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Chlorine & Bromine | Dynamics, Transport, & Waves | Hydrogen Chemistry | Ozone (Global) | Polar Processes & Ozone | Solar Effects

Chlorine & Bromine

Atmospheric measurements of chlorine and bromine compounds are important because chemically reactive chlorine and bromine species have been the major culprits involved in chemical ozone depletion over the past few decades. The source of chlorine and bromine compounds in the upper atmosphere is the release and slow decomposition of long-lived gases emitted at the Earth's surface, from (primarily) industrial products since the 1950s. The most striking consequence of these emissions was the discovery of an ozone hole over Antarctica in the early 1980s. Once the long-lived source gases (chlorofluorocarbons or CFCs, containing chlorine, and halons, containing bromine), used in refrigeration, aerosol propellants, foam material, solvents, dry cleaning, agriculture, or fire extinguishers, have been transported into the stratosphere (above about 15 km), they decompose, mainly from slow destruction by sunlight. This leads to reservoirs of chlorine (such as HCl, hydrogen chloride, measured by Aura MLS) and bromine (such as HBr), as well as more reactive ozone-depleting gases such as ClO (chlorine monoxide) and BrO (bromine monoxide), both measured by Aura MLS. Global measurements of ClO were also carried out from 1991 to 2000 by the Upper Atmosphere Research Satellite (UARS) MLS experiment; other very useful data have been provided by aircraft and ground-based measurements. A more minor species (HOCl, hypochlorous acid) is also measured by Aura MLS. The abundance of these gases is measured in amounts typically below a few parts per billion, down to parts per trillion, but the reactive radicals (ClO, BrO) can destroy ozone rapidly (at the rate of 1 percent/day in the lower stratosphere, during polar winter/spring) via catalytic reactions whereby one atom of chlorine (or bromine) can lead to the destruction of more than 100000 molecules of ozone. The total chlorine abundance at the Earth's surface has been decreasing since about 1993, and the sum of bromine abundances at the surface has decreased since the late 1990s. It is important to track the decrease (at a rate of about -0.5 to -1 %/yr) of total upper atmospheric chlorine (as seen in HCl near 50 km) as well as bromine; such measurements help to confirm the beneficial impact of the Montreal Protocol (which curtailed harmful surface emissions), and provide a database for comparison with atmospheric models and for improved projections of future change. For more specifics about chlorine and bromine scientific publications tied to the MLS observations, see the MLS publications list.

For an overview (targeted to the general public) of the impact of chlorine and bromine species on ozone destruction, see (for example) See also the MLS website information regarding Global Ozone and Polar Ozone.

Dynamics, Transport, & Waves

Dynamics and Transport are among the fundamental processes controlling the composition of the middle atmosphere (stratosphere and mesosphere) and its connections with the lower atmosphere (troposphere). Dynamics comprises the fundamental physical processes determining the characteristics and evolution of a fluid (the atmosphere) on rotating sphere externally forced by solar radiation. The study of atmospheric dynamics uses observations and mathematical models to characterize and explain the evolution of temperatures and winds, as well as products derived from them (e.g., geopotential heights, streamfunctions, vorticity and potential vorticity). An important aspect of this behavior is characterization of wave motions. Of particular importance in the middle atmoshpere are planetary scale (1 to 3 cycles around a latitude circle) waves that are forced by upper tropospheric weather systems or can be generated internally by instabilities in the background wind fields, and gravity waves.

In the stratosphere, the dominant feature of the circulation is the winter polar vortex, consisting of a band of strong winds westerly winds roughly encircling the pole (the polar night jet) that forms as a result of the fundamental balance between the Coriolis forcing and radiative forcing (air moves upward and poleward as a result of solar heating in the tropics, is deflected eastward by the Coriolis force, and descends in the polar winter because of very low temperatures there resulting from the absence of sunlight).

