Changing atmospheric acidity alters the delivery of nutrients to the ocean and affects marine productivity and ecology.
Anthropogenic emissions to the atmosphere have increased the flux of nutrients, ...especially nitrogen, to the ocean, but they have also altered the acidity of aerosol, cloud water, and precipitation over much of the marine atmosphere. For nitrogen, acidity-driven changes in chemical speciation result in altered partitioning between the gas and particulate phases that subsequently affect long-range transport. Other important nutrients, notably iron and phosphorus, are affected, because their soluble fractions increase upon exposure to acidic environments during atmospheric transport. These changes affect the magnitude, distribution, and deposition mode of individual nutrients supplied to the ocean, the extent to which nutrient deposition interacts with the sea surface microlayer during its passage into bulk seawater, and the relative abundances of soluble nutrients in atmospheric deposition. Atmospheric acidity change therefore affects ecosystem composition, in addition to overall marine productivity, and these effects will continue to evolve with changing anthropogenic emissions in the future.
Anthropogenic nitrogen (N) emissions to the atmosphere have increased significantly the deposition of nitrate (NO
) and ammonium (NH
) to the surface waters of the open ocean, with potential impacts ...on marine productivity and the global carbon cycle. Global-scale understanding of the impacts of N deposition to the oceans is reliant on our ability to produce and validate models of nitrogen emission, atmospheric chemistry, transport and deposition. In this work, ~2900 observations of aerosol NO
and NH
concentrations, acquired from sampling aboard ships in the period 1995 - 2012, are used to assess the performance of modelled N concentration and deposition fields over the remote ocean. Three ocean regions (the eastern tropical North Atlantic, the northern Indian Ocean and northwest Pacific) were selected, in which the density and distribution of observational data were considered sufficient to provide effective comparison to model products. All of these study regions are affected by transport and deposition of mineral dust, which alters the deposition of N, due to uptake of nitrogen oxides (NO
) on mineral surfaces. Assessment of the impacts of atmospheric N deposition on the ocean requires atmospheric chemical transport models to report deposition fluxes, however these fluxes cannot be measured over the ocean. Modelling studies such as the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), which only report deposition flux are therefore very difficult to validate for dry deposition. Here the available observational data were averaged over a 5° × 5° grid and compared to ACCMIP dry deposition fluxes (ModDep) of oxidised N (NO
) and reduced N (NH
) and to the following parameters from the TM4-ECPL (TM4) model: ModDep for NO
, NH
and particulate NO
and NH
, and surface-level particulate NO
and NH
concentrations. As a model ensemble, ACCMIP can be expected to be more robust than TM4, while TM4 gives access to speciated parameters (NO
and NH
) that are more relevant to the observed parameters and which are not available in ACCMIP. Dry deposition fluxes (CalDep) were calculated from the observed concentrations using estimates of dry deposition velocities. Model - observation ratios, weighted by grid-cell area and numbers of observations, (R
) were used to assess the performance of the models. Comparison in the three study regions suggests that TM4 over-estimates NO
concentrations (R
= 1.4 - 2.9) and under-estimates NH
concentrations (R
= 0.5 - 0.7), with spatial distributions in the tropical Atlantic and northern Indian Ocean not being reproduced by the model. In the case of NH
in the Indian Ocean, this discrepancy was probably due to seasonal biases in the sampling. Similar patterns were observed in the various comparisons of CalDep to ModDep (R
= 0.6 - 2.6 for NO
, 0.6 - 3.1 for NH
). Values of R
for NH
CalDep - ModDep comparisons were approximately double the corresponding values for NH
CalDep - ModDep comparisons due to the significant fraction of gas-phase NH
deposition incorporated in the TM4 and ACCMIP NH
model products. All of the comparisons suffered due to the scarcity of observational data and the large uncertainty in dry deposition velocities used to derive deposition fluxes from concentrations. These uncertainties have been a major limitation on estimates of the flux of material to the oceans for several decades. Recommendations are made for improvements in N deposition estimation through changes in observations, modelling and model - observation comparison procedures. Validation of modelled dry deposition requires effective comparisons to observable aerosol-phase species concentrations and this cannot be achieved if model products only report dry deposition flux over the ocean.
