Open questions on the chemical composition of airborne particles

Abstract

Airborne particles have significant impacts on health, visibility, and climate. Here, an overview of what is known about particle chemical composition is presented, along with open questions and challenges that are central to relating composition to life cycles and impacts. Airborne particles have been known for at least 800 years to negatively impact visibility and health. For example, Moses Ben Maimonides (1135-1204) described air in Cairo as "stagnant, turbid, thick, misty, and foggy", and attributed "dullness of understanding, failure of intelligence, and defect of memory" to "spoilage of the air". A recent estimate is that exposure to outdoor particulate matter (PM) currently leads to 3.3 million premature deaths per year worldwide, and this could double by the year 2050 1. It is known that these particles impact climate as well 2. The chemical composition of the particles is an important determinant of all of these atmospheric impacts. Hence, understanding the composition and processes affecting composition is critical for developing cost-effective control strategies. A first step is to distinguish "primary particles", those that are directly emitted from natural and anthropogenic sources such as sea spray, transportation, industry, forest fires, etc. from "secondary particles", those formed from chemical reactions of gaseous precursors in air. The latter include both inorganics such as sulfuric acid/sulfate from SO 2 oxidation and nitrate from NO x oxidation, as well as a host of volatile organics, which make up a major fraction of the particles around the world 3. Because there are thousands of potential organic precursors in air, and a number of different oxidation reactions involving O 3 and OH, Cl, and NO 3 radicals (Fig. 1), the organic component of particles becomes very complex 4. Further adding to the complexity of this secondary organic aerosol (SOA) are reactions in the condensed phase that can also form new products after the particle has formed 5. Composition, size, and phase A critical property of atmospheric particles is their size. Particles must grow from sub-nm seeds formed by the gas phase chemistry to~100 nm in order to scatter light efficiently. This reduces visibility and affects climate both directly through light scattering and indirectly through their impacts on clouds. This~100 nm size also efficiently reaches the alveolar region of the lung.

Coupled interfacial and bulk kinetics govern the timescales of multiphase ozonolysis reactions

“…[1][2][3] Multiphase aerosol chemical transformations govern the fate of particle phase species, including organic pollutants, 4,5 and both climate and health-relevant aerosol properties. [6][7][8] The rates and mechanisms of these reactions are governed not only by the concentrations of atmospheric oxidants (e.g., O3, OH radicals), but also by the variable composition of organic and inorganic aerosol components and the often significant fraction of liquid water. [9][10][11] To understand these chemical transformations, we need to consider multiphase reaction dynamics in concentrated to dilute aqueous phases relevant to aqueous aerosol and cloud water, which may not reflect the purely organic aerosol systems that are often the focus of mechanistic laboratory studies.…”

Phase Behavior of Hydrocarbon-like Primary Organic Aerosol and Secondary Organic Aerosol Proxies Based on Their Elemental Oxygen-to-Carbon Ratio

“…Atmospheric organic aerosols comprise a wide range of compounds with different volatilities, molecular weights, polarities, oxidation states, and functional groups. ,− In general, they can be broadly classified as primary organic aerosol (POA) and secondary organic aerosol (SOA) particles. POA denotes organic aerosols that are directly emitted into the atmosphere as particles.…”

Impact of Vehicle Light on Physical Properties of Particulate Matters Emitted from Vehicles

“…On the one hand, small PMs have the potential to penetrate more deeply into the human's and animals' lung tissues through the respiratory passageways causing more damaging the lung tissues compared to the larger PMs [13,15]. On the other hand, large PMs have the potential on the changes in the global climate and environment via absorbing solar radiation, melting snow and ice placed on mountains, as well as reacting with other types of atmospheric constituents [16][17][18]. In addition, PMs are a threat to the underwater environment and animals due to their solubility potential in water [18].…”