Observations of mercury-containing aerosols.

In situ analyses with a laser ionization mass spectrometer have shown that a large fraction of aerosols in the bottom few kilometers of the stratosphere contain small amounts of mercury (1). Electron microscopy of particles collected near the tropopause has also detected mercury. The distribution of mercury onto many particles, including those less than 20 nm in diameter, indicates that the mercury is from local condensation of mercury compounds onto particles rather than transport of mercury-rich aerosols from surface sources. Although the results are only semiquantitative, they suggest that most of the mercury in the lower stratosphere is converted into the particulate phase. Mercury-containing particles were present at both middle latitudes and the tropics in two seasons. There is therefore good reason to believe that particulate mercury above the tropopause is global and could affect the atmospheric lifetime of mercury. There are indications that bromine and/ or iodine may be involved in the conversion of mercury from the gas to particle phase. Measurements at altitudes below 5 km did not find mercury in any particles despite sampling some particles that clearly originated in the stratosphere. This indicates that the particulate mercury from the lower stratosphere may be volatile enough to evaporate or decompose once particles reach warmer temperatures.

Evidence that nitric acid increases relative humidity in low-temperature cirrus clouds.

In situ measurements of the relative humidity with respect to ice (RHi) and of nitric acid (HNO3) were made in both natural and contrail cirrus clouds in the upper troposphere. At temperatures lower than 202 kelvin, RHi values show a sharp increase to average values of over 130% in both cloud types. These enhanced RHi values are attributed to the presence of a new class of HNO3-containing ice particles (Delta-ice). We propose that surface HNO3 molecules prevent the ice/vapor system from reaching equilibrium by a mechanism similar to that of freezing point depression by antifreeze proteins. Delta-ice represents a new link between global climate and natural and anthropogenic nitrogen oxide emissions. Including Delta-ice in climate models will alter simulated cirrus properties and the distribution of upper tropospheric water vapor.

Chemistry and microphysics of polar stratospheric clouds and cirrus clouds.

Ice particles found within polar stratospheric clouds (PSCs) and upper tropospheric cirrus clouds can dramatically impact the chemistry and climate of the Earth's atmosphere. The formation of PSCs and the subsequent chemical reactions that occur on their surfaces are key components of the massive ozone hole observed each spring over Antarctica. Cirrus clouds also provide surfaces for heterogeneous reactions and significantly modify the Earth's climate by changing the visible and infrared radiation fluxes. Although the role of ice particles in climate and chemistry is well recognized, the exact mechanisms of cloud formation are still unknown, and thus it is difficult to predict how anthropogenic activities will change cloud abundances in the future. This article focuses on the nucleation, chemistry, and microphysical properties of ice particles composing PSCs and cirrus clouds. A general overview of the current state of research is presented along with some unresolved issues facing scientists in the future.