Coated-wall Flow Tube Research Articles

Heterogeneous reaction of NO2 with hematite: The effects of UV irradiation, O2 and relative humidity

Heterogeneous reactions on mineral dust surfaces are increasingly considered important in the removal of gaseous pollutants and the formation of secondary aerosols. Although the heterogeneous reaction of NO2 on the hematite surface has been investigated in many previous studies, little is known about the reaction of NO2 with hematite under ambient conditions. In this work, heterogeneous reactions of NO2 with hematite at 298 K were investigated via a coated-wall flow tube reactor and in situ diffuse reflectance Fourier transformed infrared spectroscopy (DRIFTS). The influence of UV illumination, relative humidity (RH) and O2 on the uptake coefficients and adsorption amount of NO2, as well as the nitrate formation on the hematite surface, has been analyzed comprehensively. UV irradiation shows a significant effect on the true uptake coefficient (γBET), which increases from 2.00 × 10−6 to 4.76 × 10−6 in the N2 stream and 1.32 × 10−6 to 4.07 × 10−6 in the air stream under dry conditions (∼0.3 % RH). RH (in the range of 0–67 %) exhibits an inhibitory effect on the adsorption of NO2 on the hematite surface because of the competition between NO2 and water molecules, that is, γBET and adsorption amount of NO2 decrease with an increase in RH under both the dark and light reaction conditions. Meanwhile, both the γBET and adsorption amount of NO2 on hematite decrease in the air stream compared to those in N2 conditions. In addition, the results from the DRIFTS experiments indicate that the presence of UV irradiation promotes the conversion of NO2 to nitrate and both the RH and O2 suppress the nitrate formation. From this research, the heterogeneous reactions between NO2 with mineral dust under ambient conditions will be better understood.

Unexpected electrophiles in the atmosphere - anhydride nucleophile reactions and uptake to biomass burning emissions.

Biomass burning is a significant contributor to atmospheric pollution, its emissions have been found to have adverse impacts on climate and human health. Largely, these impacts are dictated by how the composition of the emissions changes once emitted into the atmosphere. Recently, anhydrides have been identified as a significant fraction of biomass burning emissions, however, little is known about their atmospheric evolution, or their interactions within the burn plume. Without this understanding, it is challenging to predict the impact of anhydrides on biomass burning emissions, and by extension, their influence on climate and health. In this study, we investigate anhydrides as potentially unrecognized electrophiles in the atmosphere. Firstly, by exploring their reactivity towards important biomass burning emitted nucleophiles, and secondly, by measuring their uptake on the emissions themselves. Our results show that phthalic and maleic anhydride can react with a wide range of nucleophiles, including hydroxy and amino-containing compounds, such as levoglucosan or aniline. Additionally, using a coated-wall flow tube setup, we demonstrate that anhydrides reactively uptake to biomass burning films and influence their composition. The anhydride nucleophile reaction was found to be irreversible, proceeding without sunlight or free radicals and indicating it may occur during the day or nighttime. Furthermore, the reaction products were found to be water-stable and contain functional groups which enhance their mass and likely contribute to the formation of secondary organic aerosol, with knock-on climate effects. Overall, our study sheds light on the fundamental chemistry of anhydrides and their potential impacts in the atmosphere.

Reactive uptake coefficients for multiphase reactions determined by a dynamic chamber system

Abstract. Dynamic flow-through chambers are frequently used to measure gas exchange rates between the atmosphere and biosphere on the Earth's surface such as vegetation and soils. Here, we explore the performance of a dynamic chamber system in determining the uptake coefficient γ of exemplary gases (O3 and SO2) on bulk solid-phase samples. After characterization of the dynamic chamber system, the derived γ is compared with that determined from a coated-wall flow tube system. Our results show that the dynamic chamber system and the flow tube method show a good agreement for γin the range of 10−8 to 10−3. The dynamic chamber technique can be used for liquid samples and real atmospheric aerosol samples without complicated coating procedures, which complements the existing techniques in atmospheric kinetic studies.

