Dissecting the contributions of organic nitrogen aerosols to global atmospheric nitrogen deposition and implications for ecosystems

Responses of dissolved organic nitrogen to varied nitrogen deposition: A 16-year nitrogen-addition and cessation experiment in a subtropical forest

The leaching of dissolved organic nitrogen (DON) signals N loss in forest ecosystems, connecting terrestrial and aquatic ecosystems. The response of DON to varied inorganic N (Nin) deposition remains unclear. A 16-year continuous monitoring of DON in throughfall, soil water, and stream water was conducted under a field N manipulation (10-year Nin addition and subsequent cessation of Nin addition) in a subtropical forest in China. Under reference conditions, the average organic N (Nor) deposition was 23.0 ± 4.9 kg N ha-1 yr-1, with approximately one-third leaching at a soil depth of 30 cm, a value that was higher than those reported for temperate forests. Over a 16-year period, DON concentrations tended to increase in throughfall, soil solutions and stream water. The long-term increase in DON concentrations in soil solutions was primarily driven by increasing Nor deposition and indirectly influenced by decreasing acid deposition. Due to a doubling of monthly Nin input, either as sodium nitrate or as ammonium nitrate, the soil DON leaching doubled, causing the soil to transition from a DON sink to a DON source. After Nin addition ceased, the DON leaching returned to natural levels, and the soil reverted to being a DON sink. This study reveals the importance of Nor deposition and decreasing acid deposition as drivers of long-term trends in DON concentrations in soil solutions, in addition to elucidating the response patterns of DON to increasing and decreasing Nin deposition.

Nitrogen dominates global atmospheric organic aerosol absorption

Atmospheric organic aerosols (OAs) influence Earth's climate by absorbing sunlight. However, the link between their evolving composition and their absorptive effects is unclear. We demonstrate that brown nitrogen (BrN), the absorptive nitrogenous component of OAs, dominates their global absorption. Using a global model, we quantified BrN abundance, tracked its optical evolution with chemical aging, and assessed its radiative absorption. BrN contributes 76% of OAs' surface light absorption over the US and 61% of their global absorptive optical depth. Moreover, the observed variability of OAs' absorptive capacity is primarily driven by the sources and aging of BrN. BrN represents 18% of the global absorptive direct radiative effect of carbonaceous aerosols, with biomass burning being the largest contributor. Our research establishes a nitrogen-centric framework for attributing the climate impacts of OAs.

Brown Carbon in East Asia: Seasonality, Sources, and Influences on Regional Climate and Air Quality

Brown carbon (BrC) has been recognized as an important light-absorbing carbonaceous aerosol, yet understanding of its influence on regional climate and air quality has been lacking, mainly due to the ignorance of regional coupled meteorology-chemistry models. Besides, assumptions about its emissions in previous explorations might cause large uncertainties in estimates. Here, we implemented a BrC module into the WRF-Chem model that considers source-dependent absorption and avoids uncertainties caused by assumptions about emission intensities. To our best knowledge, we made the first effort to consider BrC in a regional coupled model. We then applied the developed model to explore the impacts of BrC absorption on radiative forcing, regional climate, and air quality in East Asia. We found notable increases in aerosol absorption optical depth (AAOD) in areas with high OC concentrations. The most intense forcing of BrC absorption occurs in autumn over Southeast Asia, and values could reach around 4 W m-2. The intensified atmospheric absorption modified surface energy balance, resulting in subsequent declines in surface temperature, heat flux, boundary layer height, and turbulence exchanging rates. These changes in meteorological variables additionally modified near-surface dispersion and photochemical conditions, leading to changes of PM2.5 and O3 concentrations. These findings indicate that BrC could exert important influence in specific regions and time periods. A more in-depth understanding could be achieved later with the developed model.

Atmospheric deposition of pollutants at three altitudes on Mount Emei, Sichuan Basin, southwestern China

Comparing the atmospheric deposition chemistry between above and below the planetary boundary layer (PBL) may help understand the impacts from inter-regional air pollutant transport and local emissions to air pollutant deposition. In this study, we monitored ions, soluble and insoluble potentially toxic elements (PTEs), and dissolved organic nitrogen and carbon at the base (M-base: 551 m asl), middle (M-middle: 2400 m asl), and summit (M-summit: 3077 m asl) on Mount Emei. The annual volume-weighted mean (VWM) concentrations of all measured components were substantially higher at M-base than at M-summit, except for Na+, Cl-, and NO2- as Na+ and Cl- at M-summit may be largely from ocean and NO2- to NO3- transformation may be faster at M-base. Not all components had their highest annual deposition fluxes at M-base mainly due to the differences in precipitation. Organic nitrogen and organic carbon accounted for 19 %-26 % and 63 %-71 % of the annual total dissolved nitrogen and carbon at the three sites, respectively. The contributions of soluble and insoluble fractions showed a large variability among the nine PTEs measured (3 %-75 % and 97 %-25 %, respectively) but they were similar among the three sites, as the PETs' solubility largely depended on pH. From low to high elevations, the contributions to the air pollutant deposition fluxes within and outside the SCB decreased and increased, respectively. Even south and southeast Asia were important source regions for some pollutants at M-summit. In sum, this study revealed the basin's large effects on air pollutant accumulation and deposition and the importance of non-SCB emissions above the SCB's PBL; However, either above or within the PBL, the inorganic, organic, soluble, and insoluble portions of air pollutants (particularly nitrogen and PTEs) should be considered together for a better understanding on air pollutants' transport, deposition, and ecological risks.

