Steady-state Uptake Coefficient Research Articles

Heterogeneous photochemical uptake of SO2 on typical brown carbon species: A significant sulfate source

The heterogeneous reactions of SO2 on organic aerosols (OA) have been proposed as a significant source of sulfates. The photochemical conversion of SO2 to sulfates on the surface of light-absorbing OA, i.e., brown carbon (BrC), remained slight understanding. Herein, the photochemical uptake of SO2 on typical BrC species, including aromatic carbonyl compounds (ACs), polycyclic aromatic hydrocarbons (PAHs) and phenols (PHs), was systematically investigated by a flow tube reactor. Light can significantly promote the uptake of SO2 on all BrC species used here, especially for ACs. The initial and steady-state uptake coefficients (γi and γss) of SO2 exhibited positive relationships with the light intensity, whereas they negatively correlated with the initial SO2 concentration. Significant sulfate formation was clearly observed with in situ attenuated total internal reflection infrared (ATR-IR) spectrometer, and H2O and O2 had obvious enhancement effects on the sulfate production. Photoinduced •OH, which primarily originated from •O2− that was generated through an electron transfer process on BrC under irradiation, was determined to be a key driver for the oxidation of SO2 to sulfates. Finally, SO2 lifetime and sulfate formation rate were evaluated on the basis of experimental results, highlighting potential contributions of BrC photochemistry to atmospheric SO2 loss and sulfate sources.

Kinetics of Heterogeneous Reaction of H2O2 and SO2 on Coal Fly Ash: Temperature Effect and Their Synergistic Effects

Coal-derived fly ash is a major aerosol composition in the atmosphere and presents a major challenge in understanding the atmospheric environment. In this study, the heterogeneous uptake of hydrogen peroxide and sulfur dioxide on coal fly ash was investigated using a Knudsen cell reactor coupled with a quadrupole mass spectrometer. The uptake coefficients were measured as a function of the initial mixing ratio of H2O2 and SO2 from 10 to 60 ppbv, and the temperature dependence of the uptake coefficients was also carried out over a range from 253 to 313 K. The mixing ratio of H2O2 and SO2 showed little effect on the uptake coefficients for these heterogeneous processes. As a function of temperature, the initial uptake coefficients of H2O2 and SO2 on coal fly ash were (0.48–1.65) × 10−4 and (1.50–8.54) × 10−5, respectively, which decreased with an increase in temperature. The steady-state uptake coefficients of H2O2 on coal fly ash were in the range of 2.46 × 10−5 to 4.84 × 10−5, which increased with an increase in temperature. Furthermore, we examined the synergistic effects between SO2 and H2O2 in their reaction on coal fly ash and found the existence of gaseous H2O2 appeared to enhance the ability of SO2 uptake on coal fly ash. Therefore, this finding suggests that the oxidation of SO2 on the surface of coal fly ash by gaseous H2O2 would promote SO2 consumption and transformation.

Heterogeneous Chemistry of Mercuric Chloride on Inorganic Salt Surfaces.

Gaseous oxidized mercury (GOM) is a major chemical form responsible for deposition of atmospheric mercury, but its interaction with environmental surfaces is not well understood. To address this knowledge gap, we investigated the uptake of gaseous HgCl2, used as a GOM surrogate, by several inorganic salts representative of marine and urban aerosols. The process was studied in a fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer, where gaseous HgCl2 was quantitatively detected as HgCl2·NO3-. Uptake curves showed a common behavior, where upon exposure of the salt surface to HgCl2, the gas-phase concentration of the latter dropped rapidly and then recovered gradually. None of the salts produced a full recovery of HgCl2, indicating the presence of an irreversible chemical reaction in addition to reversible adsorption, and all salts showed reactive behavior consistent with the presence of surface sites of a high and a low reactivity. On the basis of the decrease in the uptake coefficient with increasing concentration of gaseous HgCl2, we conclude that the interaction follows the Langmuir-Hinshelwood mechanism. The reactivity of a deactivated salt surface after uptake could be partially restored by cycling through an elevated relative humidity at atmospheric pressure. The overall surface reactivity decreased in the series Na2SO4 > NaCl > (NH4)2SO4 > NH4NO3. The uptake on NH4NO3 was nearly fully reversible, with low values of the initial (0.4 × 10-2) and steady-state (3.3 × 10-4) uptake coefficients, whereas Na2SO4 was significantly more reactive (3.1 × 10-2 and 1.7 × 10-3). Depending on the aerosol loading, the lifetimes of gaseous HgCl2 on dry urban and marine particles (as pure (NH4)2SO4 and NaCl, respectively) were estimated to range from half an hour to about a day.

