Unveiling the underestimated direct emissions of nitrous acid (HONO).

Gaseous nitrous acid (HONO) is a critical source of hydroxyl radicals (OH) in the troposphere. While both direct and secondary sources contribute to atmospheric HONO, direct emissions have traditionally been considered minor contributors. In this study, we developed δ15N and δ18O isotopic fingerprints to identify six direct HONO emission sources and conducted a 1-y case study on the isotopic composition of atmospheric HONO at rural and urban sites. Interestingly, we identified that livestock farming is a previously overlooked direct source of HONO and determined its HONO to ammonia (NH3) emission ratio. Additionally, our results revealed that spatial and temporal variations in atmospheric HONO isotopic composition can be partially attributed to direct emissions. Through a detailed HONO budget analysis incorporating agricultural sources, we found that direct HONO emissions accounted for 39~45% of HONO production in rural areas across different seasons. The findings were further confirmed by chemistry transport model simulations, highlighting the significance of direct HONO emissions and their impact on air quality in the North China Plain. These findings provide compelling evidence that direct HONO emissions play a more substantial role in contributing to atmospheric HONO than previously believed. Moreover, the δ15N and δ18O isotopic fingerprints developed in this study may serve as a valuable tool for further research on the atmospheric chemistry of reactive nitrogen gases.

Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy.

Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-µm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

Widespread Pesticide Distribution in the European Atmosphere Questions their Degradability in Air.

Risk assessment of pesticide impacts on remote ecosystems makes use of model-estimated degradation in air. Recent studies suggest these degradation rates to be overestimated, questioning current pesticide regulation. Here, we investigated the concentrations of 76 pesticides in Europe at 29 rural, coastal, mountain, and polar sites during the agricultural application season. Overall, 58 pesticides were observed in the European atmosphere. Low spatial variation of 7 pesticides suggests continental-scale atmospheric dispersal. Based on concentrations in free tropospheric air and at Arctic sites, 22 pesticides were identified to be prone to long-range atmospheric transport, which included 15 substances approved for agricultural use in Europe and 7 banned ones. Comparison between concentrations at remote sites and those found at pesticide source areas suggests long atmospheric lifetimes of atrazine, cyprodinil, spiroxamine, tebuconazole, terbuthylazine, and thiacloprid. In general, our findings suggest that atmospheric transport and persistence of pesticides have been underestimated and that their risk assessment needs to be improved.

Photo-Oxidation of α-Pinene Oxidation Products in Atmospheric Waters - pH- and Temperature-Dependent Kinetic Studies.

The atmospheric α-pinene oxidation leads to three carboxylic acids: norpinonic acid (NPA), pinic acid (PA), and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA). In this study, the OH radical kinetics in the aqueous phase of these carboxylic acids were investigated at different temperatures and pH values of solutions. Activation parameters and the corresponding atmospheric lifetimes of the acids in the troposphere were derived. The overall second-order rate constants for the individual speciation forms of the acids (AH and A- for NPA; AH2, AH- and A2- for PA; and AH3, AH2-, AH2- and A3- for MBTCA) were determined. At 298 K, the rate constants for reactions of protonated forms (AHx) of NPA, PA, and MBTCA with •OH, were (1.5 ± 0.2) × 109 L mol-1 s-1, (2.4 ± 0.1) × 109 L mol-1 s-1, and (4.1 ± 0.6) × 108 L mol-1 s-1, respectively. For the fully deprotonated forms (Ax-) of studied acids, the second-order rate constants were (2.2 ± 0.2) × 109 L mol-1 s-1, (2.8 ± 0.1) × 109 L mol-1 s-1, and (10.2 ± 0.7) × 108 L mol-1 s-1 at 298 K, respectively. It was found that the reactions of NPA and PA with OH radicals are faster than with MBTCA. For MBTCA, the reaction rate depends on pH more strongly at elevated temperatures (>298 K). The atmospheric lifetimes of the acids considered due to their reactivity with •OH were calculated for different model scenarios at a temperature of 283 K and pH = 2 in the aqueous phase. For this purpose, liquid water content (LWC) was used for aerosols and clouds under storm conditions and at various aqueous-phase concentrations of OH radicals. The lifetimes decreased with increasing LWC (from 10-12 m3 m-3 in aerosol to 10-5 m3 m-3 in storms), indicating that the acids undergo significant aqueous processing under realistic atmospheric conditions. Besides, the aerosol systems appeared less effective in removing PA and NPA, with lifetimes ranging from hundreds of days to tens and hundreds of hours, respectively. Clouds were more effective, with lifetimes ranging from tens of hours to a single second or less. MBTCA, which dissolves better in water, was effectively removed in all systems, with the longest lifetime of approximately 90 min.

