Terms and Definitions

activation See chlorine activation.

activation energy The minimum kinetic energy possessed by the reactants in a bimolecular reaction in order for a collision to result in a reaction. See Equation (2.10) and accompanying discussion.

adiabatic A thermodynamic process occurring at constant entropy and thus without gain or loss of heat. In the atmosphere such a process conserves potential temperature and potential vorticity.

aerosols See sulfate aerosols.

association reaction A reaction that involves the association of two atoms or molecules to form a single reaction. Sometimes called a termolecular reaction.

CFC See chlorofluorocarbons.

chemically perturbed region That region of the polar vortex containing air which has been exposed to PSCs and, in general, contains high abundances of CI,.

chlorofluorocarbons (CFCs) A class of organic molecules containing chlorine and fluorine. This term generally applies to man-made chemicals.

CLAES The Cryogenic Limb Array Etalon Spectrometer, an instrument onboard the UARS. See Roche et al. [278] for more details.

chlorine activation When reactions on PSCs lead to the conversion of the chlorine reservoir species HC1 and ClONO, into CI,.

CI, CI + CIO + 2 x ClOOCI. CI, is the sum of the chlorine species that are involved in the catalytic cycles that destroy odd oxygen. CI, is also known as "reactive chlorine".

CIy HC1 + ClONO, + HOC1 + CIO + CI + 2 x ClOOCI. Cl„ is total inorganic chlorine. It is the sum of all of the chlorine that has been released from CFCs and other chlorine-bearing organic molecules.

CCly The concentration of chlorine atoms in an air mass that are bound into long-lived organic molecules such as CFCs. The sum of CI, and CCI,. is a conserved quantity and is equal to the total amount of chlorine that entered the stratosphere in the air mass.

deactivation The opposite of chlorine activation: the net conversion of enhanced amounts of CI, back into the chlorine reservoir species HC1 and CI0N02.

denitrification The removal of NO, from the gas phase by incorporation into solid or liquid particles. If the particles formed are large enough, they will sediment out of the stratosphere and the denitrification is irreversible. If the particles formed are small, then they do not sediment and when the air warms, the particles release the NOy back into the gas phase. In this case, the denitrification is temporary, diurnal cycle A cycle over 24 h.

diurnal steady-state Photochemical production is equal to photochemical loss when averaged over 24 h. diabatic A thermodynamic process in which heat is gained or lost and entropy therefore changes. In the atmosphere such a process does not conserve potential temperature or potential vorticity. Dobson unit (DU) The unit of measure for column 0 ;. If you were to take all the O, in a column of air stretching from the surface of the Earth to space, and bring it to standard temperature (273 K) and pressure (1013 mb), the O, would be 0.25-0.50 cm thick (except in the Antarctic "ozone hole"). The "Dobson unit" is defined to be this thickness in milli-centimeters, so 0.25-0.50 cm corresponds to 250-500 DU. Alternatively, 1 DU = 2.69 x 10lh molecules cm \ DU see Dobson Unit.

^-folding time The time for some geophysical parameter to decrease by a factor Me = 0.368. Often referred to as the lifetime or time scale of the parameter/process, easterly Coming from the east.

equivalent latitude The equivalent latitude of a point in the atmosphere is the latitude circle that encloses the same area as the PV contour that runs through that point.

first-order reaction A reaction in which a reuctanl is transformed into one or more products. Examples of this include photolysis, thermal dissociation, radioactive decay, and isomerization. free radical See radical.

HCFC A CFC that contains a hydrogen atom. This shortens the lifetime of the molecule. As a result the molecule is destroyed lower in the atmosphere, where its chlorine and bromine will have less of an effect on stratospheric Oa. hectopascal (hPa) A unit of pressure, equal to I mb or 100 N m Surface pressure is generally taken to be 1013 hPa, and the stratosphere runs from -300-100 hPa to ~1 hPa. F10 CC14. Fll CC13P. F12 CC1,F2.

global lifetime Total amount of a molecule in the atmosphere divided by the loss rate of the molecule integrated over the entire atmosphere. hPa See hectopascal.

Junge layer The region of the atmosphere—generally speaking between 16 and 20 km—that contains most of the stratosphere's burden of sulfate aerosol.

local lifetime The reciprocal of the chemical loss frequency (1 !L).

local solar time (1st) The time based on the position of the Sun. Noon LST occurs each day when the Sun is at its highest point in sky. loss rate The rate of loss of a chemical constituent, with units of amount/unit time (most often molecules cm ! s ' or VMR s '). It is generally the product of a loss frequency L and the abundance of the species in question, lowermost stratosphere That part of the stratosphere located between the ~380 K potential temperature surface and the tropopause; it exists only in mid- and high latitudes.

isentrope A line or surface of constant potential temperature (and therefore constant entropy).

isopleth A line or surface of constant mixing ratio of a constituent. M In a chemical reaction, M refers to "any molecule": in practice, it refers to N, or 0;. [M] is the number density of all molecules in a volume. A useful shortcut to calculating total number density is: [M] (in molecules cm ') = 7.22 x MY'P/T. where pressure P is in hectopascals (hPa) and temperature T is in Kelvin (K). The VMR of M is, of course, 1. meridional North-south, toward or away from the pole or equator, meridional plane The latitude-height plane. It can be obtained by zonally averaging a three-dimensional field, millibar (mb) Also known as the hectopascal (hPa).

