Previous experiments have demonstrated that the aqueous OH radical oxidation of methylglyoxal produces low volatility products including pyruvate, oxalate and oligomers. These products are found ...predominantly in the particle phase in the atmosphere, suggesting that methylglyoxal is a precursor of secondary organic aerosol (SOA). Acetic acid plays a central role in the aqueous oxidation of methylglyoxal and it is a ubiquitous product of gas phase photochemistry, making it a potential "aqueous" SOA precursor in its own right. However, the fate of acetic acid upon aqueous-phase oxidation is not well understood. In this research, acetic acid (20 μM–10 mM) was oxidized by OH radicals, and pyruvic acid and methylglyoxal experimental samples were analyzed using new analytical methods, in order to better understand the formation of SOA from acetic acid and methylglyoxal. Glyoxylic, glycolic, and oxalic acids formed from acetic acid and OH radicals. In contrast to the aqueous OH radical oxidation of methylglyoxal, the aqueous OH radical oxidation of acetic acid did not produce succinic acid and oligomers. This suggests that the methylgloxal-derived oligomers do not form through the acid catalyzed esterification pathway proposed previously. Using results from these experiments, radical mechanisms responsible for oligomer formation from methylglyoxal oxidation in clouds and wet aerosols are proposed. The importance of acetic acid/acetate as an SOA precursor is also discussed. We hypothesize that this and similar chemistry is central to the daytime formation of oligomers in wet aerosols.
We report a new synthesis of best estimates of the inputs of fixed nitrogen to the world ocean via atmospheric deposition and compare this to fluvial inputs and dinitrogen fixation. We evaluate the ...scale of human perturbation of these fluxes. Fluvial inputs dominate inputs to the continental shelf, and we estimate that about 75% of this fluvial nitrogen escapes from the shelf to the open ocean. Biological dinitrogen fixation is the main external source of nitrogen to the open ocean, i.e., beyond the continental shelf. Atmospheric deposition is the primary mechanism by which land‐based nitrogen inputs, and hence human perturbations of the nitrogen cycle, reach the open ocean. We estimate that anthropogenic inputs are currently leading to an increase in overall ocean carbon sequestration of ~0.4% (equivalent to an uptake of 0.15 Pg C yr−1 and less than the Duce et al. (2008) estimate). The resulting reduction in climate change forcing from this ocean CO2 uptake is offset to a small extent by an increase in ocean N2O emissions. We identify four important feedbacks in the ocean atmosphere nitrogen system that need to be better quantified to improve our understanding of the perturbation of ocean biogeochemistry by atmospheric nitrogen inputs. These feedbacks are recycling of (1) ammonia and (2) organic nitrogen from the ocean to the atmosphere and back, (3) the suppression of nitrogen fixation by increased nitrogen concentrations in surface waters from atmospheric deposition, and (4) increased loss of nitrogen from the ocean by denitrification due to increased productivity stimulated by atmospheric inputs.
Key Points
A new estimate of total atmospheric fixed nitrogen inputs to the ocean (39 Tg N yr−1) and its spatial distribution is presented
The effects of atmospheric deposition on the oceans are estimated as an increase of productivity equivalent to 0.15 Pg C yr−1
Four key uncertainties in these estimates are identified
Emissions of anthropogenic nitrogen (N) to the atmosphere have increased tenfold since preindustrial times, resulting in increased N deposition to terrestrial and coastal ecosystems. The sources of N ...deposition to the ocean, however, are poorly understood. Two years of event‐based rainwater samples were collected on the island of Bermuda in the western North Atlantic, which experiences both continent‐ and ocean‐influenced air masses. The rainwater ammonium concentration ranged from 0.36 to 24.6 μM, and the ammonium δ15N from −12.5 to 0.7‰; and neither has a strong relationship with air mass history (6.0 ± 4.2 μM, −4.1 ± 2.6‰ in marine air masses and 5.9 ± 3.2 μM, −5.8 ± 2.5‰ in continental air masses; numerical average ± standard deviation). A simple box model suggests that the ocean can account for the concentration and isotopic composition of ammonium in marine rainwater, consistent with the lack of correlation between ammonium δ15N and air mass history. If so, ammonium deposition reflects the cycling of N between the ocean and the atmosphere, rather than representing a net input to the ocean. The δ15N data appear to require that most of the ammonium/a flux to the ocean is by dissolution in surface waters rather than atmospheric deposition. This suggests that the atmosphere and surface ocean are near equilibrium with respect to air/sea gas exchange, implying that anthropogenic ammonia will equilibrate near the coast and not reach the open marine atmosphere. Whereas ~90% of the ammonium deposition to the global ocean has previously been attributed to anthropogenic sources, the evidence at Bermuda suggests that the anthropogenic contribution could be much smaller.
