UNIVERSITI PUTRA MALAYSIA

METHOD DEVELOPMENT, SPECIATED MEASUREMENTS AND CALCULATED REACTIVITIES OF BENZENE, TOLUENE, ETHYLBENZENE AND XYLENES FROM VEIDCLE EXHAUST

NORASALWA ZAKARIA

FSAS 2001 10

METHOD DEVELOPMENT, SPECIATED MEASUREMENTS AND CALCULATED REACTIVITIES OF BENZENE, TOLUENE, ETHYLBENZENE AND XYLENES FROM VEIDCLE EXHAUST

By

NORASALWAZAKARIA <'

Thesis Submitted in FuIfUment of the Requirement for the Degree of.Master of Science in the Faculty of Science and Environmental Studies

Univeniti Putra Malaysia

December 2001

For my lovely little girl;

Qistina Saf.fWah Mohd. Qayyum

11

Abstract of thesis presented to the Senate ofUniversiti Putra Malaysia in fulfilment of the requirement for the degree of Master of Science

METHOD DEVELOPMENT, SPECIATED MEASUREMENTS AND CALCULATED REACTIVITIES OF BENZENE, TOLUENE, ETHYLBENZENE AND XYLENES FROM VEmCLE EXHAUST

By

NORASALWA ZAKARIA

December 2001

Chairperson: Dr. Puziah Abd. Latif

Faculty: Science and Environmental Studies

Mathematical modeling of ambient air photochemistry requires comprehensive

speciation of hydrocarbons from mobile source emission. The objective of this study is

to develop a simple and reliable method for analyzing tailpipe emission focusing on

benzene, toluene, ethylbenzene and xylenes (BTEX). The method consists of sampling,

qualitative analysis and quantitative analysis. The samples were collected in Tedlar bags

at cold start and hot start conditions and were injected manually using a gas-tight syringe

into Gas Chromatograph-Mass Spectrometer (GC-MS) operated on Electron Impact

Ionization (EI) mode.

The method developed has demonstrated ability to produce rapid and reliable

separations of exhaust hydrocarbons. An approximate of 50 hydrocarbon compounds

were identified in the exhaust ranging from C4 to e}2. It was found difficult to analyze

111

C 1 to some of the C4 hydrocarbons without the required accessories for volatile organic

gases analysis.

The emISSIOn rate of BTEX was emphasized because of its potential

carcinogenicity and toxicity. High concentration of BTEX was observed during cold

start and hot start. The mean concentrations of BTEX at cold start were as follows:

benzene (55.4 ppm), toluene (184.7 ppm), ethylbenzene (50.2 ppm), m-xylene (143.9

ppm), p-xylene (59.0 ppm) and o-xylene (65.4 ppm). The mean concentrations ofBTEX

at hot start were as follows: benzene (82.4 ppm), toluene (198.3 ppm), ethylbenzene

(40.0 ppm), m-xylene (184.0 ppm), p-xylene (62.1 ppm) and o-xylene (50.3 ppm).

The concentrations of BTEX in weighted percentages for all cars were fairly

constant. The weighted percentages concentration was used to estimate the

photochemical ozone reactivity by applying the Maximum Incremental Reactivity (MIR)

factors to the concentrations. The ozone forming potential as a result ofBTEX emission

from vehicle exhaust were estimated as follows: 3.07 g O-J g benzene, 74.64 g O-J g

toluene, 13.0 g 03/ g ethylbenzene, 145.8 g O-J g m- and p-xylene, and 50.97 g O-J g 0-

xylene. The results suggested that BTEX emitted from the vehicle exhaust increases the

formation of photochemical ozone in the atmosphere significantly.

IV

Abstrak tesis yang dikemukakan kepada Senat Universiti Putra Malaysia sebagai memenuhi keperluan untuk ijazah Master Sains

