Smog is derived from the combination of smoke and fog. It was normally seen in industrial cities due to the use of coal and factory emissions. These sulfurous (sulfur dioxide) emissions are called grey smog.
Smog is a type of air pollution that is characterized by a haze or fog-like appearance. It is a mixture of pollutants that can have adverse effects on human health and the environment. There are two main types of smog: photochemical smog and industrial smog.
Photochemical smog is a type of smog that is formed by the reaction of sunlight with pollutants in the air, such as nitrogen oxides and volatile organic compounds (VOCs). It is most commonly found in urban areas with high levels of traffic and industrial activity. Photochemical smog is a major contributor to air pollution in cities and is characterized by a brownish-gray haze. It can have adverse effects on human health, including respiratory problems, and it can also damage plants and crops.
The formation of photochemical smog begins with the emission of nitrogen oxides and VOCs from sources such as vehicles, industrial processes, and power plants. These pollutants react with sunlight to form ozone, which is a highly reactive gas. The reaction between sunlight, nitrogen oxides, and VOCs also produces a range of other pollutants, including particulate matter, aldehydes, and peroxyacyl nitrates (PANs). These pollutants contribute to the formation of photochemical smog.Photochemical smog is most commonly found in urban areas with high levels of traffic and industrial activity. It is typically more severe during the summer months when temperatures are higher and there is more sunlight. Photochemical smog can have adverse effects on human health, including respiratory problems, and it can also damage plants and crops.
Industrial smog is a type of smog that is formed by the emission of pollutants from industrial processes and the burning of fossil fuels. It is characterized by a thick, yellowish haze, and it is most commonly found in areas with heavy industrial activity, such as power plants and factories. Industrial smog can have adverse effects on human health, including respiratory problems, and it can also damage plants and crops.The formation of industrial smog begins with the emission of pollutants from industrial processes and the burning of fossil fuels. These pollutants, which include sulfur dioxide, particulate matter, and nitrogen oxides, react with each other and with other chemicals in the air to form a range of pollutants, including sulfuric acid, particulate matter, and nitrogen dioxide. These pollutants contribute to the formation of industrial smog.
Formation
Photochemical smog (brown smog) is formed by the reaction of nitrogen oxides and VOCs with tropospheric ozone.
Nitrogen dioxide is from both natural and anthropogenic sources. Sunlight breaks a bond and releases one oxygen forming nitrogen oxide (NO). The lone oxygen then bonds to atmospheric oxygen (O2) with the assistance of sunlight. This forms tropospheric ozone (O3). As the sun goes down, ozone undergoes a reaction with the recently made NO resulting in NO2 and O2.
Volatile organic compounds (VOCs) are also natural and anthropogenic. When they come into contact with the NO produced in the earlier equation, photochemical oxidants are formed. This disrupts the breakdown of the O3. The combination of the photochemical oxidants and ozone create photochemical smog. It is commonly known as brown smog due to the nitrogen compounds.
As many VOCs are from gasoline fumes, it should be understood that there is more brown smog in larger cities and warmer, sunny days. Trees are also emitters of VOCs so forested areas are also likely candidates of brown smog, though their ambient temperature is lower.
Health Concerns
Photochemical smog can have a range of adverse effects on human health.
The pollutants that contribute to photochemical smog, including ozone and particulate matter, can have a range of negative effects on human health. Ozone can irritate the respiratory system and cause coughing, wheezing, and shortness of breath. It can also reduce lung function and exacerbate existing respiratory problems, such as asthma. Particulate matter can also irritate the respiratory system and cause coughing, wheezing, and shortness of breath. It can also increase the risk of cardiovascular disease, such as heart attacks and stroke.
In addition to the direct effects on human health, photochemical smog can also have indirect effects by damaging plants and crops. Ozone and other pollutants can reduce crop yields and quality, which can have economic and social impacts.
Overall, photochemical smog can have serious health consequences, including respiratory problems and cardiovascular disease. It is important to reduce the levels of photochemical smog in the air to protect public health and the environment. This can be achieved through measures such as reducing vehicle emissions and increasing the use of renewable energy.
Many people in heavily affected cities and regions in China will opt to wear surgical face masks in an attempt to lessen these health effects.
🎥 Watch: AP Environmental Science - Air pollution
Frequently Asked Questions
What is photochemical smog and how does it form?
