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The "Essence"To make roadside measurements of Hydrogen Sulphide (H2S) levels in the atmosphere and to investigate, with reference to peaks as well as long term averages, its role in the "pollution chain".
Background
Following the introduction of catalytic converters into the manufacture of an increasing number of modern cars, a greater incidence of the characteristic "bad eggs" smell of H2S has been observed. This is reputedly due to the chemical reduction of sulphur dioxide from a poorly tuned engine on passing through the catalytic converter.
Sulphur compounds are believed to be temporarily "stored" in the converter and then released "en masse" during abrupt acceleration or following prolonged periods of high speed driving (eg on motorways).
Sulphur release is not (apparently) a problem in the United States of America where nickel (Ni) is a constituent of the converters. The European Economic Community (EEC) however currently ban the use of nickel, because of a perceived, but unproven, risk of releasing nickel compounds into the environment.
On the other hand sulphur compounds are known to have effects on the respiratory system. Hydrogen Sulphide is a respiratory inhibitor , ie it affects the part of the brain which controls the lungs, and is similar in action to hydrogen cyanide - used in American gas chambers.
Hydrogen Sulphide is however highly reactive and will combine rapidly with other elements to form eg, Sulphur Dioxide which has been heavily researched in recent years in connection with environmental problems such as acid rain and increasing respiratory problems. Sulphur dioxide is a respiratory sensitiser which can worsen other conditions as well as causing medical problems in its own right.
The emission of Hydrogen Sulphide from car exhausts is (arguably) worse than some of the emissions the converters were designed to eradicate and the intention of this project was therefore to obtain absolute evidence of this problem against a background of increasing converter use.
Ambient H2S Concentrations and the Contribution of Major Roads
During the course of this project, ambient concentrations of H2S were found to be of the order of 1-2 ppb (v/v) in all rural environments investigated. It was found that vehicular pollution increased these concentrations by a further 1-2 ppb alongside major roads such as motorways and dual carriageways.
H2S Peaks and the Driver's Perspective
A new instrument, the Hydrogen Sulphide Peak Gauge (HSPG) was developed specifically to measure, almost continually, any rapidly changing concentrations in H2S. A typical unconditioned output of this instrument is illustrated below. The final design eliminated the baseline drift evident here.
A typical example of H2S monitoring output, against a stable baseline, is shown below. Note that the points marked 'Z' in the following three graphs show brief periods when the HSPG was going through a zero-check cycle.
It is now believed that the slight elevation of H2S concentration, measured at the roadside and associated with road vehicles, is actually an integration of many peaks, as typified above.
Another example, showing the cumulative effect of moderate traffic and numerous peaks, is shown below.
A more extreme example, showing the effect of very heavy traffic, is shown below. This journey included a number of busy intersections - the most notable being around 2300 seconds. (Note that the five extreme peaks at the time of the zero-checks are purely spurious and not actual H2S concentrations.)
It is important to note that the graphs above all show H2S concentrations measured within vehicles travelling along a road. Roadside measurement of gas concentration have been shown to be several times smaller.
References
CADLE, S.H. and MULAWA, P.A. (1978) Sulfide emissions from catalyst-equipped cars. In: Congress and exposition of the Society of Automobile Engineers, pp. 1-9. Society of Automobile Engineers, Detroit, Michigan.CLIFFORD, M.J., CLARKE, R. and RIFFAT, S.B. (1997) Driver's exposure to carbon monoxide in Nottingham UK. Atmospheric Environment 31, 7, 1003-1009.
DEUCHAR, C.N., COLLS, J.J. and YOUNG, S.D. (1999) Hydrogen sulphide from vehicle exhaust: exposure dynamics. Atmospheric Environment 33, 3077-3079.
DEUCHAR, C.N., COLLS, J.J. and YOUNG, S.D. (2002) A technique for the passive monitoring of hydrogen sulphide on landfill sites. In: Lencioni,E. and Dhanda,R., (Eds.) Integrated Waste Management and Pollution Control: Research, Policy and Practice, pp. 473-483. Waste2002, Warwick.
WATTS, S.F. and ROBERTS, C.N. (1999) Hydrogen sulfide from car catalytic converters. Atmospheric Environment 33, 169-170.
U.S.ENVIRONMENTAL PROTECTION AGENCY (1974) Evaluation of Catalytic Converters, U.S. Environmental Protection Agency, New York.
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