High-resolution measurements of sulfur dioxide (SO2), nitric acid (HNO3), and hydrochloric acid (HCl) were conducted in Athens, Greece, from 2014 to 2016 via a wet rotating annular denuder system ...paired with an ion chromatograph. Decreased mean annual levels of SO2 and HNO3 (equal to 3.3 ± 4.8 μg m−3 and 0.7 ± 0.6 μg m−3, respectively) were observed relative to the past, whereas for HCl (mean of 0.4 μg m−3 ) no such comparison was possible as the past measurements are very scarce. Regional and local emission sources regulated the SO2 levels and contributed to both the December and the July maxima of 6.6 μg m−3 and 5.5 μg m−3, respectively. Similarly, the significant enhancement at noon and during the winter nighttime was due to transported SO2 and residential heating, respectively. The oxidation of NO2 by OH radicals and the heterogeneous reactions of HNO3 on sea salt seemed to drive the HNO3 and HCl formation, respectively, whereas nighttime biomass burning affected only the former by almost 50%. During summer, the sulfate anions dominated over the SO2, in contrast to the chloride and nitrate ions that prevailed during the winter and were linked to the aerosol acidity that influences their lifetime as well as their impact on ecosystems.
This work documents and evaluates the tropospheric
gas-phase chemical mechanism MOGUNTIA in the three-dimensional chemistry
transport model TM5-MP. Compared to the modified CB05 (mCB05) chemical
...mechanism previously used in the model, MOGUNTIA includes a detailed
representation of the light hydrocarbons (C1–C4) and isoprene, along with a
simplified chemistry representation of terpenes and aromatics. Another
feature implemented in TM5-MP for this work is the use of the Rosenbrock
solver in the chemistry code, which can replace the classical Euler backward
integration method of the model. Global budgets of ozone (O3), carbon
monoxide (CO), hydroxyl radicals (OH), nitrogen oxides (NOx), and
volatile organic compounds (VOCs) are analyzed, and their mixing ratios are
compared with a series of surface, aircraft, and satellite observations for
the year 2006. Both mechanisms appear to be able to satisfactorily represent
observed mixing ratios of important trace gases, with the MOGUNTIA chemistry
configuration yielding lower biases than mCB05 compared to measurements in
most of the cases. However, the two chemical mechanisms fail to reproduce
the observed mixing ratios of light VOCs, indicating insufficient primary
emission source strengths, oxidation that is too fast, and/or a low bias in the
secondary contribution to C2–C3 organics via VOC atmospheric oxidation.
Relative computational memory and time requirements of the different model
configurations are also compared and discussed. Overall, the MOGUNTIA scheme
simulates a large suite of oxygenated VOCs that are observed in the
atmosphere at significant levels. This significantly expands the possible
applications of TM5-MP.
Atmospheric iron (Fe) deposition to the open ocean affects net primary productivity, nitrogen fixation, and carbon uptake. We investigate changes in soluble Fe (SFe) deposition from the ...pre‐industrial period to the late 21st century using the EC‐Earth3‐Iron Earth System model. EC‐Earth3‐Iron considers various sources of Fe, including dust, fossil fuel combustion, and biomass burning, and features comprehensive atmospheric chemistry, representing atmospheric oxalate, sulfate, and Fe cycles. We show that anthropogenic activity has changed the magnitude and spatial distribution of SFe deposition by increasing combustion Fe emissions and atmospheric acidity and oxalate levels. We report that SFe deposition has doubled since the early industrial era, using the Coupled Model Intercomparison Project Phase 6 emission inventory. We highlight acidity as the main solubilization pathway for dust‐Fe and oxalate‐promoted processing for the solubilization of combustion‐Fe. We project a global SFe deposition increase of 40% by the late 21st century relative to present day under Shared Socioeconomic Pathway (SSP) 3–7.0, which assumes weak climate change mitigation policies. Conversely, SSPs with stronger mitigation pathways (1–2.6 and 2–4.5) result in 35% and 10% global decreases, respectively. Despite these differences, SFe deposition increases over the equatorial Pacific and decreases in the Southern Ocean (SO) for all SSPs. We further observe that deposition over the equatorial Pacific and SO are highly sensitive to future changes in dust emissions from Australia and South America, as well as from North Africa. Future studies should focus on the potential impact of climate‐ and human‐induced changes in dust and wildfires combined.