The influence of long-range transported Saharan dust on the inflammatory potency of ambient PM2.5 and PM10.

Although desert dust promotes morbidity and mortality, it is exempt from regulations. Its health effects have been related to its inflammatory properties, which can vary between source regions. It remains unclear which constituents cause this variability. Moreover, whether long-range transported desert dust potentiates the hazardousness of local particulate matter (PM) is still unresolved. We aimed to assess the influence of long-range transported desert dust on the inflammatory potency of PM2.5 and PM10 collected in Cape Verde and to examine associated constituents. During a reference period and two Saharan dust events, 63 PM2.5 and PM10 samples were collected at four sampling stations. The content of water-soluble ions, elements, and organic and elemental carbon was measured in all samples and endotoxins in PM10 samples. The PM-induced release of inflammatory cytokines from differentiated THP-1 macrophages was evaluated. The association of interleukin (IL)-1ß release with PM composition was assessed using principal component (PC) regressions. PM2.5 from both dust events and PM10 from one event caused higher IL-1ß release than PM from the reference period. PC regressions indicated an inverse relation of IL-1ß release with sea spray ions in both size fractions and organic and elemental carbon in PM2.5. The PC with the higher regression coefficient suggested that iron and manganese may contribute to PM2.5-induced IL-1ß release. Only during the reference period, endotoxin content strongly differed between sampling stations and correlated with inflammatory potency. Our results demonstrate that long-range transported desert dust amplifies the hazardousness of local air pollution and suggest that, in PM2.5, iron and manganese may be important. Our data indicate that endotoxins are contained in local and long-range transported PM10 but only explain the variability in inflammatory potency of local PM10. The increasing inflammatory potency of respirable and inhalable PM from desert dust events warrants regulatory measures and risk mitigation strategies.

Gas-Phase Formation of Sulfurous Acid (H2SO3) in the Atmosphere.

Sulfurous acid (H2SO3) is known to be thermodynamically instable decomposing into SO2 and H2O. All attempts to detect this elusive acid in solution failed up to now. Reported H2SO3 formation from an experiment carried out in a mass spectrometer as well as results from theoretical calculations, however, indicated a possible kinetic stability in the gas phase. Here, it is shown experimentally that H2SO3 is formed in the OH radical-initiated gas-phase oxidation of methanesulfinic acid (CH3S(O)OH) at 295±0.5 K and 1 bar of air with a molar yield of 53 - 17 + 7 ${{53}_{-17}^{+\ 7}}$ %. Further main products are SO2, SO3 and methanesulfonic acid. CH3S(O)OH represents an important intermediate product of dimethyl sulfide oxidation in the atmosphere. Global modeling predicts an annual H2SO3 production of ∼8 million metric tons from the OH+CH3S(O)OH reaction. The investigated H2SO3 depletion in the presence of water vapor results in k(H2O+H2SO3) <3×10-18 cm3 molecule-1 s-1, which indicates a lifetime of at least one second for atmospheric humidity. This work provides experimental evidence that H2SO3, once formed in the gas phase, is kinetically stable enough to allow its characterization and subsequent reactions.

Daytime Atmospheric Halogen Cycling through Aqueous-Phase Oxygen Atom Chemistry.

