Atmospheric Chemistry in the Urban Air

Atmospheric chemistry in urban settings is important because it drives the formation of a wide variety of gaseous and particulate air pollution, from London fog to Los Angeles photochemical smog to Beijing haze. Air pollution in cities not only constitutes a public health crisis for over half of the world’s population but also exerts a large regional climate forcing. However, the chemical processes governing pollution formation in the urban atmosphere are not well understood, due to the scarce measurements at the molecular level and the poor constraint of complex synergy between various pollutants. These pollutants are likely to follow different trajectories under future air-pollution controls. Therefore, in urban air, a better understanding of atmospheric chemistry at the molecular level can inform policies for reducing urban air pollution, as well as guide global models in predicting how the climate will respond to future mitigation of pollution.

In this Research Topic, we are interested in addressing urban complex air pollution caused by ozone (O₃) and fine particles (PM_2.5) together. In the urban atmosphere, O₃ production mainly involves photochemical oxidation of volatile organic compounds (VOCs) (or recently identified volatile chemical products, VCPs) in the presence of nitrogen oxides (NOx). However, VOCs emitted from conventional traffic sources have declined rapidly from tailpipe emission regulations. Other sources are growing in relative importance, such as the recently identified volatile chemical products (VCPs), which could be important O₃ precursors in the present-day and the future urban atmosphere.

Further, VOC oxidation also leads to the formation of oxygenated organic molecules (OOMs). OOMs that have low enough volatility can contribute to new particle formation and subsequent rapid growth, together with other nucleation precursors (e.g., sulfuric acid, amines, ammonia, and nitric acid), significantly increasing particle number concentration in urban air. However, it is highly obscure what chemical mechanisms and molecular properties control the gas-to-particle conversion processes in the urban atmosphere. Recent developments in atmospheric measurement techniques may provide tools to characterize such processes at the molecular level. It is thus pivotal to timely collect original research and address these critical scientific questions.

This Research Topic welcomes Original Research and Reviews that address atmospheric chemical processes and their impacts on air quality in the urban atmosphere. Original model studies that inform policies for reducing urban air pollution are also welcome.

Subtopics of interest include, but are not limited to, the following:
• Precursors and mechanisms for urban new particle formation;
• Speciation of volatile organic compounds (VOCs) and oxygenated organic molecules (OOMs) in cities;
• Oxidation mechanisms and product yields for anthropogenic VOCs;
• Formation of ozone and secondary aerosols in urban air;
• Photochemical aging and heterogenous reactions on aerosol surface;
• Urban halogen chemistry;