formation mechanisms of organic aerosol (OA) based on bulk analysis, yet the source and formation process of individual organic particles may be quite different due to the diversity of chemical composition and mixing state in single particles. Here we present the observation results of chemical composition and mixing state of carbonaceous single particles at an urban site in Guangzhou. The carbonaceous particles accounted for 74.6% of the total detected single particles, and were grouped into four types including elemental carbon-aged (EC-aged), elemental and organic carbon (ECOC), organic carbon-rich (OC-rich) and secondary ions-rich (SEC) particles. The formation of EC-aged particles was closely associated with the absorption of organics onto fresh EC particles from primary sources, and the further enrichment of organics in EC-aged particles resulted in the production of ECOC particles. In the daytime OC-rich and SEC particles were mainly produced from the photochemical reactions, while in the nighttime their sharp increases were found along with the enrichment of nitrate and organic nitrogen fragments, suggesting the heterogeneous formation of nitrate and organic nitrogen in OC-rich and SEC particles. The production rates of carbonaceous particles were also investigated in an episodic event, and the EC-aged particles showed the highest production rate compared to the other carbonaceous particles both in the daytime and nighttime, suggesting a significant role of EC in the formation and aging process of carbonaceous particles. The results from this work have revealed different formation processes and production rates of carbonaceous particles due to their diversity in mixing state, providing further insights into the formation mechanisms of OA in field studies.

Contribution of Particulate Nitrate Photolysis to Heterogeneous Sulfate Formation for Winter Haze in China.

Nitrate and sulfate are two key components of airborne particulate matter (PM). While multiple formation mechanisms have been proposed for sulfate, current air quality models commonly underestimate its concentrations and mass fractions during northern China winter haze events. On the other hand, current models usually overestimate the mass fractions of nitrate. Very recently, laboratory studies have proposed that nitrous acid (N(III)) produced by particulate nitrate photolysis can oxidize sulfur dioxide to produce sulfate. Here, for the first time, we parameterize this heterogeneous mechanism into the state-of-the-art Community Multi-scale Air Quality (CMAQ) model and quantify its contributions to sulfate formation. We find that the significance of this mechanism mainly depends on the enhancement effects (by 1-3 orders of magnitude as suggested by the available experimental studies) of nitrate photolysis rate constant ( J NO 3 - ) in aerosol liquid water compared to that in the gas phase. Comparisons between model simulations and in-situ observations in Beijing suggest that this pathway can explain about 15% (assuming an enhancement factor (EF) of 10) to 65% (assuming EF = 100) of the model-observation gaps in sulfate concentrations during winter haze. Our study strongly calls for future research on reducing the uncertainty in EF.

Multiphase Photochemistry of Iron-Chloride Containing Particles as a Source of Aqueous Chlorine Radicals and Its Effect on Sulfate Production.

Photolysis of iron chlorides is a well-known photolytic source of Cl• in environmental waters. However, the role of particulate chlorine radicals (Cl• and Cl2•-) in their multiphase oxidative potential has been much less explored. Herein, we examine the effect of Cl•/Cl2•- produced from photolysis of particulate iron chlorides on atmospheric multiphase oxidation. As a model system, experiments on multiphase oxidation of SO2 by Cl•/Cl2•- were performed. Fast sulfate production from SO2 oxidation was observed with reactive uptake coefficients of ∼10-5, comparable to the values necessary for explaining the observations in the haze events in China. The experimental and modeling results found a good positive correlation between the uptake coefficient, γSO2, and the Cl• production rate, d[Cl•]/dt, as γSO2 = 5.3 × 10-6 × log(d[Cl•]/dt) + 4.9 × 10-5. When commonly found particulate dicarboxylic acids (oxalic acid or malonic acid) were added, sulfate production was delayed due to the competition of Fe3+ between chloride and the dicarboxylic acid for its complexation at the initial stage. After the delay, comparable sulfate production was observed. The present study highlights the importance of photochemistry of particulate iron chlorides in multiphase oxidation processes in the atmosphere.

Relative Humidity History Affects Hygroscopicity of Mixed Particles of Glyoxal and Reduced Nitrogenous Species.