Photoenhanced heterogeneous reaction of O3 with humic acid: Focus on O3 uptake and changes in the composition and optical property.

Heterogeneous photochemical reaction of O3 with humic acid (HA) under simulated sunlight was performed using a flow tube reactor coupled to an O3 analyzer at ambient pressure. It was confirmed that light significantly enhanced the uptake of O3 on HA. The initial uptake coefficient (γi) and the steady-state uptake coefficient (γss) of O3 under irradiation increased by 1.6 and 3.8 times compared to those in the dark, respectively. The γi and γss on HA varied in the range of 0.76-2.77×10-5 and 1.50-9.55×10-6, respectively, which were dependent on various environmental factors including HA mass, total irradiance, initial O3 concentration, O2 content, temperature, relative humidity (RH) and HA solution pH. Both γi and γss showed linear dependence on the total irradiance (0-2.07×1016 photons/(cm2⋅s)) of the light source, and increased with the HA mass (0-3.2μg/cm2), temperature (278-298K) and HA solution pH (4.0-9.6). However, they showed negative correlations with the initial O3 concentration and O2 content. The γi remained constant in the RH range of 7%-60%, while γss exhibited the maximum value at RH=20%. During the ozonization of HA under irradiation, some functional groups were consumed, including CH2, CH3, aromatic CC, OH, CO, COOH and COO-. HA aged by O3 exhibited a decrease in the mass absorption efficiency (MAE) and a small increase in the absorption Ångström exponent between 300 and 600nm wavelength (AAE300,600), which was ascribed to changes in the composition of HA during the photochemical ozonization process.

Reactive uptake of NO2 on volcanic particles: A possible source of HONO in the atmosphere

The heterogeneous degradation of nitrogen dioxide (NO2) on five samples of natural Icelandic volcanic particles has been investigated. Laboratory experiments were carried out under simulated atmospheric conditions using a coated wall flow tube (CWFT). The CWFT reactor was coupled to a blue light nitrogen oxides analyzer (NOx analyzer), and a long path absorption photometer (LOPAP) to monitor in real time the concentrations of NO2, NO and HONO, respectively. Under dark and ambient relative humidity conditions, the steady state uptake coefficients of NO2 varied significantly between the volcanic samples probably due to differences in magma composition and morphological variation related with the density of surface OH groups. The irradiation of the surface with simulated sunlight enhanced the uptake coefficients by a factor of three indicating that photo-induced processes on the surface of the dust occur. Furthermore, the product yields of NO and HONO were determined under both dark and simulated sunlight conditions. The relative humidity was found to influence the distribution of gaseous products, promoting the formation of gaseous HONO. A detailed reaction mechanism is proposed that supports our experimental observations. Regarding the atmospheric implications, our results suggest that the NO2 degradation on volcanic particles and the corresponding formation of HONO is expected to be significant during volcanic dust storms or after a volcanic eruption.

Heterogeneous reaction of ozone with syringic acid: Uptake of O3 and changes in the composition and optical property of syringic acid

Syringic acid, which is a typical methoxyphenol emitted from wood combustion, can provide heterogeneous reaction sites for gaseous active components, influencing the concentrations of trace gases and the compositions of syringic acid. The heterogeneous uptake of O3 on syringic acid was investigated using a flow tube reactor under ambient pressure. The initial uptake coefficient (γi) and the steady-state uptake coefficient (γss) of O3 linearly increased with syringic acid mass (0–0.16 μg cm−2) and temperature (278–328 K), while they decreased with increasing the O3 concentration and the O2 content. The γi was independent of relative humidity (20%–70%), whereas γss decreased with relative humidity (7%–70%). The compositional changes of syringic acid by the ozonization were analyzed by the Fourier transform infrared spectrometer (FT-IR) and the gas chromatography-mass spectrometry (GC-MS), confirming the generation of 2,6-dimethoxy-1,4-benzoquinone. In addition, compared to that of fresh syringic acid, the mass absorption efficiency of syringic acid aged by O3 exhibited an increase in the range of 290–320 nm.