One of the most dramatic dynamical phenomena in the middle atmosphere is the midwinter stratospheric sudden warming (SSW). SSWs occur frequently (historically about once every two years on average) but unpredictably as a result of waves propagating from the upper troposphere under conditions that result in the waves breaking -- depositing their momentum -- in the upper stratosphere and leading to a breakdown of the winter stratospheric polar vortex. A similar process results in the "final warming" by which the polar vortex breaks down in spring, but SSWs are notable in that they disrupt and reverse the circulation in midwinter, leading to a period of easterly winds followed by a recovery to westerlies and re-establishment of the polar vortex. The most dramatic SSWs (three of which have occurred very recently, in Jan 2004, 2006, and 2009) (Manney et al, 2005, JGR; 2008, JGR, ACP; 2009, ACP, GRL) result in a complete disruption of the typical middle atmosphere high-latitude temperature structure (minimum near the tropopause, ~8-10km, maximum at the stratopause, ~50km), such that the conventional distinction between stratosphere and mesosphere is rendered somewhat meaningless (Manney et al 2008, JGR; 2009 GRL, and references therein). While SSWs are thought of as being controlled primarily by planetary scale wave motions, recent work has shown that gravity waves are also important, especially the re-establishment of the vortex after strong SSWs (e.g., Siskind et al, 2007).

The evolution of the circulation and wave motions in the middle atmosphere have also been shown to extend through the mesosphere (Lee et al, 2009, GRL, and references therein), and to affect the weather patterns in the upper troposphere, especially during extreme events such as SSWs. Dynamical processes and wave motions are also important in the tropics: Phenomena such as the annual cycle of upward transport of water vapor (and other trace gases, the so-called "tape-recorder" effect), the Quasi-biennial and semiannual oscillations, and Kelvin wave motions play a large role in determining the structure of the equatorial middle atmosphere.

The dynamics of the polar middle atmosphere is instrumental in determining the extent and timing of polar processing and ozone loss, since those processes are strongly dependent on temperature and vortex containment. In addition, dynamics play a large role in determining the composition of the middle atmosphere via transport processes. The relative roles of transport and chemistry in determining the distribution of trace gases depend on the time scales for chemical and dynamical changes; these time scales vary dramatically with season, altitude, latitude and species. Measurements of trace gases with long chemical lifetimes (e.g., N2O, in some regions CO, H2O, O3) provide information to quantify transport processes; this information is critical for assessing the ability of models to correctly reproduce atmospheric transporteg, (eg, Jin et al, 2009, ACP; Manney et al, 2009, ACP) and for calculations aimed at separating the contribution of chemical and dynamical processes to changes in atmospheric composition, especially for ozone (e.g., Manney et al, JAS, 1995a, b; Singleton et al, 2007, JGR; and references therein).

Prior to datasets from Aura MLS (and other instruments on that satellite), measurements of many species (including long-lived tracers) in the middle atmosphere were sporadic and limited in both spatial and temporal coverage. Aura MLS now provides measurements of long-lived tracers from the upper troposphere through the mesosphere with global daily coverage. CO, HNO3, O3, and H2O measurements are useful in the upper troposphere/lower stratosphere as in many situations they behave as tracers of transport there. N2O is a long-lived tracer useful in studying transport in the lower through the middle stratosphere. CO and H2O provide information on transport in the upper stratosphere and into the mesosphere. Measurements from Aura MLS and other recent satellite instruments have been and continue to be instrumental in improving our understanding of middle atmosphere dynamics and transport.

Hydrogen Chemistry

The odd-hydrogen radicals, OH and HO2 (collectively known as HOx), play fundamental roles in the middle atmospheric photochemistry due to their ability to destroy odd oxygen (O3 and O). In particular, at altitudes above ~40 km, the major catalytic O3 loss is controlled by reactions involving HOx species. A better understanding of the trend in HOx thus helps to properly identify the recovery of the O3 layer. In this region of the atmosphere, HOx mainly come from the photolysis of O3 (i.e., the break down of O3 by sunlight) followed by reaction with H2O. In addition, chemical destruction of H2 and direct photolysis of H2O are also sources of HOx, especially above 80 km. The distribution of HOx species and the chemistry involved are sensitive to solar forcing such as the diurnal cycle, the seasonal cycle, the solar 11-year and 27-day cycles, and solar proton events (SPEs).

Before the launch of Aura, the investigation of hydrogen chemistry had been greatly limited due to the lack of multi-year systematic observations on a global scale. The only available long-term records were a number of ground-based measurements of OH total column abundance. Large discrepancies among measurements and between model predictions and observations were reported. MLS provides the first global daily measurements of HOx vertical profiles in the stratosphere and the mesosphere, making it possible to resolve some of these discrepancies, and significantly improve our understanding of the chemistry involved and the response to external forcing. In particular, MLS observations will help to investigate the discrepancy between modeled and observed O3 in the upper stratosphere, known as the "O3 deficit", which implies that there could be a gap in the current understanding of hydrogen chemistry. In addition, the magnitude of the solar effects on hydrogen chemistry is currently unclear, given the large discrepancies between observed and modeled response of O3 and OH to the solar cycle.