Halogen atoms are important atmospheric oxidants that have unidentified daytime sources from photochemical halide oxidation in sea salt aerosols. Here, we show that the photolysis of nitrate in aqueous chloride solutions generates nitryl chloride (ClNO2) in addition to Cl2 and HOCl. Experimental and modeling evidence suggests that O(3P) formed in the minor photolysis channel from nitrate oxidizes chloride to Cl2 and HOCl, which reacts with nitrite to form ClNO2. This chemistry is different than currently accepted mechanisms involving chloride oxidation by OH and could shift our understanding of daytime halogen cycling in the lower atmosphere.

Field Evidence for Asian Outflow and Fast Depletion of Total Gaseous Mercury in the Polluted Coastal Atmosphere.

Atmospheric mercury (Hg) cycling in polluted coastal atmosphere is complicated and not fully understood. Here, we present measurements of total gaseous mercury (TGM) monitored at a coastal mountaintop in Hong Kong downwind of mainland China. Sharp TGM peaks during cold front passages were frequently observed due to Asian pollution outflow with typical TGM/CO slopes of 6.8 ± 2.2 pg m-3 ppbv-1. Contrary to the daytime maximums of other air pollutants, TGM exhibited a distinct diurnal variation with a midday minimum. Moreover, we observed four cases of extremely fast TGM depletion after sunrise, during which TGM concentrations rapidly dipped to 0.3-0.6 ng m-3 accompanied by other pollutants on the rise. Simulated meteorological fields revealed that morning upslope flow transporting anthropogenically polluted but TGM-depleted air masses from the mixed layer caused morning TGM depletion at the mountaintop location. The TGM-depleted air masses were hypothesized to result mainly from fast photooxidation of Hg after sunrise with minor contributions from dry deposition (5.0%) and nocturnal oxidation (0.6%). A bromine-induced two-step oxidation mechanism involving abundant pollutants (NO2, O3, etc.) was estimated to play a dominant role, contributing 55%-60% of depleted TGM and requiring 0.20-0.26 pptv Br, an amount potentially available through sea salt aerosol debromination. Our findings suggest significant effects of the interaction between anthropogenic pollution and marine halogen chemistry on atmospheric Hg cycling in the coastal areas.

Evolution of Ozone Pollution in China: What Track Will It Follow?

Increasing surface ozone (O3) concentrations has emerged as a key air pollution problem in many urban regions worldwide in the last decade. A longstanding major issue in tackling ozone pollution is the identification of the O3 formation regime and its sensitivity to precursor emissions. In this work, we propose a new transformed empirical kinetic modeling approach (EKMA) to diagnose the O3 formation regime using regulatory O3 and NO2 observation datasets, which are easily accessible. We demonstrate that mapping of monitored O3 and NO2 data on the modeled regional O3-NO2 relationship diagram can illustrate the ozone formation regime and historical evolution of O3 precursors of the region. By applying this new approach, we show that for most urban regions of China, the O3 formation is currently associated with a volatile organic compound (VOC)-limited regime, which is located within the zone of daytime-produced O3 (DPO3) to an 8h-NO2 concentration ratio below 8.3 ([DPO3]/[8h-NO2] ≤ 8.3). The ozone production and controlling effects of VOCs and NOx in different cities of China were compared according to their historical O3-NO2 evolution routes. The approach developed herein may have broad application potential for evaluating the efficiency of precursor controls and further mitigating O3 pollution, in particular, for regions where comprehensive photochemical studies are unavailable.

Humic Substance Photosensitized Degradation of Phthalate Esters Characterized by 2H and 13C Isotope Fractionation.

