1/ Aerosols
I am seeing a lot of what seems like tension and chatter about aerosols vs. airborne and disciplinary differences in how the latter is defined.
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2/ My "go to" source for all things aerosols is WC Hinds' great text - Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles.
Anyone who studies aerosols should know of Hind's text. I am old enough to have a tattered copy of the (cough) 1st ed. more
3/ Note the title of the book carefully.
Please also note that aerosols are a binary system consisting of particles AND the gas in which they are suspended. Aerosol particles refers to the particle (liquid or solid) phase of aerosols.
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4/ On page 1 an aerosol is defined as a "solid or liquid particles suspended in a gas" (obviously - in the case of indoor air that gas is air). Hinds then writes that "Aerosols are also referred to as suspended particulate matter, aerocolloidal systems, and disperse systems."
5/ The term "aerosol" was coined 100 years ago (in 1920 - right after the 1918 flu) as the air analogy to hydrosol, a stable suspension of particles in a liquid.
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6/ Take note. A key word in all of this is "suspended"
Aerosol particles are dynamic. They can shrink by evaporation and grow by coagulation (particle collide and adhere) and condensation.
7/ Their chemical composition of aerosol particles also changes by adsorption and absorption of gases as well as condensation onto particle surfaces and chemical reactions on and in the particles.
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8/ Aerosol particles do NOT stay suspended forever. While aerosol particles remain in air far longer than large droplets (what many are referring to as ballistic droplets), some fraction does deposit on indoor surfaces, often very far from the source.
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9/ Indoors, some fraction deposits onto surfaces by a combination of gravitational settling, inertial impaction, interception, Brownian motion (for very small particles), electrostatic attraction, and thermophoresis.
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10/ These particle removal mechanisms lead to some removal to floors, walls, furniture, and even people inside buildings. Think of the very large surface area associated with clothing and hair of 25 high school students in a 600 sq ft classroom.
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11/ But how much deposits on indoor surfaces? I have combined bldg meta data with best-available peer-reviewed and published decay rates for particles of different sizes to estimate fractional deposition of aerosol particles in high school and university classrooms.
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12/ Using only aerosol particles of 0.5 to 4 microns, I estimate fractional collective deposition of less than 3% to over 20% by number of particles depending on degree of ventilation, mixing conditions, and appropriate surface area adjustments.
13/ Given uncertainties in human exposure and dose estimates, as well as source and particle size distributions, mixing conditions, etc., it is slightly conservative but reasonable to neglect such deposition losses for the size range I have modeled.
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14/ I hope this provides some useful information for discussions on social media about definitions, provides a reference to a valuable text on aerosols, and also opens eyes to the dynamic behavior of aerosol particles. There is so much more. Pick up a used copy of Hinds' text!
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