Plain Language Summary
Marine biota needs bioavailable (or soluble) iron (Fe) as a nutrient for photosynthesis. Given that photosynthesis captures atmospheric carbon dioxide (CO2), the amount and spatial distribution of soluble Fe (SFe) deposited regulate the capacity of the ocean to store CO2 and hence can affect the global climate. The supply of Fe to the atmosphere is dominated by desert dust aerosols created by wind erosion of arid surfaces, with a minor contribution from combustion aerosols. Freshly emitted dust‐Fe is mainly insoluble but can be partly transformed into SFe species during atmospheric transport through various dissolution mechanisms mainly affected by aerosol acidity and oxalate concentrations. We conduct a modeling study to quantitatively understand how changes in aerosol and gas‐phase species emissions from the early industrial era to the late 21st century alter SFe deposition. Our simulations indicate that SFe deposition has doubled since the beginning of the industrial era. Future estimates depend upon the projected socio‐economic scenario, with solubilization being boosted in the scenario with weaker mitigation policies, and vice versa. Results show that understanding changes in aerosol acidity and oxalate concentrations are key to constrain projections of SFe deposition.
Key Points
Global soluble iron deposition will increase (decrease) by 40% (35%) with weak (strong) climate mitigation policies
Aerosol acidity controls the dissolution of iron from dust sources and oxalate from combustion sources in the past, present and future
Future soluble iron deposition decreases (increases) over the Southern Ocean (the equatorial Pacific) regardless of the mitigation policy
The global marine organic aerosol budget is investigated by a 3-dimensional chemistry-transport model considering recently proposed parameterisations of the primary marine organic aerosol (POA) and ...secondary organic aerosol (SOA) formation from the oxidation of marine volatile organic compounds. MODIS and SeaWiFS satellite data of Chlorophyll-a and ECMWF solar incoming radiation, wind speed, and temperature are driving the oceanic emissions in the model. Based on the adopted parameterisations, the SOA and the submicron POA marine sources are evaluated at about 5 Tg yr−1 (~1.5 Tg C yr−1) and 7 to 8 Tg yr−1 (~4 Tg C yr−1), respectively. The computed marine SOA originates from the dimethylsulfide oxidation (~78%), the potentially formed dialkyl amine salts (~21%), and marine hydrocarbon oxidation (~0.1%). Comparison of calculations with observations indicates an additional marine source of soluble organic carbon that could be partially encountered by marine POA chemical ageing.
The global atmospheric iron (Fe) cycle is parameterized in the global 3-D chemical transport model TM4-ECPL to simulate the proton- and the organic ligand-promoted mineral-Fe dissolution as well as ...the aqueous-phase photochemical reactions between the oxidative states of Fe (III/II). Primary emissions of total (TFe) and dissolved (DFe) Fe associated with dust and combustion processes are also taken into account, with TFe mineral emissions calculated to amount to ~ 35 Tg-Fe yr-1 and TFe emissions from combustion sources of ~ 2 Tg-Fe yr-1. The model reasonably simulates the available Fe observations, supporting the reliability of the results of this study. Proton- and organic ligand-promoted Fe dissolution in present-day TM4-ECPL simulations is calculated to be ~ 0.175 Tg-Fe yr-1, approximately half of the calculated total primary DFe emissions from mineral and combustion sources in the model (~ 0.322 Tg-Fe yr-1). The atmospheric burden of DFe is calculated to be ~ 0.024 Tg-Fe. DFe deposition presents strong spatial and temporal variability with an annual flux of ~ 0.496 Tg-Fe yr-1, from which about 40 % (~ 0.191 Tg-Fe yr-1) is deposited over the ocean. The impact of air quality on Fe deposition is studied by performing sensitivity simulations using preindustrial (year 1850), present (year 2008) and future (year 2100) emission scenarios. These simulations indicate that about a 3 times increase in Fe dissolution may have occurred in the past 150 years due to increasing anthropogenic emissions and thus atmospheric acidity. Air-quality regulations of anthropogenic emissions are projected to decrease atmospheric acidity in the near future, reducing to about half the dust-Fe dissolution relative to the present day. The organic ligand contribution to Fe dissolution shows an inverse relationship to the atmospheric acidity, thus its importance has decreased since the preindustrial period but is projected to increase in the future. The calculated changes also show that the atmospheric DFe supply to the globe has more than doubled since the preindustrial period due to 8-fold increases in the primary non-dust emissions and about a 3-fold increase in the dust-Fe dissolution flux. However, in the future the DFe deposition flux is expected to decrease (by about 25 %) due to reductions in the primary non-dust emissions (about 15 %) and in the dust-Fe dissolution flux (about 55 %). The present level of atmospheric deposition of DFe over the global ocean is calculated to be about 3 times higher than for 1850 emissions, and about a 30 % decrease is projected for 2100 emissions. These changes are expected to impact most on the high-nutrient-low-chlorophyll oceanic regions.
