A review of atmospheric aerosol measurements

Recent developments in atmospheric aerosol measurements are reviewed. The t opics included complement those covered in the recent review by Chow (JAWMA 45: 320-382, 1995) which focuses on regulatory compliance measurements and filter measurements of particulate composition. This review focuses on mea surements of aerosol integral properties (total number concentration, CCN c oncentration, optical coefficients, etc.), aerosol physical chemical proper ties (density, refractive index, equilibrium water content, etc.), measurem ents of aerosol size distributions, and measurements of size-resolved aeros ol composition. Such measurements play an essential role in studies of seco ndary aerosol formation by atmospheric chemical transformations and enable one to quantify the contributions of various species to effects including l ight scattering/absorption, health effects, dry deposition, etc. Aerosol me asurement evolved from an art to a science in the 1970s following the devel opment of instrumentation to generate monodisperse calibration aerosols of known size, composition, and concentration. While such calibration tools pe rmit precise assessments of instrument responses to known laboratory-genera ted aerosols, unquantifiable uncertainties remain even when carefully calib rated instruments are used for atmospheric measurements. This is because in strument responses typically depend on aerosol properties including composi tion, shape, density, etc., which: for atmospheric aerosols, may vary from particle-to-particle and are often unknown. More effort needs to be made to quantify measurement accuracies that can be achieved for realistic atmosph eric sampling scenarios. The measurement of organic species in atmospheric particles requires substantial development. Atmospheric aerosols typically include hundreds of organic compounds, and only a small fraction (similar t o 10%) of these can be identified by state-of-the-art analytical methodolog ies. Even the measurement of the total particulate organic carbon mass conc entration is beset by difficulties including the unknown extent of evaporat ive losses during sampling, adsorption of gas-phase organic compounds onto sampling substrates, and the unknown relationship between carbon mass and m ass of the particulate organics. The development of improved methodologies for such measurements should be a high priority for the future. Mass spectr ometers that measure the composition of individual particles have recently been developed. It is not clear that these instruments will provide quantit ative information on species mass concentrations, and more work is needed t o routinely interpret the vast quantities of data generated during field sa mpling. Nevertheless, these instruments substantially expand the range of a tmospheric aerosol issues that can be explored experimentally. These instru ments represent the most significant advance in aerosol instrumentation in recent years. (C) 2000 Elsevier Science Ltd. All rights reserved.