Atmospheric mercury over the NE Pacific during spring 2002: Gradients, residence time, upper troposphere lower stratosphere loss, and long-range transport

Gaseous elemental mercury (GEM) was measured over the North Pacific and coastal California, Oregon and Washington in spring 2002 during the Intercontinental Transport and Chemical Transformation Experiment-2002 (ITCT2K2). Observations were made from the NOAA WP3 on 11 flights which spanned an altitude range of 0-8 km and latitudes between 35 and 55°N. GEM was determined using a Tekran 2537A mercury instrument modified for airborne use to collect and analyze samples every 2.5 min under variable ambient temperature and pressure conditions. GEM concentrations were in general at the low end of previous measurements made over continents and in fresh continental outflow over the ocean. They showed (1) a significant vertical gradient in GEM between the lower sample altitudes and the upper altitudes, with median concentrations decreasing from 1.2 to 0.5 ng m⁻³; (2) GEM was depleted in upper troposphere lower stratospheric air and indicated this region is a sink for GEM; (3) GEM concentrations were ∼1/2 at the surface to ∼1/4 at midaltitudes of those observed immediately downwind of Asia during ACE-Asia, however; (4) highest GEM concentrations (∼4 ng m⁻³) were found in plume layers at concentrations comparable to those observed off the Asian coast in ACE-Asia; and (5) the observed variance in GEM suggested a lifetime on the order of 100 days, which was shorter than the previously accepted nominal lifetime of 1 year.