Uptake of Hg0(g) on TiO2, Al2O3, and Fe2O3 Nanoparticles: Importance in Atmospheric Chemical and Physical Processes.

Mineral dust aerosols play an important role in tropospheric chemistry and aerosol-cloud interaction processes. Yet, their interactions with gaseous elemental mercury (Hg0(g)) are not currently well understood. Using a coated-wall flow tube (CWFT) reactor, we measured the uptake of Hg0(g) on some common components of mineral dust aerosols, including TiO2, Al2O3, and Fe2O3, and the effects of irradiation (dark, visible and UV-A) and relative humidity (<2% to 60%) on the uptake kinetics. Under UV-A irradiation (320-400 nm) in dry air, we measured uptake coefficients (γ) equal to >1 × 10-3 and (3 ± 1) × 10-6 on TiO2 and Al2O3, respectively. Under visible light irradiation (380-700 nm), Hg0(g) uptake was only observed on TiO2, with γ = (4 ± 3) × 10-4. Raising the relative humidity inhibited the uptake on both TiO2 and Al2O3, and the uptake coefficient at 60% RH for TiO2 under UV-A irradiation was lower by ca. 3 orders of magnitude than dry conditions. Furthermore, we observed that water vapor induced the desorption of two distinct fractions from Hg-exposed surfaces via the displacement of weakly, physisorbed Hg and the photocatalyzed reduction of chemisorbed Hg. Based on the uptake coefficients from this report, we estimate that heterogeneous interactions with mineral dust may be significant under conditions with low relative humidity (<30%) and high dust loading masses. We herein discuss the implication of this study on understanding the life cycle analysis of atmospheric mercury in nature.

Reactive uptake of ozone to azo dyes in a coated-wall flow tube.

Azo dyes are the most common colorants in consumer products, including clothing and cosmetics. Some azo dyes and their products from reductive degradation are known to be mutagenic, so dermal exposure to these species has been studied extensively. In contrast, oxidative degradation of azo dyes in consumer products has not been studied so thoroughly. In the indoor environment, ozone is ubiquitous, so reactive uptake of ozone to azo dyes could lead to dermal exposure to other classes of degradation products. Here, we report the first measurements of the reactive uptake of ozone to thin films of three widely used commercial azo dyes: sunset yellow, amaranth, and tartrazine. Steady-state uptake was observed for all three dyes, under all conditions investigated, even at the lowest relative humidity (RH) of 0%. The uptake coefficients increased with RH. For sunset yellow at 100 ppb of ozone, the value at 80% RH, (2.0 ± 0.5) × 10-7, was 2.5 times greater than that at 0% RH, (8 ± 1) × 10-8, consistent with plasticization of the thin film due to absorption of water. The uptake coefficient of sunset yellow at 80% RH exhibited an inverse dependence on the ozone mixing ratio, approaching an asymptote of 1 × 10-7 above 250 ppb. At 80% RH and 100 ppb of ozone, the uptake coefficients for the three dyes were similar, (2.0 ± 0.5) × 10-7 for sunset yellow, (2.7 ± 0.6) × 10-7 for amaranth, and (3.2 ± 0.3) × 10-7 for tartrazine, despite differences in structural parameters related to the number of reactive sites at the surface. Together, these results are consistent with ozone diffusing into the thin film and the dye molecules mixing between the layers, such that reaction is not restricted to the surface of the film. Finally, the results are suggestive of a role for azo dyes, including the occurrence of their oxidation products, in indoor chemistry.

Emerging investigator series: ozone uptake by urban road dust and first evidence for chlorine activation during ozone uptake by agro-based anti-icer: implications for wintertime air quality in high-latitude urban environments.