Uptake and surface chemistry of SO2 on natural volcanic dusts

V-dust (v-dust) is a highly variable source of natural particles in the atmosphere, and during the period of high volcanic activity it can provide a large surface for heterogeneous interactions with other atmospheric compounds. Icelandic v-dust was chosen as a case study due to frequency of volcanic eruptions and high aeolian activity in the area. In this study, we focus on the kinetics and mechanism of the reaction of sulfur dioxide (SO2) with natural v-dust samples under atmospheric conditions using coated wall flow tube reactor and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Steady state uptake coefficients determined are in the range of 10−9 to 10−8 depending on the considered v-dust. Concomitantly with SO2 uptake, both sulfites and sulfates are monitored on the surface of v-dust, with sulfates being the final oxidation product, attesting of SO2 surface reaction. Surface hydroxyl groups play a crucial role in the conversion of SO2 to sulfites as evidenced from both flow tube and DRIFTS experiments. Based on these experimental results, a mechanism for SO2 interaction with different surface sites of v-dust is proposed and discussed. This study provides original insights in the kinetics of SO2 uptake under simulated atmospheric conditions and its mechanism and transformation on volcanic material. To that regards, it brings an accurate perspective on SO2 heterogeneous sinks in the atmosphere.

Chemical composition and heterogeneous reactivity of soot generated in the combustion of diesel and GTL (Gas-to-Liquid) fuels and amorphous carbon Printex U with NO2 and CF3COOH gases

The heterogeneous reactions of nitrogen dioxide (NO2) and trifluoroacetic acid (CF3COOH) with soot produced by diesel and GTL (gas-to-liquid) fuels were investigated using a Knudsen flow reactor with mass spectrometry as a detection system for gas phase species. Soot was generated with a 4 cylinder diesel engine working under steady-state like urban operation mode. Heterogeneous reaction of the mentioned gases with a commercial carbon, Printex U, used as reference, was also analyzed. The initial and the steady-state uptake coefficients, γ0 and γss, respectively, were measured indicating that GTL soot reacts faster than diesel soot and Printex U carbon for NO2 gas reactant. According to the number of reacted molecules on the surface, Printex U soot presents more reducing sites than diesel and GTL soot. Initial uptake coefficients for GTL and diesel soot for the reaction with CF3COOH gas reactant are very similar and no clear conclusions can be obtained related to the initial reactivity. The number of reacted molecules calculated for CF3COOH reactions shows values two orders of magnitude higher than the corresponding to NO2 reactions, indicating a greater presence of basic functionalities in the soot surfaces. More information of the surface composition has been obtained using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) before and after the reaction of soot samples with gas reactants. As conclusion, the interface of diesel and GTL soot before reaction mainly consists of polycyclic aromatic hydrocarbons (PAHs), nitro-compounds as well as ether functionalities. After reaction with gas reactant, it was observed that PAHs and nitro-compounds remain on the soot surface and new spectral bands such as carbonyl groups (carboxylic acids, aldehydes, esters and ketones) are observed. Physical properties of soot from both fuels studied such as BET surface isotherm and SEM analysis were also developed and related to the observed reactivity.

Ozone Decomposition on Kaolinite as a Function of Monoterpene Exposure and Relative Humidity

Atmospheric processing of mineral aerosol by trace gases results in the formation of surface-adsorbed products that have the capacity to alter the chemical and physical properties of these airborne particulates. To investigate one potential impact of aerosol processing by biogenic volatile organic compounds (BVOCs), we investigated the heterogeneous decomposition of ozone on pure and monoterpene-processed kaolinite. We used a laminar flow reactor to measure O3 reactive uptake coefficients on kaolinite-coated tubes as a function of relative humidity, O3 concentration, and pre-exposure to gaseous limonene and α-pinene. At 26% RH, kaolinite has a near equivalent of a monolayer of adsorbed water, and the ozone steady-state uptake coefficient was γav = 2.9 × 10–9 assuming the BET surface area. Pre-exposing kaolinite to limonene and α-pinene increased O3 uptake coefficients by nearly 2 orders of magnitude to 2.1 × 10–7 and 2.5 × 10–7, respectively. At all humidities studied (10–50% RH), O3 uptake was at least 1...

