Promotive effects of marine-derived dimethyl sulfoxide on the photodegradation of phenanthrene in the atmosphere

Dimethyl sulfoxide (DMSO), a versatile medium, is a particular component in the marine atmosphere that possibly causes polycyclic aromatic hydrocarbons (PAHs) to degrade differently than they do in the continental atmosphere. In this study, phenanthrene (Phe) was used as a model PAH in batch photochemical experiments to investigate the chemical actions of DMSO and the underlying mechanisms. The photodegradation of Phe in aqueous solutions with DMSO volume fractions from 0 % to 100 % was initiated by ultraviolet (UV) radiation and promoted by singlet oxygen, which was consistent with pseudo-first-order kinetics. Phe photodegraded faster in a mixture of DMSO and water than in water or DMSO alone, and the rate constant showed a unimodal distribution over the DMSO fraction range, peaking at 33 % DMSO (0.0333 ± 0.0009 min-1) and 40 % DMSO (0.0199 ± 0.0005 min-1) under 254 nm and 302 nm UV radiation, respectively. This interesting phenomenon was attributed to the competition of DMSO for UV radiation and singlet oxygen and changes in dissolved oxygen and free water contents caused by the interaction between DMSO and water molecules. In addition, 9,10-phenanthrenequinone (9,10-PhQ) with high cytotoxicity was the main photodegradation product of Phe under various conditions. The photodegradation rate of Phe in the mixtures of DMSO and water was comparable to its reaction rate with OH radicals, suggesting that 9,10-PhQ can be rapidly generated in the marine atmosphere, driven by a mechanism different from that in the continental or urban atmosphere. Under the presented experimental conditions, UV intensity and DMSO fraction were the primary factors that affected the photodegradation rate of Phe and 9,10-PhQ and altered their integrated toxicity. The findings of this study support the conclusion that the marine atmosphere is an essential field in the atmospheric transport of PAHs, in which DMSO is an important component that affects their photodegradation.

Spatiotemporal distribution and influencing factors of secondary organic aerosols in the summer atmosphere from the Bering Sea to the western North Pacific

To better understand the formation process of biogenic and anthropogenic secondary organic aerosols (BSOA and ASOA) in the marine atmosphere under the background of global warming, aerosol samples were collected over three summers (i.e., 2014, 2016 and 2018) from the Bering Sea (BS) to the western North Pacific (WNP). The results showed that temporally, atmospheric concentrations of isoprene-derived SOA (SOA_I) tracers were the lowest in 2014 regardless of the marine region, while atmospheric concentrations of monoterpenes-derived SOA (SOA_M) tracers in this year were the highest and the aerosols were more aged than those in the other two years. In comparison, the concentrations of β-caryophyllene-derived and toluene-derived SOA (SOA_C and SOA_A) tracers were relatively low overall. Spatially, the concentrations of SOA tracers were significantly higher over the WNP than over the BS, with SOA tracers over the BS mainly coming from marine sources, while the WNP was strongly influenced by terrestrial inputs. In particular, for land-influenced samples from the WNP, NO_x-channel products of SOA_I were more dependent on O₃ and SO₂ relative to HO₂-channel product, and the high atmospheric oxidation capacity and SO₂ could promote the formation of later-generation SOA_M products. The extent of terrestrial influence was further quantified using a principal component analysis (PCA)-generalized additive model (GAM), which showed that terrestrial emissions explained more than half of the BSOA tracers' concentrations and contributed almost all of the ASOA tracer. In addition, the assessment of secondary organic carbon (SOC) highlighted the key role of anthropogenic activities in organic carbon levels in offshore areas. Our study revealed significant contributions of terrestrial natural and anthropogenic sources to different SOA over the WNP, and these relevant findings help improve knowledge about SOA in the marine atmosphere.