High-latitude urban regions provide a unique and complex range of environmental surfaces for uptake of trace pollutant gases, including winter road maintenance materials (e.g., gravel, rock salts, and anti-icer, a saline solution applied to roads during winter). In an effort to reduce the negative environmental and economic impacts of road salts, many municipalities have turned to agro-based anti-icing materials that are rich in organic material. To date, the reactivity of both anti-icer and saline road dust with pollutant gases remain unexplored, which limits our ability to assess the potential impacts of these materials on air quality in high-latitude regions. Here, we used a coated-wall flow tube to investigate the uptake of ozone, an important air pollutant, by road dust collected in Edmonton, Canada. At 25% relative humidity (RH) and 50 ppb ozone, γBET for ozone uptake by this sample is (8.0 ± 0.7) × 10-8 under dark conditions and (2.1 ± 0.1) × 10-7 under illuminated conditions. These values are 2-4× higher than those previously obtained by our group for natural mineral dusts, but are not large enough for suspended road dust to influence local ozone mixing ratios. In a separate set of experiments, we also investigated the uptake of ozone by calcium chloride (i.e., road salt) and commercial anti-icer solution. Although ozone uptake by pure calcium chloride was negligible, ozone uptake by anti-icer was significant, which implies that the reactivity of anti-icer is conferred by its organic content. Importantly, ozone uptake by anti-icer-and, to a lesser extent, road dust doped with anti-icer-leads to the release of inorganic chlorine gas, which we collected using inline reductive trapping and quantified using ion chromatography. To explain these results, we propose a novel pathway for chlorine activation: here, ozone oxidation of the anti-icer organic fraction (in this case, molasses) yields reactive OH radicals that can oxidize chloride. In summary, this study demonstrates the ability of road dust and anti-icer to influence atmospheric oxidant mixing ratios in cold-climate urban areas, and highlights previously unidentified air quality impacts of winter road maintenance decisions.

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.

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.

Halogen activation and radical cycling initiated by imidazole-2-carboxaldehyde photochemistry

Abstract. Atmospheric aerosol particles can contain light-absorbing organic compounds, also referred to as brown carbon (BrC). The ocean surface and sea spray aerosol particles can also contain light-absorbing organic species referred to as chromophoric dissolved organic matter (CDOM). Many BrC and CDOM species can contain carbonyls, dicarbonyls or aromatic carbonyls such as imidazole-2-carboxaldehyde (IC), which may act as photosensitizers because they form triplet excited states upon UV–VIS light absorption. These triplet excited states are strong oxidants and may initiate catalytic radical reaction cycles within and at the surface of atmospheric aerosol particles, thereby increasing the production of condensed-phase reactive oxygen species (ROS). Triplet states or ROS can also react with halides, generating halogen radicals and molecular halogen compounds. In particular, molecular halogens can be released into the gas phase, which is one halogen activation pathway. In this work, we studied the influence of bromide and iodide on the photosensitized production and release of hydroperoxy radicals (HO2) upon UV irradiation of films in a coated wall flow tube (CWFT) containing IC in a matrix of citric acid (CA) irradiated with UV light. In addition, we measured the iodine release upon irradiation of IC ∕ CA films in the CWFT. We developed a kinetic model coupling photosensitized CA oxidation with condensed-phase halogen chemistry to support data analysis and assessment of atmospheric implications in terms of HO2 production and halogen release in sea spray particles. As indicated by the experimental results and confirmed by the model, significant recycling of halogen species occurred via scavenging reactions with HO2. These prevented the full and immediate release of the molecular halogen (bromine and iodine) produced. Recycling was stronger at low relative humidity, attributed to diffusion limitations. Our findings also show that the HO2 production from BrC or CDOM photosensitized reactions can increase due to the presence of halides, leading to high HO2 turnover, in spite of low release due to the scavenging reactions. We estimated the iodine production within sea salt aerosol particles due to iodide oxidation by ozone (O3) at 5.0×10-6 M s−1 assuming O3 was in Henry's law equilibrium with the particle. However, using an O3 diffusion coefficient of 1×10-12 cm2 s−1, iodine activation in an aged, organic-rich sea spray is estimated to be 5.5×10-8 M s−1. The estimated iodine production from BrC photochemistry based on the results reported here amounts to 4.1×10-7 M s−1 and indicates that BrC photochemistry can exceed O3 reactive uptake in controlling the rates of iodine activation from sea spray particles under dry or cold conditions where diffusion is slow within particles.

