Field Evidence of Fe-Mediated Photochemical Degradation of Oxalate and Subsequent Sulfate Formation Observed by Single Particle Mass Spectrometry

In this work, we deployed a single particle aerosol mass spectrometer (SPAMS) at a suburban coastal site in Hong Kong from February 04 to April 17, 2013 to study individual oxalate particles and a monitor for aerosols and gases in ambient air (MARGA) to track the bulk oxalate concentrations in particle matter smaller than 2.5 μm in diameter (PM2.5). A shallow dip in the bulk oxalate concentration was consistently observed before 10:00 am in the morning throughout the observation campaign, corresponding to a 20% decrease in the oxalate concentration on average during the decay process. Such a decrease in PM oxalate was found to be coincident with a decrease in Fe-containing oxalate particles, providing persuasive evidence of Fe-mediated photochemical degradation of oxalate. Oxalate mixed with Fe and Fe_NaK particles, from industry sources, were identified as the dominant factors for oxalate decay in the early morning. We further found an increase of sulfate intensity by a factor of 1.6 on these individual Fe-containing particles during the oxalate decomposition process, suggesting a facilitation of sulfur oxidation. This is the first report on the oxalate-Fe decomposition process with individual particle level information and provides unique evidence to advance our current understanding of oxalate and Fe cycling. The present work also indicates the importance of anthropogenic sourced iron in oxalate-Fe photochemical processing. In addition, V-containing oxalate particles, from ship emissions, also showed evidence of morning photodegradation and need further attention since current models rarely consider photochemical processing of oxalate_V particles.