The North Pacific Ice Core Record of Atmospheric Pollution

Atmospheric concentrations of sulfate, nitrate and heavy metals have largely been declining in the United States and Western Europe since the implementation of emission restrictions in the mid-late 20^th century (e.g. the U.S. Clean Air Act of 1970). However, air pollution remains a significant environmental and health concern in many regions, particularly in Asia. In China alone, air pollution accounts for over 1 million premature deaths each year. Ice core records of acid and metal concentrations provide pre-industrial context for recent pollution levels and trends, and can elucidate natural and anthropogenic sources. Here we evaluate the history of air pollution in the North Pacific region as recorded in three ice cores: Mount Logan PR Col (Yukon Territory, Canada), Eclipse Icefield (Yukon), and the recent Denali ice core from Mount Hunter (Alaska, USA). In addition to their separate geographic locations in the North Pacific, these three ice cores represent atmospheric concentrations across a range of elevations from 3017 m to 5300 m. All three cores were sampled by continuous melter system and analyzed by ion chromatography and inductively coupled plasma mass spectrometry. We find that the Denali ice core record (3900 m elevation) reflects a mixture of Asian, Eurasian and North American heavy metal and acid emissions, similar to the Eclipse ice core (3017 m elevation). Lead concentrations and enrichment factors (EFs) in the Denali ice core increased 890-950% in the 20^th and 21^st centuries relative to pre-industrial levels, while Bi, Zn, and Cd concentrations and EFs increased 60-90%. In time series, Pb, Bi, Cd and Zn show initial pollution peaks in the 1960s-70s attributed to North American pollution, followed by a decline and subsequent stronger rise since the 1990s attributed to Asian pollution. Decreases in Denali ice core Pb levels since 2005 are consistent with reported vehicle Pb emission reductions in China. However, Denali concentrations of Cd and Zn remained elevated or continued to increase through the end of the record in 2012, indicating that emissions of those toxic metals had not yet appreciably declined.