The relative humidity (RH) history that manifests the cycling of dehydration (water evaporation) and hydration (water uptake) may affect particle-phase reactions, products from which have strong influences on the physical properties and thus climatic effects of atmospheric particles. Using single-trapped particles, we show herein hygroscopic growths of mixed particles with reactive species undergoing three types of RH cycles, simulating different degrees of particle-phase reactions in the atmosphere. The reactive species are the widely known α-dicarbonyl glyoxal (GLY), and five reduced nitrogenous species, ammonium sulfate (AS), glycine (GC), l-alanine (AL), dimethylamine (DMA), and diethylamine (DEA). The results showed that the mixed particles after reactions generally had altered efflorescence relative humidity (ERH) and deliquescence relative humidity (DRH) values and reduced hygroscopic growths at moderately high RH (>80%) conditions. For example, with an additional slow drying step, the mean mass growth factors at 90% RH during dehydration dropped from 2.56 to 2.02 for GC/GLY mixed particles and from 2.45 to 1.23 for AL/GLY mixed particles. The reduced hygroscopicity with more RH cycling will thus lead to less efficient light scattering of the mixed particles, thereby resulting in less cooling and exacerbating direct heating due to light absorption by the products formed.

Reconciling Measurement and Prediction of Free and Solvated Water in Solution.

In 2019, Wexler showed that, by applying Raoult's law rigorously to aqueous solutions and properly accounting for hydration of solutes, equations can be derived that accurately describe solute and solvent activity over the full range of concentrations from infinitely dilute to pure solutes. One feature of this theory is that it also predicts the amount of water that is free and the amount that is bound to the solute. In 2004 and 2005, Choi and co-workers used an electrodynamic balance to measure (i) the concentration of a range of organic and electrolyte solutes as a function of water activity and (ii) the amount of free and bound water in these solutions using the fluorescence of pyranine as a probe. In the current work, we compare the predictions of Wexler's theory to the measurements of Choi. After the adjustments to the amount of free water obtained by these measurements, the predictions of Wexler's theory match the measurements of Choi. The potential reasons for the adjustments are discussed.

Enhanced Sulfate Production by Nitrate Photolysis in the Presence of Halide Ions in Atmospheric Particles.

Heterogeneous oxidation of SO2 is an effective production pathway of sulfate in the atmosphere. We recently reported a novel pathway for the heterogeneous oxidation of SO2 by in-particle oxidants (OH, NO2, and NO2-/HNO2) produced from particulate nitrate photolysis (Environ. Sci. Technol. 2019, 53, 8757-8766). Particulate nitrate is often found to coexist with chloride and other halide ions, especially in aged sea-salt aerosols and combustion aerosols. Reactive uptake experiments of SO2 with UV-irradiated nitrate particles showed that sulfate production rates were enhanced by a factor of 1.4, 1.3, and 2.0 in the presence of Cl-, Br-, and I-, respectively, compared to those in the absence of halide ions. The larger sulfate production was attributed to enhanced nitrate photolysis promoted by the increased incomplete solvation of nitrate at the air-particle interface due to the presence of surface-active halide ions. Modeling results based on the experimental data showed that the nitrate photolysis rate constants increased by a factor of 2.0, 1.7, and 3.7 in the presence of Cl-, Br-, and I-, respectively. A linear relation was found between the nitrate photolysis rate constant, jNO3-, and the initial molar ratio of Cl- to NO3-, [Cl-]0/[NO3-]0, as jNO3- = 9.7 × 10-5[Cl-]0/[NO3-]0 + 1.9 × 10-5 at [Cl-]0/[NO3-]0 below 0.2. The present study demonstrates that the presence of halide ions enhances sulfate production produced during particulate nitrate photolysis and provides insights into the enhanced formation of in-particle oxidants that may increase atmospheric oxidative capacity.

Dynamics of lubricious, concentrated PMMA brush layers studied by surface forces and resonance shear measurements.

