question: Why is car smog much more visible in colder weather?
comment: There is moisture in automobile exhaust. The condensation of the warm exhaust’s moisture in cold air is what you’re actually seeing. A lot of components of gasoline exhaust aren’t very visible in the air. Naturally, diesel smoke and things that are high-carbon smoke (burnt oil, for example) are visible.
passages: As a result, air pollution from the burning of solid fuel can be of a greater concern in smaller towns in Ireland (Dublin City Council, 2009).
  
  Section::::Irish legislation.
   BULLET::::- Carbon Dioxide: Carbon dioxide is one of the most prominent greenhouse gasses emitted by motor vehicles. In 2006, 23.6% of the total inventory of U.S. greenhouse gasses were derived from motor vehicles. The compound is generated as a byproduct of the combustion of any fuel source containing carbon.
   The major culprits from transportation sources are carbon monoxide (CO), nitrogen oxides (NO and NO), volatile organic compounds, sulfur dioxide, and hydrocarbons. (Hydrocarbons are the main components of petroleum fuels such as gasoline and diesel fuel.) These molecules react with sunlight, heat, ammonia, moisture, and other compounds to form the noxious vapours, ground level ozone, and particles that comprise smog.
  
  Section::::Commuting trends in the United States.
   Low-carbon emission
  
  The main components of automobile exhaust are carbon dioxide (CO) and water vapour (HO). Carbon dioxide is the most important anthropogenic greenhouse gas (GHG) and the most significant Greenhouse Gas emitted in the U.S. (with 82-84% of all U.S. emissions). Increases in carbon dioxide concentration are due primarily to fossil fuel use and land-use change.
  
  The CO emission standards can be referred to the fuel or to the vehicle :
  
  BULLET::::- The European Union is moving towards fuel mandatory CO standards and USA has reflected it in the Greenhouse Gas Score.
   Section::::Causes.:Transportation emissions.
  
  Traffic emissions – such as from trucks, buses, and automobiles– also contribute to the formation of smog. Airborne by-products from vehicle exhaust systems cause air pollution and are a major ingredient in the creation of smog in some large cities.
   According to the Corporate Average Fuel Economy standard (CAFE) regulation, which was enacted in 1975, every seller of automobiles in the US had to achieve by 1985 a minimum sales-weighted average fuel efficiency of 27.5 miles per gallon (MPG). This standard had to be achieved for domestically produced and imported cars separately. Failure to meet the prescribed standard incurred a penalty of $5 per car per 1/10 of a gallon that the corporate average fuel economy fell below the standard. The first idea about the environmental impact of the CAFE regulation can be obtained by examining its effects on the average fuel efficiency of domestic and foreign firms; these effects are largest for the domestic production of US manufacturers, whose corporate average fuel efficiency would be lower by 1.2 MPG in the absence of CAFE standards. CAFE standards also lead to approximately fuel consumption savings per year. Contrary to the CAFE standards, gasoline taxes affect not only new but also used cars, so that there is no reason to expect any substitution towards less fuel efficient used cars when taxes are raised. Small tax increases are insufficient to induce fuel cost savings of the same order of magnitude as CAFE.
   Motor vehicle emissions contribute air pollution. Concentrations of air pollutants and adverse respiratory health effects are greater near the road than at some distance away from the road. Road dust kicked up by vehicles may trigger allergic reactions. In addition, on-road transportation greenhouse gas emissions are the largest single cause of climate change, scientists say.
  
  Section::::Regulation.
  
  Section::::Regulation.:Right- and left-hand traffic.
   Transportation is considered as large contributor to global climate change and especially in the US 80 percent of carbon dioxide emissions arise from moving cars and trucks moving goods.
  