The photosensitized transformation of organic chemicals is an important degradation mechanism in natural surface waters, aerosols, and water films on surfaces. Dissolved organic matter including humic-like substances (HS), acting as photosensitizers that participate in electron transfer reactions, can generate a variety of reactive species, such as OH radicals and excited triplet-state HS (3HS*), which promote the degradation of organic compounds. We use phthalate esters, which are important contaminants found in wastewaters, landfills, soils, rivers, lakes, groundwaters, and mine tailings. We use phthalate esters as probes to study the reactivity of HS irradiated with artificial sunlight. Phthalate esters with different side-chain lengths were used as probes for elucidation of reaction mechanisms using 2H and 13C isotope fractionation. Reference experiments with the artificial photosensitizers 4,5,6,7-tetrachloro-2',4',5',7'-tetraiodofluorescein (Rose Bengal), 3-methoxy-acetophenone (3-MAP), and 4-methoxybenzaldehyde (4-MBA) yielded characteristic fractionation factors (-4 ± 1, -4 ± 2, and -4 ± 1‰ for 2H; 0.7 ± 0.2, 1.0 ± 0.4, and 0.8 ± 0.2‰ for 13C), allowing interpretation of reaction mechanisms of humic substances with phthalate esters. The correlation of 2H and 13C fractions can be used diagnostically to determine photosensitized reactions in the environment and to differentiate among biodegradation, hydrolysis, and photosensitized HS reaction.

Apportioning Atmospheric Ammonia Sources across Spatial and Seasonal Scales by Their Isotopic Fingerprint.

Mitigating ammonia (NH3) emissions is a significant challenge, given its well-recognized role in the troposphere, contributing to secondary particle formation and impacting acid rain. The difficulty arises from the highly uncertain attribution of atmospheric NH3 to specific emission sources, especially when accounting for diverse environments and varying spatial and temporal scales. In this study, we established a refined δ15N fingerprint for eight emission sources, including three previously overlooked sources of potential importance. We applied this approach in a year-long case study conducted in urban and rural sites located only 40 km apart in the Shandong Peninsula, North China Plain. Our findings highlight that although atmospheric NH3 concentrations and seasonal trends exhibited similarities, their isotopic compositions revealed significant distinctions in the primary NH3 sources. In rural areas, although agriculture emerged as the dominant emission source (64.2 ± 19.5%), a previously underestimated household stove source also played a considerably greater role, particularly during cold seasons (36.5 ± 12.5%). In urban areas, industry and traffic (33.5 ± 15.6%) and, surprisingly, sewage treatment (27.7 ± 11.3%) associated with high population density were identified as the major contributors. Given the relatively short lifetime of atmospheric NH3, our findings highlight the significance of the isotope approach in offering a more comprehensive understanding of localized and seasonal influences of NH3 sources compared to emissions inventories. The refined isotopic fingerprint proves to be an effective tool in distinguishing source contributions across spatial and seasonal scales, thereby providing valuable insights for the development of emission mitigation policies aimed at addressing the increasing NH3 burden on the local atmosphere.

Temperature-Dependent Oxidation of Hydroxylated Aldehydes by •OH, SO4•-, and NO3• Radicals in the Atmospheric Aqueous Phase.

T-dependent aqueous-phase rate constants were determined for the oxidation of the hydroxy aldehydes, glyceraldehyde, glycolaldehyde, and lactaldehyde, by the hydroxyl radicals (•OH), the sulfate radicals (SO4•-), and the nitrate radicals (NO3•). The obtained Arrhenius expressions for the oxidation by the •OH radical are: k(T,GLYCERALDEHYDE+OH•) = (3.3 ± 0.1) × 1010 × exp((-960 ± 80 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+OH•) = (4.3 ± 0.1) × 1011 × exp((-1740 ± 50 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+OH•) = (1.6 ± 0.1) × 1011 × exp((-1410 ± 180 K)/T)/L mol-1 s-1; for the SO4•- radical: k(T,GLYCERALDEHYDE+SO4•-) = (4.3 ± 0.1) × 109 × exp((-1400 ± 50 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+SO4•-) = (10.3 ± 0.3) × 109 × exp((-1730 ± 190 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+SO4•-) = (2.2 ± 0.1) × 109 × exp((-1030 ± 230 K)/T)/L mol-1 s-1; and for the NO3• radical: k(T,GLYCERALDEHYDE+NO3•) = (3.4 ± 0.2) × 1011 × exp((-3470 ± 460 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+NO3•) = (7.8 ± 0.2) × 1011 × exp((-3820 ± 240 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+NO3•) = (4.3 ± 0.2) × 1010 × exp((-2750 ± 340 K)/T)/L mol-1 s-1, respectively. Targeted simulations of multiphase chemistry reveal that the oxidation by OH radicals in cloud droplets is important under remote and wildfire influenced continental conditions due to enhanced partitioning. There, the modeled average aqueous •OH concentration is 2.6 × 10-14 and 1.8 × 10-14 mol L-1, whereas it is 7.9 × 10-14 and 3.5 × 10-14 mol L-1 under wet particle conditions. During cloud periods, the aqueous-phase reactions by •OH contribute to the oxidation of glycolaldehyde, lactaldehyde, and glyceraldehyde by about 35 and 29%, 3 and 3%, and 47 and 37%, respectively.

