Interfacial photochemistry at the ocean surface is a global source of organic vapors and aerosols

Aqueous VOCs in complex water environment of oil exploitation sites: Spatial distribution, migration flux, and risk assessment

Pollution of the aqueous environment by volatile organic compounds (VOCs) has caused increasing concerns. However, the occurrence and risks of aqueous VOCs in oil exploitation areas remain unclear. Herein, spatial distribution, migration flux, and environmental risks of VOCs in complex surface waters (including River, Estuary, Offshore and Aquaculture areas) were investigated at a typical coastal oil exploitation site. Among these surface waters, River was the most polluted area, and 1,2-Dichloropropane-which emerges from oil extraction activities-was the most prevalent VOC. Positive matrix factorization showed that VOCs pollution sources changed from oil exploitation to offshore disinfection activities along River, Estuary, Offshore and Aquaculture areas. Annual volatilization of VOCs to the atmosphere was predicted to be ∼34.42 tons, and rivers discharge ∼23.70 tons VOCs into the Bohai Sea annually. Ecological risk assessment indicated that Ethylbenzene and Bromochloromethane posed potential ecological risks to the aquatic environment, while olfactory assessment indicated that VOCs in surface waters did not pose an odor exposure risk. This study provides the first assessment of the pollution characteristics of aqueous VOCs in complex aqueous environments of oil exploitation sites, highlighting that oil exploitation activities can have nonnegligible impacts on VOCs pollution profiles.

Effects of dissolved organic matter and halogen ions on phototransformation of pharmaceuticals and personal care products in aquatic environments

Photochemical reactions contribute to the attenuation and transformation of pharmaceuticals and personal care products (PPCPs) in surface natural waters. Nevertheless, effects of DOM and halogen ions on phototransformation of PPCPs remain elusive. This work selected disparate PPCPs as target pollutants to investigate their aquatic phototransformation processes. Results show that PPCPs containing multiple electron-donating groups (-OH, -NH2, -OR, etc.) are more reactive with photochemically produced reactive intermediates (PPRIs) such as triplet DOM (3DOM*), singlet oxygen (1O2), and reactive halogen species (RHSs), relative to PPCPs containing electron-withdrawing groups (-NOR, -COOR, -OCR, etc.). The generation of RHSs as a result of the coexistance of DOM and halide ions changed the contribution of PPRIs to the photochemical conversion of PPCPs during their migration from fresh water to seawater. For PPCPs (AMP, SMZ, PN, NOR, CIP, etc) with highly reactive groups toward RHSs, the generation of RHSs facilitated their photolysis in halide ion-rich waters, where Cl- plays a critical role in the photochemical transformation of PPCPs. Density functional theory (DFT) calculations showed that single electron transfer and H-abstraction are main reaction pathways of RHSs with the PPCPs. These results demonstate the irreplaceable roles of PPRIs and revealing the underlying reaction mechanisms during the phototransformation of PPCPs, which contributes to a better understanding of the environmental behaviors of PPCPs in complex aquatic environments.

Atmospheric isoprene measurements reveal larger-than-expected Southern Ocean emissions

Isoprene is a key trace component of the atmosphere emitted by vegetation and other organisms. It is highly reactive and can impact atmospheric composition and climate by affecting the greenhouse gases ozone and methane and secondary organic aerosol formation. Marine fluxes are poorly constrained due to the paucity of long-term measurements; this in turn limits our understanding of isoprene cycling in the ocean. Here we present the analysis of isoprene concentrations in the atmosphere measured across the Southern Ocean over 4 months in the summertime. Some of the highest concentrations ( >500 ppt) originated from the marginal ice zone in the Ross and Amundsen seas, indicating the marginal ice zone is a significant source of isoprene at high latitudes. Using the United Kingdom Earth System Model we show that current estimates of sea-to-air isoprene fluxes underestimate observed isoprene by a factor >20. A daytime source of isoprene is required to reconcile models with observations. The model presented here suggests such an increase in isoprene emissions would lead to >8% decrease in the hydroxyl radical in regions of the Southern Ocean, with implications for our understanding of atmospheric oxidation and composition in remote environments, often used as proxies for the pre-industrial atmosphere.