Photochemical smog is a type of urban air pollution formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in sunlight and heat to make secondary pollutants—especially tropospheric ozone, peroxyacyl nitrates (PANs), and formaldehyde. Early in the day vehicle emissions release NOx; under strong sunlight (usually late morning–afternoon, highest in summer) NOx + VOCs → O3 and other photochemical products, so ozone peaks in the afternoon. Weather and geography (traffic congestion, temperature inversions, sunny seasons, and urban basins) increase formation. Impacts include respiratory irritation, worsened asthma, eye irritation, and crop damage. You reduce it by cutting NOx and VOCs—vehicle emission controls (catalytic converters), cleaner fuels, reduced driving, and industrial VOC limits. This topic appears in Unit 7 on the AP exam; focus on the chemical role of NOx/VOCs, afternoon ozone peak, health effects, and mitigation (CED EK STB-2.B.*). For the Fiveable study guide on this topic, see (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD). For more unit review/practice, see (https://library.fiveable.me/ap-environmental-science/unit-7) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Why does smog get worse in the afternoon instead of the morning?
Smog gets worse in the afternoon because the chemistry that makes photochemical smog needs sunlight, heat, and time. Morning traffic emits lots of NOx (nitrogen oxides) and VOCs (volatile organic compounds). Early in the day NOx is produced, but sunlight-driven photochemistry (photolysis) and higher afternoon temperatures speed reactions that convert NOx + VOCs into tropospheric ozone and secondary pollutants like PANs. Ozone concentrations therefore build through the day and usually peak in the afternoon (EK STB-2.B.1 & STB-2.B.3). Also, stronger sunlight and warmer air increase reaction rates and can reduce vertical mixing, concentrating pollutants near the surface. For AP review, remember “NOx produced early, ozone peaks afternoon” and check the Topic 7.2 study guide on Fiveable for diagrams and practice (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD). For extra practice, try the unit problems (https://library.fiveable.me/practice/ap-environmental-science).
What's the difference between regular smog and photochemical smog?
“Regular” smog (classic or London/industrial smog) is mostly sulfur dioxide (SO2) and particulate matter from burning coal and other fuels under cool, humid conditions; it’s a mix of smoke and SO2 that reduces visibility and irritates lungs. Photochemical smog, by contrast, forms in sunny, warm urban settings when nitrogen oxides (NOx) and volatile organic compounds (VOCs) from vehicle and industrial emissions react in sunlight to produce tropospheric ozone, peroxyacyl nitrates (PANs), and other oxidants. Key differences: photochemical smog peaks in the afternoon and summer (ozone formation needs sunlight), is driven by NOx + VOC + sunlight chemistry, and causes ozone-related respiratory irritation and eye problems; classic smog is sulfur/particulate–dominated and tied to coal combustion and temperature inversions. For APES, memorize the NOx + VOC → ozone mechanism, afternoon ozone peak, and control strategies (reduce NOx/VOC; catalytic converters). Review Topic 7.2 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
I'm confused about VOCs - what are volatile organic compounds and where do they come from?
VOCs (volatile organic compounds) are carbon-containing chemicals that easily evaporate or sublimate at room temperature—examples on the CED are formaldehyde, gasoline vapors, and biogenic compounds like isoprene from trees. They come from both human and natural sources: vehicle exhaust and gasoline evaporation, solvents, paints, industrial processes, and fuel combustion are big anthropogenic sources; many plants (especially trees) emit VOCs naturally. In photochemical smog chemistry, VOCs react with nitrogen oxides (NOx) in the presence of heat and sunlight to form tropospheric (ground-level) ozone and other secondary pollutants like PANs, which peak in the afternoon and harm respiratory health (EK STB-2.B.1, STB-2.B.3–5). Reducing VOC emissions (better fuel controls, catalytic converters, low-VOC products) is a key way to reduce smog (EK STB-2.B.6). For the AP review, see the Topic 7.2 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD) and extra practice (https://library.fiveable.me/practice/ap-environmental-science).
How do nitrogen oxides and sunlight create ozone pollution?
Photochemical smog (tropospheric ozone) forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in sunlight. Early in the day, combustion (cars, power plants) emits NO and NO2. Sunlight breaks NO2 into NO + free O atom (photolysis). That free O quickly bonds with O2 to make ozone (O3). VOCs (like gasoline vapors or isoprene from trees) react with NO to make peroxy radicals that convert NO back to NO2 without consuming ozone, so more O3 is produced overall. Because sunlight and heat drive these reactions, ozone peaks in the afternoon and in summer—especially in urban areas with lots of traffic. Ozone irritates lungs and eyes. To reduce photochemical smog, cut NOx and VOC emissions (car emission controls like catalytic converters, cleaner fuels, and reduced vehicle use). For more AP-aligned review, see the Topic 7.2 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD), the full Unit 7 overview (https://library.fiveable.me/ap-environmental-science/unit-7), and extra practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Why is photochemical smog worse in summer than winter?