Saharan dust induces NLRP3-dependent inflammatory cytokines in an alveolar air-liquid interface co-culture model.

BACKGROUND: Epidemiological studies have related desert dust events to increased respiratory morbidity and mortality. Although the Sahara is the largest source of desert dust, Saharan dust (SD) has been barely examined in toxicological studies. Here, we aimed to assess the NLRP3 inflammasome-caspase-1-pathway-dependent pro-inflammatory potency of SD in comparison to crystalline silica (DQ12 quartz) in an advanced air-liquid interface (ALI) co-culture model. Therefore, we exposed ALI co-cultures of alveolar epithelial A549 cells and macrophage-like differentiated THP-1 cells to 10, 21, and 31 µg/cm² SD and DQ12 for 24 h using a Vitrocell Cloud system. Additionally, we exposed ALI co-cultures containing caspase (CASP)1-/- and NLRP3-/- THP-1 cells to SD. RESULTS: Characterization of nebulized DQ12 and SD revealed that over 90% of agglomerates of both dusts were smaller than 2.5 µm. Characterization of the ALI co-culture model revealed that it produced surfactant protein C and that THP-1 cells remained viable at the ALI. Moreover, wild type, CASP1-/-, and NLRP3-/- THP-1 cells had comparable levels of the surface receptors cluster of differentiation 14 (CD14), toll-like receptor 2 (TLR2), and TLR4. Exposing ALI co-cultures to non-cytotoxic doses of DQ12 and SD did not induce oxidative stress marker gene expression. SD but not DQ12 upregulated gene expressions of interleukin 1 Beta (IL1B), IL6, and IL8 as well as releases of IL-1ß, IL-6, IL-8, and tumor necrosis factor α (TNFα). Exposing wild type, CASP1-/-, and NLRP3-/- co-cultures to SD induced IL1B gene expression in all co-cultures whereas IL-1ß release was only induced in wild type co-cultures. In CASP1-/- and NLRP3-/- co-cultures, IL-6, IL-8, and TNFα releases were also reduced. CONCLUSIONS: Since surfactants can decrease the toxicity of poorly soluble particles, the higher potency of SD than DQ12 in this surfactant-producing ALI model emphasizes the importance of readily soluble SD components such as microbial compounds. The higher potency of SD than DQ12 also renders SD a potential alternative particulate positive control for studies addressing acute inflammatory effects. The high pro-inflammatory potency depending on NLRP3, CASP-1, and IL-1ß suggests that SD causes acute lung injury which may explain desert dust event-related increased respiratory morbidity and mortality.

Unraveling the O3-NOX-VOCs relationships induced by anomalous ozone in industrial regions during COVID-19 in Shanghai.