Secondary inorganic aerosols (SIAs) are major components of fine particulate matter (PM2.5), having substantial implications for climate and air quality in an urban environment. In this study, a ...state-of-the-art thermodynamic model has been coupled to the source dispersion and photochemistry city-scale chemistry transport model EPISODE–CityChem, which is able to simulate pollutants at a horizontal resolution of 100m×100m, to determine the equilibrium between the inorganic gas and aerosol phases over the greater Athens area, Greece, for the year 2019. In agreement with in situ observations, sulfate (SO42-) is calculated to have the highest annual mean surface concentration (2.15 ± 0.88 µgm-3) among SIAs in the model domain, followed by ammonium (NH4+; 0.58 ± 0.14 µgm-3) and fine nitrate (NO3-; 0.24 ± 0.22 µgm-3). Simulations denote that NO3- formation strongly depends on the local nitrogen oxide emissions, along with the ambient temperature, the relative humidity, and the photochemical activity. Additionally, we show that anthropogenic combustion sources may have an important impact on the NO3- formation in an urban area. During the cold period, the combined effect of decreased temperature in the presence of non-sea-salt potassium favors the partitioning of HNO3 in the aerosol phase in the model, raising the NO3- formation in the area. Overall, this work highlights the significance of atmospheric composition and the local meteorological conditions for the equilibrium distribution of nitrogen-containing semi-volatile compounds and the acidity of inorganic aerosols, especially in urban areas where atmospheric trace elements from natural and anthropogenic sources coexist.
Ice-nucleating particles (INPs) enable ice formation,
profoundly affecting the microphysical and radiative properties, lifetimes,
and precipitation rates of clouds. Mineral dust emitted from arid ...regions,
particularly potassium-containing feldspar (K-feldspar), has been shown to
be a very effective INP through immersion freezing in mixed-phase clouds.
However, despite the fact that quartz has a significantly lower ice nucleation activity,
it is more abundant than K-feldspar in atmospheric desert dust and
therefore may be a significant source of INPs. In this contribution, we test
this hypothesis by investigating the global and regional importance of
quartz as a contributor to INPs in the atmosphere relative to K-feldspar. We
have extended a global 3-D chemistry transport model (TM4-ECPL) to predict
INP concentrations from both K-feldspar and quartz mineral dust particles
with state-of-the-art parameterizations using the ice-active surface-site approach for immersion freezing. Our results show that, although
K-feldspar remains the most important contributor to INP concentrations
globally, affecting mid-level mixed-phase clouds, the contribution of quartz
can also be significant. Quartz dominates the lowest and the highest
altitudes of dust-derived INPs, affecting mainly low-level and high-level
mixed-phase clouds. The consideration of quartz INPs also improves the
comparison between simulations and observations at low temperatures. Our
simulated INP concentrations predict ∼ 51 % of the
observations gathered from different campaigns within 1 order of magnitude
and ∼ 69 % within 1.5 orders of magnitude, despite
the omission of other potentially important INP aerosol precursors like
marine bioaerosols. Our findings support the inclusion of quartz in addition
to K-feldspar as an INP in climate models and highlight the need for further
constraining their abundance in arid soil surfaces along with their
abundance, size distribution, and mixing state in the emitted dust
atmospheric particles.
Atmospheric new particle formation (NPF) is a common phenomenon all over
the world. In this study we present the longest time series of NPF records in
the eastern Mediterranean region by analyzing 10 ...years of aerosol number size
distribution data obtained with a mobility particle sizer. The measurements
were performed at the Finokalia environmental research station on Crete,
Greece, during the period June 2008–June 2018. We found that NPF took place
on 27 % of the available days, undefined days were 23 % and non-event
days 50 %. NPF is more frequent in April and May probably due to the
terrestrial biogenic activity and is less frequent in August. Throughout the
period under study, nucleation was observed also during the night. Nucleation
mode particles had the highest concentration in winter and early spring,
mainly because of the minimum sinks, and their average contribution to the
total particle number concentration was 8 %. Nucleation mode particle
concentrations were low outside periods of active NPF and growth, so there
are hardly any other local sources of sub-25 nm particles. Additional
atmospheric ion size distribution data simultaneously collected for more than
2 years were also analyzed. Classification of NPF events based on ion
spectrometer measurements differed from the corresponding classification
based on a mobility spectrometer, possibly indicating a different
representation of local and regional NPF events between these two measurement
data sets. We used the MALTE-Box model
for simulating a case study of NPF in the eastern Mediterranean region.
Monoterpenes contributing to NPF can explain a large fraction of the observed
NPF events according to our model simulations. However the adjusted
parameterization resulting from our sensitivity tests was significantly
different from the initial one that had been determined for the boreal
environment.
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