Physicochemical uptake and release of volatile organic compounds by soil in coated-wall flow tube experiments with ambient air

Abstract. Volatile organic compounds (VOCs) play a key role in atmospheric chemistry. Emission and deposition on soil have been suggested as important sources and sinks of atmospheric trace gases. The exchange characteristics and heterogeneous chemistry of VOCs on soil, however, are not well understood. We used a newly designed differential coated-wall flow tube system to investigate the long-term variability of bidirectional air–soil exchange of 13 VOCs under ambient air conditions of an urban background site in Beijing. Sterilized soil was investigated to address physicochemical processes and heterogeneous/multiphase reactions independently from biological activity. Most VOCs revealed net deposition with average uptake coefficients (γ) in the range of 10−7–10−6 (referring to the geometric soil surface area), corresponding to deposition velocities (Vd) of 0.0013–0.01 cm s−1 and soil surface resistances (Rc) of 98–745 s cm−1, respectively. Formic acid, however, was emitted at a long-term average rate of ∼6×10-3 nmol m−2 s−1, suggesting that it was formed and released upon heterogeneous oxidation of other VOCs. The soil–atmosphere exchange of one individual VOC species can be affected by both its surface degradation/depletion caused by surface reactions and by competitive uptake or heterogeneous formation/accommodation of other VOC species. Overall, the results show that physicochemical processing and heterogeneous oxidation on soil and soil-derived dust can act as a sink or as a source of atmospheric VOCs, depending on molecular properties and environmental conditions.

Contrary Role of H2O and O2 in the Kinetics of Heterogeneous Photochemical Reactions of SO2 on TiO2.

Although the heterogeneous conversion of SO2 on TiO2 has been considered a potential source of sulfate in the troposphere, little is known about the kinetics of SO2 uptake on TiO2 under ambient conditions. In this study, heterogeneous reactions of SO2 on a TiO2 surface at 298 K were investigated by a coated-wall flow tube reactor and in situ diffuse reflectance Fourier transformed infrared spectroscopy (DRIFTS). The effect of UV irradiation, relative humidity (RH), and O2 on the uptake coefficients and adsorption amount of SO2 have been analyzed comprehensively. UV irradiation exhibits a slight effect on the initial uptake coefficient (γBET), which increases from 1.23 × 10-6 to 1.48 × 10-6 in air conditions and 5.89 × 10-7 to 6.42 × 10-7 in a N2 stream under dry conditions. Surface water exhibits a competition effect on the adsorption of SO2 on TiO2. An increase in RH (in the range of 0-75%) led to a decrease in the γBET and adsorption amount of SO2 both in the dark and the light reaction. In contrast, oxygen plays an important role in the uptake of SO2. Both the uptake coefficients and adsorption amount of SO2 on TiO2 decreased in the N2 stream compared to those in air conditions. Only sulfite was formed during the adsorption of SO2 in dark and dry conditions. The presence of UV irradiation was found to promote the conversion of SO2 to sulfate and the adsorption amount of sulfur species on the TiO2 surface. These results suggest that heterogeneous reaction on TiO2 as a sink for SO2 is not important in the atmosphere.

Microphysics of the aqueous bulk counters the water activity driven rate acceleration of bromide oxidation by ozone from 289-245 K.