The COVID-19 pandemic promoted strict restrictions to human activities in China, which led to an unexpected increase in ozone (O3) regarding to nitrogen oxides (NOx) and volatile organic compounds (VOCs) co-abatement in urban China. However, providing a quantitative assessment of the photochemistry that leads to O3 increase is still challenging. Here, we evaluated changes in O3 arising from photochemical production with precursors (NOX and VOCS) in industrial regions in Shanghai during the COVID-19 lockdowns by using machine learning models and box models. The changes of air pollutants (O3, NOX, VOCs) during the COVID-19 lockdowns were analyzed by deweathering and detrending machine learning models with regard to meteorological and emission effects. After accounting for effects of meteorological variability, we find increase in O3 concentration (49.5%). Except for meteorological effects, model results of detrending the business-as-usual changes indicate much smaller reduction (-0.6%), highlighting the O3 increase attributable to complex photochemistry mechanism and the upward trends of O3 due to clear air policy in Shanghai. We then used box models to assess the photochemistry mechanism and identify key factors that control O3 production during lockdowns. It was found that empirical evidence for a link between efficient radical propagation and the optimized O3 production efficiency of NOX under the VOC-limited conditions. Simulations with box models also indicate that priority should be given to controlling industrial emissions and vehicle exhaust while the VOCs and NOX should be managed at a proper ratio in order to control O3 in winter. While lockdown is not a condition that could ever be continued indefinitely, findings of this study offer theoretical support for formulating refined O3 management in industrial regions in Shanghai, especially in winter.

Nontarget Approach to Identify Complexing Agents in Atmospheric Aerosol and Rainwater Samples.

Atmospheric particles and droplets contain numerous organic substances, some of which form complexes with metal ions, significantly affecting bulk physicochemical properties and chemical reactivity. However, the detection and identification of complexing agents and their corresponding metal complexes remains an analytical challenge. In this study, we developed an LC/HRMS nontarget screening (NTS) approach which allows the selective detection of complexing agents in aerosol particle extracts and rainwater. To achieve this, a T-junction is installed between the LC outlet and the ion source, and a FeCl3 solution is added for postcolumn complexation. The resulting mass spectra are screened for the three characteristic iron(III)-complexes [M - H + FeCl3]-, [M - 2H + FeCl2]-, and [M - 3H + FeCl]- with mass differences (Δm/z) between the complexing agent and the iron complex of 160.8416, 124.8648, and 89.8959, respectively. Up to 29 di- or tricarboxylic acids were identified as complexing agents in aerosol particle samples from two different sites (Melpitz, Germany, and Wangdu, China) at concentrations as low as 50 nM. Thirteen complexing agents were detected even in measurements without postcolumn iron addition from complexation with background Fe3+ traces from the analytical system. At least for the highest concentrated complexing agents, the proposed screening approach can thus be exploited in a NTS approach without any device modification. Besides carboxylic acids, 4-nitrophenol and 4-nitrocatechol were identified as further complexing agents, demonstrating the applicability of the approach to other matrices and to a range of different complexing agents.

Aerosol Hygroscopicity and its Link to Chemical Composition in a Remote Marine Environment Based on Three Transatlantic Measurements.

The hygroscopicity of marine aerosols may largely impact particle optical properties, cloud activation ability, and consequently the global climate system. This study highlights findings from real-time hygroscopicity and chemical composition measurements in three open-ocean cruises over the Atlantic Ocean. Spatial variations in hygroscopicity (κ) for marine boundary layer particles (≤300 nm) were provided for the first time covering nearly 100° of the latitude over the Atlantic Ocean, ranging from 0.14 to 1.06. Externally mixed particles with remarkably low hygroscopicity (0.14-0.16) were observed near the equator influenced by biomass burning emissions transported from Africa. For marine aerosols, a positive linear correlation evidently existed between κ and wind speed within a range of 5-15 m/s even for nanometer particles. A closure study shows that the measured κ of 300 nm particles is well explained by the bulk chemical composition. A good negative correlation between measured κ and the organic mass fraction in PM1 for marine aerosols was found (slope = -2.26, R2 = 0.44), while a different linear relationship appeared for continental aerosols at several sites (slope = -0.47, R2 = 0.77). Accordingly, we provide a parameterization method to estimate bulk aerosol hygroscopicity both in continental and marine environments using particulate organic fractions.

