Peatland Wildfires Enhance Nitrogen-Containing Organic Compounds in Marine Aerosols over the Western Pacific.

Peatland wildfires contribute significantly to the atmospheric release of light-absorbing organic carbon, often referred to as brown carbon. In this study, we examine the presence of nitrogen-containing organic compounds (NOCs) within marine aerosols across the Western Pacific Ocean, which are influenced by peatland fires from Southeast Asia. Employing ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in electrospray ionization (ESI) positive mode, we discovered that NOCs are predominantly composed of reduced nitrogenous bases, including CHN+ and CHON+ groups. Notably, the count of NOC formulas experiences a marked increase within plumes from peatland wildfires compared to those found in typical marine air masses. These NOCs, often identified as N-heterocyclic alkaloids, serve as potential light-absorbing chromophores. Furthermore, many NOCs demonstrate pyrolytic stability, engage in a variety of substitution reactions, and display enhanced hydrophilic properties, attributed to chemical processes such as methoxylation, hydroxylation, methylation, and hydrogenation that occur during emission and subsequent atmospheric aging. During the daytime atmospheric transport, aging of aromatic N-heterocyclic compounds, particularly in aliphatic amines prone to oxidation and reactions with amine, was observed. The findings underscore the critical role of peatland wildfires in augmenting nitrogen-containing organics in marine aerosols, underscoring the need for in-depth research into their effects on marine ecosystems and regional climatic conditions.

Molecular transformation of organic nitrogen in Antarctic penguin guano-affected soil.

Organic nitrogen (ON) is an important participant in the Earth's N cycle. Previous studies have shown that penguin feces add an abundance of nutrients including N to the soil, significantly changing the eco-environment in ice-free areas in Antarctica. To explore the molecular transformation of ON in penguin guano-affected soil, we collected guano-free weathered soil, modern guano-affected soil from penguin colonies, ancient guano-affected soil from abandoned penguin colonies, and penguin feces from the Ross Sea region, Antarctica, and Fourier transform ion cyclotron mass spectrometry (FT-ICR MS) was used to investigate the chemical composition of water-extractable ON. By comparing the molecular compositions of ON among different samples, we found that the number of ON compounds (>4,000) in weathered soil is minimal, while carboxylic-rich alicyclic-like molecules (CRAM-like) are dominant. Penguin feces adds ON into the soil with > 10,000 CHON, CHONS and CHN compounds, including CRAM-like, lipid-like, aliphatic/ peptide-like molecules and amines in the guano-affected soil. After the input of penguin feces, macromolecules continue to degrade, and other ON compounds tend to be oxidized into relatively stable CRAM-like molecules, this is an important transformation process of ON in guano-affected soils. We conclude the roles of various forms of ON in the N cycle are complex and diverse. Combined with previous studies, ON eventually turns into inorganic N and is lost from the soil. The lost N ultimately returns to the ocean and the food web, thus completing the N cycle. Our study preliminarily reveals the molecular transformation of ON in penguin guano-affected soil and is important for understanding the N cycle in Antarctica.

Acidification impacts on the molecular composition of dissolved organic matter revealed by FT-ICR MS.

Solid-phase extraction (SPE) is a traditional pretreatment procedure widely used for dissolved organic matter (DOM) desalination and enrichment prior to the Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis, and the extracts are usually acidified to pH = 2 with hydrochloric acid (HCl) before passing through the cartridge. However, little is known about the effects of acidification on the integrity and molecular composition of DOM. Here, the differences in the molecular compositions in acidified and nonacidified DOM samples of soil, seawater and atmospheric aerosol were performed by FT-ICR MS. The results showed that the quantity and intensity of aromatic compounds with high oxygen content (e.g., polyphenols, tannin-like and highly oxygenated organic compounds) were greatly enhanced after acidification, while highly saturated compounds (lipid-like and aliphatic/peptide-like) were absent. The underlying reason is the variation of solubility and hydrolysis of DOM under acidic conditions. Meanwhile, the effect of acidification on the molecular composition of DOM was also dependent on their original environmental media. Based on these results, we suggest that the extracts of soil samples are selectively acidified according to the focus of research, while the extract is acidified for seawater samples and the pH of the extract can be unadjusted for aerosol samples before the SPE procedure. These findings provide a reference for the selection of suitable pretreatment methods for different experimental purposes and for the comprehensive characterization of samples with different properties.