Isoprene Heterogeneous Uptake and Reactivity on TiO2 : A Kinetic and Product Study

The heterogeneous interaction of isoprene with TiO2 surfaces was studied under dark and UV light irradiation conditions. The experiments were conducted at room temperature, using zero air as bath gas, in a flow reactor coupled with a SIFT-MS (selected-ion flow-tube mass spectrometer) and a FTIR spectrometer for the gas-phase monitoring of reactants and products. The steady-state uptake coefficient and the yields of the products formed were measured as a function of TiO2 mass (9–120 mg), light intensity (37–112 W m−2), isoprene concentration (36–12000 ppb), and relative humidity (0.01–90% of RH). Under dark and dry conditions, isoprene was efficiently and reversibly adsorbed on TiO2. In contrast, under humid conditions, isoprene uptake was diminished, pointing to competitive adsorption with water molecules. In the presence of UV light irradiation, isoprene reacted on the surface of TiO2. The reactive steady-state uptake coefficient, γss, was independent of RH under most ambient relative humidity conditions (>50%). However, γss was strongly dependent on isoprene initial concentration according to the empirical expression: γss = (2.0 × 10−4) × [isoprene]0−n with n = 0.35 and 0.28 for 37 and 112 W m−2 irradiation conditions, respectively. In addition to the kinetics, a detailed product study was performed. The gas-phase oxidation products were mostly CO2 (ca. 90% of the carbon mass balance) and a large variety of carbonyl compounds (methyl vinyl ketone, acetone, methacrolein, formaldehyde, acetaldehyde, propanal, traces of butanal, and pentanal), the distribution of which was investigated as a function of mineral oxide mass, isoprene concentration, and RH. Furthermore, the surface-adsorbed products were determined employing off-line HPLC chromatography; their concentrations were inversely dependent on RH and decreased to background levels at RH greater than 30%. Finally, the reaction mechanism and possible implications of isoprene reaction on TiO2 are briefly discussed.

Heterogeneous uptake of gaseous hydrogen peroxide on mineral dust

The heterogeneous uptake processes of hydrogen peroxide on Arizona test dust and two types of authentic Chinese mineral dusts, i.e., Inner Mongolia desert dust and Xinjiang calciferous dust, were investigated using a Knudsen cell reactor coupled with a quadrupole mass spectrometer. The uptake coefficients were measured as a function of the initial concentration of H2O2 from 2.6×1011 to 1.2×1012molecules/cm3, and the temperature dependence of the uptake coefficients was investigated over a range from 253 to 313K. The concentration of H2O2 showed little effect on the uptake coefficients of these heterogeneous processes. As a function of temperature, the initial uptake coefficients decrease with increasing temperature, whereas the steady state uptake coefficients of Arizona test dust and Inner Mongolia desert dust increase with increasing temperature. Implications for the understanding of the uptake processes onto mineral dust samples were also discussed.

Investigation of the Photochemical Reactivity of Soot Particles Derived from Biofuels Toward NO2. A Kinetic and Product Study

