Source and mixing state of iron-containing particles in Shanghai by individual particle analysis

Bioavailable iron (Fe) is an essential nutrient that can control oceanic productivity, thereby impacting the global carbon budget and climate. Therefore it is of vital importance to identify chemical species and mixing state of Fe-containing particles in the air, which are demonstrated to pose substantial impact on bioavailability of Fe. Using a single particle aerosol mass spectrometer (SPAMS), ~2,000,000 individual particles with mass spectra were collected in Shanghai for nearly 22d during the winter of 2011. Number fraction of Fe-containing particles (NfFe) varied in a wide range (<1-15%) throughout the measurement. Fe-containing particles were mainly clustered into four chemical groups, comprising of Fe-rich, K-rich, Dust and V-containing particle types. Analysis of mass spectra and mixing state suggests that Fe-containing particles correspond to various sources in Shanghai, especially anthropogenic sources iron/steel industrial activities, and fly ashes from both biomass burning and coal combustion, accounting for ~55% and ~18%, respectively. However, invasion of dust from northern desert areas is suspected to be more responsible for the spikes of NfFe (>10%), when Dust particle type contributed to >50% of Fe-containing particles. It is also revealed that Fe-containing particles were internally mixed with secondary species (e.g., sulfate and nitrate). Anthropogenic K-rich and Fe-rich particles tended to associate with both sulfate and nitrate, and thus might lead to more fraction of soluble Fe, compared to Dust particles. These results imply that atmospheric processing of Fe-containing particles from various sources might vary and thus would change the bioavailability of atmospheric Fe.