METR 3513: Section 2 Kinetics

Chemical Kinetics

the study of how fast chemical reactions occur and the factors that control reaction rates

Mixing ratio (parts per X)

how much of the atmos is this gas (units: ppm, ppb, ppt, pphm)

Number Density (Molecules per Unit Volume)

How many molecules are present (units: molecules cm^-3)

Mass concentration (mass per unit volume)

how much mass of pollutant is present (ugm^-3,mgm^-3)

ppm means

10^-6

pphm means

10^-8

ppb means

10^-9

ppt means

10^-12

Dalton's Law

px=XxP

Px

Partial Pressure

Xx

Mole fraction

P

total atmospheric pressure

Total atmospheric pressure (P) at sea level

1 atm

Mole fraction (Xx) at sea level

1 * 10^-6

partial pressure (Px) at sea level

10^-6 atm

number density of air (nair)

NA P / (R T) in molecules com^-3

Why do we use mg m^-3

EPA PM2.5 and PM10 standards and health regulations

ppm and/or ppb are used for

atmospheric abundance

molecules cm^-3 are used for

chemical reaction rates

molecules ug m^-3 are used for

air quality and regulations

why do some atmos species disappear in seconds while others survive for years?

chemical kinetics

what is a chemical reaction

a process in which reactants are transformed into products

reaction rate units

concentration per unit time (molecules cm^-3 s^-1)

when do faster reactions occur

concentrations are higher, more collisions occur, and the rate constant is larger

what controls reaction speed

concentrations, the number of collisions, and the rate constant

what is a rate constant

k, it tells how efficient a reaction is, smaller the k, the slower the reaction, larger the k, the faster the reaction

rate of a hypothetical elementary reaction (aA + bB + cC -> products)

Kn is defined as Raten = kn [A]^a [B]^b * [C}^c

approximate lifetime of OH

seconds because it is extremely reactive

approximate lifetime of NO

minutes to hours because it oxidizes rapidly

examples of removal

chemical reactions, photolysis, dry deposition, wet deposition, OH reactions, and ozone reactions

what controls lifetime?

faster removal = shorter lifetime; slower removal = longer lifetime