Field Methods of Monitoring Atmospheric Systems

Particulate Matter

Copyright © 2006 by DBS

Introduction

Diameter (μm)

Commonly used term

< 0.1 fume

0.1-10 smoke

10-100 dust

> 100 grit

Importance in Atmospheric Chemistry: Reaction and Dispersal

• Many atmospheric reactions occur either – In gas/solid phase - on surface of PM– in liquid phase – in water adsorbed onto the

surface of particles

e.g. London smog of 1952 PM provided surface for oxidation of SO2

• Dispersal mechanism for many pollutants

Sourcese.g NH4Cl, SO4

2- / NO3- salts

Natural: forest fires, volcanoes etc.

Man-made: fossil-fuel combustion, industry

Mineral dust from weathering of rocks and soils

Chemical composition can be used to ID source and fate

Typical Analysis

• Total particulate mass concentratione.g 70 μg m-3 rural

300 μg m-3 urban10 mg m-3 factory workshop100 mg m-3 power station flue gases

• Chemical Composition– Metals - highly insoluble silicates require

dissolution– Inorganic ions– Organics– Size distribution– Total suspended particulate matter (TSP) Very small amounts

means an exacting analytical task

As a function of particle size

Question

Why is particle size important?

1. Transport and removal – residence time depends on size

2. Physiological properties – smaller particles penetrate deeper into lungs

3. Distribution of chemical species

4. Effect on atmospheric reactions

Sampling Methods

A. Filtration and Impaction– High-Volume– PM10– Dichotomous (divided)– Polyurethane Foam

B. Beta Attenuator Methods (BAM)

C. Tapered Element Oscillating Microbalance (TEOM) Methods

D. Nephelmeter

An advantage of continuous monitoring is that it provides additional information, such as the time of day that peak concentrations occurred

A. High-Volume (Hi-Vol) Samplers

• ‘Hi-Vol’ sampler

• Determines mg/m3

• Pump up to 90 cfm (~150 m3 h-1)

• Draws large volume of air over 24 hr period

– Glass fiber or membrane filter

– Weighed before and after

– 0.3 to ~100 μm particle size

Reeve, 2002

Question

Convert 90 cfm to m3/min

90 cfm = 90 f3/min

= 90 x (12 in/ft)3 x (2.54 cm/in)3 x (m/100 cm)3

= 90 x 0.0283… m3/min = 2.55 m3/min

PM-10 Sampler

• ‘Add-on’ to standard Hi-Vol sampler

• Remove large particles via impaction or settling chambers

Filters• Choice of filter depends on analyis– Glass fiber – finely spun borosilicate

fibers with binder

– Cellulose

– Membrane - microporous plastic films made from a variety of polymers

Filters

• Chose correct filter for analyses– Retain correct size– Absence of impurities– Compatible with

analytical procedure

Source: Wight, 1994

FiltersPros Cons Use

Glass FIber

- Low water uptake

- High efficiency for particles > 0.3 μm

- Resistant to high T

Organics

Cellulose

- Low head loss

- Low metallic contamination level

- Easily ashed

- HydrophillicMetals + inorganic ions

Membrane

- Flat for microscopic analysis

- Hydrophobic

- Chemically resistant

- Large head loss

- Brittle

- Not resistant high T

Question

Calculate the PM-10 concentration for the following conditions:

Filter mass gain = 0.000670 mg

Sample time = 1446 min

Initial sampler flow rate = 1.875 cm3 min-1

Final sampler flow rate = 1.807 cm3 min-1

Average flow rate = 1.841 cm/min

Volume of air = 1.841 cm3/min x 1446 min = 2662 cm3 = 2.662 x10-3 m3

PM concentration = 6.70 x 10-1 μg / 2.662 x 10-3 m3 = 251.7 μg m-3

Polyurethane Foam Sampler (PUF)

• For organics need both solid and vapor phases• Vapor cartridge is placed in-line with quartz fiber

filter for semi-volatile organics– PUF plug– Adsorbent resin (XAD-2)

• If vp is high VOC’s may be in aerosol form

- Pesticides- PCB’s- Dioxins- PAH’s

Cascade Impactors

• Uses adhesion of PM rather than filtration

• Flow is constricted, velocity increases

• Successively smaller particles adhere to each successive surface

• Fractionated according to mass

• 2.5 – 10 μm / < 2.5 μm

Flow Calibration

Source: Wight, 1994

B. Beta Attenuator Methods (BAM)

• Beta particles from 14C source are attenuated (lose signal strength) as they pass through particulate deposits on a filter tape

• Absorption of radiation is proportional to mass of PM– Contunuous real-time

measurement (no weighing required!)

• Range: 30 - 300 µg/m3 • Temperature: -30° to +45°C

• New version is heated to remove moisture effects

http://www.thermo.com/

C. Tapered Element Oscillating

Microbalance (TEOM) Methods • Air is drawn through a filter at the end

of a a tapered oscillating glass tube

• Change in frequency is directly related to the mass of PM accumulated

• Range: 30 - 300 µg/m3 • Temperature: -30° to +45°C

Reeve, 2002

D. Nephelmeter

• Optical method– Similar to turbidimeter

• Scattering of light by suspended PM, detection by photomultiplier tube. Baffles minimize reflected light

Question

There are currently several investigations to compare the results of different PM analyzers. What reasons could contribute to this concern?

Measurements are dependent on atmospheric conditions

Absorbed water may be difficult to control during operationsMeteorological conditions may affect flow rate

Each technique responds differently to individual particle sizes

Some components are volatile and may be lost due to heat (TEOM)

MASA: 501

• Principle: air is drawn through a 8 x 10” filter at a known flow rate– Mass of particles is determined by the difference in filter

mass prior to and after sampling– Concentration of suspended particulate matter is calculated

by diving the mass gain of the filter by the volume of air sampled

Lodge, 1988

Further Reading

• Journal articles

Text Books

• Hesketh, H.D. (1994) Air and Waste Management: A Laboratory/Field Handbook. CRC Press, Florida.

• Lodge, J.P.A. ed. (1988) Methods of Air Sampling and Analysis, 3rd Edition. CRC Press, Florida.

• Wight, G.D. (1994) Fundamentals of Air Sampling. CRC Press, Florida.

• Winegar, E.D. and Keith, L.A. eds. (1993) Sampling and Analysis of Airborne Pollutants. CRC Press, Florida.