Key Points
Stable isotope ratios of rainwater ammonium were measured at BermudaRainwater ammonium isotopes do not vary with air mass historyIsotopes and simple steady state box model suggest ocean ammonia source
Atmospheric water soluble organic nitrogen (WSON) is a subset of the complex organic matter in aerosols and rainwater, which impacts cloud condensation processes and aerosol chemical and optical ...properties and may play a significant role in the biogeochemical cycle of N. However, its sources, composition, connections to inorganic N, and variability are largely unknown. Rainwater samples were collected on the island of Bermuda (32.27° N, 64.87° W), which experiences both anthropogenic and marine influenced air masses. Samples were analyzed by ultra-high resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry to chemically characterize the WSON. Elemental compositions of 2281 N containing compounds were determined over the mass range m/z+ 50 to 500. The five compound classes with the largest number of elemental formulas identified, in order from the highest number of formulas to the lowest, contained carbon, hydrogen, oxygen, and nitrogen (CHON+), CHON compounds that contained sulfur (CHONS+), CHON compounds that contained phosphorus (CHONP+), CHON compounds that contained both sulfur and phosphorus (CHONSP+), and compounds that contained only carbon, hydrogen, and nitrogen (CHN+). Compared to rainwater collected in the continental USA, average O:C ratios of all N containing compound classes were lower in the marine samples whereas double bond equivalent values were higher, suggesting a reduced role of secondary formation mechanisms. Despite their prevalence in continental rainwater, no organonitrates or nitrooxy-organosulfates were detected, but there was an increased presence of organic S and organic P containing compounds in the marine rainwater. Cluster analysis showed a clear chemical distinction between samples collected during the cold season (October to March) which have anthropogenic air mass origins and samples collected during the warm season (April to September) with remote marine air mass origins. This, in conjunction with patterns identified in van Krevelen diagrams, suggests that the cold season WSON is a mixture of organic matter with both marine and anthropogenic sources while in the warm season the WSON appears to be dominated by marine sources. These findings indicate that, although the concentrations and percent contribution of WSON to total N is fairly consistent across diverse geographic regions, the chemical composition of WSON varies strongly as a function of source region and atmospheric environment.