PEMBENTUKAN KAEDAH, PENll..AIAN SPESIS DAN PENGUKURAN KEREAKTIFAN BENZENA, TOLUENA, ETILBENZENA DAN XILENA DARI

EKZOS KENDERAAN

Oleh

NORASALWAZAKARIA

Disember 2001

Pengerusi: Dr. Puziab Abd. Latif

Fakulti: Sains Dan Pengajian Alam Sekitar

Pembentukan model matematik untuk fotokimia udara rnemerlukan penilaian

yang komprehensif terhadap spesis hidrokarbon dari sumber yang bergerak. Tujuan

kajian ini adalah untuk membentuk satu kaedah yang ringkas dan berkesan untuk

menjalankan analisis ke atas hidrokarbon dari ekzos kenderaan terutamanya benzena,

toluena, etilbenzena dan xilena (BTEX). Kaedah ini terdiri daripada teknik peyampelan,

analisis kualitatif dan anal isis kuantitatif. Semua sarnpel ekzos diperangkap di dalam beg

Tedlar dan disuntik secara manual menggunakan jarum kedap udara ke dalarn Gas Kromatografi-Spektrometer Jisim (GC-MS) yang beroperasi pada mod Pengionan

Impak Elektron.

Kaedah yang dibentuk telah berjaya mengasingkan konponen-komponen

hidrokarbon di dalam ekzos. Secara kasar, 50 spesis hidrokarbon telah dikesan iaitu dari

v

C.$ hingga e\2. Komponen hidrokarbon dari Cl hingga sebahagian dari C4 tidak berjaya

dikesan oleh alat GC-MS yang digunakan kerana ia memerlukan aksesori tambahan

untuk tujuan tersebut.

Kadar pelepasan BTEX perlu dititikberatkan kerana ia bersimt karsinogen dan

toksik. Hasil kajian menunjukkan kepekatan BTEX yang tinggi semasa kenderaan

dihidupkan pada permulaan sejuk dan pada permulaan panas. Kepekatan min BTEX

pada keadaan permulaan sejuk adalah seperti berikut: benzena (55.4 ppm), toluena

(184.7 ppm), etilbenzena (50.2 ppm), m-xilena (143.9 ppm), p-xilena (59.0 ppm) and 0-

xilena (65.4 ppm). Kepekatan min BTEX pada keadaan permulaan panas adalah seperti

berikut: benzena (82.4 ppm), toluena (198.3 ppm), etilbenzena (40.0 ppm), m-xilena

(184.0 ppm), p-xilena (62.1 ppm) and o-xilena (50.3 ppm).

Kesan BTEX terhadap pembentukan ozon fotokimia dianggar menggunakan

faktor 'Peningkatan Reaktiviti Maksimum' (MIR). Potensi pembentukan ozon akibat

pelepasan BTEX dari ekzos kenderaan adalah dianggarkan seperti berikut: 3.07 g OJ! g

benzena, 74.64 g OJ! g toluena, 13.0 g 031 g etilbenzena, 145.8 g OJ! g m- and p-xilena,

dan 50.97 g 031 g o-xilena. Ini menunjukkan bahawa kandungan BTEX di dalam ekzos

kenderaan mempunyai kesan yang ketara terhadap pembentukan ozon fotokimia di

atmosfera.

VI

ACKNOWLEDGEMENTS

My deepest appreciation goes to Dr. Puziah Abdul Latif who has given

generous advice and constructive comment throughout the study. Her understanding

and invaluable guidance is truly treasured.

I would like to thank En. Azman and Dr. Azizi for the ongoing feedback,

which helps to sharpening my idea. Thank you for your invaluable time.

A special acknowledgement goes to Pn. Nahariah and En. Sujak for their

competent assistance in the laboratories works.

Finally, to my lovely husband and parents, thank you for the immense

understanding and support.

Vll

I certify that an Examination Committee met on 8th December 2001 to conduct the final examination of Norasalwa Zakaria on her Master of Science thesis entitled "Method Development, Speciated Measurements and Calculated Reactivities of Benzene, Toluene, Ethylbenzene and Xylenes from Vehicle Exhaust" in accordance with Universiti Pertanian Malaysia (Higher Degree) Act 1980 and Universiti Pertanian Malaysia (Higher Degree) Regulations 1981. The Committee recommends that the candidate be awarded the relevant degree. Members of the Examination Committee are as follows:

Anuar Kassim, Ph.D. Associate Professor Faculty of Science and Environmental Studies Universiti Putra Malaysia (Chairman)

Puziah Abd. Latif, Ph.D. Faculty of Science and Environmental Studies Universiti Putra Malaysia (Member)

Azman Zainal Abidin, M.Sc. Faculty of Science and Environmental Studies Uruversiti Putra Malaysia (Member)

Azizi Hj. Muda, Ph.D. Associate Professor Faculty of Science and Environmental Studies Universiti Putra Malaysia (Member)

AINI IDERIS, Ph.D Professor/ Dean of Graduate School Universiti Putra Malaysia

Date: - a JAN 2002

Vlll

The thesis submitted to the Senate of Universiti Putra Malaysia has been accepted as fulfilment of the requirement for the degree of Master of Science.

