Field Methods of Monitoring Atmospheric Systems

Measurement of Air Pollution

Copyright © 2009 by DBS

Introduction

• Usually carried out at fixed site monitoring stations

• Routine monitoring is carried out primarily for the protection of public health

Methods of Analysis

• Can be broken down into two broad categories:

(i) Chemical measurements – best suited for 24hr average concentrations or longer

(ii) Procedures based on the physical properties of the pollutant – continuous fast-response measurements

Sulfur Dioxide (SO2)

• Gas phase fluorescence instrument

• Air is drawn continuously through a cell in which it is irradiated by light (λ = 214 nm)

• Causes fluorescence, intensity of which is proportional to the SO2 concentration

• Measures down to 1 ppb (~1 min)

Particulates

• Hi-Volume sampler has been used for many years

• Size selective impactor inlets allow collection of PM-10 and PM-2.5

• 24 hr collection period

Particulates

• Hi-Vol has been replaced by tapered element oscillating microbalance (TEOM)

• Continuous measurements (~15 mins) in real-time

• Air is drawn through a PM-10 or 2.5 size selective inlet and heated to remove water vapor

• Dried particles are collected on a filter attached to the vibrating element of an oscillating microbalance

Carbon Monoxide (CO)

Gas filter correlation infra-red• Broad band IR passing alternately

through 2 gas cells containing CO and N2 as they rotate beneath the IR source on a spinning wheel

• The IR pulses then travel through the multipass sample cell

• Sample beam is produced when IR passes through N2 cell

• Reference beam when IR passes through CO

• Difference in signal is the result of absorption by CO within the sample

Measures 0.1 - 50ppm(~2 mins)

Nitrogen Oxides (NOx)

• Chemiluminescence analyzer

• Based on light emission from chemiluminescent reaction between nitric oxide (NO) in air sample with ozone generated by the instrument

O3 + NO NO2* + O2

• Measures down to 1 ppb (~1 min)

• NO Mode: Air sample is passed directly to reaction chamber measuring NO

• NOx Mode: Air sample is passed over a catalyst, converts NOx to NO

• NO2 is calculated as the difference

Volatile Organic Compounds (VOCs)

• Difficult and expensive – not very common

• Due to wide range of compounds and low concentrations

• Methods require pre-concentration using an absorbent

• Analyzed using gas chromatography

Volatile Organic Compounds 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

Ozone (O3)

• UV photometric analyzers – utilize absorption of UV by O3 at 254 nm

• Measured in long path absorption cell through which air sample is drawn

• Air passes through a direct inlet and an inlet which passes through an ozone scrubber

• Difference in UV absorption between the two inlet routes is solely due to ozone (~1 min)

• Can also use chemiluminescence with ethylene

Instrument Deployment and Use

• Careful calibration and zeroing required on a weekly basis

• Quality assurance

• Massive volumes of data require data-logging

Location of Monitoring Stations

• Inlets need to be well exposed to air from all directions

• Should be in open areas not close to high buildings

– Background locations

– Hot spots

Science center inlet

Monitoring Networks

• Instruments usually form part of a network

• Network design varies according to purpose

http://www.epa.gov/air/data/repsst.html?st~PA~Pennsylvania

Further Reading

Journal articles

• Lodge (1989)

• Dollard (1995)

• From: Harrison, R.M. (1999) Measurements of Concentrations of Air Pollutants, in: Holgate, S.T., Koren, H.S., Samet, J.M. and Maynard, R.L. (eds.), Air Pollution and Health. Academic Press, NY.

Text Books

• Finlayson-Pitts, B.J., and Pitts, J.N. (2000) Chemistry of the Lower Atmosphere: Theory, Experiments and Applications. Academic Press, San Diego.

• Harrison, R.M. (1999) Measurements of Concentrations of Air Pollutants, in: Holgate, S.T., Koren, H.S., Samet, J.M. and Maynard, R.L. (eds.), Air Pollution and Health. Academic Press, NY.

• Heard, D.E. (ed.) (2006) Analytical Techniques for Atmospheric Measurement. Blackwell Publishing.

• Hollas, J.M. (1996) Modern Spectroscopy. John Wiley & Sons Ltd., New York.