Trans-Pacific transport of mercury

We examine the trans-Pacific transport of mercury with a global chemical transport model. Using existing anthropogenic inventories, the model underestimates the observed Hg/CO ratio in Asian long-range transport events observed at ground-based sites in Okinawa, Japan and Mount Bachelor, Oregon, by 18-26%. This is in contrast with previous studies that inferred a factor of two underestimate in Asian anthropogenic emissions. We find that mercury from land emissions and re-emissions, which are largely colocated with anthropogenic emissions, account for a significant fraction of the observed Hg/CO ratio. Increasing Asian anthropogenic Hg⁰ emissions by 50% while holding land emissions constant, or further increasing anthropogenic emissions while decreasing land emissions, corrects the remaining model bias in the Hg/CO ratio. We thus find that a total Asian source of 1260-1470 Mg/a Hg⁰ is consistent with observations. Hg⁰ emissions from Asia are transported northeastward across the Pacific, similar to CO. Asian anthropogenic emissions of mercury contribute 18% to springtime Hg⁰ concentrations at Mount Bachelor. Asian RGM is not directly transported to North America in the lower troposphere but contributes to a well-mixed pool at high altitude. Asian and North American sources each contribute approximately 25% to deposition to the United States, with Asian anthropogenic sources contributing 14% and North American anthropogenic sources contributing 16%.