The reaction of ozone with bromide is an initiation process in bromine activation resulting in the formation of reactive bromine species with impacts on the fate of compounds in the lower atmosphere. Environmental halide sources often contain organics, which are known to influence aqueous bulk reactivity. Here, we present a study investigating the temperature dependence of bromide oxidation by ozone using a coated wall flow tube reactor coated with an aqueous mixture of citric acid, as a proxy for oxidized secondary organic matter, and sodium bromide. Using the resistor model formulation, we quantify changes in the properties of the aqueous bulk relevant for the observed reactivity. The reactive uptake coefficient decreased from 2 × 10-6 at 289 K to 0.5 × 10-6 at 245 K. Our analysis indicates that the humidity-driven increase in concentration with a corresponding increase in the pseudo-first order reaction rate was countered by the colligative change in ozone solubility and the effect of the organic fraction via increased viscosity and decreased diffusivity of ozone as the temperature decreased. From our parameterization, we provide an extension of the temperature dependence of the reaction rate coefficients driving the oxidation of bromide, and assess the temperature-dependent salting effects of citric acid on ozone solubility. This study shows the effects of the organic species at relatively mild temperatures, between the freezing point and eutectic temperature of sea as is typical for the Earth's cryosphere. Thus, this study may be relevant for atmospheric models at different scales describing halogen activation in the marine boundary layer or free troposphere including matrices such as sea-spray aerosol and brine in sea ice, snow, and around mid-latitude salt lakes.

Ozone Uptake by Clay Dusts under Environmental Conditions

Clay is the most emitted type of dust in the atmosphere and offers a large surface for heterogeneous interactions with atmospheric compounds. In this study, we focus on the uptake of ozone (O3) by montmorillonite and kaolinite dust samples at atmospheric pressure in a coated-wall flow tube reactor. The influence of relevant environmental parameters, such as O3 concentration, relative humidity, and temperature, is determined. A mechanism for O3 interaction with the surface sites of clays is described, and atmospheric implications are discussed. Although the impact of O3 uptake by fresh clay dust seems limited in the atmosphere, this work highlights the need to consider relevant and thoroughly defined uptake coefficients in models. Moreover, the presence of a pressure-dependence in the mechanism of O3 uptake by dust suggested in this work helps reconciling measurements made at atmospheric pressure with those performed at low pressure in Knudsen cells.

Technical note: Influence of surface roughness and local turbulence on coated-wall flow tube experiments for gas uptake and kinetic studies

Abstract. Coated-wall flow tube reactors are frequently used to investigate gas uptake and heterogeneous or multiphase reaction kinetics under laminar flow conditions. Coating surface roughness may potentially distort the laminar flow pattern, induce turbulence and introduce uncertainties in the calculated uptake coefficient based on molecular diffusion assumptions (e.g., Brown/Cooney–Kim–Davis (CKD)/Knopf–Pöschl–Shiraiwa (KPS) methods), which has not been fully resolved in earlier studies. Here, we investigate the influence of surface roughness and local turbulence on coated-wall flow tube experiments for gas uptake and kinetic studies. According to laminar boundary theory and considering the specific flow conditions in a coated-wall flow tube, we derive and propose a critical height δc to evaluate turbulence effects in the design and analysis of coated-wall flow tube experiments. If a geometric coating thickness δg is larger than δc, the roughness elements of the coating may cause local turbulence and result in overestimation of the real uptake coefficient (γ). We further develop modified CKD/KPS methods (i.e., CKD-LT/KPS-LT) to account for roughness-induced local turbulence effects. By combination of the original methods and their modified versions, the maximum error range of γCKD (derived with the CKD method) or γKPS (derived with the KPS method) can be quantified and finally γ can be constrained. When turbulence is generated, γCKD or γKPS can bear large difference compared to γ. Their difference becomes smaller for gas reactants with lower uptake (i.e., smaller γ) and/or for a smaller ratio of the geometric coating thickness to the flow tube radius (δg ∕ R0). On the other hand, the critical height δc can also be adjusted by optimizing flow tube configurations and operating conditions (i.e., tube diameter, length, and flow velocity), to ensure not only unaffected laminar flow patterns but also other specific requirements for an individual flow tube experiment. We use coating thickness values from previous coated-wall flow tube studies to assess potential roughness effects using the δc criterion. In most studies, the coating thickness was sufficiently small to avoid complications, but some may have been influenced by surface roughness and local turbulence effects.

Heterogeneous reaction of NO2 with soot at different relative humidity.