IX

AINI IDERIS, Ph.D.

Professor/ Dean of Graduate School Universiti Putra Malaysia

Date: .1 4 MAR iwtj2

DECLARATION

I hereby declare that the thesis is based on my original work except quotations and citations, which have been duly acknowledged. I also declare that it has not been previously or concurrently submitted for any other degree at UPM or other institutions.

x

TABLE OF CONTENTS

DEDICATION ABSTRACT ABSTRAK ACKNOWLEDGEMENTS APPROVAL DECLARATION LIST OF TABLES LIST OF FIGURES LIST OF ABBREVIATIONS

CHAPTER

1

2

3

INTRODUCTION 1.1 Introduction 1.2 Statement of the Problems 1.3 Objectives of the Study 1.4 Scope and Limitation of the Study 1.5 Significance of the Study

LITERATURE REVIEW 2.1 Introduction 2.2 Sources ofBTEX 2.3 BTEX as Air Toxics 2.4 Health Effects ofBTEX 2.5 Impact on Air Quality

2.5.1 BTEX as Photochemical Ozone Precursors 2.5.2 BTEX as Photochemical Smog Precursors 2.5.3 BTEX Acid precursors

2.6 Techniques ofBTEX Sampling 2.7 The Method ofBTEX Analysis

2.7.1 BTEX in Ambient Air 2.7.2 BTEX in Vehicle Exhaust

2.8 Analytical Instrument for BTEX Analysis 2.8.1 Introduction 2.8.2 Principal Operation of Gas Chromatograph 2.8.3 Principal Operation of Mass Spectrometer

2.9 GC-MS Conditions for VOC Analysis 2.10 Qualitative Analysis ofBTEX 2.11 Quantitative Analysis ofBTEX

METHODOLOGY 3.1 Development of Sampling Train 3.2 Sources of Vehicle Exhal.st and Ambient Air

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Page 11 III V V11 Vlll X Xl11 xv XVll

1 1 3 4 4 5

7 7 8 9 10 11 11 13 14 14 20 20 21 22 22 23 24 26 30 32

36 36 38

4

5

3.3 Specification of The Instrument 40 3.4 Chemicals and Reagents 41 3.5 Preparation of Working Standard Solutions 41

3.5.1 Preparation of Liquid Working Standard 41 3.5.2 Preparation of Gas Standard Mixture 42

3.6 Method Development for BTEX Analysis 43 3.7 Sample Analysis Procedure 48

3.7.1 Instrument Tuning 48 3.7.2 Blank Sample Run 49 3.7.3 Working Standard Run 50 3.7.4 Sample Run 50

3.8 Qualitative Analysis of the Exhaust 51 3.9 Quantitative Analysis of the Exhaust 52

3.9.1 External Standardization Method 53 3.9.2 Area Normalization Method 53 3.9.3 Other Calibrant Mixtures Used For Calibrating

Exhaust Hydrocarbons 54

RESULTS AND DISCUSSIONS 55 4.1 Method Development For BTEX Analysis 55 4.2 Method of Vehicle Exhaust Sampling 70 4.3 Method of Ambient Air Sampling 74 4.4 Qualitative Analysis of Vehicle Exhaust 75

4.4.1 Qualitative Analysis of Target Analyte 75 4.4.2 Qualitative Analysis of Hydrocarbons Other Than

BTEX in the Vehicle Exhaust 83 4.5 Quantitative Analysis of the Exhaust 92

4.5.1 Calibrant Mixture 92 4.5.2 Concentration ofBTEX in the Exhaust 93 4.5.3 Quantification Using Area Normalization Method 99

4.6 Photochemical Ozone Reactivity 103

CONCLUSIONS AND RECOMMENDATIONS 5.1 Findings of the Study 5.2 Recommendations For Improving the Results of