Sunlight Induces the Production of Atmospheric Volatile Organic Compounds (VOCs) from Thermokarst Ponds

Ground subsidence caused by permafrost thawing causes the formation of thermokarst ponds, where organic compounds from eroding permafrost accumulate. We photolyzed water samples from two such ponds in Northern Quebec and discovered the emission of volatile organic compounds (VOCs) using mass spectrometry. One pond near peat-covered permafrost mounds was organic-rich, while the other near sandy mounds was organic-poor. Compounds up to C10 were detected, comprising the atoms of O, N, and S. The main compounds were methanol, acetaldehyde, and acetone. Hourly VOC fluxes under actinic fluxes similar to local solar fluxes might reach up to 1.7 nmol C m-2 s-1. Unexpectedly, the fluxes of VOCs from the organic-poor pond were greater than those from the organic-rich pond. We suggest that different segregations of organics at the air/water interface may partly explain this observation. This study indicates that sunlit thermokarst ponds are a significant source of atmospheric VOCs, which may affect the environment and climate via ozone and aerosol formation. Further work is required for understanding the relationship between the pond's organic composition and VOC emission fluxes.

Impurity contribution to ultraviolet absorption of saturated fatty acids

Saturated fatty acids are abundant organic compounds in oceans and sea sprays. Their photochemical reactions induced by solar radiation have recently been found as an abiotic source of volatile organic compounds, which serve as precursors of secondary organic aerosols. However, photoabsorption of wavelengths longer than 250 nanometers in liquid saturated fatty acids remains unexplained, despite being first reported in 1931. Here, we demonstrate that the previously reported absorption of wavelengths longer than 250 nanometers by liquid nonanoic acid [CH3(CH2)7COOH)] originates from traces of impurities (0.1% at most) intrinsically contained in nonanoic acid reagents. Absorption cross sections of nonanoic acid newly obtained here indicate that the upper limit of its photolysis rate is three to five orders of magnitude smaller than those for atmospherically relevant carbonyl compounds.

Characteristics of dissolved black carbon in riverine surface microlayer

Black carbon (BC) is produced by the incomplete combustion of biomass and fossil fuels. The dissolved form of BC (DBC), which is transported through rivers into the oceans, is of great significance for the carbon cycling on the planet due to its refractory features. However, the characteristics and sources of DBC in riverine water are poorly constrained. Here, we analyzed DBC contents and stable carbon isotope (δ13C) signatures in surface microlayer (SML) from the upper, middle and lower reaches of Pearl River (PR) in the first study of its kind. The DBC contents (100.9 to 166.6 μg L-1) in SML were lower than the global average for riverine water following a trend of upper > middle > lower reaches in PR. The molecular markers of DBC (BPCAs) and their δ13C values showed no statistical differences between the sampling sites (p > 0.05), suggesting biomass burning as the dominant source.

Interfacial photochemistry of marine diatom lipids: Abiotic production of volatile organic compounds and new particle formation

The global importance of abiotic oceanic production of volatile organic compounds (VOCs) still presents a source of high uncertainties related to secondary organic aerosol (SOA) formation. A better understanding of the photochemistry occurring at the ocean-atmosphere interface is particularly important in that regard, as it covers >70% of the Earth's surface. In this work, we focused on the photochemical VOCs production at the air-water interface containing organic material from authentic culture of marine diatom Chaetoceros pseudocurvisetus. Abiotic VOCs production upon irradiation of material originating from total phytoplankton culture as well as the fraction containing only dissolved material was monitored by means of PTR-ToF-MS. Furthermore, isolated dissolved lipid fraction was investigated after its deposition at the air-water interface. All samples acted as a source of VOCs, producing saturated oxygenated compounds such as aldehydes and ketones, as well as unsaturated and functionalized compounds. Additionally, a significant increase in surfactant activity following irradiation experiments observed for all samples implied biogenic material photo-transformation at the air-water interface. The highest VOCs flux normalized per gram of carbon originated from lipid material, and the produced VOCs were introduced into an atmospheric simulation chamber, where particle formation was observed after its gas-phase ozonolysis. This work clearly demonstrates abiotic production of VOCs from phytoplankton derived organic material upon irradiation, facilitated by its presence at the air/water interface, with significant potential for affecting the global climate as a precursor of particle formation.