Dwindling aromatic compounds in fine aerosols from chunk coal to honeycomb briquette combustion.

With the implementation of clean coal policy in China, the chunk coal has been gradually replaced by honeycomb briquette in domestic energies. In this study, the molecular composition of fine particles (PM2.5) from chunk coal and honeycomb briquette combustion is characterized using the Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). More than 6000 molecular formulae were detected in each PM2.5 sample. A remarkable decrease in unsaturation and aromatic compounds was found from chunk coal to honeycomb briquette derived aerosols. Around 73.6% of the unique CHON compounds in chunk coal are considered to have aromatic structures, while it decreased to 7.3% in honeycomb briquette. Most of these nitroaromatics detected only in chunk coal are highly carcinogenic and mutagenic with 4-6 rings. Moreover, the aromatic compounds in sulfur-containing compounds also showed a significant decrease. Meanwhile, because of the perforated shape and the additives added during the production of honeycomb briquettes, there are more heteroatoms-containing molecules released from honeycomb briquette combustion, which are highly functional compounds with high molecular weight, high degree of oxidation, and low volatility. Our results provide molecular level evidence that the transformation from chunk coal to honeycomb briquette can effectively reduce the emission of aromatic compounds, which is beneficial to assessing and reducing the impacts to climate change as well as human health.

Source and formation process impact the chemodiversity of rainwater dissolved organic matter along the Yangtze River Basin in summer.

Rainwater dissolved organic matter (DOM) plays an important role in the biogeochemical cycle and evolution of organic matter in the land-atmosphere interface. To better understand their sources and molecular composition in the atmosphere, rainwater samples were collected at six different locations along the Yangtze River Basin. Based on the application of a combined approach including excitation-emission matrix (EEM) fluorescence and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), various sources (terrestrial, anthropogenic, and autochthonous sources) of rainwater DOM were revealed. Results show that the derivatives of biogenic volatile organic compounds were widely distributed and contributed to rainwater DOM along the Yangtze River Basin. In the up-river city Batang, rainwater DOM was affected by the long-range atmospheric transport due to the Indian summer monsoon. Lijiang, a city on the southeastern edge of Tibetan plateau, was related to strong local biomass burning. The industrial cities of Panzhihua and Luzhou showed large differences in organic composition due to distinct industrial types. Fuling, a district in Chongqing Municipality, was significantly contributed by aged organics from biomass burning. While rainwater DOM in Shanghai, a coastal megacity, contained a high fraction of sea spray organics. Further, more than 70% of rainwater DOM molecules are associated with 36 typical transformation mechanisms during rainwater-scavenging processes, e.g., oxidation reactions, dealkylation and decarboxylation. Our study demonstrates that local natural and anthropogenic emissions and climatic conditions strongly shaped the chemodiversity and possible precursor-product pairs of rainwater DOM along the Yangtze River Basin, which helps to better understand the biogeochemical cycles of organic matter in a large-scale watershed under the influence of human activities.

Cable-car measurements of vertical aerosol profiles impacted by mountain-valley breezes in Lushan Mountain, East China.

In-situ field observations of vertical aerosol profiles for one month in complex terrain (Lushan Mountain, China) were carried out using a cable car, which resolved detailed vertical distributions of mountain aerosols with low-cost operation. Cable-car observations were conducted during the early morning and late afternoon, when mountain and valley winds dominated, respectively. The diurnal aerosol variations at the top and foot of Lushan Mountain were analyzed based on environmental and meteorological stations. The observations indicated that the mountain-valley breezes notably impacted the mountain-area aerosol distribution under weak weather conditions. More uniform aerosol profiles for the afternoon than the morning, with their decreasing rates of PM2.5 (particles with diameters less than 2.5 µm) were 1.64 and 2.28 µg m-3/100 m, respectively. The PM2.5/PM10 ratio at the mountain top increased from 0.69 to 0.81, and that at the mountain base decreased from 0.75 to 0.70 from morning to afternoon. The PM2.5 concentration decreased in and around Lushan Mountain from daytime to nighttime, with the impacted diameter of the 300-m topography line being smaller than ~5 km, while the concentration increased in Jiujiang City. The relative decreasing rate of PM2.5 was higher at the mountain top site (~20%) than at the base site (~2%) from daytime to nighttime. Moreover, uniform aerosol profiles could have been caused by regional transport through a relatively strong low-level synoptic flow (~5 m s-1) and the mountain's dynamic lifting effect.