A global analysis of plant nutrient limitation affected by atmospheric nitrogen and phosphorous deposition

Uncovering the response of plant functional types (PFTs) to nutrient limitation caused by atmospheric deposition is critical for assessing the health of terrestrial ecosystems under climate change conditions. However, it remains unclear how atmospheric deposition and underlying ecological factors affect PFTs globally. To address this, we compiled a global dataset of four PFTs, i.e., herb, evergreen broad-leaf (EB), deciduous broad-leaf (DB), and conifer (CO), and utilized both linear mixed-effects models and structural equation models to describe the thresholds of their net primary productivity (NPP), and tested the relationships between their NPP and potential environmental drivers based on the N/P threshold hypothesis. We found that atmospheric N and P deposition non-linearly affected NPP and the effects were most pronounced for the EB, DB, and CO categories, with tipping points in the ranges of 8.32-9.33 kg N·ha-1·yr-1 and 0.20-0.30 kg P·ha-1·yr-1, respectively. Atmospheric N and P deposition negatively affected the NPP of approximately 53.68% and 43.88% of terrestrial ecosystem plants, respectively, suggesting increased P limitation and N saturation in most terrestrial ecosystems worldwide. We further determined that the N/P threshold hypothesis is applicable in assessing the effects of atmospheric N and P deposition on the growth of woody plants (EB, DB, and CO) through nutrient limitation. The results of this study will contribute to more effective landscape management in changing environments.

Impacts of atmospheric particulate matter deposition on phytoplankton: A review

In many rapidly urbanizing and industrializing countries, atmospheric pollution causes severe environmental problems and compromises the health of humans and ecosystems. Atmospheric emissions, which encompass gases and particulate matter, can be transported back to the earth's surface through atmospheric deposition. Atmospheric deposition supplies chemical species that can serve as nutrients and/or toxins to aquatic ecosystems, resulting in wide-ranging responses of aquatic organisms. Among the aquatic organisms, phytoplankton is the basis of the aquatic food web and is a key player in global primary production. Atmospheric deposition alters nutrient availability and thus influences phytoplankton species abundance and composition. This review provides a comprehensive overview of the physiological responses of phytoplankton resulting from the atmospheric deposition of trace metals, nitrogen-containing compounds, phosphorus-containing compounds, and sulfur-containing compounds in particulate matter into aquatic ecosystems. Knowledge gaps and critical areas for future studies are also discussed.

Rapid aqueous-phase dark reaction of phenols with nitrosonium ions: Novel mechanism for atmospheric nitrosation and nitration at low pH

Dark aqueous-phase reactions involving the nitrosation and nitration of aromatic organic compounds play a significant role in the production of light-absorbing organic carbon in the atmosphere. This process constitutes a crucial aspect of tropospheric chemistry and has attracted growing research interest, particularly in understanding the mechanisms governing nighttime reactions between phenols and nitrogen oxides. In this study, we present new findings concerning the rapid dark reactions between phenols containing electron-donating groups and inorganic nitrite in acidic aqueous solutions with pH levels <3.5. This reaction generates a substantial amount of nitroso- and nitro-substituted phenolic compounds, known for their light-absorbing properties and toxicity. In experiments utilizing various substituted phenols, we demonstrate that their reaction rates with nitrite depend on the electron cloud density of the benzene ring, indicative of an electrophilic substitution reaction mechanism. Control experiments and theoretical calculations indicate that the nitrosonium ion (NO+) is the reactive nitrogen species responsible for undergoing electrophilic reactions with phenolate anions, leading to the formation of nitroso-substituted phenolic compounds. These compounds then undergo partial oxidation to form nitro-substituted phenols through reactions with nitrous acid (HONO) or other oxidants like oxygen. Our findings unveil a novel mechanism for swift atmospheric nitrosation and nitration reactions that occur within acidic cloud droplets or aerosol water, providing valuable insights into the rapid nocturnal formation of nitrogen-containing organic compounds with significant implications for climate dynamics and human health.

Characteristics and source apportionment of water-soluble organic nitrogen (WSON) in PM2.5 in Hong Kong: With focus on amines, urea, and nitroaromatic compounds

Water-soluble organic nitrogen (WSON) is ubiquitous in fine particulate matter (PM2.5) and poses health and environmental risks. However, there is limited knowledge regarding its comprehensive speciation and source-specific contributions. Here, we conducted chemical characterization and source apportionment of WSON in 65 PM2.5 samples collected in Hong Kong during a 1-yr period. Using various mass-spectrometry-based techniques, we quantified 22 nitrogen-containing organic compounds (NOCs), including 17 nitroaromatics (NACs), four amines, and urea. The most abundant amine and NACs were dimethylamine and 4-nitrocatechol, respectively. Two secondary (i.e., secondary formation and secondary nitrate) and five primary sources (i.e., sea salt, fugitive dust, marine vessels, vehicle exhaust, and biomass burning) of WSON and these three categories of NOCs were identified. Throughout the year, secondary sources dominated WSON formation (69.0%), while primary emissions had significant contributions to NACs (77.1%), amines (75.9%), and urea (83.7%). Fugitive dust was the leading source of amines and urea, while biomass burning was the main source of NACs. Our multi-linear regression analysis revealed the significant role of sulfate, NO3, nitrate, liquid water content, and particle pH on WSON formation, highlighting the importance of nighttime NO3 processing and heterogeneous and aqueous-phase formation of NOCs in the Hong Kong atmosphere.