The Smell of the City

by Bill Schlesinger

Even before COVID-19 cleared the air in major world’s cities—New York, Los Angeles, Beijing and Delhi—the urban sky is much cleaner in the United States than it was a few decades ago. When I fly into Los Angeles, I no longer notice the orange haze of smog that once blanketed the city. Regulations against the emission of organic chemicals, from dry cleaning, outdoor grills, and gasoline vapors, have helped a lot. All of these pollutants fell into the general category of volatile organic carbon (VOC) compounds. They are precursors to the formation of ozone and aerosols in urban air, which have detrimental effects on human health.

Within VOCs are two categories of compounds—chain hydrocarbons, in which the carbon atoms are bound to one another in a long chain. Propane is a good example of a chain hydrocarbon with 5 carbon atoms, and octane is another, with eight carbon atoms. The second class of VOC compounds is known as aromatic compounds, in which the carbon atoms are bound in a ring, usually with six carbon atoms with some double bonds between them (unsaturated). Benzene is the classic cyclic carbon compound. When the rings themselves are bound together, they are known as PAH—polycyclic aromatic hydrocarbons. Both simple and PAH compounds have a somewhat distinctive, pleasant scent.

With the removal of many of the obvious sources of VOCs in urban air, especially sources associated with transportation (e.g., gasoline vapors), organic compounds of chemical origin now contribute up to 50% of the VOCs in the air of many large cities, such as Los Angeles. These include emissions from pesticides, coatings, drying paints, adhesives, and personal care products. For some of these, nearly the entire product is expected to evaporate—100% volatilization.

Use of hot asphalt paving materials also contributes significant VOCs, especially benzene, in many areas. In hot climates, such as the Southwest, these VOC emissions are likely to continue for some time after paving is complete and add to the inventory of VOCs in the urban atmosphere.

These inadvertent releases of VOCs, both chain hydrocarbons and PAHs, are now a large source of compounds that are precursors to the formation of ozone and small particles known as aerosols in urban air. Both are associated with poor respiratory health and they exasperate infections such as COVID-19. Many carbonaceous aerosols have carcinogenic effects. Exposure is greatest indoors, where the majority of these products are used and people spend the most time.

We breathe more than 10,000 liters of air each day. With the shutdown of much of the economy, we can see the human impact on air quality, and the underlying volatilization of chemicals from our activities. Especially in these troubled times, we should expect that it is clean.

References

Al-Naiema, I.M., and 6 others. 2020. Secondary organic aerosols from aromatic hydrocarbons and their contribution to fine particulate matter in Atlanta, Georgia. Atmospheric Environment 223: 10.1016/j.atmosenv.2019.117227

Cui, P., G. Schito, and Q.B. Cui. 2020. VOC emissions from asphalt pavement and health risks to construction workers. Journal of Cleaner Production 244: doi: 10.1016/jclepro.2019.118757

DeMarini, D.M. and J. Lewtas. 2008. Mutagenicity and carcinogenicity of complex combustion emissions: Emerging molecular data to improve risk assessment. Toxicology and Environmental Chemistry 49: 157-166.

McDonald, B.C., et al. 2018. Volatile chemical products emerging as largest petrochemical source of urban organic emissions. Science 359: 760-764.