The influences of relative humidity (RH) on the heterogeneous reaction of NO2 with soot were investigated by a coated wall flow tube reactor at ambient pressure. The initial uptake coefficient (γ initial) of NO2 showed a significant decrease with increasing RH from 7 to 70%. The γ initial on "fuel-rich" and "fuel-lean" soot at RH=7% was (2.59±0.20)×10-5 and (5.92±0.34)×10-6, respectively, and it decreased to (5.49±0.83)×10-6 and (7.16±0.73)×10-7 at RH=70%, respectively. Nevertheless, the HONO yields were almost independent of RH, with average values of (72±3)% for the fuel-rich soot and (60±2)% for the fuel-lean soot. The Langmuir-Hinshelwood mechanism was used to demonstrate the negative role of RH in the heterogeneous uptake of NO2 on soot. The species containing nitrogen formed on soot can undergo hydrolysis to produce carboxylic species or alcohols at high RH, accompanied by the release of little gas-phase HONO and NO.

Heterogeneous photochemistry of imidazole-2-carboxaldehyde: HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; radical formation and aerosol growth

Abstract. The multiphase chemistry of glyoxal is a source of secondary organic aerosol (SOA), including its light-absorbing product imidazole-2-carboxaldehyde (IC). IC is a photosensitizer that can contribute to additional aerosol ageing and growth when its excited triplet state oxidizes hydrocarbons (reactive uptake) via H-transfer chemistry. We have conducted a series of photochemical coated-wall flow tube (CWFT) experiments using films of IC and citric acid (CA), an organic proxy and H donor in the condensed phase. The formation rate of gas-phase HO2 radicals (PHO2) was measured indirectly by converting gas-phase NO into NO2. We report on experiments that relied on measurements of NO2 formation, NO loss and HONO formation. PHO2 was found to be a linear function of (1) the [IC] × [CA] concentration product and (2) the photon actinic flux. Additionally, (3) a more complex function of relative humidity (25 % &lt; RH &lt; 63 %) and of (4) the O2 ∕ N2 ratio (15 % &lt; O2 ∕ N2 &lt; 56 %) was observed, most likely indicating competing effects of dilution, HO2 mobility and losses in the film. The maximum PHO2 was observed at 25–55 % RH and at ambient O2 ∕ N2. The HO2 radicals form in the condensed phase when excited IC triplet states are reduced by H transfer from a donor, CA in our system, and subsequently react with O2 to regenerate IC, leading to a catalytic cycle. OH does not appear to be formed as a primary product but is produced from the reaction of NO with HO2 in the gas phase. Further, seed aerosols containing IC and ammonium sulfate were exposed to gas-phase limonene and NOx in aerosol flow tube experiments, confirming significant PHO2 from aerosol surfaces. Our results indicate a potentially relevant contribution of triplet state photochemistry for gas-phase HO2 production, aerosol growth and ageing in the atmosphere.

Uptake of gaseous formaldehyde by soil surfaces: a combination of adsorption/desorption equilibrium and chemical reactions

Abstract. Gaseous formaldehyde (HCHO) is an important precursor of OH radicals and a key intermediate molecule in the oxidation of atmospheric volatile organic compounds (VOCs). Budget analyses reveal large discrepancies between modeled and observed HCHO concentrations in the atmosphere. Here, we investigate the interactions of gaseous HCHO with soil surfaces through coated-wall flow tube experiments applying atmospherically relevant HCHO concentrations of ∼ 10 to 40 ppbv. For the determination of uptake coefficients (γ), we provide a Matlab code to account for the diffusion correction under laminar flow conditions. Under dry conditions (relative humidity = 0 %), an initial γ of (1.1 ± 0.05) × 10−4 is determined, which gradually drops to (5.5 ± 0.4) × 10−5 after 8 h experiments. Experiments under wet conditions show a smaller γ that drops faster over time until reaching a plateau. The drop of γ with increasing relative humidity as well as the drop over time can be explained by the adsorption theory in which high surface coverage leads to a reduced uptake rate. The fact that γ stabilizes at a non-zero plateau suggests the involvement of irreversible chemical reactions. Further back-flushing experiments show that two-thirds of the adsorbed HCHO can be re-emitted into the gas phase while the residual is retained by the soil. This partial reversibility confirms that HCHO uptake by soil is a complex process involving both adsorption/desorption and chemical reactions which must be considered in trace gas exchange (emission or deposition) at the atmosphere–soil interface. Our results suggest that soil and soil-derived airborne particles can either act as a source or a sink for HCHO, depending on ambient conditions and HCHO concentrations.