Secondary organic aerosol (SOA) is a substantial component of total atmospheric organic particulate matter, but little is known about the composition of SOA formed through cloud processing. We ...conducted aqueous phase photo-oxidation experiments of methylglyoxal and hydroxyl radical to simulate cloud processing. In addition to predicted organic acid monomers, oligomer formation from methylglyoxal–hydroxyl radical reactions was detected by electrospray ionization mass spectrometry (ESI-MS). The chemical composition of the oligomers and the mechanism of their formation were investigated by ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and LCQ DUO ion trap mass spectrometry (ESI-MS-MS). Reaction products included 415 compounds detected in the mass range 245–800
Da and the elemental composition of all 415 compounds were determined by ultra-high resolution FT-ICR MS. The ratio of total organic molecular weight per organic carbon weight (OM:OC) of the oligomers (1.0–2.5) was lower than the OM:OC of the organic acid monomers (2.3–3.8) formed, suggesting that the oligomers are less hygroscopic than the organic acid monomers formed from methylglyoxal–hydroxyl radical reaction. The OM:OC of the oligomers (average=2.0) is consistent with that of aged atmospheric aerosols and atmospheric humic-like substances (HULIS). A mechanism is proposed in which the organic acid monomers formed through hydroxyl radical reactions oligomerize through esterification. The mechanism is supported by the existence of series of oligomers identified by elemental composition from FT-ICR MS and ion fragmentation patterns from ESI-MS-MS. Each oligomer series starts with an organic acid monomer formed from hydroxyl radical oxidation, and increases in molecular weight and total oxygen content through esterification with a hydroxy acid (C
3H
6O
3) resulting in multiple additions of 72.02113
Da (C
3H
4O
2) to the parent organic acid monomer. Methylglyoxal is a water-soluble product of both gas phase biogenic (i.e., isoprene) and anthropogenic (i.e., aromatics, alkenes) hydrocarbon oxidation. The varied and multiple sources of methylglyoxal increase the potential for these low volatility cloud processing products (e.g., oxalic acid and oligomers) to significantly contribute to SOA. Aqueous phase oligomer formation investigated here and aerosol phase oligomer formation appear to be more similar than previously realized, which may simplify the incorporation of oligomers into atmospheric SOA models.
Wet deposition is an important removal mechanism for atmospheric organic matter, and a potentially important input for receiving ecosystems, yet less than 50% of rainwater organic matter is ...considered chemically characterized. Precipitation samples collected in New Jersey, USA, were analyzed by negative ion ultra-high resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Elemental compositions of 552 unique molecular species were determined in the mass range 50–500 Da in the rainwater. Four main groups of organic compounds were identified: compounds containing carbon, hydrogen, and oxygen (CHO) only, sulfur (S) containing CHOS compounds, nitrogen (N) containing CHON compounds, and S- and N- containing CHONS compounds. Organic acids commonly identified in precipitation were detected in the rainwater. Within the four main groups of compounds detected in the rainwater, oligomers, organosulfates, and nitrooxy-organosulfates were assigned based on elemental formula comparisons. The majority of the compounds identified are products of atmospheric reactions and are known contributors to secondary organic aerosol (SOA) formed from gas phase, aerosol phase, and in-cloud reactions in the atmosphere. It is suggested that the large uncharacterized component of SOA is the main contributor to the large uncharacterized component of rainwater organic matter.
Increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for up to about a third of the ocean's external (nonrecycled) nitrogen supply and up to ∼30% of ...the annual new marine biological production, ∼0.3 petagram of carbon per year. This input could account for the production of up to ∼1.6 teragrams of nitrous oxide ($\text{N}_{2}\text{O}$) per year. Although ∼10% of the ocean's drawdown of atmospheric anthropogenic carbon dioxide may result from this atmospheric nitrogen fertilization, leading to a decrease in radiative forcing, up to about two-thirds of this amount may be offset by the increase in$\text{N}_{2}\text{O}$emissions. The effects of increasing atmospheric nitrogen deposition are expected to continue to grow in the future.
Emissions of anthropogenic nitrogen (N) to the atmosphere have increased tenfold since preindustrial times, resulting in increased N deposition to terrestrial and coastal ecosystems. The current ...sources of N deposition to the ocean, however, are poorly understood. To investigate the sources of nitrate in rainwater deposited to the ocean, two years of daily rainwater samples were collected on the island of Bermuda in the western North Atlantic. Air mass back trajectories were computed for each sample and two dominant regimes were identified: slow moving events that originate over the ocean and occur all year, and fast moving events that originate over the continental USA and occur primarily during the cool season (October–March). Marine‐influenced air masses result in rainwater nitrate with lower concentrations, higher average δ15N, and lower average δ18O (4.4 μM, −1.1‰ versus N2 in air, and 69.0‰ versus Vienna SMOW, respectively) than those influenced by North American air masses (6.3 μM, −5.4‰, and 75.0‰). The δ15N decrease and concentration increase from marine to continental air masses are due to a change in NOx source, with increased anthropogenic inputs associated with continental air. We suggest that heterogeneous halogen chemistry in the marine boundary layer leads to isotopic fractionation. This causes higher δ15N‐NO3− to be deposited near the coast and lower δ15N‐NOx to be transported over the open ocean, yielding a low δ15N for anthropogenic NO3− deposition. It is possible that this process also contributes to variations in δ15N‐NO3− from marine air masses. There is a negative linear correlation (r2 = 0.58) between δ15N and δ18O which is driven by the seasonal change in trajectory influencing both the source NOx and the nitrate formation pathways.