  Many people understand that what contributes to greenhouse gas emissions is style of driving and length of a trip. Less known fact when considering measurement of emissions is how it changes with changing speed of vehicle. 
   Section::::Air pollution research.
   Smog can form in almost any climate where industries or cities release large amounts of air pollution, such as smoke or gases. However, it is worse during periods of warmer, sunnier weather when the upper air is warm enough to inhibit vertical circulation. It is especially prevalent in geologic basins encircled by hills or mountains. It often stays for an extended period of time over densely populated cities or urban areas, and can build up to dangerous levels.
  
  Section::::Areas affected.:Canada.
   Among these diesel emission components, particulate matter has been a serious concern for human health due to its direct and broad impact on the respiratory organs. In earlier times, health professionals associated PM10 (diameter < 10 μm) with chronic lung disease, lung cancer, influenza, asthma, and increased mortality rate. However, recent scientific studies suggest that these correlations be more closely linked with fine particles (PM2.5) and ultra-fine particles (PM0.1).
  
  Long-term exposure to urban air pollution containing soot increases the risk of coronary artery disease.
   Traffic gridlock in Delhi and other Indian cities is extreme. The average trip speed on many Indian city roads is less than 20 kilometers per hour; a 10 kilometer trip can take 30 minutes, or more. At such speeds, vehicles in India emit air pollutants 4 to 8 times more than they would with less traffic congestion; Indian vehicles also consume a lot more carbon footprint fuel per trip, than they would if the traffic congestion was less. Emissions of particles and heavy metals increase over time because the growth of the fleet and mileage outpaces the efforts to curb emissions.
   Smog is often categorized as being either summer smog or winter smog. Summer smog is primarily associated with the photochemical formation of ozone. During the summer season when the temperatures are warmer and there is more sunlight present, photochemical smog is the dominant type of smog formation. During the winter months when the temperatures are colder, and atmospheric inversions are common, there is an increase in coal and other fossil fuel usage to heat homes and buildings. These combustion emissions, together with the lack of pollutant dispersion under inversions, characterize winter smog formation. While photochemical smog is the main smog formation mechanism during summer months, winter smog episodes are still common. Smog formation in general relies on both primary and secondary pollutants. Primary pollutants are emitted directly from a source, such as emissions of sulfur dioxide from coal combustion. Secondary pollutants, such as ozone, are formed when primary pollutants undergo chemical reactions in the atmosphere.
   Increases in asthma rates have occurred despite improvements in air quality produced by the passage and enforcement of clean air legislation, such as the U.S. Clean Air Act of 1963 and the Clean Air Act of 1990. existing legislation and regulation have not kept pace with changing climatic conditions and their health consequences. Compounding the problem of air quality is the fact that air-borne pollens have been found to attach themselves to diesel particles from truck or other vehicular exhaust floating in the air, resulting in heightened rates of asthma in areas where busy roads bisect densely populated areas, most notably in poorer inner city areas.
   When oxides of nitrogen (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight, ground level ozone is formed, a primary ingredient in smog. A 2005 U.S. EPA report gives road vehicles as the second largest source of VOCs in the U.S. at 26% and 19% are from non road equipment which is mostly gasoline and diesel stations. 27% of VOC emissions are from solvents which are used in the manufacturer of paints and paint thinners and other uses.
  
  Section::::Main motor vehicle emissions.:Ozone.
   The major culprits from transportation sources are carbon monoxide (CO),nitrogen oxides (NO and NO),volatile organic compounds, and hydrocarbons (hydrocarbons are the main component of petroleum fuels such as gasoline and diesel fuel). Transportation emissions also include sulfur dioxides and particulate matter but in much smaller quantities than the pollutants mentioned previously. The nitrogen oxides and volatile organic compounds can undergo a series of chemical reactions with sunlight, heat, ammonia, moisture, and other compounds to form the noxious vapors, ground level ozone, and particles that comprise smog.
  