Photochemical smog is worse in summer because its key ingredients—nitrogen oxides (NOx) and volatile organic compounds (VOCs)—react with strong sunlight and heat to form tropospheric ozone and other photochemical pollutants. In summer you get more intense sunlight and higher temperatures, more VOC evaporation (gasoline, solvents, and natural VOCs like isoprene from trees), and faster photochemical reaction rates, so ozone concentrations peak in the afternoon and are higher overall in summer (EK STB-2.B.1, EK STB-2.B.3, EK STB-2.B.4). Urban traffic supplies lots of early-day NOx, and heat + sun later produce the worst ozone episodes (EK STB-2.B.5). To reduce summer smog, cut NOx and VOC emissions (better catalytic converters, lower vehicle use, control industrial VOCs) so there’s less fuel for sunlight-driven chemistry (EK STB-2.B.6). For a quick topic review, see Fiveable’s photochemical smog study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD) and more practice at (https://library.fiveable.me/practice/ap-environmental-science).
Can trees actually contribute to air pollution through VOCs?
Yes—trees can contribute. Many plants emit biogenic VOCs (like isoprene and formaldehyde) naturally; the CED even lists trees as a natural VOC source (EK STB-2.B.4). Those VOCs by themselves aren’t the whole problem, but when they mix with nitrogen oxides (NOx) from vehicles and heat + sunlight, they fuel the photochemical reactions that form tropospheric ozone and PANs (EK STB-2.B.1, EK STB-2.B.3). In urban or downwind areas, biogenic VOCs can therefore raise afternoon ozone peaks, worsen respiratory irritation, and boost smog. For AP responses, emphasize the interaction: biogenic VOCs + anthropogenic NOx + sunlight = smog; reduction strategies target NOx and human VOCs first (EK STB-2.B.6). For a focused review, see the Topic 7.2 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD) and try practice problems (https://library.fiveable.me/practice/ap-environmental-science).
What are the main health effects of breathing photochemical smog?
Breathing photochemical smog (tropospheric ozone plus related pollutants like PANs from NOx + VOCs) mainly irritates and damages the respiratory system. Short-term effects: coughing, throat irritation, chest tightness, reduced lung function, worsening of asthma and bronchitis, and more ER visits. It also causes eye irritation and headaches. Repeated or long-term exposure can lead to chronic respiratory disease, increased susceptibility to lung infections, and can stress the cardiovascular system (higher risk of heart problems). Sensitive groups—kids, elderly, and people with asthma—are most affected. These impacts are exactly what the CED lists under EK STB-2.B.7 (respiratory problems and eye irritation). For AP review, see the Topic 7.2 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD) and practice questions (https://library.fiveable.me/practice/ap-environmental-science) to prep for related exam items.
Why do cities have more smog problems than rural areas?
Cities get more photochemical smog because they have the ingredients and conditions that drive the chemistry in EK STB-2.B. Lots of motor vehicles and industry produce high NOx and VOC emissions (EK STB-2.B.1, EK STB-2.B.5). In the daytime heat and strong sunlight—especially in summer—those NOx and VOCs react to form tropospheric ozone and secondary pollutants (EK STB-2.B.3). Urban traffic congestion and higher temperatures (urban heat island) boost emissions and reaction rates, so ozone usually peaks in the afternoon (keywords: vehicle emissions, VOCs, nitrogen oxides, tropospheric ozone). Weather and topography (like calm conditions or temperature inversions) can trap pollutants over a city, making smog worse (EK STB-2.B.2; see related Topic 7.3). For AP review, study the causes/effects and controls (reduce NOx/VOC—catalytic converters, emission controls) in the Topic 7.2 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD) or the full Unit 7 overview (https://library.fiveable.me/ap-environmental-science/unit-7). For extra practice, try the practice problems (https://library.fiveable.me/practice/ap-environmental-science).
How can we reduce photochemical smog formation?
Photochemical smog forms when NOx and VOCs react in sunlight (EK STB-2.B.1). To reduce it, focus on lowering NOx and VOC emissions (EK STB-2.B.6): - Cut vehicle emissions: catalytic converters, stricter tailpipe standards, routine vehicle maintenance, and switching to electric or hybrid cars. - Reduce driving: improve public transit, carpooling, telecommuting, and congestion pricing to lower traffic in urban areas (EK STB-2.B.5). - Control VOCs from fuels/industry: use low-VOC gasoline, vapor recovery systems at gas stations, better leak detection/repair at refineries and chemical plants. - Choose urban vegetation wisely: plant low-isoprene tree species because some trees emit VOCs (EK STB-2.B.4). - Time-based actions: restrict high-emission activities on hot, sunny days and during afternoon ozone peak (EK STB-2.B.3). These align with AP expectations to explain causes/effects and propose reductions (STB-2.B). For a quick review, see the Topic 7.2 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD) and Unit 7 overview (https://library.fiveable.me/ap-environmental-science/unit-7). For extra practice, try the APES practice problems (https://library.fiveable.me/practice/ap-environmental-science).
