Question

The following observations have been made on a typical day in Los Angeles: The hydrocarbon content in the atmosphere starts to rise at about 6 A.M. It peaks at 8 A.M. and dies off at 10 A.M. A similar peaked curve occurs at 6 P.M. About 2 hours after each peak, the ozone concentration rises. How can you explain these observations?

Short answer

The observed peaks in hydrocarbon content at 8 A.M. and 6 P.M. can be attributed to increased vehicular emissions during rush hours in Los Angeles. Hydrocarbons react with nitrogen oxides in the presence of sunlight to form ozone. The 2-hour delay in ozone concentration increase after the hydrocarbon peaks is due to the time needed for the photochemical reactions to occur, ultimately resulting in a rise in atmospheric ozone levels.

Step-by-step solution

Understand the data

First, let us analyze the given observations: 1. Hydrocarbon content in the atmosphere starts to rise at 6 A.M., peaks at 8 A.M., and dies off at 10 A.M. 2. A similar peaked curve of hydrocarbon content happens at 6 P.M. 3. About 2 hours after each peak, the ozone concentration rises.

Relationship between hydrocarbons and ozone

To understand these observations, we must be familiar with the relationship between hydrocarbons and ozone. Hydrocarbons are volatile organic compounds (VOCs) that participate in the formation of ground-level ozone when they react with nitrogen oxides (NOx) in the presence of sunlight. This reaction is more likely to occur during hours of intense sunlight.

Identify possible sources of hydrocarbon emissions

Possible sources of hydrocarbon emissions include motor vehicle exhaust, evaporation of solvents and fuels, industrial facilities, and biogenic sources from plants. Considering the pattern seen in Los Angeles (peaks at 8 A.M. and 6 P.M), it is reasonable to assume that motor vehicle emissions during daily commuting hours are the primary source of these hydrocarbon emissions.

Explaining the peaks in hydrocarbon emissions

The hydrocarbon peaks at 8 A.M. and 6 P.M. may be attributed to the increased vehicular activity during morning and evening rush hours, respectively. This is when the highest number of vehicles is on the road, which translates to more hydrocarbon emissions into the atmosphere.

Explaining the rise in ozone concentration later

After roughly 2 hours from the peak in hydrocarbon emissions, the ozone concentration rises. Given that hydrocarbons contribute to ozone formation through a reaction with nitrogen oxides in the presence of sunlight, it is understandable that there's a delay in ozone concentration increase. This additional time is needed for the necessary reactions to occur and generate the increased ozone levels in the atmosphere. In conclusion, the observations of hydrocarbon content and ozone concentration in Los Angeles can be explained by the daily traffic patterns and subsequent photochemical reactions involving hydrocarbons, nitrogen oxides, and sunlight.

Key concepts

Hydrocarbons and Ozone Relationship

Understanding the relationship between hydrocarbons and ozone is crucial to explaining the fluctuating air quality in urban areas like Los Angeles. Hydrocarbons, a significant component of volatile organic compounds (VOCs), interact with nitrogen oxides (NOx) in the presence of sunlight to produce ozone—a primary constituent of photochemical smog.

During the early hours, as the city stirs to life, the number of vehicles on the road increases, leading to a higher emission of hydrocarbons. The presence of sunlight catalyzes a series of chemical reactions that involve these hydrocarbons.

Sunlight-Driven Reactions

When hydrocarbons are exposed to ultraviolet (UV) light, they break down and interact with the nitrogen oxides released by vehicles and industrial processes. This process leads to the formation of various oxidants, including ground-level ozone. Improving our understanding of this phenomenon helps us in crafting better pollution control policies and in instilling awareness among the populace about the importance of minimizing emissions during peak hours.

• Peak vehicular traffic correlates with higher hydrocarbon emissions.
• These emissions require sunlight to trigger the chemical reactions that produce ozone.
• A delay is observed between hydrocarbon peaks and ozone concentration increases due to the reaction times.

This sequence of events explains the observed lag between the spike in hydrocarbons and the subsequent rise in ozone levels.

Ground-Level Ozone

Ground-level ozone is a colorless and highly irritating gas that forms just above the earth's surface. It is a secondary pollutant, meaning it is not directly emitted by any source. Instead, it is the result of chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of heat and sunlight.

Ozone at ground level is a critical component of photochemical smog, which can harm human health, damage crops, and affect the overall ecosystem. It is most commonly produced in urban areas where the concentration of vehicles and industrial activities is high.

Impacts of Ozone

Exposure to elevated levels of ozone can lead to respiratory problems, trigger asthma, reduce lung function, and cause lung diseases. That's why it's pivotal to understand the conditions under which ozone is formed to effectively predict air quality and issue appropriate health warnings. Moreover, strategies to reduce the precursors of ozone, mainly NOx and VOCs, can significantly decrease the presence of ground-level ozone.

• Formed by the reaction of NOx and VOCs in the presence of sunlight and heat.
• Acts as a significant part of photochemical smog, affecting health and the environment.
• Predictive measures and emission control can lower ground-level ozone concentrations.

Insights into the formation and impacts of ground-level ozone can help in developing more effective air quality management programs.

Nitrogen Oxides and VOCs Reactions

Nitrogen oxides (NOx) and volatile organic compounds (VOCs), under the right conditions, participate in a series of complex reactions that result in the formation of photochemical smog, with ground-level ozone being a primary component.

Nitrogen oxides, particularly nitrogen dioxide (NO2), play a significant role in the formation of ozone. NO2 can absorb sunlight and split into nitrogen oxide (NO) and a free oxygen atom, which can then react with oxygen in the air to form ozone (O3).

Chemical Reaction Complexity

The overall process is highly complex and involves chain reactions that can multiply the effects of the initial pollutants. This is why vehicular emissions in the morning can lead to poor air quality several hours later as the reactions unfold throughout the day.

• NOx and VOCs are the primary precursors for ozone formation.
• The sunlight-driven breakdown of NO2 is a key step in ozone production.
• Chain reactions amplify the impact of the original emissions on air quality.

Reducing NOx and VOC emissions is crucial for controlling ground-level ozone formation and improving air quality. Effective strategies include implementing stricter emission standards for vehicles, using cleaner fuels, and promoting public transportation to reduce the number of cars on the road during peak times.