Oceanic emissions of methyl halides and effect of nutrients concentration on their production: A case of the western Pacific Ocean (2°N to 24°N)

Methyl halides are important greenhouse gases responsible for the majority of the ozone layer depletion. This study investigated atmospheric and seawater methyl halides (CH₃Cl, CH₃Br, and CH₃I) in the western Pacific Ocean between 2°N and 24°N. Increases in methyl halides in the atmosphere were likely to have originated from Southeast Asian regions. Elevated CH₃I concentrations in seawater were mainly produced photochemically from dissolved organic carbon. Maximum methyl halide and chlorophyll a levels in the upper water column (0-200 m) were linked to biological activity and downwelling or upwelling caused by cold and warm eddies. Ship-based incubation experiments showed that nutrient supplementation promoted methyl halide emissions. The elevated methyl halide production was associated with increases in phytoplankton such as diatoms. The mean fluxes of CH₃Cl, CH₃Br, and CH₃I in study area of during the cruise were 82.91, 4.70, and 3.50 nmol m^-2 d^-1, respectively. The estimated emissions of CH₃Cl, CH₃Br, and CH₃I in the western Pacific Ocean accounted for 0.67%, 0.79% and 0.09% of global oceanic emissions, respectively, indicating that the open sea contribute insignificantly to the global oceanic emissions of these gases.

Bromoform, dibromochloromethane, and dibromomethane over the East China Sea and the western Pacific Ocean: Oceanic emission and spatial variation

Spatial distributions of bromocarbons, including bromoform (CHBr₃), dibromochloromethane (CHBr₂Cl), and dibromomethane (CH₂Br₂), and influential oceanographic parameters that determine their concentrations were measured in the marine atmosphere and seawater of the East China Sea (ECS) and western Pacific Ocean during two cruises from 14 to 24 September, 2017 and from 5 October to 3 December, 2018. The atmospheric concentrations of CHBr₃, CHBr₂Cl, and CH₂Br₂ were 0.33-3.02, 0.16-1.96, and 0.85-1.75 pptv over the western Pacific Ocean and 2.23-4.92, 0.26-1.52, and 0.24-7.47 pptv over the ECS, respectively. There was significant spatial variability in atmospheric bromocarbon concentrations in the study region, with higher concentration over the ECS. The atmospheric mixing ratios of bromocarbons were significantly correlated to the surface seawater bromocarbon concentrations and wind speed. In the ECS, input from terrestrial sources also significantly influenced the distributions of bromocarbons in air. PCA analysis revealed that seawater bromocarbon concentrations were correlated with both water mass and chlorophyll a. Generally lower CH₂Br₂/CHBr₃ ratios were observed in the ECS, which was indicative of mixing and/or dilution in coastal areas. The estimated average sea-to-air fluxes of CHBr₂Cl, CH₂Br₂, and CHBr₃ were 46.86, -3.77, and -6.71 nmol m^-2 d^-1 in the western Pacific Ocean and 111.49, 0.89, and 321.74 nmol m^-2 d^-1 in the ECS, respectively. These results of the net sea-to-air fluxes indicated oceanic net uptake of CH₂Br₂ and CHBr₃ for the western Pacific Ocean and oceanic emission of bromocarbons for the ECS.

Spatial distributions and sea-to-air fluxes of non-methane hydrocarbons in the atmosphere and seawater of the Western Pacific Ocean