The Role of Iron-Bearing Minerals in NO2 to HONO Conversion on Soil Surfaces.

Nitrous acid (HONO) accumulates in the nocturnal boundary layer where it is an important source of daytime hydroxyl radicals. Although there is clear evidence for the involvement of heterogeneous reactions of NO2 on surfaces as a source of HONO, mechanisms remain poorly understood. We used coated-wall flow tube measurements of NO2 reactivity on environmentally relevant surfaces (Fe (hydr)oxides, clay minerals, and soil from Arizona and the Saharan Desert) and detailed mineralogical characterization of substrates to show that reduction of NO2 by Fe-bearing minerals in soil can be a more important source of HONO than the putative NO2 hydrolysis mechanism. The magnitude of NO2-to-HONO conversion depends on the amount of Fe(2+) present in substrates and soil surface acidity. Studies examining the dependence of HONO flux on substrate pH revealed that HONO is formed at soil pH < 5 from the reaction between NO2 and Fe(2+)(aq) present in thin films of water coating the surface, whereas in the range of pH 5-8 HONO stems from reaction of NO2 with structural iron or surface complexed Fe(2+) followed by protonation of nitrite via surface Fe-OH2(+) groups. Reduction of NO2 on ubiquitous Fe-bearing minerals in soil may explain HONO accumulation in the nocturnal boundary layer and the enhanced [HONO]/[NO2] ratios observed during dust storms in urban areas.

SO2 Uptake on Oleic Acid: A New Formation Pathway of Organosulfur Compounds in the Atmosphere

Organosulfates are tracers for secondary organic aerosol (SOA) formation. We propose a new mechanism of organosulfur product formation in the atmosphere, in which sulfur dioxide (SO2) reacts directly with alkenes. The experiments were conducted at the gas–liquid interface with a coated-wall flow tube reactor. It was shown, for the first time, that SO2 reacts efficiently with the unsaturated bond in oleic acid under atmospheric conditions (without ozone), leading to the formation of C9 and C18 organosulfur products. The associated uptake coefficients were in excess of 10–6, decreasing with initial SO2 concentration and increasing with humidity. These results might explain a fraction of organosulfur products detected in atmospheric particles. This work tends to elucidate the role of organosulfates’ interfacial chemistry as a potentially unrecognized pathway for their contribution to SOA formation; however, it remains to be determined how significant this pathway is to the overall organosulfate abundances measured in ambient aerosol.

Protein Cross-Linking and Oligomerization through Dityrosine Formation upon Exposure to Ozone.

Air pollution is a potential driver for the increasing prevalence of allergic disease, and post-translational modification by air pollutants can enhance the allergenic potential of proteins. Here, the kinetics and mechanism of protein oligomerization upon ozone (O3) exposure were studied in coated-wall flow tube experiments at environmentally relevant O3 concentrations, relative humidities and protein phase states (amorphous solid, semisolid, and liquid). We observed the formation of protein dimers, trimers, and higher oligomers, and attribute the cross-linking to the formation of covalent intermolecular dityrosine species. The oligomerization proceeds fast on the surface of protein films. In the bulk material, reaction rates are limited by diffusion depending on phase state and humidity. From the experimental data, we derive a chemical mechanism and rate equations for a kinetic multilayer model of surface and bulk reaction enabling the prediction of oligomer formation. Increasing levels of tropospheric O3 in the Anthropocene may promote the formation of protein oligomers with enhanced allergenicity and may thus contribute to the increasing prevalence of allergies.