Inhibition of typical phenolic compounds entering into condensed freshwater by surfactants during solar-driven seawater distillation

Recently, solar-driven seawater desalination based on air-water interfacial heating has triggered significant research interest due to its high water evaporation rate, high photothermal conversion efficiency, low energy consumption, simple operation and low cost. However, as the air-water interface temperature reaches as high as 40-70 °C, volatile organic compounds (VOCs) will volatilize into the condensed desalinated water and results in the polluted freshwater. In this work, anionic, cationic, and nonionic surfactants were applied for the first time to inhibit the phenolic compounds such as phenol, p-methylphenol and p-chlorophenol entering into the condensed freshwater. Results showed that the concentration of phenol could be reduced by the addition of cetyl trimethyl ammonium bromide (CTAB). The phenol's distillation concentration ratio (R_D) reduced from 76% to 35% due to the electrostatic interaction and the micellar encapsulation between the CTAB and phenol. Moreover, parameters including CTAB dose, initial phenol concentration, solar intensity, pH, and salinity that affecting the R_D were also investigated. Finally, a real seawater solar-driven distillation experiment also revealed that the water quality of freshwater was improved by the addition of CTAB. This work revealed that the surfactants such as CTAB can be potentially used to inhibit VOCs entering into the condensed freshwater during solar-driven seawater distillation.

Ambient volatile organic compounds in tropical environments: Potential sources, composition and impacts - A review

Volatile organic compounds (VOCs) are widely recognized to affect the environment and human health. This review provides a comprehensive presentation of the types and levels of VOCs, their sources and potential effects on human health and the environment based on past and current observations made at tropical sites. Isoprene was found to be the dominant biogenic VOC in the tropics. Tropical broad leaf evergreen trees are the main emitters of isoprene, making up more than 70% of the total emissions. The VOCs found in the tropical remote marine atmosphere included isoprene (>100 ppt), dimethyl sulfide (≤100 ppt) and halocarbons, i.e. bromoform (≤8.4 ppt), dibromomethane (≤2.7 ppt) and dibromochloromethane (≤1.6 ppt). VOCs such as benzene, toluene, ethylbenzene and xylene (BTEX) are the most monitored anthropogenic VOCs and are present mainly due to motor vehicles emissions. Additionally, biomass burning contributes to anthropogenic VOCs, especially high molecular weight VOCs, e.g. methanol and acetonitrile. The relative contributions of VOC species to ozone are determined through the level of the Ozone Formation Potential (OFP) of different species. Emissions of VOCs (e.g. very short-lived halogenated gases) in the tropics are capable of contributing to stratospheric ozone depletion. BTEX has been identified as the main types of VOCs that are associated with the cancer risk in urban areas in tropical regions. Finally, future studies related to VOCs in the tropics and their associated health risks are needed to address these concerns.

Ocean Aerobiology

Ocean aerobiology is defined here as the study of biological particles of marine origin, including living organisms, present in the atmosphere and their role in ecological, biogeochemical, and climate processes. Hundreds of trillions of microorganisms are exchanged between ocean and atmosphere daily. Within a few days, tropospheric transport potentially disperses microorganisms over continents and between oceans. There is a need to better identify and quantify marine aerobiota, characterize the time spans and distances of marine microorganisms’ atmospheric transport, and determine whether microorganisms acclimate to atmospheric conditions and remain viable, or even grow. Exploring the atmosphere as a microbial habitat is fundamental for understanding the consequences of dispersal and will expand our knowledge of biodiversity, biogeography, and ecosystem connectivity across different marine environments. Marine organic matter is chemically transformed in the atmosphere, including remineralization back to CO₂. The magnitude of these transformations is insignificant in the context of the annual marine carbon cycle, but may be a significant sink for marine recalcitrant organic matter over long (∼10⁴ years) timescales. In addition, organic matter in sea spray aerosol plays a significant role in the Earth’s radiative budget by scattering solar radiation, and indirectly by affecting cloud properties. Marine organic matter is generally a poor source of cloud condensation nuclei (CCN), but a significant source of ice nucleating particles (INPs), affecting the formation of mixed-phase and ice clouds. This review will show that marine biogenic aerosol plays an impactful, but poorly constrained, role in marine ecosystems, biogeochemical processes, and the Earth’s climate system. Further work is needed to characterize the connectivity and feedbacks between the atmosphere and ocean ecosystems in order to integrate this complexity into Earth System models, facilitating future climate and biogeochemical predictions.