Ozone (Global)

The stratospheric ozone (O3) layer (near 20 km altitude) is an absorber of ultraviolet light from the sun; this absorption protects humans from the potentially deadly effects of skin cancer, and can also shield animals and the marine food chain, as well as plants, from undesirable UV-related consequences. Health effects tied to excessive UV exposure include skin cancer, cataracts, and a decline of the immune response system; while there are significant changes in average UV exposure from low to high latitudes, additional exposure (at any latitude) represents some increase in the risks. Decreases in the ozone layer have therefore been a cornerstone of atmospheric research for the past several decades, motivated by the realisation that industrial release of chlorofluorocarbon (CFC) gases at the Earth's surface were linked to a gradual depletion of the ozone layer, as well as the seasonal "ozone hole" phenomenon over Antarctica caused by enhancements in the ozone-destroying forms of chlorine and bromine in the northern hemisphere during the cold winter and spring periods. The different variations in ozone between the northern and southern hemispheres are related to the interplay of dynamical and chemical effects. A circling whirlpool of winds isolates the so-called polar vortex region at high latitudes in winter. More vigorous wave activity in the North leads to a shorter-lived winter polar vortex than in the South, and this reduces the net ozone loss in the northern hemisphere. On a global scale, ozone depletion is typically measured with respect to pre-1980 abundances; values of overhead (column) ozone abundances in the past few years have been lower than the pre-1980 levels by 3 to 6% (for mid- to high latitudes in the North and South, respectively). Thanks to internationally-agreed reductions in CFC emissions after the 1987 Montreal Protocol on Subtances that Deplete the Ozone Layer (with its many subsequent amendments), global ozone is expected to recover to pre-1980 levels in the 2nd half of the 21st century. The slow recovery process arises because of the very long lifetime of the main CFC gases in the upper atmosphere (sunlight destroys these compounds very slowly). There is mounting evidence that a slow path towards such a recovery is being achieved, although continued attention to unexpected chemistry and the variations in ozone is still a very useful endeavor.

NASA's ozonewatch website provides up-to-date information on the status of the ozone layer and the ozone hole over the South Pole. A summary of the history of ozone research and the relationship between UV radiation and ozone can be found here. Research and results regarding environmental effects of ozone depletion can be found, for example, in the following location: here. Based on accumulated research of the Earth's atmospheric composition, scientific consensus reports are published every four years about the state of the ozone layer and the chemical and dynamical effects surrounding the variations in stratospheric ozone. See the following link for access to the 2006 Scientific Assessment of ozone depletion, with "Twenty questions and answers" and an Executive Summary, in addition to the more extensive and detailed research and results: here.

Besides health-related concerns, the study of ozone and the distribution of other gases in the Earth's atmosphere is an important process to help understand atmospheric behavior and to help predict future global change. The Aura satellite is continuing its measurements of ozone and many other gases on a global scale, using both the MLS and OMI instruments (since August, 2004). MLS provides a vertically-resolved view of changes in ozone, with profiles being measured on a roughly 3 km vertical grid, every 170 km around the orbit (for about 3500 profiles every 24 hours). Ozone and related datasets are also being connected to previous global satellite measurements, so that data records can be constructed and preserved for studies of long-term changes in atmospheric composition. More detailed MLS-derived information and scientific publications relating to ozone can be found on the MLS website; this includes the use of MLS data for investigations of polar ozone loss, and the inference of changes in tropospheric ozone through the combined usage of OMI (total column ozone) and MLS (stratospheric column) measurements.

Polar Processes & Ozone

One of the overarching goals of the Aura mission is to track the stability of the stratospheric ozone layer. At issue is whether global stratospheric ozone will recover as anticipated as the abundances of ozone-depleting substances decline in response to international regulations. The primary agent responsible for the formation of the ozone hole that forms over Antarctica every austral spring is anthropogenic chlorine. Stratospheric chlorine loading is presently near its peak but waning; assuming compliance with existing protocols, it should return to pre-1980 levels by about 2050. Although detection and attribution of small changes in chlorine-catalyzed ozone loss are challenging problems, some abatement of lower stratospheric polar ozone depletion may become apparent during the Aura timeframe. There are, however, important linkages between climate change and ozone depletion that could delay recovery of the ozone layer. Changes in stratospheric temperature, humidity, and circulation patterns brought about by climate change could exacerbate polar ozone destruction processes. These issues are of particular concern in the Arctic, where wintertime temperatures often hover near the thresholds at which the processes leading to severe chlorine-catalyzed ozone destruction are triggered.

