Wavelength Dependence of Fe(II) Photoformation in the Water-Soluble Fraction of Aerosols Collected in Okinawa, Japan

We studied photoformation of Fe(II) in the water-soluble fractions (WSFs) of bulk aerosol particles collected in Okinawa, Japan, using radiation at wavelengths of 313, 334, 366, and 405 nm. Fe(II) photoformation quickly reached a steady state within 5 min of irradiation at all wavelengths. The steady-state Fe(II) concentrations were 85 ± 13% (n = 39) of the total dissolved Fe (TDFe) concentrations in the WSF solutions. Apparent quantum yields of Fe(II) photoformation were determined based on total absorbance of the WSF solutions, and the means (±1 S.D.) were 0.019 (±0.034), 0.021 (±0.031), 0.014 (±0.023), and 0.010 (±0.025) at 313, 334, 366, and 405 nm, respectively. Comparison of the observed rates of Fe(II) photoformation for the WSF solutions and the calculated rates from the known Fe(II)-forming compounds suggested that Fe(oxalate)₂^- could account for the observed Fe(II) photoformation rates if the Fe(oxalate)₂^- concentration is sufficiently high (>20% of [Fe(III)]₀). Furthermore, our study showed that the calculated wavelength dependence of Fe(II) photoformation from Fe(oxalate)₂^- was consistent with that of Fe(II) photoformation observed in the WSF solutions. The results obtained here have implications to daytime Fe(III)/Fe(II) cycles in the atmospheric water droplet.