Investigation of the Acid-Mediated Photosensitized Reactions of Amphiphilic α-Keto Acids at the Air-Water Interface Using Field-Induced Droplet Ionization Mass Spectrometry

The photochemistry of α-keto acids has been of great interest due to its implications in atmospheric and prebiotic chemistries. α-Keto acids with long alkyl chains are amphiphilic in nature, and they tend to partition at the air-water interface of atmospheric water droplets and add to the complexity of the chemistries therein. The air-water interface is a unique environment that plays a vital role in overall atmospheric processes. However, existing studies mostly focus on the photochemistry of α-keto acids in the bulk solution and neglect the reactions that occur at the interface. In this study, using the field-induced droplet ionization mass spectrometry methodology that is capable of selectively sampling amphiphilic molecules that reside at the air-water interface, we show that the acid-mediated photochemistry of 2-oxooctanoic acid and 2-oxoheptoic acid is highly different from those of previously reported reactions in the bulk and contributes to the formation of humic-like substances (HULIS). This work emphasizes the uniqueness of the photochemistry at the air-water interface. We anticipate that studies of atmosphere-relevant photochemistry at the air-water interface will be an avenue rich with opportunities.

Photo-induced degradation and toxicity change of decabromobiphenyl ethers (BDE-209) in water: Effects of dissolved organic matter and halide ions

BDE-209 is a widely used brominated flame retardant that is ubiquitous in the aquatic environment, especially in marine water. However, photodegradation of BDE-209 in seawater is still not fully understood. In this work, the photodegradation kinetics of BDE-209 in water was studied and the effects of seawater dissolved organic matter (S-DOM) and halide ions (Cl^-, Br^-) were evaluated. S-DOM inhibited the degradation of BDE-209 through dynamic quenching and light shielding effect. However, with the coexistence of S-DOM, Cl^- and Br^-, the photodegradation of BDE-209 was significantly promoted. The promotional effect is attributed to the generation of excited triplet state S-DOM, singlet oxygen, and reactive halogen radicals. The results of density functional theory calculation showed that •Cl addition reaction on C-Br sites of BDE-209 is the main reaction pathway of BDE-209 with chlorine radical, which leads to the generation of mixed Cl/Br substituted intermediates. The acute toxicity and estrogenic effects of BDE-209 solution were enhanced during simulated sunlight irradiation. These results indicate that the environmental factors in seawater play important roles in the photodegradation of BDE-209, and contribute to the potential ecological risks of PBDEs in the marine environment.

Impact of Gas-Liquid Interface on Photochemical Vapor Generation

Interfacial effect has attracted increasing interest as the inherent asymmetric environment of a gas-liquid interface leads to different chemical and physical properties between this region and the bulk phase, resulting in enhanced chemical processes, specific reactions, and mass transfer at the interface. Photochemical vapor generation (PVG) is regarded as a simple and green sample introduction method in atomic spectrometry. However, the photochemical behavior of elements with the interface is not known. Herein, we report the PVG of elements at the gas-liquid interface along with a possible mechanism investigated for the first time. Enhancement and/or suppression effects from the gas-liquid interface were observed on the PVG of 17 elements, which was correlated with the properties of analytes and the generated intermediate substances/products of PVG and the applied conditions. Enhancement from 1.1- to 7.3-fold in analytical sensitivity was found for 12 elements in the system with gas-liquid interface(s) compared to the results obtained in previous reports of PVG using traditional flow injection with inductively coupled plasma mass spectrometry measurement. The introduction of gas-liquid interface(s) and the resultant elevated temperature inside the PVG reactor likely facilitated the generation of radicals, the subsequent radical-based reactions, and the separation/transport/detection of volatile species of elements. In contrast, intermediate substances/products generated in PVG with poor thermostability will readily decompose at elevated temperatures, leading to a decreased signal response of analytes. The finding is helpful to understand the transport of elements under UV irradiation in the environment and has potential for analysis of trace elements in environmental and biological samples.

