ENVR101 Atmospheric Pollution
Introduction to Atmospheric Pollution
This section elaborates on the geochemical cycling of nutrients, focusing particularly on carbon, within the Earth system. It provides a comprehensive view of atmospheric functions, detailing various pollution sources and examining local pollution issues specific to Christchurch. The course adopts a global perspective, incorporating local examples to highlight and contextualize broader environmental phenomena.
Atmosphere Structure
The Earth's atmosphere, a thin yet crucial layer of gases, is essential for sustaining life. The usable portion of the atmosphere extends up to 15 km at the Equator and 10 km at the poles, with the bottom 3-4 km being the most critical for weather and life processes. The lecture provides an in-depth coverage of atmospheric structure, pressure dynamics, stability and instability factors, local pollution issues in Christchurch (including detailed analysis of inversions), and global wind field patterns.
Structure and Density
The atmosphere is structured into distinct layers: troposphere, stratosphere, mesosphere, and thermosphere, each with unique characteristics.
Troposphere: This is the layer closest to Earth's surface, characterized by a temperature that decreases with altitude. The surface is warmer due to the re-emission of energy from the sun-heated ground, maintaining an average surface temperature of approximately .
Tropopause: Located at the top of the troposphere, it is marked by extremely cold temperatures, typically ranging from minus 50 to -60 degrees Celsius.
Stratosphere: This layer contains the critical ozone layer, which absorbs high-energy particles from the sun, leading to heat generation and protecting the Earth from harmful radiation.
Mesosphere: In this layer, temperatures decrease again with altitude.
Thermosphere: A high-altitude layer with low-density and high-speed particles, resulting in very high temperatures. It's the region where spacecraft experience significant heat, often leading to burn up.
The troposphere is most relevant for weather and climate patterns, primarily driven by the interaction of solar energy with the Earth's surface. These interactions dictate much of our daily weather phenomena.
Air Parcels and Stability
When an air parcel rises, it expands due to reduced pressure and cools; conversely, descending air compresses and warms. The rate at which air cools or warms is defined by:
Dry Adiabatic Lapse Rate: This rate specifies that dry air cools by per 100 meters of ascent (approximately per 100 meters). This is a key concept in understanding atmospheric thermodynamics.
Moist Adiabatic Lapse Rate: This rate is applicable when water vapor condenses within the air parcel, releasing latent heat. The release of latent heat offsets some of the cooling, making the moist adiabatic lapse rate lower than the dry rate. The exact rate depends on the moisture content of the air.
The stability of air masses is crucial in determining weather patterns. Stable air resists vertical movement, preventing rising and the formation of clouds and precipitation. Conversely, unstable air encourages rising motions, leading to cloud development. On hot days, unstable air is a significant factor in the formation of towering clouds and thunderstorms.
Pollution Traps
During the day, the ground heats the air, prompting it to rise and mix with the air above. However, at night, the ground cools more rapidly than the overlying air. This leads to temperature inversions, where a layer of warmer air sits above a layer of colder air near the ground.
This inversion prevents vertical mixing of the air, trapping pollutants close to the ground, a phenomenon particularly pronounced during winter.
Christchurch historically suffered from severe smog as a result of these inversions combined with emissions from domestic fires.
Ironically, earthquakes inadvertently improved air quality in the region by destroying many chimneys, thereby reducing emissions from domestic heating.
Air Pollution Composition
Air pollution is a complex mixture comprising photochemical smog (produced from vehicles and fires), particulate matter (including smoke and ash), and gases like carbon dioxide. Particulate matter, particularly PM10 and PM2.5, poses significant health risks.
PM2.5 particles are especially dangerous because they can penetrate deep into the lungs and even enter the bloodstream, leading to severe respiratory and cardiovascular issues.
Air quality indices are used to communicate health risks associated with air pollution levels. High levels of pollutants often necessitate avoiding outdoor activities to minimize exposure.
Winds
Winds are generated by differential heating of the Earth's surface and are influenced by the Earth's spin (Coriolis effect). New Zealand's characteristic windiness is largely due to its geographical location in the Southern Hemisphere westerlies, a zone of prevailing strong winds.