T- and pH-dependent OH radical reaction kinetics with glycine, alanine, serine, and threonine in the aqueous phase.

Glycine, alanine, serine, and threonine are essential amino acids originating from biological activities. These substances can be emitted into the atmosphere directly. In the present study, the aqueous phase reaction kinetics of hydroxyl radicals (˙OH) with the four amino acids is investigated using the competition kinetics method under controlled temperature and pH conditions. The following T-dependent Arrhenius expressions are derived for the ˙OH reactions with glycine, k(T, H2A+) = (9.1 ± 0.3) × 109 × exp[(-2360 ± 230 K)/T], k(T, HA±) = (1.3 ± 0.1) × 1010 × exp[(-2040 ± 240 K)/T]; alanine, k(T, H2A+) = (1.4 ± 0.1) × 109 × exp[(-1120 ± 320 K)/T], k(T, HA±) = (5.5 ± 0.2) × 109 × exp[(-1300 ± 200 K)/T]; serine, k(T, H2A+) = (1.1 ± 0.1) × 109 × exp[(-470 ± 150 K)/T], k(T, HA±) = (3.9 ± 0.1) × 109 × exp[(-720 ± 130 K)/T]; and threonine, k(T, H2A+) = (5.0 ± 0.1) × 1010 × exp[(-1500 ± 100 K)/T], k(T, HA±) = (3.3 ± 0.1) × 1010 × exp[(-1320 ± 90 K)/T] (in units of L mol-1 s-1). The energy barriers of the ˙OH-induced H atom abstractions were simulated by the density functional theory (DFT) calculation performed with GAUSSIAN using the method of M06-2X and the basis set of 6-311++G(3df,2p). According to the calculation results, the -COOH and -NH3+ groups with strong negative inductive effects increase the energy barriers and thus decrease the ˙OH reaction rate constants. In contrast, the presence of a -OH or -CH3 group with weak negative or positive inductive effects can reduce energy barriers and hence increase the ˙OH reaction rate constants. To improve the previous structure-activity relationship, the contribution factors of -NH3+ at Cα-atom and Cß-atom are determined as 0.07 and 0.15, respectively. Aqueous phase ˙OH oxidation acts as an important sink of the amino acids in the atmosphere, and can be accurately described by the obtained Arrhenius expressions under atmospheric conditions.

Temperature- and pH- Dependent OH Radical Reaction Kinetics of Tartaric and Mucic Acids in the Aqueous Phase.

Tartaric acid and mucic acid are dicarboxylic acids (DCAs), a substance class often found in atmospheric aerosols and cloud droplets. The hydroxyl radical (•OH)-induced oxidation in the aqueous phase is known to be an important loss process of organic compounds such as DCAs. However, the study of •OH kinetics of DCAs in the aqueous phase is still incomplete. In the present study, the rate constants of the •OH reactions of tartaric acid and mucic acid in the aqueous phase were determined by the thiocyanate competition kinetics method as a function of temperature and pH. The following T-dependent Arrhenius expressions (in units of L mol-1 s-1) were first derived for the •OH reactions with tartaric acid─k(T, H2A) = (3.3 ± 0.1) × 1010 exp[(-1350 ± 110 K)/T], k(T, HA-) = (3.6 ± 0.1) × 1010 exp[(-580 ± 110 K)/T], and k(T, A2-) = (3.3 ± 0.1) × 1010 exp[(-1190 ± 170 K)/T]─as well as mucic acid─k(T, H2A) = (2.2 ± 0.1) × 1010 exp[(-1140 ± 150 K)/T], k(T, HA-) = (4.8 ± 0.1) × 1010 exp[(-1280 ± 170 K)/T], and k(T, A2-) = (2.1 ± 0.1) × 1010 exp[(-970 ± 70 K)/T]. A general trend of the •OH rate constant is found as kA2- > kHA- > kH2A. The pH- and temperature-dependent rate constants of the OH radical reactions allow an accurate description of the source and sink processes in the tropospheric aqueous phase.