Constraining remote oxidation capacity with ATom observations

The global oxidation capacity, defined as the tropospheric mean concentration of the hydroxyl radical (OH), controls the lifetime of reactive trace gases in the atmosphere such as methane and carbon monoxide (CO). Models tend to underestimate the methane lifetime and CO concentrations throughout the troposphere, which is consistent with excessive OH. Approximately half of the oxidation of methane and non-methane volatile organic compounds (VOCs) is thought to occur over the oceans where oxidant chemistry has received little validation due to a lack of observational constraints. We use observations from the first two deployments of the NASA ATom aircraft campaign during July-August 2016 and January-February 2017 to evaluate the oxidation capacity over the remote oceans and its representation by the GEOS-Chem chemical transport model. The model successfully simulates the magnitude and vertical profile of remote OH within the measurement uncertainties. Comparisons against the drivers of OH production (water vapor, ozone, and NO y concentrations, ozone photolysis frequencies) also show minimal bias, with the exception of wintertime NO y . The severe model overestimate of NO y during this period may indicate insufficient wet scavenging and/or missing loss on sea-salt aerosols. Large uncertainties in these processes require further study to improve simulated NO y partitioning and removal in the troposphere, but preliminary tests suggest that their overall impact could marginally reduce the model bias in tropospheric OH. During the ATom-1 deployment, OH reactivity (OHR) below 3 km is significantly enhanced, and this is not captured by the sum of its measured components (cOHRobs) or by the model (cOHRmod). This enhancement could suggest missing reactive VOCs but cannot be explained by a comprehensive simulation of both biotic and abiotic ocean sources of VOCs. Additional sources of VOC reactivity in this region are difficult to reconcile with the full suite of ATom measurement constraints. The model generally reproduces the magnitude and seasonality of cOHRobs but underestimates the contribution of oxygenated VOCs, mainly acetaldehyde, which is severely underestimated throughout the troposphere despite its calculated lifetime of less than a day. Missing model acetaldehyde in previous studies was attributed to measurement uncertainties that have been largely resolved. Observations of peroxyacetic acid (PAA) provide new support for remote levels of acetaldehyde. The underestimate in both model acetaldehyde and PAA is present throughout the year in both hemispheres and peaks during Northern Hemisphere summer. The addition of ocean sources of VOCs in the model increases cOHRmod by 3% to 9% and improves model-measurement agreement for acetaldehyde, particularly in winter, but cannot resolve the model summertime bias. Doing so would require 100 Tg yr-1 of a long-lived unknown precursor throughout the year with significant additional emissions in the Northern Hemisphere summer. Improving the model bias for remote acetaldehyde and PAA is unlikely to fully resolve previously reported model global biases in OH and methane lifetime, suggesting that future work should examine the sources and sinks of OH over land.

Distribution and Drivers of Marine Isoprene Concentration across the Southern Ocean

Isoprene is a biogenic trace gas produced by terrestrial vegetation and marine phytoplankton. In the remote oceans, where secondary aerosols are mostly biogenic, marine isoprene emissions affect atmospheric chemistry and influence cloud formation and brightness. Here, we present the first compilation of new and published measurements of isoprene concentrations in the Southern Ocean and explore their distribution patterns. Surface ocean isoprene concentrations in November through April span 1 to 94 pM. A band of higher concentrations is observed around a latitude of ≈40 ∘ S and a surface sea temperature of 15 ∘ C. High isoprene also occurs in high productivity waters near islands and continental coasts. We use concurrent measurements of physical, chemical, and biological variables to explore the main potential drivers of isoprene concentration by means of paired regressions and multivariate analysis. Isoprene is best explained by phytoplankton-related variables like the concentrations of chlorophyll-a, photoprotective pigments and particulate organic matter, photosynthetic efficiency (influenced by iron availability), and the chlorophyll-a shares of most phytoplankton groups, and not by macronutrients or bacterial abundance. A simple statistical model based on chlorophyll-a concentration and a sea surface temperature discontinuity accounts for half of the variance of isoprene concentrations in surface waters of the Southern Ocean.

Emerging investigator series: exploring the surface properties of aqueous aerosols coated with mixed surfactants

Mixed Langmuir monolayers of cholesterol with both saturated and unsaturated fatty acids, stearic acid (SA), and oleic acid (OA) spread at the air-seawater surface were studied. The phase behavior, molecular interaction, and conformational order of the monolayers were investigated by surface pressure-area (π-A) isotherms and infrared reflection-absorption spectroscopy (IRRAS) measurements. The thermodynamic parameters of the mixed films, including excess molecular area and excess Gibbs free energy were calculated by using the isotherm data. The interaction between SA (or OA) and cholesterol varied with the molar fraction of the fatty acids and surface pressure. OA/chol monolayers showed the characteristics of miscibility, but they acted as nonideal systems. Cholesterol has been observed to have a stabilizing effect on OA monolayers. The negative values of the excess Gibbs free energy in the entire composition range demonstrated that mixed OA/chol monolayers were thermodynamically stable. IRRAS spectra showed that mixing with cholesterol changes the ordering of fatty acid monolayers at the air-seawater surface. The findings provide general information regarding the structural changes in the monolayer induced by lateral packing. These results help in the understanding of the mixing behavior of fatty acids and cholesterol and provide insights into the fate of the mixed-monolayer-coated sea salt aerosol in the ocean environment.