Atmospheric Aerosols: The Mystery Floating in the Air

By Dr. Charles Ichoku, NASA Scientist for GLOBE Student Research Campaign on Climate

The following two pictures of the same place were taken on different days. Can you explain why the upper picture is clear but the lower one is not?

Two pictures taken from the same place at Texas Tech University, Lubbock, Texas, USA show: (upper) the bluish sky on a clear spring day in 1998, and (lower) the hazy sky on a dusty day (6 April 2001) at about 5:30 PM local time (pictures downloaded from http://www.atmo.ttu.edu/dust.html)

As we all probably learned from our science classes, the air within our atmosphere is naturally composed of gases, including nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and several others. The atmosphere also frequently contains water vapor (H2O), which is water in its gaseous form. In addition to gases, the atmosphere contains very small particles in solid or liquid form, called “aerosols”. Liquid aerosol particles are in the form of viscous (or oily) droplets rather than water droplets.  Individually, aerosol particles are practically invisible to the human eye, because most of them are 10 microns or less in size (1 micron or micrometre = 1 metre divided by 1000000). By comparison, the human hair has a diameter of between 17 and 181 microns. Certain types of atmospheric aerosols (typically on the order of 0.2 microns in size) can serve as a nucleus upon which water vapor condenses to form clouds. Such aerosols are referred to as cloud condensation nuclei.  However, when there are high concentrations of aerosols in the air, especially near the surface of the earth where people can breathe them, we say that the air is polluted. In fact, the agents of air pollution (or pollutants) can occur either as unhealthy gases mixed up with the air or as aerosols floating in the air. People that monitor the air quality in different places often report the aerosol content of the air to the public in terms of the concentration of particles by mass per unit volume of air (typically expressed in units of micrograms per cubic meter). For air-quality purposes, aerosols are often referred to as PM10, which means all particulate matter (PM) in the atmosphere whose aerodynamic diameter (apparent diameter while floating in the air) is 10 microns or less. A subgroup of the PM10 often identified in air-quality monitoring is called PM2.5, which means all particulate matter whose aerodynamic diameter is 2.5 microns or less. There are several different types of aerosols depending on the materials or chemicals they are made of and where the aerosols come from. People and animals inhale aerosols in the air they breathe. The tinier the particles are, the easier they can enter the lungs and cause serious harm to our health. Therefore, for a given aerosol type, those in the PM2.5 size group are more harmful than the larger size group.

Aerosols can come from many different sources, some of which are natural and others anthropogenic (i.e. caused by human activities). Some of the main aerosol types and their sources are: (i) chemical pollution aerosols from industries, cars, trucks, and other modes of transportation, (ii) smoke from large and small fires, (iii) dust blown by wind from bare ground surfaces, (iv) sea salt from ocean sprays caused by waves resulting from the action of the wind and other forces that cause sea motion, and (v) volcanic aerosols from eruptions of volcanoes. As you may have guessed, chemical pollution aerosols are almost all caused by people, because of many of the things we do to enjoy life and move around. Smoke aerosols are to a large extent caused by people who set fires to forests, bushes, trash, or anything that produces smoke, although in certain places smoke originate from fires caused by lightning strikes or large accidental events. Dust aerosols are mostly generated by wind, but sometimes people produce dust while moving or conducting certain activities in dusty places. In fact, when we do anything to destroy vegetation anywhere and leave the land bare, we are also helping to provide favorable conditions for dust generation. Sea salt aerosols are mostly natural, and only a very tiny proportion is indirectly produced from human activities that cause waves in the ocean, such as fast moving boats and ship. Volcanic aerosols are entirely natural and often lofted very high in the atmosphere away from where people can inhale them. Ironically, aerosols caused mainly by people, such as chemical pollution and smoke, are mostly in the PM2.5 size range, which are the most harmful to people.

How can we know when there is a high concentration of aerosols in the atmosphere? One simple way is to look up in the sky when the sun is up. If there are no clouds, the sky should look bluish (that is, sky blue) when the air is clean. If the sky is hazy (that is, not bluish) when there are no clouds, then there must be a high concentration of aerosols in the atmosphere. In this case, the color of the sky will depend on the source, type, and amount of the aerosol along our line of sight to the sky. The reason for this is that the Sun’s light is made up of (electromagnetic) waves distributed across a wide range of wavelengths forming a spectrum. Only light whose wavelength is in the visible range (approximately 0.38 to 0.75 microns) of the spectrum can be seen by the human eye, and represent the different colors of the rainbow (violet, indigo, blue, green, yellow, orange, and red: as arranged in ascending order of wavelength). When the Sun’s light is travelling through a clean atmosphere that has no cloud, the air molecules scatter the shorter wavelengths, of which the eyes are most sensitive to the blue light, because air molecules are smaller than visible light wavelengths. This phenomenon was discovered by the 1904 Nobel Laureate in physics, Lord Rayleigh, and is known as Rayleigh scattering. This is why clear sky looks blue during the daytime, except during sunrise or sun set. When the sky is cloudy, the clouds appear white. Since the cloud droplets are much larger than the wavelengths of visible light, the cloud scatters all visible wavelengths almost equally and appears white because the sum of all colors in the rainbow is white. Aerosols in the atmosphere can scatter and/or absorb lights of different wavelengths to various degrees depending on the aerosol type, amount, and size distribution. Therefore, large amounts of aerosols in the atmosphere cause the sky to look hazy in different shades of grey or pale yellowish to brownish colors, depending on the position of the sun relative to the observer.

As a practical exercise, look up the sky one or more times a day for at least a week and take pictures of what you see. Remove the pictures that contain thick clouds. Out of those that have no clouds, or with just a few clouds and much free sky space, try to separate the pictures in which the sky is blue from those in which it is not. Those where the sky is hazy must contain large amounts of aerosols. Try to identify the photo with the largest amount of aerosols. How much aerosols were present on this day with respect to the other days?  Compare this picture carefully with that of the blue sky day and discuss it with your friends, teachers, family, and possibly community, to try to identify the possible source(s) of the aerosols. What could all of you do (if anything) to reduce or stop such aerosol at its source(s)? We know that high concentrations of aerosols in the atmosphere are harmful to people’s health in many ways and it is the responsibility of us all to keep the air clean. In future blogs, we shall discuss many other aspects of aerosols, such as: how far they can travel from their sources, how long they can stay in the atmosphere, how they leave the atmosphere and where they go, how they are measured from the ground, or from aircraft or satellite, how they relate to clouds, and what they can do to the weather and climate.