The Study 5.3 Recommendation For Future Studies

106 106

108 109

REFERENCES APPENDICES

Appendix A AppendixB

VITA

111 117 118 129 131

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LIST OF TABLES Table Page

1.1 Comparison Between Recommended Malaysian Guideline (RMG) and National Ambient Air Quality 2

2.1 Categories of Organic Compounds 7

2.2 Physical Properties of Benzene, Toluene, Ethylbenzene and Xylenes 8

2.3 Health Effects on Human and The Exposure Limit ofBTEX 10

2.4 Column Flow and Total Flowrate Correlated with Column Diameter 29

3.1 Profile of the Test Vehicle 38

3.2 Sampling Locations of Ambient Air 39

3.3 Conditions That Were Varied During Developing Method For BTEX Analysis 44

3.4 Sequence of the Method Development Process 45

3.5 GC-MS Operating Parameters For Speciating BTEX and Other Hydrocarbons In The Vehicle Exhaust 51

4.1 Results of Varying GC-MS Conditions in Developing BTEX Analysis Method 56

4.2 Name of Compounds And Retention Time of the Peaks Labeled in Chromatogram in Figure 4.19 Above 82

4.3 Peak Report for Figure 4.20 84

4.4 Retention Time, Relative Retention Time And Compounds Identified In the Exhaust 86

4.5 Concentration Of Benzene and Toluene Calculated Using Three Different Cali brant Mixtures 92

4.6 Concentration OfBTEX (ppm) For Car 1, Car 2, Car 3 and Car 4

Tested At Cold Start 94

4.7 Concentration OfBTEX (ppm) For Car 1, Car 2, Car 3 and Car 4 Tested At Hot Start 94

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4.8 Statistical Analysis Of The BTEX Concentration In the Exhaust Samples Tested At Cold Start And Hot Start 97

4.9 Concentration OfBTEX (ppm) For Car 5, Car 6 and Car 7 Tested At 3000rpm 98

4.10 Normalized Area For BTEX In the Exhaust Sample For Car 1, Car 2, Car 3 And Car 4, Tested At Cold Start 99

4.11 Normalized Area ForBTEX In the Exhaust Sample For Car 1, Car 2, And Car 4, Tested At Hot Start 99

4.12 Normalized Area For BTEX In the Exhaust Sample For Car 5, Car 6, And Car 7, Tested At 3000rpm 100

4.13 Calculated Reactivities of Benzene, Toluene, Ethylbenzene and Xylenes From Vehicle Exhaust 103

XlV

LIST OF FIGURES

Figures Page

2.1 Schematic Diagram Of Gas Chromatograph-Mass Spectrometer 23

2.2 Total Ion Chromatogram of a BTEX Mixture 24

2.3 Mass Spectrum ofEthylbenzene 25

3.1 Block Diagram of The Sampling Train 37

4.1 Sample Mix TOC 1A Analyzed Using BTXSCN7.Met 66

4.2 The Effect of Insufficient Solvent Cut 67

4.3 Circled Areas of The Chromatogram Illustrate Split Peak, Tailing and Broaden Peaks 70

4.4 A Chromatogram Showing No Split Peaks, Tailing and Broaden Peaks 70

4.5 Sample TLF16, Motor Exhaust Collected in Hexane 73

4.6 Air Sample Collected at The Outside Corridor of labatan A1am Sekitar, UPM 74

4.7 Mass Spectrum of Benzene From The Exhaust Samples Run Under BTXSCN12.Met 76

4.8 Mass Spectrum of Benzene Acquired From the System's Public Library 76

4.9 Mass Spectrum of Toluene From The Exhaust Samples Run Under BTXSCN12.Met 77

4.10 Mass Spectrum of Toluene Acquired From the System's Public Library 77

4.11 Mass Spectrum of Ethylbenzene From The Exhaust Samples Run Under BTXSCN12.Met 78

4.12 Mass Spectrum of Ethyl benzene Acquired From the System's Public Library 78

4.13 Mass Spectrum of m-xylene From The Exhaust Samples Run Under BTXSCN12.Met 79

4.14 Mass Spectrum ofm-xylene Acquired From the System's Public Library 79

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4.15 Mass Spectrum ofp-xylene From The Exhaust Samples Run Under BTXSCN12.Met 80

4 . 16 Mass Spectrum ofp-xylene Acquired From the System's Public Library 80

4. 17 Mass Spectrum of o-xylene From The Exhaust Samples Run Under BTXSCN12.Met 81