We employed surface forces and resonance shear measurement (RSM) for studying the structure and properties of typical concentrated polymer brushes (CPBs) of poly(methylmethacrylate) (PMMA) in toluene, which are known to show very low friction. The surface forces measured between the silica surfaces bearing PMMA brush layers showed a steric repulsive force at distances between the silica surfaces of less than ca. 1050 nm (Donset). Upon retraction after compression of the PMMA brush layers, no adhesive force was observed. This indicated that the interpenetration of the polymer chain was not induced by the normal load. Based on the resonance shear measurement, the elastic (k2) and damping (viscous) (b2) parameters, which represent the dynamic properties, of the PMMA brush layers were obtained by analyzing the resonance curves. At distances below the Donset, the b2 value significantly increased and slightly decreased at the higher normal loads, and the k2 value monotonically increased with increasing load. These k2 and b2 values were greater than those obtained for a PMMA brush layer and a bare silica surface (PMMA-silica). This indicated that the mobility of the polymer chains for the PMMA-PMMA brush layers was more suppressed compared to that for the PMMA-silica, due to the interpenetration of the polymer chains. The interpenetration of the polymer chains, commonly not observed for CPBs, could be most probably induced by the application of both the normal load and oscillating shear motion. With increasing shear amplitude on the compressed PMMA-PMMA brushes (at L = 0.84, 1.34 and 4.28 mN), the b2 value first increased then decreased whilst the k2 value monotonically decreased. These tendencies can be explained by the change from the sticking condition due to interpenetration (high k2), small sliding under interpenetration (increase in b2, decrease in k2), and then smooth sliding by pulling out of interpenetrated polymer chains (decrease in b2 and k2). The obtained results indicated that the operating conditions are quite important for using polymer brush layers as tribological materials because they can exhibit both a high and low friction depending on the conditions such as the load and shear amplitude.

Heterogeneous Oxidation of SO2 in Sulfate Production during Nitrate Photolysis at 300 nm: Effect of pH, Relative Humidity, Irradiation Intensity, and the Presence of Organic Compounds.

Heterogeneous oxidation of SO2 is one of the promising mechanisms to account for high loading of sulfate during severe haze periods in China. Our earlier work reported on the SO2 oxidation by OH and NO2 produced during 250 nm nitrate photolysis (Environ. Sci. Technol. Lett. 2019, 6, 86-91). Here, we extend that work to examine sulfate production during nitrate photolysis at 300 nm irradiation, which can additionally generate NO2- or HNO2, N(III). Flow cell/in situ Raman experiments showed that the reactive uptake coefficient of SO2, γSO2, can be expressed as γSO2 = 1.64 × pNO3-, where pNO3- is the nitrate photolysis rate in the range of (1.0-8.0) × 10-5 M s-1. Our kinetic model with the pNO3- predicts that N(III) is the main contributor to the SO2 oxidation, followed by NO2 contribution. Furthermore, the addition of OH scavengers (e.g., glyoxal or oxalic acid) does not suppress the sulfate production because of the reduced N(III)-consuming reactions and the high particle pH sustained by their presence. Our calculations illustrate that under characteristic haze conditions, the nitrate photolysis mechanism can produce sulfate at ∼1 µg m-3 h-1 at pH 4-6 and pNO3- = 10-5 M s-1. The present study highlights the importance of in-particle nitrate photolysis in heterogeneous oxidation of SO2 by reactive nitrogen (NO2-/HNO2 and NO2) under atmospherically relevant actinic irradiation. However, the nitrate photolysis rate constant needs to be better constrained for ambient aerosols.

Reactive Uptake of Glyoxal by Ammonium-Containing Salt Particles as a Function of Relative Humidity.

Reactions between dissolved ammonia and carbonyls, which form light-absorbing species in atmospheric particles, can be accelerated by actively removing water from the reaction system. Here, we examine the effects of relative humidity (RH) on the reactive uptake of glyoxal (Gly) by aqueous particles of ammonium sulfate (AS), ammonium bisulfate, sodium sulfate, magnesium sulfate, ammonium nitrate (AN), and sodium nitrate. In situ Raman analysis was used to quantify particle-phase Gly and a colored product, 2,2'-biimidazole (BI), as a function of uptake time. Overall, the Gly uptake rate increases with decreasing RH, reflecting the "salting-in" effect. The BI formation rate increases significantly with decreasing RH or aerosol liquid water (ALW). Compared to that at 75% RH, the BI formation rate is enhanced by factors of 29 at 60% RH and 330 at 45% RH for AS particles and 65 at 60% RH, 210 at 45% RH, and 460 at 30% RH for AN particles. These enhancement factors are much larger than those estimated from increased reactant concentrations due to decreases in RH and ALW alone. We postulate that the reduction in ALW at low RH increases the Gly uptake rate via the "salting-in" effect and the BI formation rate by facilitating dehydration reactions.