Aura MLS measures vertical profiles of many of the key species involved in polar processing and ozone loss in the lower stratosphere. In addition to temperature and ozone itself, MLS is providing the first simultaneous, collocated daily global measurements of both ClO, the primary form of reactive (ozone-destroying) chlorine in the stratosphere, and HCl, the main stratospheric chlorine reservoir (relatively inactive) species. MLS also measures two other minor chlorine species, HOCl and (in version 3) CH3Cl (the only significant natural source of organic chlorine). In addition, MLS measures H2O and HNO3, the main components of the polar stratospheric clouds that form in the very low temperatures in the winter polar regions in both hemispheres; these cloud particles provide surfaces on which the heterogeneous chemical reactions that convert reservoir chlorine to reactive forms can take place, thus priming the atmosphere for severe ozone destruction. N2O and CO are "tracers" of stratospheric air motions; MLS measurements of these species provide critical information needed to disentangle the effects of transport and mixing from those of chemical loss on the observed ozone distributions. Other MLS measurements that are arguably relevant for lower stratospheric polar processes and ozone loss include volcanic SO2 (for helping diagnose the influence of major volcanic eruptions on the ozone layer), and HCN, CH3CN, and cloud ice water content in the upper troposphere (for helping assess the impact of pollutants and other species lofted from below that may affect ozone chemistry in the stratosphere).

Solar Effects on the Atmosphere

The best-known solar effects that lead to variability of atmospheric composition and temperature include the 11-year solar cycle, the 27-day solar cycle, and the solar energetic particle (SEP) events.

The solar 11-year cycle, characterized by the change in the frequency of sunspots, is the major periodic solar variation. Although the corresponding change in the total solar irradiance (TSI) is only about 0.1%, the change in the solar UV flux increases rapidly with decreasing wavelength (from a few percent at 200 nm to several tens of percent at 120nm, the Lyman α region) and greatly affects Earth's atmosphere. The change of solar flux over the solar 27-day cycle, which originates from the rotation of the Sun, has a similar wavelength-dependence but a smaller magnitude. The observations of various atmospheric responses such as trace gases, temperature, humidity, and circulation to both solar cycles have been reported. Solar variability over other time scales has also been reported but with much weaker signals. The corresponding impacts on atmospheric chemistry, dynamics, and the climate are not fully understood, e.g., given the discrepancies between the observed and modeled magnitudes of the atmospheric responses. MLS observations of the middle atmosphere, e.g., O3, H2O, OH, HO2, CO, temperature, and geopotential height, have made a significant contribution to our understanding of solar effects on Earth's atmosphere. Signals of both the 11-year solar cycle and the 27-day solar cycle have been extracted from MLS observations from the mesosphere to the upper troposphere. Ongoing investigations should lead to a better understanding of the mechanisms by which solar activity influences the natural variability of Earth's atmosphere.

In particular, we are currently in the midst of a prolonged solar minimum between solar cycles 23 and 24. Compared to previously recorded solar minimums (1976, 1986, and 1996), the current solar minimum (2008-2009) is unusually prolonged, with a record number of sunspot-free days in the history of satellite measurements since the 1970s. Anomalously low upper atmospheric temperature and low levels of O3, CO, and OH in MLS observations during the current solar minimum also point to exceptionally low solar output. Reduced solar UV irradiance and the corresponding low levels of O3 could affect the recovery from O3 depletion by anthropogenic CFCs. These changes due to solar effects should be factored in when studying the climate change related to accumulated greenhouse gases in the upper atmosphere.

During solar energetic particle (SEP) events, which are also known as the energetic particle precipitation (EPP), high-energy protons, electrons, and ions originating from strong solar activity (e.g. solar flares) reach Earth's atmosphere, leading to changes in atmospheric composition. In particular, solar proton events (SPEs) deposit energetic protons at high geomagnetic latitudes and trigger short-term changes in the distribution of many chemical species such as O3, OH, HO2, HNO3, N2O, and potentially ClO and HOCl. For example, OH responds rapidly to the proton forcing due to its short chemical lifetime and the negligible impact from transport. MLS mesospheric observations have shown strong enhancement of nighttime OH in polar regions during SPEs and the corresponding rapid decrease of O3 due to catalytic O3 destruction reactions.

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