Ambient Tropospheric Particles
Atmospheric particulate matter (PM) is a complex mixture of solid and liquid particles suspended in ambient air (also known as the atmospheric aerosol). Ambient PM arises from a wide range of sources and/or processes, and consists of particles of different shapes, sizes, and compositions, with an array of physicochemical properties (1,2). Because particle size best relates to particle aerodynamics, diameter is most often used when describing ambient PM, most notably with regard to its atmospheric transport, lung deposition, and sampling for scientific or regulatory purposes. Particle number concentration, surface area, and volume (where particle volume particle density ¼ mass) are typically used to describe the size distributions of particles comprising ambient PM (Fig. 1). When coupled with chemical composition and PM optical properties (and sometimes charge), one obtains a more complete appreciation of the complex physicochemical nature of ambient PM.