Photochemical smog (smoke + fog) is a type of pollution that is created when ultraviolet light reacts with NOx (nitrogen oxides) and VOC’s (volatile organic compounds) which are found in the atmosphere. This can only occur when there is sunlight and is most prominent in the morning and afternoon. Warm sunny days with gentle winds and a lower level inversion combined with the pollutants derived from the combustion of hydrocarbon and NOx cause this to happen. Even though the combustion of hydrocarbons and NOx is mostly omitted from the burning of fossil fuels, these pollutants can also be sourced from volcanic eruption, and forest fires.

When broken down, photochemical smog occurs when nitrogen and oxygen combine in the combustion process to form nitric oxide. The nitric oxide created oxidizes in the atmosphere to produce nitrogen dioxide. Sunlight breaks down nitrogen dioxide into nitric oxide and an oxygen radical. The oxygen radical combines with molecular oxygen to form ozone. This ozone produced is the main component of photochemical smog. 

The result of this photochemical fog is that it visibly lingers in the air causing a haze in the air. This smog can cause detrimental effects on the human respiratory system and can have serious strain on the heart and circulatory system. These effects cause humans to become mildly sick but can also be as serious as contributing to death. 

Photochemical smog can also adversely affect the environment. It can decrease crop production, the termination of photosynthetic plants, and have poor effects on any animal dependent on clean air. Photochemical smog can even corrode hard surfaces that make up buildings and cities. 

LA is an ideal place for photochemical smog to occur. It’s low valley creates a good place for the smog to settle once the emissions of its overcrowded car population are created and put into the atmosphere to interact with sunlight. Instead of burning coal to create electricity a better alternative is to combust natural gas because it has cleaner emissions. By limiting the emissions of electricity production, you can directly reduce the photochemical smog occurring. LA created a phasing out of certain pollutants that can be held responsible for the decrease in smog in the area. 

Sources 

Chameides, W. L., et al. “The role of biogenic hydrocarbons in urban photochemical smog: Atlanta as a case study.” Science241.4872 (1988): 1473-1475.

Tiao, G. C., G. E. P. Box, and W. J. Hamming. “Analysis of Los Angeles photochemical smog data: a statistical overview.” Journal of the Air Pollution Control Association 25.3 (1975): 260-268.

Forming in densely populated urban areas, photochemical smog manifests itself as a brown cloud of fog lingering along the troposphere, subjecting those living in and around the air space to inhale the dense and toxic air it creates. Photochemical smog forms when nitrogen oxides and volatile organic compounds (VOCs) interact with sunlight, forming a light brown haze and creating tropospheric ozone. Photochemical smog in specific comes from automobile emissions, and although ozone is naturally occurring, it is only in the upper atmosphere: if ozone forms at ground-level, it comes at the risk of health issues to humans and other organisms, as well as harming our infrastructure and agriculture.

In terms of human health, nitrogen oxides on their own can cause heart and lung issues, while reducing the immune system response to infections. VOCs cause eye irritation and act as potential human carcinogens. Tropospheric ozone also causes eye irritation, but proves to be harmful to those with pre-existing respiratory conditions: not only does it cause coughing and wheezing, but it can add to the respiratory issues a person is already experiencing, especially for people with asthma. When it comes to infrastructure though, photochemical smog can cause significant damages through weathering. This is possible through acid rain, which is caused by the evaporation of pollutants into the hydrologic cycle. Acid rain has a lower pH balance than normal water, causing it to be more acidic. When acid rain occurs, it corrodes metals and used for buildings and weather stonework, leaving immense damage over a period of time. These damages are expensive and are difficult to repair. 

So what can be done? When it comes to photochemical smog, the city of Los Angeles has been able to find solutions to this issue. L.A. is notorious for its photochemical smog because of the vast number of automobiles in the area. To combat this issue, the California Motor Vehicle Pollution Control Board was created by the California government in 1959, and from there they passed laws allowing for proper emissions tests. Cars were also re-modeled by the 1960s to have a “blowby” in their crankcase that allowed hydrocarbons to re-burn, preventing them from being released into the atmosphere at such a big rate. Later in the 1970s, fuel was also changed. By removing lead from fuel, reduced further pollutants from being emitted, and reduced the risk of further health issues. They also promoted the use of alternative fuels like natural gas or methanol to reduce gasoline usage and emissions. 

References:

Britannica, T. Editors of Encyclopaedia (2019, December 17). Smog. Encyclopedia Britannica. https://www.britannica.com/science/smog

Canada, E. A. C. C. (2010, June 2). Air pollution damage to infrastructure and industry – Canada.ca. Government of Canada. https://www.canada.ca/en/environment-climate-change/services/air-pollution/quality-environment-economy/economic-issues/damage-infrastructure-industry.html

Deziel, C. (2019, March 2). How Is Photochemical Smog Formed? Sciencing. https://sciencing.com/photochemical-smog-formed-6505511.html

Photochemical Smog- what it means for us. (2004, March). U.S. EPA. https://www.epa.sa.gov.au/files/8238_info_photosmog.pdf

The Southland’s War on Smog: Fifty Years of Progress Toward Clean Air (through May 1997). (n.d.). South Coast AQMD. Retrieved December 3, 2021, from https://www.aqmd.gov/home/research/publications/50-years-of-progress#Early%20Smog%20Control%20Efforts