Fundamentals of Air Sampling

Gregory D. Wight

• Editorial: CRC Press
• Año: 2018
• ISBN:
  9781351446747
• Edición: 1
• Categoría: Scienze biologiche
• Categoría: Conservazione e Tutela dell'Ambiente

There is a growing need for environmental measurement personnel who possess a solid understanding of the techniques of air pollutant sampling. This essential book explains the fundamentals of air sampling, develops the theory of gas measurement, and presents several "how-to" examples of calibration and use of air and gas sampling devices. Other topics covered range from the basics of pressure measurement and units conversion to specific discussions regarding the use of a Volatile Organic Sampling Train or a SUMMA-polished canister sampling system.

1    INTRODUCTION TO AIR SAMPLING

Since the earliest versions of the U.S. Clean Air Act in the 1960s established maximum safe pollutant concentrations, scientists and engineers have been developing methods to quantify air pollutant concentrations and rates of release of pollutants into the air from sources. Early crude methods such as smoke density assessment tools (Ringelmann Charts), dustfall buckets, and soiling index have largely been replaced by continuous emission monitors, inhalable particulate samplers, Fourier Transform Infrared detectors (FTIDs), cryogenic trapping sampling and analysis techniques, etc. The essentials, however, of pollutant collection technologies have not changed. To quantify air pollutant levels, the pollutant concentration must be accurately known. In many air quality or emission assessment applications, concentration is found as the quotient of the mass of the pollutant of interest and the volume of air or gas sampled that contained that amount of pollutant.

The quantification of pollutant mass is primarily the job of the analytical laboratory or of the lab personnel. The quantification of the volume of gas sampled, however, is the job of the air sampling or field personnel. It is this air sampling portion of the overall air quality assessment process that this text addresses.

There are hundreds of techniques and types of instruments that allow the quantification of mass of collected air pollutant — some that require a few milligrams for accurate assessment, like Total Suspended Particulate samplers, others that respond to fantastically small quantities of the pollutant (e.g., gas chromatography/mass spectroscopy, (GC/MS), perhaps even in the range of a few molecules (e.g., alpha track radon detectors).

The volume of air or exhaust gas in which the pollutant of interest is mixed must be as accurately measured as the pollutant mass. A number of volume, gas velocity, and flow rate measuring or controlling devices are employed in the process of air and gas sampling. The choice of technique depends greatly upon the properties of the gas to be sampled and the pollutant to be collected. Fundamental concepts of physics, thermodynamics, and fluid mechanics form the foundation for calibration and use of these devices and for computation of gas volume.

This text concentrates primarily on methods to accurately collect the air or gas sample and accurately quantify the volume of gas collected.

The term air sampling as used here refers to five distinct sampling categories:

1.  Source or emission sampling — the measurement of gas flow rate, physical characteristics, and composition and pollutant concentration in exhaust gas streams leaving a process, factory, chimney, or ventilation system and entering the atmosphere.

2.  Ambient sampling — the measurement of outdoor air pollutant levels, generally in attempts to characterize fairly broad area pollutant levels to assess health effects or to forecast effects of a proposed new source of pollutants.

3.  Industrial hygiene air sampling — usually indoor measurement in workplaces, either stationary or area monitoring or personal monitoring (carried by an employee) for worker protection.

4.  Residential indoor sampling — to evaluate healthfulness of indoor air in living areas.

5.  Process or quality control — gas sampling in order to monitor the makeup of a production process or a manufactured product.

Standards — maximum allowable concentrations of contaminants in the air or gas — are set by the Environmental Protection Agency (EPA), state and local environmental agencies, manufacturers, or professional organizations. The American Society for Testing and Materials (ASTM), American Society of Mechanical Engineers (ASME), American Council of Governmental Industrial Hygienists (ACGIH), American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE), and governmental agencies like the Occupational Safety and Health Administration (OSHA) or its counterparts at the state level, and the National Institute of Occupational Safety and Health (NIOSH) as well as numerous private consultants have participated in the development of standards and refinement of methods for sampling air and gas over the past 30 or more years.

An important distinction should be made between air sampling and air monitoring. This book stresses techniques for sampling — the proper techniques to acquire a representative sample of possibly contaminated air or gas for later evaluation or analysis of the quantity of pollutant present. Monitoring, on the other hand, generally refers to methods and equipment designed to give real-time or near-immediate information on air or gas characteristics, especially pollutant concentrations. Continuous or intermittent instruments are in wide-ranging use as stack gas monitors (continuous emission monitors, CEMS⁴⁸) and as ambient, industrial, indoor, or process control monitoring devices. While the sampling concepts and techniques introduced in this book are essential for air and gas monitoring, considerable additional information, beyond the scope of this text, is needed for a full understanding of continuous air monitoring.

Air sampling, the collection of a sample for later analysis, has become a good deal more important in recent years due to three developments:

1.  Widespread awareness of the dangers of even small quantities of organic or other toxic pollutants in the air

2.  The development of toxic air pollutant regulations by state air pollution control agencies, under the guidance of the EPA

3.  The development of highly sophisticated laboratory techniques for measurement of minute quantities of pollutants collected from air or exhaust gas streams

As a result, there is more demand for air sampling, particularly emission sampling for toxic pollutants and indoor sampling. As well, there is more public scrutiny, and a consequently greater need for accuracy and carefully documented procedures and quality assurance programs.¹⁹

Continuous monitors often do not have the required sensitivity to measure toxic air pollutants at very low concentrations. Sampling techniques must be used to collect and, in some cases, concentrate a sample for later laboratory analysis by gravimetric techniques or by GC/MS, flame ionization detection (FID), photoionization detection (PID), electron capture detection (ECD), or atomic absorption spectrometry (AA), ion chromatography, or other techniques.^22,30

The collection of a representative sample of contaminated air or gas requires attention to a number of parameters of the gas: for example, temperature, pressure, major constituent volume percentages, average molecular weight, humidity, and sampling rate, as well as volume collected. Chapter 2 provides a brief review of some important concepts and their application to air sampling.

2 BASIC GAS CONCEPTS

2.1 INTRODUCTION

This chapter provides a brief review of some essential principles of physics, thermodynamics, and fluid mechanics.

2.2 PRESSURE MEASUREMENT

Fluid pressure measurement is nearly always made relative to the surroundings or atmospheric pressure by a mercury or water manometer, or by a pressure transducer or mechanical gauge. Such relative pressure is called “gage” pressure (while “gauge” is the preferred spelling, “gage” is an accepted alternate) and is positive if higher than surroundings, negative if lower. Relative to atmospheric pressure, a negative gage pressure is called “vacuum”.

The most commonly used units for gage pressure are pounds per square inch, gage (psig), inches or millimeters of mercury or water, or Pascals or kiloPascals (Pa, kPa; 1 Pa = 1 Newton per square meter.) See Table 2.1 for conversions.

Table 2.1 Pressure Units

1 atm	= 14.69 psi (lbf/in^2)
	= 760 mm Hg
	= 101.325 kPa
	= 1013.25 mB
	= 29.92″ Hg	density ratio: ρHg = 13.6 ρH2O
1″ Hg	= 13.6″ H2O
	= 25.4 mm Hg
	= (13.6 × 25.4...