In the current study, the heterogeneous reaction of NO2 with soot and biosoot surfaces was investigated in the dark and under illumination relevant to atmospheric conditions (J(NO2) = 0.012 s(-1)). A flat-flame burner was used for preparation and collection of soot samples from premixed flames of liquid fuels. The biofuels were prepared by mixing 20% v/v of (i) 1-butanol (CH3(CH2)3OH), (ii) methyl octanoate (CH3(CH2)6COOCH3), (iii) anhydrous diethyl carbonate (C2H5O)2CO and (iv) 2,5 dimethyl furan (CH3)2C4H2O additive compounds in conventional kerosene fuel (JetA-1). Experiments were performed at 293 K using a low-pressure flow tube reactor (P = 9 Torr) coupled to a quadrupole mass spectrometer. The initial and steady-state uptake coefficients, γ0 and γ(ss), respectively, as well as the surface coverage, N(s), were measured under dry and humid conditions. Furthermore, the branching ratios of the gas-phase products NO (∼80-100%) and HONO (<20%) were determined. Soot from JetA-1/2,5-dimethyl furan was the most reactive [γ0 = (29.1 ± 5.8) × 10(-6), γ(ss)(dry) = (9.09 ± 1.82) × 10(-7) and γ(ss)(5.5%RH) = (14.0 ± 2.8)(-7)] while soot from JetA-1/1-butanol [γ0 = (2.72 ± 0.544) × 10(-6), γ(ss)(dry) = (4.57 ± 0.914) × 10(-7), and γ(ss)(5.5%RH) = (3.64 ± 0.728) × 10(-7)] and JetA-1/diethyl carbonate [γ0 = (2.99 ± 0.598) × 10(-6), γ(ss)(dry) = (3.99 ± 0.798) × 10(-7), and γ(ss)(5.5%RH) = (4.80 ± 0.960) × 10(-7)] were less reactive. To correlate the chemical reactivity with the physicochemical properties of the soot samples, their chemical composition was analyzed employing Raman spectroscopy, NMR, and high-performance liquid chromatography. In addition, the Brunauer-Emmett-Teller adsorption isotherms and the particle size distributions were determined employing a Quantachrome Nova 2200e gas sorption analyzer. The analysis of the results showed that factors such as (i) soot mass collection rate, (ii) porosity of the particles formed, (iii) aromatic fraction, and (iv) pre-existence of nitro-containing species in soot samples (formed during the combustion process) can be used as indicators of soot reactivity with NO2.

Heterogeneous Interaction of H2O2 with Arizona Test Dust

The heterogeneous interaction of H2O2 with solid films of Arizona Test Dust (ATD) was investigated under dark conditions and in presence of UV light using a low pressure flow tube reactor coupled with a quadrupole mass spectrometer. The uptake coefficients were measured as a function of the initial concentration of gaseous H2O2 ([H2O2]0 = (0.18 - 5.1) × 10(12) molecules cm(-3)), irradiance intensity (JNO2 = 0.002 - 0.012 s(-1)), relative humidity (RH = 0.002 - 69%), and temperature (T = 268 - 320 K). The initial uptake coefficient was found to be independent of the concentration of H2O2 and UV irradiation intensity and to decrease with increasing RH and temperature according to the following expressions: γ0 = 4.8 × 10(-4)/(1+ RH(0.66)) at T = 275 K and γ0 = 3.2 × 10(-4)/(1 + 2.5 × 10(10)exp(-7360/T)) at RH = 0.35% (calculated using BET surface area, estimated conservative uncertainty of 30%). By contrast, the steady state uptake coefficient was found to be independent of temperature, to increase upon UV irradiation of the surface, and to be inversely (γSS ∼ [H2O2](-0.6)) dependent on the concentration of H2O2. The RH independent steady state uptake coefficient was measured under dark and UV irradiation conditions: γSS(dark) = (0.95 ± 0.30) × 10(-5) and γSS(UV) = (1.85 ± 0.55) × 10(-5), for RH = (2 - 69)% and [H2O2]0 ≅ 1.0 × 10(12) molecules cm(-3). The present experimental data support current considerations that uptake of H2O2 on mineral aerosol is potentially an important atmospheric process.

