Dust and pollution: A recipe for enhanced ocean fertilization?

For Fe contained in aeolian dust to act as a micronutrient for oceanic phytoplankton, some fraction of it must first be transformed (mobilized) into a form soluble in ocean water. Fe solubilization in deliquesced mineral dust aerosols emanating from East Asia during the springtime outflow conditions is investigated here with a Lagrangian box model of the gas and aqueous-phase chemistry. The model simulates the scavenging of soluble and reactive gaseous species by mineral dust aerosols, chemical reactions of these species and mineral dust ions in deliquesced solution, and dissolution of Fe-III oxides that occurs as a result of the acid mobilization. The calculations indicate that mineral calcite (CaCO3) strongly buffers deliquesced dust aerosols with a pH that remains close to 8 until the amount of acid added to the aerosol solution exceeds CaCO3 alkalinity. SO2 pollutant emissions are a potential source of acidity to advecting dust from East Asia. The model is used to simulate the chemical evolution of dust plumes formed from two contrasting documented Gobi-desert storms that advected dust to Fe-limited regions of the North Pacific Ocean (NPO). These calculations indicate that only plumes with relatively high initial SO2-to-dust ratios are capable of delivering significant amounts of bioavailable Fe to the NPO. The estimated change in phytoplankton population inferred from the model-calculated inputs of bioavailable Fe for these two episodes is consistent with the satellite-measured chlorophyll a concentrations in the NPO. Model simulations indicate that (1) large dust advection episodes, that can export vast amounts of mineral dust to the open ocean, should have insignificant dissolved iron fraction (DIF) as the amount of SO2 required to acidify such dust plumes is about an order of magnitude higher than what can typically be entrained in the plume during its advection; (2) smaller dust plumes will generally have higher DIFs because they require lower amounts of SO2 and, even if such small plumes may not cause algae blooms, they could still be important sources of dissolved Fe to the NPO. The calculations suggest that future changes in SO2-pollutant emissions from East Asia, either up or down, may affect the productivity of the NPO. Copyright 2005 by the American Geophysical Union.