I don't understand why ozone is bad at ground level but good in the atmosphere - can someone explain?
Ozone’s effect depends on where it is. In the stratosphere (about 10–50 km up) ozone forms the ozone layer that absorbs harmful UV-B and UV-C radiation, protecting life and reducing skin cancer and DNA damage—so stratospheric ozone is “good.” At ground level (troposphere), ozone is a major component of photochemical smog: it’s produced when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in sunlight and heat (EK STB-2.B.1, EK STB-2.B.3). Tropospheric ozone irritates eyes and lungs, worsens asthma, and reduces plant growth (EK STB-2.B.7). That’s why ozone can be both beneficial (blocking UV high up) and harmful (a pollutant near you). For AP exam review, focus on the chemistry and sources (vehicle NOx, VOCs), the afternoon/summer ozone peak, and control strategies (reduce NOx and VOCs) covered in Topic 7.2 (see the Fiveable study guide: https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD). For extra practice, try problems at https://library.fiveable.me/practice/ap-environmental-science.
What time of day are nitrogen oxide levels highest and why?
Nitrogen oxide (NOx) levels are highest in the morning—especially during the morning rush hour. That’s because NOx is emitted directly from combustion sources (cars, trucks, power plants) and commuter traffic spikes early in the day, producing a fresh pulse of NO and NO2. Sunlight then drives photochemical reactions: over the day NOx + VOCs + strong sunlight produce ozone, so ozone typically peaks in the afternoon while NOx has already been emitted earlier (CED EK STB-2.B.3). For APES, remember: NOx = primary pollutant produced early; ozone = secondary pollutant formed later. For a concise review, see the Topic 7.2 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD). For extra practice on timing and effects, check the Unit 7 overview (https://library.fiveable.me/ap-environmental-science/unit-7) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
How does traffic congestion make photochemical smog worse?
Traffic congestion makes photochemical smog worse because it increases the sources and conditions that drive the sunlight-driven chemistry that forms tropospheric ozone. Idling and stop-and-go cars emit more nitrogen oxides (NOx) and volatile organic compounds (VOCs) per mile than free-flowing traffic; those NOx + VOCs react in heat and sunlight to form ozone and secondary pollutants like PANs (EK STB-2.B.1, EK STB-2.B.5). Congestion also concentrates emissions near street level and reduces dispersion, so pollutant concentrations build up—especially on hot, sunny afternoons and in summer when ozone peaks (EK STB-2.B.2, EK STB-2.B.3). That increases respiratory irritation and eye problems (EK STB-2.B.7). Solutions tested on the AP include reducing NOx/VOC emissions through cleaner engines/catalytic converters, traffic management, and promoting transit/car-sharing (EK STB-2.B.6). For more review, see the Topic 7.2 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD) and Unit 7 resources (https://library.fiveable.me/ap-environmental-science/unit-7). Practice problems: (https://library.fiveable.me/practice/ap-environmental-science).
What weather conditions make photochemical smog formation more likely?
Photochemical smog is most likely when you have strong sunlight, high temperatures, and stagnant air. Sunlight + heat drive the photochemistry that turns morning NOx (from traffic) and VOCs into tropospheric ozone and PANs, so ozone concentrations peak in the afternoon and are higher in summer. Light winds and a temperature inversion (a warm layer above cooler air) trap pollutants near the ground, concentrating smog in urban areas with lots of vehicles. So think: sunny, hot afternoons in congested cities with calm or stable air (often in summer) = highest smog risk. For the AP exam, be ready to link NOx + VOCs + sunlight + weather (inversion, low wind, heat) to ozone peaks and health effects, and name reductions like cutting NOx/VOC emissions (CED EK STB-2.B.1–.6). Review the Topic 7.2 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD) and try practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Why does formaldehyde count as a VOC and how does it get into the air?
Formaldehyde counts as a VOC because it’s a volatile organic compound—a small organic molecule that evaporates at room temperature (low boiling point), so it exists as a gas in the atmosphere. On the AP CED list it’s given as an example of a VOC that participates in photochemical smog formation. It gets into the air from both natural and human sources: incomplete combustion (vehicle exhaust, power plants, wildfires), gasoline vapors, industrial processes, and even from trees and other vegetation. It can also form secondarily when larger hydrocarbons are oxidized in sunlight (so it’s both a primary and secondary VOC). Once airborne, formaldehyde and other VOCs react with NOx in sunlight to make tropospheric ozone, PANs, and other smog components (EK STB-2.B.1, EK STB-2.B.4). For a quick AP review, check the Topic 7.2 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/photochemical-smog/study-guide/PWfwfAuylO1PBZeNUKkD) and more unit/practice resources (https://library.fiveable.me/ap-environmental-science/unit-7; https://library.fiveable.me/practice/ap-environmental-science).