Presentation on theme: "Measurement and modeling of aerosol Fe speciation Jingqiu Mao (GFDL/NOAA), Songmiao Fan (GFDL/NOAA), Ying Chen (Fudan U, China)"— Presentation transcript:
Measurement and modeling of aerosol Fe speciation Jingqiu Mao (GFDL/NOAA), Songmiao Fan (GFDL/NOAA), Ying Chen (Fudan U, China)
A dominant source of nutrient iron to open ocean, critical for plankton in surface waters: -Biological pump of CO2 -DMS production->sulfate->marine clouds->climate Why do we care about aerosol Fe? “Give me a half a tanker of iron and I'll give you the next ice age”- John Martin Phytoplankton blooms in the South Atlantic Ocean. (MODIS) Ocean Fe is mainly supplied by dust (95%)
Fe(II) is the bioavailable form of aerosol Fe Crystal structure of hematite Fe(II) solubilities ~ 0.1% Solubilization of dust Fe
Solubilization of dust Fe by atmospheric processing Soil has low Fe solubilities ~ 0.1% Solubilities of aerosol Fe in remote regions: up to 80% Fine aerosols (<2.5 µm) tend to yield larger iron solubilities than coarse aerosols (Siefert et al., 1999; Baker et al., 2006) (Baker et al., 2006) Aerosol mass Solubility
Solubilization of dust Fe Dust Fe(III)= Fe 3+ + Fe(OH) 2 + + Fe(OH) 2+ + Fe(SO 4 ) + + … Fe(II)= Fe 2+ + Fe(OH) + + Fe(SO 4 ) + …
Aqueous Fe chemistry Fe(II) + H 2 O 2 → Fe(III) + OH + OH − Fe(II) + OH → Fe(III) + OH − Fe(II)Fe(III) ??? Fe(III) + H 2 O 2 /HO 2 are too slow to be important
Current mechanisms for Fe(III)→Fe(II) (Zhuang et al., 1992, Nature) (1) Enhanced photolysis of Fe(III) by cloud processing Cloud: pH~4 Aerosols: pH<3 Cannot maintain the steady state of Fe(II)/Fe(III) after clouds evaporate. cloud aerosol ? Fe(OH) 2+ + hv
(2) Enhanced photolysis by organic acids (photolysis rate ~ 10 -2 s -1 ) (Zuo and Hoigné, 1992) Limitation: need continuous supply of oxalic acid in aerosols. We need something to sustain the steady state of Fe(II)/Fe(III)! Current mechanisms for Fe(III)→Fe(II) Fe 2+ + CO 2
Fe(II) + H 2 O 2 → Fe(III) + OH + OH − Lifetime of Fe(II)< 1hr for 1ppb H2O2 (Zhu et al., 1997) N-nighttime D-daytime Current mechanisms cannot explain nighttime Fe(II) measurements!! aerosol Fe(II) measurements in marine boundary layer O 2 - is the only electron donor we can think of at night, but Fe(III) + O 2 − is too slow. However, Cu(II) + O 2 − is faster by a factor of 300 Cu(I) + Fe(III) is very fast Fe(III) → Fe(II) ???????
A new driver for aqueous Fe(II) production-HO 2 uptake Cu(I) + Fe(III) → Cu(II) + Fe(II) electron transfer reaction (very fast) Cu(II) + HO 2 → Cu(I) + O 2 + H + Fe(II) + H 2 O 2 → Fe(III) + OH + OH − Fe(II) + OH → Fe(III) + OH − Cu-Fe redox coupling Fe(II) is sustained by gas-phase HO2!!!!
HO 2 measurement over remote ocean (Kanaya et al., 2000) HO 2 >0 Unique role of HO2 in heterogeneous chemistry: (1) its lifetime (~ 1-10 min), long enough for het chem (OH lifetime only ~1 s). (2) high polarity in its molecular structure. (very soluble compared to OH/CH 3 O 2 /NO/NO 2 ). (3) very reactive in aqueous phase (a major reason for DNA damage and cancer, superoxide). Nighttime Fe(II) can be supplied by nighttime HO2
Gas phase HO 2 uptake by particles HO 2 aerosol HO 2 (aq) NH 4 + SO 4 2- HSO 4 - Aqueous reactions NH 4 + HSO 4 - ④①②③ γ(HO 2 ) defined as the fraction of HO 2 collisions with aerosol surfaces resulting in reaction. ① ② ③④
Modeling framework for HO 2 aerosol uptake HO 2 aerosol [HO 2 ] surf R in [HO 2 ] surf [HO 2 ] bulk R out [HO 2 ] surf is higher than [HO 2 ] bulk because of its short lifetime. provides a relationship between [HO 2 ] surf and [HO 2 ] bulk. The diffusion equation with chemical loss (k I [HO 2 ]) and production (P HO2 ) Aqueous chemistry include Cu, Fe, Cu- Fe coupling, odd hydrogen and photolysis. Uptake rate Volatilization rate Chemical loss rate
Chemical budget for NH 4 HSO 4 aerosols at RH=85%, T=298 K Cu/Fe = 0.05, HO 2 (g) = 10 pptv, H 2 O 2 (g) = 1 ppb Fe(III) reduction is dominated by Fe(III) + Cu(I), instead of photoreduction (implications for dust iron to ocean…) This process is entirely driven by HO 2 ( γ(HO 2 ) = 0.7) OH budget is controlled by TMI.
Fe(II)/Fe ratio modulated by gas-phase HO 2 concentrations Field measurements of Fe(II)/Fe_total in MBL Higher Cu/Fe ratio leads to higher Fe(II)/Fe_total Fe(II)/Fe_total
Cu and Fe are ubiquitous in crustal and combustion aerosols Cu/Fe ratio is between 0.01-0.1 IMPROVE Cu is fully dissolved in aerosols. Fe solubility is 80% in combustion aerosols, but much less in dust.
Ionic strength correction for aerosol aqueous chemistry Non-ideal behavior due to the electrostatic interactions between the ions. 1.Use Aerosol Inorganic Model (AIM) to calculate the ionic strength and activity coefficients for major ions (i.e. NH 4 +, H +, HSO 4 -, SO 4 2- ). 2.Calculate activity coefficients for trace metal ions and neutral species based on specific ion interaction theory. 3.Account for salting-out effect on Henry’s law constant. A i is activity coefficient for any species and also a function of ionic strength. + - + - Ideal solution (cloud droplets) Non-ideal solution (aqueous aerosol) + + ++ + + - - - - - - - - - - - - - - - - - - - - - - -- - - -
Dependence on aerosol pH and Cu concentrations (A)γ(HO 2 ) in the range 0.4-1 at T = 298 K, should be close to 1 at lower T, due to higher solubility. (B)H 2 O 2 yield is more likely to be negative than positive. (C)HO 2 uptake is limited by aqueous diffusion until Cu = 5 x 10 -4 M. Cu/Fe=0.1 Cu/Fe=0.01 typical rural site
Organic aerosols (insoluble organic) Organic-electrolyte mixtures tend to have liquid-liquid phase separation state. (Zuend et al., ACP, 2012) (Furukawa et al., ACP, 2010) Water soluble organic aerosols Fe(III)C 2 O 4 and Fe(II)C 2 O 4 complexes are very unstable. Cu complexes can also be a significant sink for aqueous HO 2 (Voelker et al., EST, 2000)