During an oceanographic campaign in the western Pacific Ocean from 12 August to 3 October 2014, the concentrations of five non-methane hydrocarbons (NMHCs) were measured in marine atmosphere and seawater. The average mixing ratios of ethane, ethylene, propane, propylene, and isoprene were 1.109 ± 0.359, 0.658 ± 0.137, 0.711 ± 0.377, 0.429 ± 0.139, and 0.255 ± 0.201 ppbv, respectively. In general, atmospheric concentrations of ethane and propane showed significant decrease from the inshore area to the open ocean, while ethylene and propylene exhibited decreasing trends from low latitudes to high latitudes. The results suggested that atmospheric ethane and propane with long lifetimes were more likely influenced by the air mass transported from continent, while ethylene, propylene and isoprene with short lifetimes were mainly derived from the surface seawater. The average concentrations of ethane, ethylene, propane, propylene, and isoprene in the surface seawater were 6.6 ± 5.8, 51.9 ± 23.5, 15.4 ± 4.3, 17.2 ± 3.8, and 23.5 ± 8.6 pmol L^-1, respectively. A significant positive correlation was observed between ethane and propane (R² = 0.45, n = 39, p < 0.001), implying that their production and removal pathways in the surface seawater were similar. High concentrations of isoprene were observed in the waters with high Chl-a values, suggesting that the biological process was a controlling factor. The estimated sea-to-air fluxes of ethane, propane, ethylene, propylene, and isoprene were in the range of 0.1-24.9, 4.2-235, 1.0-43.8, 1.5-90.2, and 2.1-149 nmol m^-2 d^-1, respectively. This study is of great importance to the contribution to the atmospheric NMHCs from the western Pacific Ocean and provides data supporting for global NMHCs emission estimates.

Spatiotemporal distributions of halocarbons in the marine boundary air and surface seawater of the Changjiang estuary and its adjacent East China Sea

Spatiotemporal distributions of volatile halogenated organic compounds (VHOCs) were investigated in the marine boundary air and surface seawater of the Changjiang (Yangtze River) estuary and its adjacent East China Sea in two cruises from March 11, 2015 to March 21, 2015 and from July 9, 2015 to July 20, 2015. Results revealed that the concentrations of released chlorofluorocarbons (CFCs) such as CFC-12, CFC-11, and CFC-114 in China decreased, suggesting that limitations set by the Chinese government on CFCs production and consumption have taken effect. Atmospheric concentrations of CFCs were affected by local industrial sources of emission and transport of terrestrial pollutants from coastal areas to varying degrees. Seasonal variations in atmospheric VHOCs were probably due to seasonal differences in prevalent monsoon and biogenic production. In the study periods, the investigated area was an essential source of atmospheric CH₃Br and CH₃I but was a net sink of CFC-12, CFC-11, and CH₃Cl.

Particulate PAHs and n-alkanes in the air over Southern and Eastern Mediterranean Sea

Particulate polycyclic aromatic hydrocarbons, n-alkanes and polar organic compounds were investigated in the marine atmosphere of Southern and Eastern Mediterranean Sea, in the frame of the scientific cruise of Urania ship between 27 July and 11 August 2013. The PM10 fraction of aerosol to which most organic substances are associated, were collected daily; contemporarily, gaseous regulated toxicants (ozone, nitrogen oxides and carbon oxide) and carbonyls were recorded. Samplings were carried out in front of Palermo and Messina, respectively the start and end harbors, and along the cruise, both in movement (transects, N = 14) and at stops (N = 11). Total PAHs ranged from 0.06 ng/m(3) up to 1.8 ng/m(3), with the maximums observed close to harbors. Unlike total concentrations that were in general comparable, the percent composition of PAHs was distinct for harbors, transects and stops, which allowed to draw insights about the pollution sources impact. Concentrations of n-alkanes (C18-C35) ranging from 6.7 to 43 ng/m(3) were quantified. The carbonyls evaluation revealed relatively high concentrations of formaldehyde (∼4-24 μg/m(3)) and acetone (∼5-35 μg/m(3)) near harbors, and of acrolein (up to 12 μg/m(3)) offshore, while benzaldehyde was quite independent of the site type (≈0.5 μg/m(3)). Nicotine and caffeine were detected, at different extents (0.0-2.2 ng/m(3) and 0.01-0.17 ng/m(3), respectively), in ca. 70% and 100% of samples. Alkyl phthalates ranged from 2.7 to 67 ng/m(3) and showed variable percentages in the samples. Finally, traces of N,N-diethyl-meta-toluene amide (up to 0.4 ng/m(3)) were found at all sites.