4.18 Mass Spectrum of o-xylene Acquired From the System's Public Library 81

4.19 Chromatogram Of Working Gas Standard Analyzed Using BTXSCN12.Met 82

4.20 Total Ion Chromatogram Of An Exhaust Sample Tested At Cold Start 83

4.21 Chromatogram of a Liquid Fuel Analysis 91

4.22 Concentration ofBTEX in The Exhaust Sample Tested at Cold Start 95

4.23 Concentration ofBTEX in The Exhaust Sample Tested at Hot Start 95

4.24 Concentration of BTEX In The Exhaust Sample Tested At 3000rpm 98

4.25 Concentration ofBTEX in The Exhaust of Motorcycle (Yamaha 7Occ) 101

XVI

LIST OF ABBREVIATIONS

arnu atomic mass unit

BTEX benzene, toluene, ethylbenzene and xylenes

CS cold start

EI Positive Electron Ionization

eV electro Volt

FID Flame Ionization Detector

FTP Federal Test Procedure

GC Gas Chromatograph

GC-MS Gas Chromatograph-Mass Spectrometer

HPLC High Performance Liquid Chromatograph

HS Hot Start

ID Internal Diameter

K Kelvin

nun minute

MIR Maximum Incremental Reactivity

MS Mass Spectrometer

mlz mass to charge ratio

NMHC Non-Methane Hydrocarbon

ppm part per million

ppb part per billion

RI Relative Intensities

rpm rotation per minute

xvu

r.t retention time

r.r.t relative retention time

T Temperature

TIC Total Ion Chromatogram

V Volt

XVlll

1.1 Introduction

CHAPTER 1

INTRODUCTION

Motor vehicles are recognized as a major contributor of hydrocarbons to the

atmosphere in urban area and have been associated with serious environmental problems

such as photochemical smog, photochemical ozone and acid rain in addition to adverse

effects on human health. Although emissions from individual vehicle are generally low,

the combined emissions from the total number of vehicle and the increasing mileage

traveled, proved them to be the principal source of anthropogenic pollutant. In Malaysia,

an increase in the number of vehicles on the road was reported from 8,510976 in 1997,

to 9,141 ;357 at the end of 1998 (JPJ, 2000).

Traditionally, measurements of auto-vehicle emiSSion were limited to total

hydrocarbon (THC), carbon monoxide (CO) and oxides of nitrogen. Other primary

pollutants in the exhaust such as hydrocarbons were not emphasized. Hydrocarbons in

the exhaust like benzene, toluene, ethylbenzene and xylenes (BTEX) should be

emphasized since they react with hydroxyl radicals in the atmosphere to form secondary

pollutants that propagate the formation of photochemical ozone. Hence, detailed

composition and characterization of vehicular emission is required simply because the

individual hydrocarbons has different inherent toxicities and ozone-forming reactivity.

1

2

Vehicle exhaust comprises of many hydrocarbons ranging from C1-CI2. The

identification of the full spectrum of the compounds present in vehicle exhaust and air is

only possible using chromatographic techniques. However, due to instrumentation

limitation and costing, the study was limited to C, to Cl2 hydrocarbons with particular

interest on light aromatic hydrocarbons like benzene, toluene, ethylbenzene and xylene

(BTEX). BTEX was chosen because it constitutes a significant fraction of the organic

compounds in the exhaust and fuel as well as in the troposphere. Furthennore, benzene

is known as carcinogen and others are considered as air toxicants. Table 1.1 shows the

comparison between the Recommended Malaysian Guideline and the United States

National Ambient Air Quality Standards (NAAQS) of common air pollutants.

Table 1.1: Comparison Between Recommended Malaysian Guideline (RMG) And National Ambient Air Quality Standards (NAAQS)

Pollutant Averaging Time

Ozone 1 br 8br

CO· Ibr 8br

N02 Ibr 24br

S02 1 0 min 1 hr 2 4 br

Particle 24br TSP lyr PM 1 0 24br

lyr 3 ... measurement In mg/m

Source:�an, 2000

(ppm) 0.10 0. 1 1 30 9 0. 1 7

0. 1 9 0. 13 0.04

RMG NAAQS (,ug/m3) (pg/m)

