The Fe(III) in marine aerosols and rainwaters can be reduced to Fe(II) by photochemical processes and by reactions with sulfite. In this paper, measurements of the rates of reduction of nanomolar levels of Fe(III) with sulfite (without O2) have been determined in NaCl and seawater solutions as a function of temperature (0¿ to 40 ¿C), pH (2 to 6.8), ionic strength (I=0.1 to 6 M), and composition (Na+, Mg2+, Ca2+, F-, Cl-, Br-, HCO, SO2-4). The overall rate constant (k, M-1 min-1) for the reaction, Fe(III)+S(IV)→products, is given by d/dt=-k. The reaction was found to be first order with respect to Fe(III) and S(IV). The rate constants as a function of pH increased from a pH=2 to 4 and decreased at higher pH. The effect of temperature and ionic strength on the rates could be represented by log k=log k0+AI0.5/(1+I0.5), where A=-1.1 in NaCl and -2.2 in seawater and log k0=25.39-6,323/T. The energy of activation was found by 121¿6 kJ mol-1. The measured rates in seawater as a function of salinity were lower than the rates in NaCl at the same ionic strength. Measurements in NaCl solutions with added sea-salt ions (Mg2+, Ca2+, F-, Br-, and SO2-4) at pH=3.5 indicate that the formation of inert FeF2+ may be responsible for the lower rates.

The effect of changes in the composition on the rates was interpreted by examining the speciation of Fe(III) and S(IV). This analysis indicates that the rate-determining steps from a pH of 2.5 to 4.0 are FeOH2++HSO-3⇆HOFeSO3H+ and HOFeSO3H+→k1FeOH++HSO3 and at pH of 4 to 6, the reactions Fe(OH)+2+HSO-3⇆(HO)2FeSO3H and (HO)2FeSO3H→k2Fe(OH)2+HSO3 become important. The changes in the concentration of FeOH2+ and HSO-3 as a function of pH and composition can account for most of the changes in the rates. These kinetic studies indicate that the rates of reduction of Fe(III) with S(IV) in acidic water droplets at natural levels of S(IV) may be an important source of Fe(II). ¿ American Geophysical Union 1995