Abstract
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.
Highlights
[2] Mercury, Hg, is the only significant metallic atmospheric vapor and is typically present in air as gaseous elemental mercury, Gaseous elemental mercury (GEM), with a typical concentration between 1 to 2 ng mÀ3
GEM concentrations are usually reported in nanograms per standard cubic meter where standard temperature and pressure are 0°C and 1013.25 hPa, respectively
The GEM-CO slope was 0.0137 ng mÀ3 ppbÀ1, which we propose was driven by the relative loss of each going into the stratosphere
Summary
[2] Mercury, Hg, is the only significant metallic atmospheric vapor and is typically present in air as gaseous elemental mercury, GEM, with a typical concentration between 1 to 2 ng mÀ3. Deposited mercury is revolatized by biological methylation, chemical reduction, photolysis and physical processes such as wildfire [Friedli et al, 2001] It remains unclear whether the global ocean is a net. [6] Previous observations of mercury in the troposphere and the interpretation of its transport and atmospheric chemistry have lead others to conclude the lifetime of GEM is $1 year [e.g., Mason et al, 1994; Bergan and Rodhe, 2001] with estimates ranging from 0.5 to 2 years. The ITCT2K2 GEM composition of air coming ashore to North America is presented and the GEM data are contrasted with previous observations, in particular with those made off the Asian-Pacific coast in spring 2000 as part of the Asian Pacific Regional Aerosol Characterization Experiment, ACE-Asia [Friedli et al, 2004] and those from Mt Batchelor, OR [Weiss-Penzias et al, 2003, 2006]
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