200 235 1 20

35 x 101 1 0 Xl01

1 0 x 1 01

320 1 1 3 500 350 1 05 365

260 90 260 1 50 1 50 50

1.2 Statement of the Problems

3

The analysis of BTEX emitted from motor vehicle exhaust is required for two

reasons. Firstly, under the Clean Air Act (CAA) of the United States, promulgated in

1990, benzene, toluene, and the xylenes have been listed as air toxicants. Furthennore,

benzene is carcinogenic. Secondly, benzene and its derivatives have been associated

with serious environmental problems such as photochemical ozone and photochemical

smog. Each of the compounds has a different ozone reactivity or ozone fonning

potential. Therefore, the concentration of benzene, toluene, ethyl benzene and xylenes

from the motor vehicles should be detennined accurately and must be monitored

continuously since the main anthropogenic source of benzene and its derivatives comes

from motor vehicles. For these to be accomplished, the analytical method for BTEX

analysis, using the available instrument, must be developed.

Accurate measurements and the information acquired from the analysis is

beneficial as the baseline for considering the requirement for regulating benzene

released from the motor vehicles as well as in the fuel. In addition, the ozone fonning

potential, as a result ofBTEX emission from the vehicle exhaust, can also be estimated.

1.3 Objectives of the Study

The objectives of the study are as follows:

4

1. To develop a rapid and reliable method for trace level analysis for routine

monitoring of the volatile organic compounds, originated from motor vehicle

exhaust and ambient air�

2. To identify and quantify BTEX in motor vehicle exhaust;

3. To identify and quantify BTEX in fuel formulations and compare them to the

emission from various motor vehicles;

4. To estimate photochemical ozone reactivity ofBTEX as a result of vehicle

exhaust emission.

1 .4 Scope and Limitation ofTbe Study

1. The main scope of the study is to develop an analytical method for BTEX

analysis using Gas Chromatograph-Mass Spectrometer. The gaseous samples

will be introduced manually into the GC, using a gas tight syringe, but the liquid

sample will be injected into the GC using an auto-injector. Other accessories

required for gaseous analysis like automatic gas sampling valve, cryogenic

concentrator and a purge and trap unit are not available at the moment.

5

2. The exhaust samples collected in this study were limited to the exhaust from the

cars only. Other gaseous samples that were collected for instance the ambient air

samples and a motorcycle's exhaust, were only used for comparison purposes

only.

1.S Significance of the Study

To the author's best knowledge, no studies have yet been conducted or published

in Malaysia involving a detailed speciation of hydrocarbons in vehicle exhaust, although

it is recognized as major anthropogenic pollutants. Thus this study will provide

infonnation on the strategy to develop an alternative method for analysis of volatile

compounds especially BTEX by direct injection of gaseous sample using gas tight

syringe into the GC-MS instead of using gas sampling valve or a purge and trap unit.

In environmental context, characterization of vehicle exhaust is needed for

several reasons. A complete speciation enable prediction of the quantity of ozone created

as a result of vehicle emission. Also, the technique and the method developed can be

used to collect data on the amount ofBTEX emitted in enclosed area like underground

car park or tunnel with no or poor ventilation so that by using these infonnation, proper

regulation can be enforced to regulate BTEX emission and proper control measure can

be installed where appropriate.

2.1 Introduction

CHAPTER 2

LITERATURE REVIEW

Organic compounds cover a wide range of groups including aliphatic

hydrocarbons, aromatics, halogenated hydrocarbons and oxygenated hydrocarbons.

Table 2. 1 below shows the categoeries for organic compounds, which are based on

their boiling point. Light aromatic hydrocarbons like benzene, toluene, ethylbenzene

and xylenes or BTEX, which have boiling points ranging from 80°C to 144°C are

categorized as volatile organic hydrocarbons (VOC).

Table 2.1 : Categories of Organic Compounds

Categories Bolling Point rC)

Very Volatile Organic Compound (WOC) < Oto 50-100

Volatile Organic Compound (VOC) 50-100 to 240-260

Semi Volatile Organic Compound (SVOC) 240-260 to 380-400

Source: Rafson, H.J. 1 988

Various aspects of BTEX were studied due to its adverse effects towards

human and to the environment. To human, benzene is a carcinogen and its

derivatives are considered as air toxicants. In air photochemistry, BTEX plays an

important role in the formation of photochemical ozone. Physical properties of

BTEX are tabulated in Table 2.2.

7