Anthropogenic Contribution to Iron in Marine Aerosol in the Northwest Pacific Ocean

Abstract:

The sources and deposition of iron in marine aerosol have been widely studied over the past few decades in order to understand its critical role and impact on marine ecosystem and global climate change through biological pump (removing CO₂ from atmosphere to ocean). Previous studies to quantify anthropogenic iron in marine aerosol mainly rely on modeling, but it is hard to verify model results due to the lack of observation and measurement data. Such information is especially needed in the Northwest Pacific Ocean, where measurement data are still much less compared to the Atlantic Ocean. In this study, the research objective is to distinguish combustion-derived Fe from mineral Fe in the Northwest Pacific Ocean. An online monitor (Xact), which can continuously collect atmospheric fine particulate matter samples and use X-ray fluorescence method to determine multiple elements in samples, was firstly used to measure iron in atmospheric fine particles (PM_2.5) during a spring cruise (from March 30 to May 5, 2015). With more than 600 data sets of multiple elements (Fe, K, Ca, Zn, Mn, Ba, Cu, Pb, V, Ni, As, Se, Cd, and Cr) from this campaign, the Positive Matrix Factorization (PMF) was applied to quantify the contribution of anthropogenic emissions to Fe in marine aerosol. Our results showed that Fe in marine aerosol was not only originated from mineral dust, but also from combustion sources such as coal combustion and industry activities. On average, non-dust sources contributed 28% of iron in the study period. This suggests that anthropogenic source of Fe still plays an important role even during spring season, when the influence of dust episodes is often the strongest. In addition, even in open sea area, episodes with the high contribution of coal combustion to Fe can be still seen. As Fe from combustion sources is generally more soluble than mineral Fe and soluble Fe can be used more efficiently as nutrient, thus anthropogenic Fe exhibits more significant impact on marine ecosystem. To our best knowledge, this is the first time to quantify the contribution of anthropogenic source to Fe in marine aerosol based on continuous cruise measurements in the Northwest Pacific Ocean, which provides important and valuable information for future model comparison and a better understanding of impacts due to human activities on marine aerosol.