Environment Chemistry Midterm #1

Impacts of UV Radiation:

• (Severe) sunburns

• skin cancers & cataracts

• DNA damage

• damage to plants and phytoplankton

Pressure _____ exponentially with height through the troposphere and into the stratosphere

decreases

temperature ______ with height in the troposphere

decreases

temperature ______ with height in the Stratosphere

increases

Trace Gases:

• carbon dioxide

• ozone

• nitrous oxide

• methane

• helium

• neon

atmospheric composition

• nitrogen

• oxygen

• argon

Gases in the atmosphere are selective absorbers of light

ozone absorbs light between 200-320 nm UV-B “sunscreen'“ in the stratosphere

“Good Ozone”

stratospheric ozone

“Bad Ozone”

tropospheric ozone

Much of UV-B and UV-C from the sun does not reach the surface because of ozone in the stratosphere

The ozone later is not 100% O3 (O3 accounts for less than 10 out of 10⁶ air molecules in the stratosphere)

How do we understand the concentration of gases in the atmosphere?

when pressure and temperature change with height

UV protection by Ozone in the stratosphere is critical for life and our health

• UV-C absorption by O2 drives an important series of chemical reactions that form O3

• O3 then absorbs a large portion of the incident UV-B

• Human-made (anthropogenic) ozone depleting substances catalytically destroy stratospheric O3

Dobson Units (DU)

type of “total column concentration”

atmospheric column concentrations:

observing changes in stratospheric ozone concentration from satellites

O3 concentrations peak in the stratosphere:

coincide with increasing temperatures above the tropopause

Chapman Cycle Reactions

natural production & destruction of O3 in the stratosphere

Chapman Cycle rxn 1:

O2 + hv (UV-C) → O + O

Chapman Cycle rxn 2:

O2 + O + M → O3 + M

Chapman Cycle rxn 3:

O3 + hv (UV-B) → O2 + O

Chapman Cycle rxn 4:

O3 + O → 2O2

The life time of O atoms (atomic oxygen) ______ strongly with altitude

increases

Why do we care about O atoms?

O atom lifetime is generally <1 second in the stratosphere, so that Ox (odd oxygen = O + O3) is mainly present as O3

CFC (ChloroFluoroCarbons)

non-toxic, non-flammable & un-reactive

Atmospheric lifetime of CFCs

• long atmospheric lifetime

• NO SINKS in the troposphere

• lifetime in the troposphere typically >50 years

Spatial distribution of chemical in the atmosphere

Within Northern or southern hemisphere troposphere ~weeks

Into the stratosphere > 3 years

Between northern or southern hemisphere troposphere ~1 year

CFCs must be chemically ____ in the troposphere, and able to travel around the global

inert

reservoir compounds for Cl radicals:

ClONO2 and HCl

The Montreal Protocol

regulates production and consumption, rather than emissions, which are harder to keep track of

CFCs are persistent in the environment:

The long lifetime of CFCs means that we are just beginning to see decreasing concentrations in the atmosphere

What is particulate matter? aka aerosol

small, nanometer - micrometer sized solid or liquid particle suspended in air

Catalyst

• a chemical compound that is used, then produced in a reaction

• lowers the activation energy for a reaction by providing an alternative pathway

Reservoir compounds

• relatively un-reactive forms of chlorine that temporarily sequester Cl radicals and prevent them from reacting with O3

primary pollutants

any pollutant that is directly emitted from a source eg, soot particles, SO2 (g) from coal burning

secondary pollutants

any pollutant that is formed in the atmosphere through chemistry eg, O3 in the troposphere

Ingredients for photochemical smog

1. hydroxyl radical (OH) → an important oxidant in the troposphere

2. volatile organic compounds (VOCs) → hydrocarbons

3. nitrogen oxides (NOx = NO + NO2) → from combustion

Volatile Organic Compounds (VOCs)

reactions to destroy OH radicals are driven by VOCs in the atmosphere

examples: carbon monoxide (CO), methane (CH4)