Mineral Oxides Change the Atmospheric Reactivity of Soot: NO2 Uptake under Dark and UV Irradiation Conditions

The heterogeneous reactions between trace gases and aerosol surfaces have been widely studied over the past decades, revealing the crucial role of these reactions in atmospheric chemistry. However, existing knowledge on the reactivity of mixed aerosols is limited, even though they have been observed in field measurements. In the current study, the heterogeneous interaction of NO2 with solid surfaces of Al2O3 covered with kerosene soot was investigated under dark conditions and in the presence of UV light. Experiments were performed at 293 K using a low-pressure flow-tube reactor coupled with a quadrupole mass spectrometer. The steady-state uptake coefficient, γ(ss), and the distribution of the gas-phase products were determined as functions of the Al2O3 mass; soot mass; NO2 concentration, varied in the range of (0.2-10) × 10(12) molecules cm(-3); photon flux; and relative humidity, ranging from 0.0032% to 32%. On Al2O3/soot surfaces, the reaction rate was substantially increased, and the formation of HONO was favored compared with that on individual pure soot and pure Al2O3 surfaces. Uptake of NO2 was enhanced in the presence of H2O under both dark and UV irradiation conditions, and the following empirical expressions were obtained: γ(ss,BET,dark) = (7.3 ± 0.9) × 10(-7) + (3.2 ± 0.5) × 10(-8) × RH and γ(ss,BET,UV) = (1.4 ± 0.2) × 10(-6) + (4.0 ± 0.9) × 10(-8) × RH. Specific experiments, with solid sample preheating and doping with polycyclic aromatic hydrocarbons (PAHs), showed that UV-absorbing organic compounds significantly affect the chemical reactivity of the mixed mineral/soot surfaces. A mechanistic scheme is proposed, in which Al2O3 can either collect electrons, initiating a sequence of redox reactions, or prevent the charge-recombination process, extending the lifetime of the excited state and enhancing the reactivity of the organics. Finally, the atmospheric implications of the observed results are briefly discussed.

Photosensitised heterogeneous oxidation kinetics of biomass burning aerosol surrogates by ozone using an irradiated rectangular channel flow reactor

Abstract. Heterogeneous reaction kinetics involving organic aerosol and atmospheric oxidants such as ozone can be enhanced under visible or UV irradiation in the presence of a photosensitiser, with subsequent implications for the climate, cloud radiative properties, air quality, and source appointment. In this study we report the steady-state reactive uptake coefficient, γ, of O3 by levoglucosan and 5-nitroguaiacol acting as surrogates for biomass burning aerosol particles, with and without the presence of Pahokee peat acting as a photosensitiser. The reactive uptake has been determined in the dark and as a function of visible and UV-A irradiation and ozone concentration. In addition, γ was determined for 1 : 1, 1 : 10, and 1 : 100 by mass mixtures of Pahokee peat and 5-nitroguaiacol, and for a 10 : 1 : 3 mixture of levoglucosan, Pahokee peat, and 5-nitroguaiacol. We developed a novel irradiated rectangular channel flow reactor (I-RCFR) that was operated under low pressures of about 2–4 hPa, and allowed for uniform irradiation of the organic substrates. The I-RCFR was coupled to a chemical ionisation mass spectrometer and has been successfully validated by measuring the kinetics between various organic species and oxidants. γ of O3 and levoglucosan in the dark and under visible and UV-A irradiation was determined to be in the range of (2–11) × 10−6 and did not change in the presence of Pahokee peat. The determined γ of O3 and 5-nitroguaiacol in the dark was 5.7 × 10−6 and was only enhanced under UV-A irradiation, yielding a value of 3.6 × 10−5. γ of the 1 : 1 Pahokee peat/5-nitroguaiacol substrate was enhanced under visible and UV-A irradiation to 2.4 × 10−5 and 2.8 × 10−5, respectively. Decreasing the amount of Pahokee peat in the 5-nitroguaiacol/Pahokee peat substrate resulted in lower values of γ under visible irradiation, however, γ was consistent under UV-A irradiation regardless of the amount of Pahokee peat. The 10 : 1 : 3 mixture by mass of levoglucosan, Pahokee peat, and 5-nitroguaiacol, under both visible and UV-A irradiation yielded γ values of 2.8 × 10−5 and 1.4 × 10−5, respectively. γ was determined as a function of photon flux for O3 with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate, yielding a linear relationship under both visible and UV-A irradiation. γ of O3 with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate was determined as a function of ozone concentration and exhibited an inverse dependence of γ on ozone concentration, commonly interpreted as a Langmuir–Hinshelwood mechanism. The reactive uptake data have been represented by a Langmuir-type isotherm. From the O3 uptake data under visible irradiation, the following fit parameters have been derived: ks = (5.5 ± 2.7) × 10−19 cm2 s−1 molecule−1 and KO3 = (2.3 ± 2.0) × 10−12 cm3 molecule−1; and under UV-A irradiation: ks = (8.1 ± 2.0) × 10−19 cm2 s−1 molecule−1 and KO3 = (1.7 ± 0.7) × 10−12 cm3 molecule−1. The oxidative power, or the product of γ and [O3], was determined for O3 with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate and was in the range of (1.2–26) × 106 molecule cm−3. Atmospheric particle lifetimes were estimated for a 0.4 μm 5-nitroguaiacol particle as a function of visible and UV-A irradiation and ozone concentration.

