The United States Environmental Protection Agency (EPA) defines certain commonly found atmospheric pollutants as "criteria pollutants" whose concentrations are used to determine the air quality of a region. High concentrations of these gases are are detrimental to human health, and the EPA has instituted a number of policies seeking to regulate their concentrations across the country. BEACON monitors three of these pollutants, namely carbon monoxide (CO), nitrogen dioxide (NO2), and ozone (O3).
Carbon monoxide, or CO, is a colorless, odorless gas produced by incomplete combustion of carbon-based fuels. It is toxic when inhaled, causing carbon monoxide poisoning and death at high concentrations. The major source of CO in urban areas is motor vehicle exhaust, which accounts for as much as 95% of all city emissions. Other sources include construction equipment, boats, and other combustion engines used off-road, as well as industry and heating. For more information on CO emissions, see the EPA's Air Quality Status and Trends though 2006, which shows how CO emissions have dropped since the introduction of cleaner engines into most of the US vehicle fleet.
In addition to being a pollutant, CO has uses as a tracer of CO2 emissions from combustion sources. Cars, trucks, and most other on-road vehicles have introduced cleaner burning engines over the last few decades, resulting in lower CO emissions for the same amount of CO2. Off-road vehicles have not experienced as much efficiency increase, and so in general have higher CO to CO2 emission ratios. Fires, either intended or not, also have much higher CO emissions than typical motor vehicle sources. We hope to use these differences in CO/CO2 ratios to identify sources pollution events observed with BEACON.
Nitrogen dioxide is one of several nitrogen oxide species found in the atmosphere. It is a reddish-brown, highly reactive toxic gas formed from the reaction of nitric oxide (NO) with ozone (O3), another of the criteria gases. This nitric oxide precursor is formed when oxygen reacts with nitrogen at very high temperatures, for example those found during combustion. Together, NO and NO2 are called NOx and contribute to a variety of air quality problems, including ozone formation, acid rain, crop degradation, and formation of atmospheric particulate. Because of these impacts, NOx emissions have been strictly regulated by a variety of policies, many of which target combustion engines and electricity generation. NOx. These policies have been extremely effective, resulting in a nearly 40% decrease in NOx concentrations across the United States. Shown below is a chart of NOx decreases in the San Francisco Bay Area.
(Figure from: A.R. Russell, L.C. Valin, E.J. Buscela, M.O. Wenig and R.C. Cohen, Space-based Constraints on Spatial and Temporal Patterns of NOx Emissions in California, 2005-2008, Env. Sci. & Tech. 44, 3608-3615, 2010.)
Ozone (O3) is an atmospheric gas that many will remember from coverage of ozone depletion and the ozone hole. When ozone is present in high concentration layers in the stratosphere high above the Earth's surface, it keeps harmful UV radiation away from the surface. When O3 is depleted by reactions with chemical radicals, a "hole" forms in the ozone layer, this UV radiation makes it to the surface, where it can cause higher rates of skin cancer.
BEACON does not measure stratospheric ozone. BEACON nodes are mounted near the Earth's surface, in a layer of the atmosphere called the troposphere. In the troposphere, O3 is a greenhouse gas and a pollutant. It is a major component of smog, and on its own is toxic, corrosive, and causes strong irritation of lungs, particularly in those with asthma or other respiratory conditions.
Unlike the other criteria gases BEACON monitors, O3 is not emitted directly, but rather formed from other pollutants and oxygen though a myriad of processes. One major pathway of ozone formation is through the NOx cycle, whereby NO2 (from combustion) reacts with oxygen in bright sunlight to form O3 and NO. The backwards process (NO to NO2) also occurs in a process that is facilitated by volatile organic compounds (VOCs), which are also targets of pollution regulations. The schematic below illustrates this process, which occurs rapidly during daylight hours.
Pollutants are are detrimental to human health
Carbon monoxide is toxic when inhaled, causing death at high concentrations
Nitrogen dioxide contributes to acid rain
Ozone is a major toxic component of smog