SO2 and benzene levels

It was like a breath of fresh air to read that SO2 and benzene levels in the air have diminished significantly in the last couple of years (January 25, error in translation, meaning to refer to SO2 and not CO2, corrected on February 3). The cause of this amelioration in air quality needs to be clearly identified in order to follow up and sustain this trend with the right policies.

The reasons given for the decrease in the two above-named pollutants, use of better quality diesel and unleaded petrol, in my opinion do not tell the full story. There is no denying that superior quality fuels have led to reduced toxic emissions.

Better quality diesel and fuel used in electricity generation have reduced sulphur content and the switch to unleaded petrol has resulted in a drastic reduction of airborne lead and benzene. The benefits in using better quality fuels may be mitigated by old technology whether found in old internal combustion engines or power generation plants.

Unleaded petrol has lower benzene content than the leaded petrol used until 2002, so the switch to the former must have resulted in a decrease of airborne benzene.

Leakage of petrol vapours from vehicles, filling stations and fuel storage facilities, especially during replenishment, is one source of airborne benzene.

The main source of this contaminant, however, is road vehicle exhaust as reported in the State of the Environment Report for Malta 2002, which quoted a 1993 report by the UK Department of the Environment. It is released as a consequence of incomplete combustion of fuel as well as being newly formed by poorly managed combustion in internal combustion engines, that is to say, older technology or poorly maintained engines.

Since 1993, engine technology has changed enormously and the introduction of three-way catalytic converters must have surely by now changed the findings of 1993, at least in the UK.

Reviewing some facts we find:

1. A drastic reduction in airborne benzene should have occurred only once between 2002 and 2003 as a consequence of the switch to unleaded petrol. What than could have caused the reductions reported in subsequent years, considering that the road vehicle population in Malta is not decreasing?

2. Statistics on public transport utilisation show that use of this service (excluding the direct service to the University) is still diminishing. This is therefore an indicator that reducing benzene levels are not caused by a shift from the use of personal road transport to public transport. There must therefore be another reason for the reducing benzene levels after 2003.

3. The massive reductions of SO2 levels reported for 2004 and 2005 cannot be explained by a corresponding reduction in road traffic. The latter has probably not occurred. The changes in SO2 levels cannot be explained simply by assuming that road transport is now using low sulphur diesel considering that diesel consumption by road transport is on the increase. What, then, has led to such a reduction of SO2?

The answers to all these questions could be found in an analysis of fuel consumption figures and to water and electricity usage. Since I am not privy to the latest and most detailed figures I will attempt to provide an explanation, and I would be most grateful if I can be proved right or wrong by whoever has more detailed information at their fingertips.

The reduction in benzene after 2003 is more likely a direct result of higher fuel prices. Lead replacement petrol (LRP) is provided to cater for older vehicles that cannot use unleaded petrol. It has also been the most expensive of all the fuels. Older vehicles that use LRP and which are used on a regular basis tend to belong to lower income groups. These citizens would also tend to have vehicles that are less well maintained and, in conjunction with their old combustion technology, release more benzene than other vehicles. The high cost of LRP would also make low income earners unable to afford as much fuel as before so causing less use of their vehicle with a reduction in benzene emissions.

Reduced use of older vehicles may also be indicated by the higher numbers of older vehicles reportedly being scrapped or taken off the road in the last few years. The increasing proportion of vehicle registrations that are diesel powered and the fact that the older technology vehicles are gradually being outnumbered by newer vehicles that have better benzene emissions may also be a contributing factor.

Higher fuel costs may have also changed driving behaviour to a more fuel efficient way of driving.

Some of the factors related to the reduced benzene emissions listed above could also lead to reduced SO2 emissions. The large drop reported for this gas in 2004 and 2005 cannot be explained by any changes in road traffic. Since electricity generating plants are the main source of SO2 emissions, the reason for lower levels of this gas should be looked for here.

The higher costs of water and electricity could have motivated many to cut down on water and energy use. It would be interesting if it is possible to identify a drop in (mains) water consumption and electricity consumption that correlates with the drop in SO2. Any reduced consumption could further be quantified by sector (domestic, commercial and industrial).

One needs to assess if there was also a year-on-year reduction in sulphur content of the fuel used for power generation and changes in generation efficiency.

If these explanations are shown to be reasonably correct, it would demonstrate that elimination of subsidies or cross subsidisation where energy saving policy is concerned is justified and instrumental in improving air quality. It also gives a clear indication that fuel price is a more effective way of controlling emissions from road vehicles rather than taxing the vehicle at purchase which actually encourages retention of older, more polluting vehicles.