6 1 Atmosphere Composition and Atmosphere Structure Motivation

6. 1 Atmosphere Composition and Atmosphere Structure 中国科学技术大学 环境地球化学概论

Motivation The objective of atmosphere chemistry is to understand the factors that control the concentrations of chemical species in the atmosphere.

Content 1 2 3 4 Atmospheric Composition Measures of Atmospheric Composition Atmospheric Structure Atmospheric Pressure

Atmosphere Composition Major and minor components Gas % by volume N 2 78. 08 O 2 20. 95 Ar 0. 93 CO 2 0. 036 All other gases 0. 004 water Variable(0. 1 -5%) } Dry air

Atmosphere Composition Mean molecular weight of dry air Is moist air heavier or lighter than dry air?

Atmosphere Composition Early (abiotic) formation of O 2 UV Photolysis of H 2 O and CO 2: H 2 O H∙ + OH∙ CO 2 CO + O∙ Radical recombination: O∙ + OH∙ O 2 + H ∙ H∙ + H∙ H 2 (lost to space. ) May have yielded an Archean O 2 at 1% of present level.

Atmosphere Composition Trace species of environmental significance

Atmosphere Composition Expressing the amount of substances in the atmosphere Mass concentration: the mass (i. e. , g, mg, μg, etc. ) of a substance in a given vol divided by that volume (i. e. , m 3, liter, etc. ) Mixing Ratio(CX): the number of moles of a substance X per mole of air. CX = n. X/nair = VX/Vair CX(ppm) = 106 ∙ n. X/nair 1 ppm = 1 μmol/mol CX(ppb) = 109 ∙ n. X/nair 1 ppb = 1 nmol/mol CX(ppt) = 1012 ∙ n. X/nair 1 ppt = 1 pmol/mol The mixing ratio of a gas does not change as T and P changes. (Example: a balloon filled with air and let it arise. ) What are the mixing ratios for N 2, O 2 and CO 2?

Atmosphere Composition Expressing the amount of substances in the atmosphere

Atmosphere Composition Expressing the amount of substances in the atmosphere

Atmosphere Composition Expressing the amount of substances in the atmosphere Example: The Hong Kong Air Quality Objective for ozone is 240 μg/m 3. the U. S. National Ambient Air Quality Standard for ozone is 120 ppb. Which standard is stricter at the same temperature (25℃) and the pressure (1 atm)?

Atmosphere Composition Expressing the amount of substances in the atmosphere

Atmosphere Composition Expressing the amount of substances in the atmosphere

Atmosphere Composition Expressing the amount of substances in the atmosphere

Atmosphere Composition Expressing the amount of substances in the atmosphere

Atmosphere Structure Atmospheric Structure Other terms: • The lower atmosphere and the upper atomphere • Ionsphere (Homework: find out the definition of ionsphere and its relation with the atmospheric layers defined on the basis of temperature structure. ) Temperature is the basis to divide the atmosphere into four different layers.

Atmosphere Structure Atmospheric Pressure The atmospheric pressure is the weight exerted by the overhead atmosphere on a unit area of surface.