Key Points
Stable isotope ratios of rainwater nitrate N and O were measured at Bermuda
Coastal MBL chemistry leads to low δ15N anthropogenic NO3‐
Negative correlation observed in N and O isotopes unique to marine rainwater
The link between pollution and poor health and mortality has been established globally. Developing countries carry most of the burden of ill health from air pollution, and urban centres like the City ...of Cape Town even more so. Effective air quality management to protect human health relies on the attainment of air quality standards. This study uses the Benefits Mapping and Analysis Program (BenMAP) along with a locally derived exposure-response function and air quality monitor data to investigate whether the consistent attainment of current or more stringent air quality standards would avoid loss of life. The results show that attaining the PM10 24-hour mean South Africa National Standard limit and the PM10 and SO2 24-hour mean World Health Organisation guidelines in Cape Town reduces levels of pollutants and does reduce excess risk of mortality in Cape Town.
Oceanic ammonia emissions are the largest natural source of ammonia globally, but the magnitude of the air‐sea flux in remote regions with minimal human influence remains uncertain. Here, we measured ...the concentration of surface ocean ammonium and atmospheric ammonia gas every two hours across a latitudinal transect (34.5°S to 61°S) of the Atlantic Southern Ocean during summer. Surface ocean ammonium concentrations ranged from undetectable to 0.36 µM and ammonia gas concentrations ranged from 0.6 to 5.1 nmol m−3. Calculated ammonia fluxes ranged from −2.8 to −75 pmol m−2 s−1, and were consistently from the atmosphere into the ocean, even in regions where surface ocean ammonium concentrations were relatively high. As expected, temperature was the dominant control on the air‐sea ammonia flux across the latitudinal transect. However, a sensitivity analysis suggests that seasonality in the surface Southern Ocean nitrogen cycle may have a major influence on the direction of the ammonia flux.
Plain Language Summary
Ammonia is the most important basic gas in the atmosphere and it plays an important role in forming new aerosols and controlling the acidity of aerosols, with implications for climate. Ammonia emissions to the atmosphere have increased significantly over time due to human activities, primarily related to agriculture. There are very few regions where one can observe the atmosphere away from these human emissions, making it difficult to quantify natural processes. Our study investigated ocean ammonia fluxes, which are thought to be the largest natural source of ammonia globally, across the Atlantic Southern Ocean where human influences should be minimal. Previous studies have suggested that in this region, the ammonia should flux into the ocean because of the cold surface seawater, but direct measurements are scarce. We found that the flux was indeed into the ocean across the latitudinal transect, although the values were small. A sensitivity analysis suggests that the flux could easily reverse as a result of seasonal changes in surface ocean biological and chemical processes. More observations are needed in this remote region of the atmosphere, particularly during the inhospitable winter season, in order to understand the natural cycling of ammonia between the ocean and atmosphere.
Key Points
Air‐sea fluxes of ammonia were calculated from ship‐board observations every two hours across the Atlantic Southern Ocean
Air‐sea fluxes were small and consistently into the ocean across the latitudinal transect
Seasonality in the biogeochemistry of the surface Southern Ocean may be the main driver of the flux direction
PDF