Uptake of isoprene, methacrylic acid and methyl methacrylate into aqueous solutions of sulfuric acid and hydrogen peroxide

Multiphase acid-catalyzed oxidation by hydrogen peroxide has been suggested to be a potential route to secondary organic aerosol formation from isoprene and its gas-phase oxidation products, but the lack of kinetics data significantly limited the evaluation of this process in the atmosphere. Here we report the first measurement of the uptake of isoprene, methacrylic acid and methyl methacrylate into aqueous solutions of sulfuric acid and hydrogen peroxide. Isoprene cannot readily partition into the solution because of its high volatility and low solubility, which hinders its further liquid-phase oxidation. Both methacrylic acid and methyl methacrylate can enter the solutions and be oxidized by hydrogen peroxide, and steady-state uptake was observed with the acidity of solution above 30 wt.% and 70 wt.%, respectively. The steady-state uptake coefficient of methacrylic acid is much larger than that of methyl methacrylate for a solution with same acidity. These observations can be explained by the different reactivity of these two compounds caused by the different electron-withdrawing conjugation between carboxyl and ester groups. The atmospheric lifetimes were estimated based on the calculated steady-state uptake coefficients. These results demonstrate that the multiphase acid-catalyzed oxidation of methacrylic acid plays a role in secondary organic aerosol formation, but for isoprene and methyl methacrylate, this process is not important in the troposphere.

Heterogeneous Interaction of H2O2 with TiO2 Surface under Dark and UV Light Irradiation Conditions

The heterogeneous interaction of H(2)O(2) with TiO(2) surface was investigated under dark conditions and in the presence of UV light using a low pressure flow tube reactor coupled with a quadrupole mass spectrometer. The uptake coefficients were measured as a function of the initial concentration of gaseous H(2)O(2) ([H(2)O(2)](0) = (0.17-120) × 10(12) molecules cm(-3)), irradiance intensity (J(NO(2)) = 0.002-0.012 s(-1)), relative humidity (RH = 0.003-82%), and temperature (T = 275-320 K). Under dark conditions, a deactivation of TiO(2) surface upon exposure to H(2)O(2) was observed, and only initial uptake coefficient of H(2)O(2) was measured, given by the following expression: γ(0)(dark) = 4.1 × 10(-3)/(1 + RH(0.65)) (calculated using BET surface area, estimated conservative uncertainty of 30%) at T = 300 K. The steady-state uptake coefficient measured on UV irradiated TiO(2) surface, γ(ss)(UV), was found to be independent of RH and showed a strong inverse dependence on [H(2)O(2)] and linear dependence on photon flux. In addition, slight negative temperature dependence, γ(ss)(UV) = 7.2 × 10(-4) exp[(460 ± 80)/T], was observed in the temperature range (275-320) K (with [H(2)O(2)] ≈ 5 × 10(11) molecules cm(-3) and J(NO(2)) = 0.012 s(-1)). Experiments with NO addition into the reactive system provided indirect evidence for HO(2) radical formation upon H(2)O(2) uptake, and the possible reaction mechanism is proposed. Finally, the atmospheric lifetime of H(2)O(2) with respect to the heterogeneous loss on mineral dust was estimated (using the uptake data for TiO(2)) to be in the range of hours during daytime, i.e., comparable to H(2)O(2) photolysis lifetime (~1 day), which is the major removal process of hydrogen peroxide in the atmosphere. These data indicate a strong potential impact of H(2)O(2) uptake on mineral aerosol on the HO(x) chemistry in the troposphere.