Kinetics and Mechanisms of Aqueous-Phase Reactions of Triplet-State Imidazole-2-carboxaldehyde and 3,4-Dimethoxybenzaldehyde with α,ß-Unsaturated Carbonyl Compounds.

Reactions in the atmospheric aqueous phase are an important source of secondary organic aerosols (SOA). Within the present study, the reactions of triplet-state imidazole-2-carboxaldehyde (32-IC*) with methyl vinyl ketone (MVK, R1), methacrolein (MACR, R2), and methacrylic acid (MAA, R3), as well as the reaction of triplet-state 3,4-dimethoxybenzaldehyde (3DMB*) with the unsaturated compounds (MVK, R4), (MACR, R5), and (MAA, R6), in the aqueous phase were investigated using laser flash excitation-laser long path absorption and ultraperformance liquid chromatography coupled with high definition electrospray ionization spectrometry. The second-order reaction constants for 32-IC* were determined to be k1 = (1.0 ± 0.1) × 109 L mol-1 s-1 at pH 4-5 and 9, k2 = (1.4 ± 0.4) × 109 L mol-1 s-1 and (1.5 ± 0.1) × 109 L mol-1 s-1 at pH 4-5 and 9, and k3 = (1.4 ± 0.4) × 109 L mol-1 s-1 and (1.1 ± 0.4) × 108 L mol-1 s-1 at pH 4-5 and 9, respectively. The main products of the [2 + 2] photocycloaddition reactions of 32-IC* with both monomer and dimer of MVK as well as MACR were characterized. Similarly, the [2 + 2] photocycloaddition of the carbonyl of the excited triplet state of 3,4-dimethoxybenzaldehyde (3DMB*) with MVK was observed. The second order rate constants for the reactions of 3DMB* were determined: k4 = (1.5 ± 0.2) × 108 L mol-1 s-1, k5 = (2.8 ± 0.5) × 108 L mol-1 s-1, and k6 = (5.2 ± 1.2) × 106 L mol-1 s-1 at pH 9. The studied reactions show that different triplet photosensitizers react with strongly varying rate constants. Advanced CAPRAM process model studies show that active photosensitizers such as 3DMB* can quickly react with unsaturated organic compounds under deliquesced aerosol conditions modifying SOA, while the quenching with oxygen dominates the excited photosensitizer loss under cloud conditions.

A statistic comparison of multi-element analysis of low atmospheric fine particles (PM2.5) using different spectroscopy techniques.

Over the past few decades, the metal elements (MEs) in atmospheric particles have aroused great attention. Some well-established techniques have been used to measure particle-bound MEs. However, each method has its own advantages and disadvantages in terms of complexity, accuracy, and specific elements of interest. In this study, the performances of inductively coupled plasma-optical emission spectrometry (ICP-OES) and total reflection X-ray fluorescence spectroscopy (TXRF) were evaluated for quality control to analyze data accuracy and precision. The statistic methods (Deming regression and significance testing) were applied for intercomparison between ICP-OES and TXRF measurements for same low-loading PM2.5 samples in Weizhou Island. The results from the replicate analysis of standard filters (SRM 2783) and field filters samples indicated that 10 MEs (K, Ca, V, Cr, Mn, Fe, Ni, Cu, Zn, and Pb) showed good accuracies and precision for both techniques. The higher accuracy tended to the higher precision in the MEs analysis process. In addition, the interlab comparisons illustrated that V and Mn all had good agreements between ICP-OES and TXRF. The measurements of K, Cu and Zn were more reliable by TXRF analysis for low-loading PM2.5. ICP-OES was more accurate for the determinations for Ca, Cr, Ni and Pb, owing to the overlapping spectral lines and low sensitivity during TXRF analysis. The measurements of Fe, influenced by low-loading PM2.5, were not able to determine which instrument could obtain more reliable results. These conclusions could provide reference information to choose suitable instrument for the determination of MEs in low-loading PM2.5 samples.