Introduction

Clouds and airborne particles play critical roles in radiative transfer through the atmosphere and in the water cycle. Particles and hydrometeors (cloud droplets, rain drops, and other types of frozen precipitation) can absorb or reflect radiation, so these particles play crucial roles in studies of climate. Particles also interact with clouds through complex mechanisms that can indirectly control the lifetime, precipitation rates, and therefore radiative properties of clouds. It is important to note that precipitation is the primary removal mechanism for water, and thus, the cloud and airborne particle measurements discussed in this lesson are useful for numerous applications.

Introduction » Learning Objectives

The broad goal of this lesson is to develop a basic ability to conduct observational studies of clouds and aerosols or airborne particles. The following learning objectives are steps toward that goal and represent the capabilities that can be acquired through the completion of the lesson:

1. List and be able to explain the standard measurable properties of clouds and airborne particles.
2. Describe the main instrument types used for measurements of clouds and airborne particles.
3. Describe the advantages and limitations of key instruments used to measure hydrometeors and airborne particles.
4. Characterize the main cloud and airborne particle instruments in terms of their operation.
5. Using cloud and airborne particle measurements, interpret plots of concentration, size distribution, and other properties.

Introduction to Hydrometeors and Aerosols

A hydrometeor is a particle in the atmosphere that consists mostly of water. Some definitions also include a particle formed at Earth's surface, such as a dewdrop. Other definitions require that the particles form via condensation and so would exclude spray generated at the ocean's surface. In this lesson, we will use only the definition focused on a hydrometeor as a particle in the atmosphere consisting of water (i.e., excluding dew but including ocean spray). A hydrometeor can include other minor constituents, for example, dissolved solutes or insoluble particles, as long as water is the major constituent.

Aerosols are colloidal systems consisting of particles dispersed in a gaseous medium. “Colloidal” means that the particles are evenly and finely dispersed and have a relatively stable size distribution. The particles are most likely not homogeneous in size, shape, or composition. Dispersed particles always fall relative to the gaseous medium to some extent and also undergo changes caused by collisions and other effects, so “relatively stable” is a qualitative statement that usually applies to particles that are smaller than 1 𝛍m in diameter, though it can sometimes also include larger particles. By convention, a cloud of water drops is normally not considered an aerosol. Strictly, the term “aerosol” should be reserved for a two-component system where the particles should be referred to as “aerosol particles” within the gaseous medium. In practice, “aerosol” is frequently used instead to refer to the particles. The usage in this lesson will be to emphasize the terms “airborne particles,” “aerosol particles,” or sometimes “particles” (where it is clear they are components of the aerosol).

Introduction to Hydrometeors and Aerosols » Hydrometeors

Introduction to Hydrometeors and Aerosols » Hydrometeors » Classification and Terminology

The following are commonly used classes of hydrometeors, and subsequent discussions of instruments will use these terms:

Term	Definition	Notes
Cloud Droplets	Water droplets with diameters about 1–500 μm	The subset with diameters from about 100–200 μm is often called drizzle and at 500 μm these droplets become raindrops. Hydrometeors with diameters below 1 μm are usually solution droplets with enough dissolved solute to affect the equilibrium water vapor pressure over the droplets.
Ice Crystals	Ice-water hydrometeors	If grown at least initially from water vapor, they are called “pristine”. If grown through continued accretion of supercooled cloud droplets, they are “rimed”.
Precipitating Hydrometeors or Precipitation	Includes raindrops, snow, graupel, and ice pellets	Raindrops are water drops with diameters larger than 500 μm. Snow consists of aggregates of ice crystals, perhaps rimed. Graupel and ice pellets are hydrometeors of ice larger than 500 μm in diameter but smaller than 5 mm. Most graupel is in the 2-5 mm range. Hail consists of dense ice hydrometeors larger than 5 mm in diameter.

The following size distribution shows measurements of hydrometeor sizes from a location over the Pacific Ocean. The nature of these plots will be discussed further in the lesson.

Introduction to Hydrometeors and Aerosols » Hydrometeors » Commonly Measured Properties of Hydrometeors

Studies of hydrometeors often require measurements of the following properties:

1. Liquid, ice, and total water content (i.e., the mass per unit volume);
2. Concentration in various size ranges (number per unit volume);
3. Size distributions and moments of those distributions;
4. Trace chemical constituents in the hydrometeors;
5. Hydrometeor shape and type (including habits of ice crystals);
6. Precipitation rate;
7. Effects on the scattering of light, including cloud albedo.

Introduction to Hydrometeors and Aerosols » Classes of Other Airborne Particles

Introduction to Hydrometeors and Aerosols » Classes of Other Airborne Particles » Classification and Terminology

Airborne particles are often classified according to their composition, origin, or size.

Airborne particles compositions include the following:

1. Soluble materials like sea salt, ammonium sulfate, or ammonium chloride, natural and anthropogenic;
2. Mineral (e.g., dust; constituents of sand or soil);
3. Carbonaceous (black carbon and organics) – e.g., smoke;
4. Biogenic (created by living organisms);
5. Volcanic (often sulfate aerosols from the oxidation of sulfur dioxide, ash);
6. Secondary organic aerosol (formed from condensed gases);
7. Mixtures of the above types.

Particle origins can be classified as:

1. Marine: arising mostly from the evaporation of sea spray;
2. Continental: dust and the result of photochemical reactions; biomass burning;
3. Volcanic;
4. Human-made: the result of combustion or photochemical reactions involving pollutants.

Sizes of airborne particle can be classified as:

1. Fine mode:
   • Aitken particles (mostly < 0.2 μm in diameter);
   • Large or accumulation-mode particles (0.2 to 2 μm diameter).
2. Coarse mode: (> 2 μm diameter )
   • Giant particles (>2 μm diameter);
   • Ultragiant (> 20 μm diameter).

Particle size is sometimes also classified according to “PM” (particulate matter) thresholds, particularly in characterizing the health effects of air pollution. PM_2.5 particles are those smaller than 2.5 μm in diameter, while PM₁₀ includes all particles smaller than 10 μm. PM₁₀ classes of particles can enter the lungs when breathing, but are often captured in the pharynx, larynx, or trachea beforehand. PM_2.5 particles are especially hazardous because they can pass deep into the lungs, where they irritate and corrode the alveolar wall, and consequently impair lung function.

Introduction to Hydrometeors and Aerosols » Classes of Other Airborne Particles » Commonly Measured Properties of Airborne Particles

Studies of airborne particles are often able to measure the following properties:

1. Concentration in various size intervals
2. The total mass and total surface area in various size intervals
3. Size distributions and moments of those distributions
4. Chemical composition
5. Radiative properties of single particles
6. Ensemble effects on the scattering of light
7. Cloud activation properties (cloud activation and ice nucleation)

Introduction to Hydrometeors and Aerosols » Hydrometeor and Airborne Particles Questions

Use the information presented thus far to complete the following questions.