Chemistry and climate interactions

Luca Pozzoli

Atmospheric trace gases and aerosols and climate interact in many ways. A quantitative assessment of the influence of trace gases and aerosols on climate can only be achieved if the interactions and feedbacks among these three major components are accounted for. The goal of this thesis was to develop, evaluate and apply the ECHAM5-HAMMOZ chemistry-aerosol-climate model. The model includes fully interactive simulations of the NOx-Ox-hydrocarbons chemistry and of the aerosol chemistry and microphysics, the two simulations being embedded into the well established ECHAM5 climate model. In particular, on-line calculation of the photolysis rates (that accounts for aerosols and clouds) and heterogeneous reactions of trace gases on aerosols are accounted for in the model. In addition, the aerosol simulation provides a prognostic representation of the mixing state of the aerosol components, a feature that is particularly important to examine the effect of the SO2 uptake and subsequent sulfate formation on aerosols. A thorough model evaluation of the model was performed using observations from the Transport and Chemical Evolution over the Pacific (TRACE-P) aircraft campaign. We also used ground based measurements from the European network EMEP (SO2 and sulfate), from the north American network IMPROVE (sulfate, black carbon and organic carbon), and from AERONET (aerosol optical depth). The coupled model is able to reproduce fairly well many of the observations over the TRACE-P region, even though some improvements are still needed. For example, we find a mean absolute bias of 20 ppbv (30%) and 40 ppbv (13%) for the simulated O3 and CO, respectively. Sulfate concentrations are represented fairly well, for all the regions considered, while SO2 is overestimated by a factor of 2 in general. Black carbon concentrations are underestimated over the TRACE-P region (mean absolute bias of 80%), most probably because of too low emissions, but well reproduced over north America. The aerosol optical depths compare well with observations at many sites in general, both in terms of annual means and seasonal variations. We show the results from a series of sensitivity simulations which goals are to assess the impact of heterogeneous reactions, photolysis reactions and sulfur chemistry on the regional and global trace gas distribution, and aerosol distribution, composition and optical properties. We found that heterogeneous reactions result in a reduction of 7% in the global O3 burden, and by up to 15% in surface O3 over regions rich in mineral dust. OH burden decreases by 10%, NO and NO2 by 20% and 29%, respectively while CO burden increases by 7%. Our numbers fall in general within the range of previous studies. We find that the effect of aerosols through the modifications of photolysis rates do not affect significantly the trace gas distributions and global burdens, while previous studies suggested larger effect. Heterogeneous reactions reduce the global mean SO2 surface concentration by 14%, while the global burden remains unchanged. This reflects the effect of competing processes. In particular, the depletion in SO2 by direct uptake on sea salt and dust is counterbalanced by the decrease in SO2 oxidation that is associated with the decrease in OH. The balance between the two competing effects differs depending on the regions (SO2 increases by more than +10% over north Africa and Indian Ocean and decreases by up to -20% at high latitudes). These processes, in turn, affect the sulfate formation. We find in general an increase (by up to 4%) in sulfate burden over the oceans because of sea salt uptake of SO2, and a decrease by up to 6% over Sahara and India, where SO2 increases. Our global burden of SO2 and sulfate amount to 0.77 and 0.87 (Tg(S)), respectively, which fall in the range of previous estimates. We find that SO2 uptake on sea salt contributes to the production of 3.69 Tg(S) yr-1 of sulfate (5% of total sulfate production) in good agreement with recent observation-based estimates. Uptake of SO2 on dust only contributes to the production of 0.55 Tg(S) yr-1 (< 1%) of sulfate, but it is an important process as it is responsible for the coating of mineral dust particles. A total of 300 Tg yr-1 of dust are transferred from the insoluble to the soluble mixed modes (56% of total emissions) because of this later process. The total burden of black carbon does not change significantly because of the heterogeneous reactions, but it is redistributed between insoluble/soluble modes. The changes in the mixing state of aerosols produce significant regional variations in aerosol optical depth and absorption. The aerosol optical depth and aerosol absorption increase by 10 to 30% and by 15%, respectively, during winter in the main polluted regions of the north hemisphere because of enhancing absorption efficiency of black carbon mixed with sulfate. On the contrary aerosol absorption decreases by up to 20% over the Atlantic ocean because of enhancing black carbon removal by wet scavenging. ECHAM5-HAMMOZ is a fully coupled model which includes many of the trace gas-aerosol and chemistry-climate interactions, and is thus a tool that allows an integrated assessment of the impact of the major short-life species that contribute to climate change and air quality.

EPFL2007

Changes in dissolved iron deposition to the oceans driven by human activity: a 3-D global modelling study

Athanasios Nenes

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. © Author(s) 2015.

Copernicus GmbH2015

Fine Particle Iron in Soils and Road Dust Is Modulated by Coal-Fired Power Plant Sulfur

Athanasios Nenes

Transition metal ions, such as water-soluble iron (WS-Fe), are toxic components of fine particles (PM2.5). In Atlanta, from 1998 to 2013, a previous study found that WS-Fe was the PM2.5 species most associated with adverse cardiovascular outcomes. We examined this data set to investigate the sources of WS-Fe and the effects of air quality regulations on ambient levels of WS-Fe. We find that insoluble forms of iron in mineral and road dust combined with sulfate from coal-fired electrical generating units were converted into soluble forms by sulfate-driven acid dissolution. Sulfate produced both the highly acidic aerosol (summer pH 1.5-2) and liquid water required for the aqueous phase acid dissolution, but variability in WS-Fe was mainly driven by particle liquid water. These processes were more pronounced in summer when particles were most acidic, whereas in winter the relative importance of WS-Fe from combustion emissions increased. Although WS-Fe constituted a minute fraction of PM2.5 mass (0.15%), the WS-Fe-PM2.5 mass correlation was high (r = 0.67) and may be explained by these formation routes, which, in part, could account for observed associations between PM2.5 mass and adverse health seen in past studies. Similar processes are expected in many regions, implying that these unexpected benefits from coal-burning reduction may be widespread.

AMER CHEMICAL SOC2020

Changes in dissolved iron deposition to the oceans driven by human activity: a 3-D global modelling study

Athanasios Nenes

Copernicus GmbH2015

Chemistry and climate interactions

Luca Pozzoli

EPFL2007