mixing ratio The mixing ratio of a constituent is defined to be the ratio of the density of a trace constituent divided by the density of air. If the number density is used, then volume mixing ratio is obtained; if the mass density is used, then mass mixing ratio is obtained. Throughout the literature on the stratosphere, volume mixing ratio is by far the more common form. Because volume mixing ratio is generally a very small number, it is typically expressed in parts per million by volume (ppmv). parts per billion by volume (ppbv), or parts per trillion by volume (pptv). Mixing ratio is always a unitless quantity. MLS The Microwave Limb Sounder is an instrument on UARS which measures

0„ CIO, H,0, HNO,. and SO,. For more information see Barath et al. [279]. National Meteorological Center (NMC) See NCEP. NCEP National Center for Environmental Prediction.

number density The number density of some constituent X- usually denoted [%\, is the number of molecules of constituent % per unit volume. Number density typically has units of molecules per cubic centimeter (molecules cm '). overworld That part of the stratosphere located above the ~380 K potential temperature surface.

photochemical steady state Photochemical production is equal to photochemical loss at each instant in time, photolysis Chemical decomposition of a molecule after absorption of a photon of radiation (usually visible or ultraviolet).

POAM Polar Ozone and Aerosol Measurement (POAM) [280], polar night That period during the winter when the high latitudes experience 24 h of darkness every day. polar stratospheric clouds (PSCs) These clouds form in the cold temperatures of the lower stratosphere of the winter and early spring polar vortex. Type I PSCs form from the co-condensation of H20 and HN03, while type II PSCs form from the condensation of water vapor. Chemistry necessary for the formation of the ozone hole occurs on the surfaces of these clouds, polar vortex The air mass that is located poleward of the polar night jet. This region is cold (so allows the formation of PSCs) and isolated from mid-latitudes by the strong zonal winds of the polar night jet. potential temperature (theta) The temperature that the parcel would have if it were moved adiabatically to a reference pressure, usually the surface pressure (1000 hPa).

potential vorticity (PV) PV may be thought of as a fluid dynamical analog of spin angular momentum. PV is normally positive in the northern hemisphere and negative in the southern hemisphere, consistent with the sense of the Earth's rotation, and increases in absolute value with latitude on isentropic surfaces. PV is conserved on time-scales similar to the time-scale over which potential temperature is conserved (see Figure 2.5). There are several definitions of potential vorticity: End's potential vorticity (EPV), quasigcostrophic potential vorticity, and modified potential vorticity (MPV). See Lait [571 for a discussion of PV. ppbv Parts per billion by volume, ppmv Parts per million by volume, pptv Parts per trillion by volume. PV See potential vorticity.

radical A radical is a molecule with an unpaired electron. Such molecules are very reactive.

rate constant The product of the rate constant of a reaction and the abundance of the reactants yields the rate of the reaction, reactive chlorine See Cf.

reactive uptake coefficient (y) The fraction of collisions between a molecule and a particle that results in a reaction, y is dimensionless and always between 0 and 1.

reservoir species A member of a chemical family that does not participate in the catalytic cycles that destroy O,. Destruction of these species will often lead to the production of 0,-dcstroying radicals. Examples of reservoir species include C10N02 and HNO,. SAD See surface area density.

surface area density (SAD) The particle surface area per unit volume, usually expressed in square centimeters per cubic centimeter (cm2 cm !) or square micrometers per cubic centimeter (pm2 cm ').

second-order reaction A reaction between two reactants that produces two or more products: X + Y-> C + D.

solar zenith angle (SZA) This is the angle between the Sun and the point located directly over the observers head (the zenith). Thus, a SZA of 0° means that the Sun is directly overhead, while a SZA of 90° means that the sun is at the horizon. SZAs > -90° correspond to night-time, steady state See photochemical steady state and diurnal steady state. stratopause The boundary between the stratosphere and mesosphere. It is located at ~1 hPa (2000 K potential temperature), sulfate aerosols Liquid particles composed primarily of H O and H,S04, and at temperatures below about 200 K, HNO,. The particles have typical radii of a few tenths of a micron. SZA See solar zenith angle. theta See potential temperature.

time-scale The characteristic length of time over which some process occurs. TOMS Total Ozone Mapping Spectrometer. This instrument measures ultraviolet light that has been back-scattered off the atmosphere, and infers the column O, from this. See McPeters and Labow [281 ] and McPeters et al. [282] for more information, total inorganic chlorine See Cly.

tropopause The boundary between the troposphere and stratosphere. It ranges from -300 hPa (310 K potential temperature) at high latitudes to 100 hPa (380 K potential temperature) over the equator. UARS Upper Atmosphere Research Satellite [283],

UKMO United Kingdom Meteorological Office. It produces a set of meteorological fields that are widely used in stratospheric analyses. See Swinbank and O'Neill [259] for more information about these fields, ultraviolet (UV) radiation Radiation with a wavelength less than -300 nm. VMR Volume mixing ratio. See mixing ratio. westerly Coming from the west, zonal East-west, along a line of constant latitude.

zonal average All of the data taken at a single latitude and altitude, but covering all longitudes, are averaged to produce a single data value for that latitude and altitude. This is based on the assumption that the atmosphere is well mixed in the zonal direction.

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