Heterogeneous Uptake Kinetics of Limonene and Limonene Oxide by Sulfuric Acid Solutions

The heterogeneous uptake of limonene and limonene oxide (also known as 1,8-cineole) by a range of different aqueous sulfuric acid (H2SO4) solutions (30%-80%(w)) was investigated to develop an understanding of the reactivity of biogenic organic compounds in the atmosphere towards acidic aerosols. Experiments were performed using a rotating wetted-wall reactor coupled to a single photon ionization time-of-flight mass spectrometer. The heterogeneous uptake of the compounds into H2SO4 followed first-order kinetics and the corresponding steady-state uptake coefficients (γ) were calculated for the first time. Limonene oxide was found to be more reactive than limonene towards H2SO4. Reactive uptake was observed for limonene oxide in acidic solution containing greater than 30%(w). The steady-state uptake coefficients of limonene oxide in 30%-50%(w) H2SO4 solutions at room temperature ranged from (7.100±0.023)×10-5 to (8.150±0.162)×10-3. Furthermore, the reactions of limonene oxide with sulfuric acid in bulk solution were investigated using gas chromatography-mass spectrometry (GC-MS) and electron spray ionization-mass spectroscopy (ESI-MS). Analysis of the products revealed the presence of monoterpenes, terpineols, terpin hydrates, and terpin hydrate diorganosulfate from the bulk solution reaction of limonene oxide with H2SO4. The formation of significantly more hydrophobic organic compounds with lower volatilities suggested that limonene oxide is a significant precursor in the formation of atmospheric secondary organic aerosols. A transformation mechanism has been proposed based on the products.

Uptake of Methacrolein into Aqueous Solutions of Sulfuric Acid and Hydrogen Peroxide

Multiphase acid-catalyzed oxidation by hydrogen peroxide has been suggested to be a potential route to secondary organic aerosol (SOA) formation from isoprene and its gas-phase oxidation products, but the kinetics and chemical mechanism remain largely uncertain. Here we report the first measurement of uptake of methacrolein into aqueous solutions of sulfuric acid and hydrogen peroxide in the temperature range of 253-293 K. The steady-state uptake coefficients were acquired and increased quickly with increasing sulfuric acid concentration and decreasing temperature. Propyne, acetone, and 2,3-dihydroxymethacrylic acid were suggested as the products. The chemical mechanism is proposed to be the oxidation of carbonyl group and C═C double bonds by peroxide hydrogen in acidic environment, which could explain the large content of polyhydroxyl compounds in atmospheric fine particles. These results indicate that multiphase acid-catalyzed oxidation of methacrolein by hydrogen peroxide can contribute to SOA mass in the atmosphere, especially in the upper troposphere.

Heterogeneous Reactions of Alkylamines with Ammonium Sulfate and Ammonium Bisulfate

The heterogeneous reactions between alkylamines and ammonium salts (ammonium sulfate and ammonium bisulfate) have been studied using a low-pressure fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS) at 293 ± 2 K. The uptake of three alkylamines, i.e., monomethylamine, dimethylamine, and trimethylamine, on ammonium sulfate shows a displacement reaction of ammonium by aminium, evidenced by the release of ammonia monitored using protonated acetone dimer as the reagent ion. For the three alkylamines, the initial uptake coefficients (γ(0)) range from 2.6 × 10(-2) to 3.4 × 10(-2) and the steady-state uptake coefficients (γ(ss)) range from 6.0 × 10(-3) to 2.3 × 10(-4) and decrease as the number of methyl groups on the alkylamine increases. A different reaction mechanism is observed for the uptake of the three alkylamines on ammonium bisulfate, which is featured by an acid-base reaction (neutralization) with irreversible alkylamine loss and no ammonia generation and occurs at a rate limited by diffusion of gaseous alkylamines to the ammonium bisulfate surface. Our results reveal that the reactions between alkylamines and ammonium salts contribute to particle growth and alter the composition of ammonium sulfate and bisulfate aerosols in the atmosphere.