  Section::::Photochemical smog.
   BULLET::::- Improvement in air quality. A reduction in energy usage would lower greenhouse emissions and air pollution (dependent on electric power fuel mix). Lower temperatures would also slow chemical reactions that create smog. In 2007, Surabi Menon and Hashem Akbari estimated that an increase of global pavement albedo of 35 to 39 percent could reduce carbon dioxide emissions worth about $400 billion.
   BULLET::::- Induced humidity and ventilation both can greatly dampen air pollution in enclosed spaces, which was found to be relatively high inside subway lines due to braking and friction and relatively less ironically inside transit buses than lower sitting passenger automobiles or subways.
  
  Section::::Reduction efforts.
   BULLET::::- Lower temperatures reduce emissions in parking lots
   The aerosol contains levels of nicotine and cancer-causing chemicals comparable to regular cigarettes. They contain comparable levels of many volatile organic compounds and greater amounts of the polycyclic aromatic hydrocarbon acenaphthene than regular cigarettes. The substances in the emissions of traditional cigarettes such as tar, nicotine, carbonyl compounds (including acetaldehyde, acrolein, and formaldehyde), and nitrosamines are also found in emissions of heat-not-burn tobacco products. A 2017 study found a 10% rise in carbon monoxide and formaldehyde air levels than compared to the background during heat-not-burn tobacco product use. A 2017 study found heat-not-burn tobacco products generated emissions of particulate metals and organic compounds as well as aldehydes. Research suggests that heat-not-burn tobacco products generate less concentrations of airborne contaminates in indoor places in comparison to a traditional cigarette. Indoor air quality is reduced from the use of heat-not-burn tobacco products.
   Studies have found that children who are exposed to higher levels of car pollutants report higher respiratory problems including wheezing, ear and throat infections and have a higher incidence of physician-diagnosed asthma.
  
  Section::::Long-term and short-term effects on babies.:Cancer.
   Air pollution from fossil (and some biofuel) powered vehicles can occur wherever vehicles are used and are of particular concern in congested city street conditions and other low speed circumstances. Emissions include particulate emissions from diesel engines, NO, volatile organic compounds, carbon monoxide and various other hazardous air pollutants including benzene. Concentrations of air pollutants and adverse respiratory health effects are greater near the road than at some distance away from the road. Road dust kicked up by vehicles may trigger allergic reactions. Carbon dioxide is non-toxic to humans but is a major greenhouse gas and motor vehicle emissions are an important contributor to the growth of CO concentrations in the atmosphere and therefore to global warming.
   Temperature reduction from shade trees in parking lots lowers the amount of evaporative emissions from parked cars. Unshaded parking lots can be viewed as miniature heat islands, where temperatures can be even higher than surrounding areas. Tree canopies will reduce air temperatures significantly. Although the bulk of hydrocarbon emissions come from tailpipe exhaust, 16% of hydrocarbon emissions are from evaporative emissions that occur when the fuel delivery systems of parked vehicles are heated. These evaporative emissions and the exhaust emissions of the first few minutes of engine operation are sensitive to local microclimate. If cars are shaded in parking lots, evaporative emissions from fuel and volatilized plastics will be greatly reduced. 
   "Cleaner" should not, however, be confused with clean. An Australian study of real-life emissions from woodheaters satisfying the current Australian standard, found that particle emissions averaged 9.4 g/kg wood burned (range 2.6 to 21.7). A heater with average wood consumption of 4 tonnes per year therefore emits 37.6 kg of PM2.5, i.e. particles less than 2.5 micrometers. This can be compared with a passenger car satisfying the current Euro 5 standards (introduced September 2009) of 0.005 g/km. So one new wood heater emits as much PM2.5 per year as 367 passenger cars each driving 20,000 km a year. A recent European study identified PM2.5 as the most health-hazardous air pollutant, causing an estimated 492,000 premature deaths. The next worst pollutant, ozone, is responsible for 21,000 premature deaths.
   Cars produce numerous harmful air pollutants in their exhaust such as Nitrogen oxides, particulate matter, low atmospheric ozone (indirectly) and in the case of leaded fuel, lead. Those pollutants are known to cause various respiratory and other health issues and cars are among the leading cause of smog in modern developed world cities.
  