mardi 22 novembre 2016

In HK, even a car that is not running is a bad car!

A particular feature of the transport system is its extreme complexity. It has multiple objectives, externalities, sectors (housing, residential, business, construction) levels (city, region, country, international) and players.  This complexity creates problems when it comes to implementing public policies in the sector.  All the externalities are interrelated, so that it is impossible to only target one of them.
However there is an effect that is completely forgotten in the economic approach, which concerns Volatile Organic Compounds (VOCs emissions when the car is not running. It has been proven (Yamada, 2013, EPA 2014 or De Gennaro et al., 2016) that VOCs emissions from cars that are parked account for a significant part of VOCs emissions. This is especially relevant for countries like Hong Kong where the diurnal temperature is rather high on average.
In Hong Kong, VOCs emissions generated by road transport accounted for 20% of total VOCs emissions in 2014. Evaporative VOCs from cars come from different source (see Table 1). According to the EPA that conducted a study on representative vehicle fleets (EPA 2014b), the diurnal source (or cold soak) is estimated to correspond to about 25-35g of emissions per day per car. They represent at least half of the total VOCs emissions from cars. The latter correspond to 34kg per year per car or are equivalent to 50 liters of liquid gasoline per year per car. De Gennaro et al. (2016) conducted a study in some Italian cities (Firenze and Modena) and found an evaporative VOCs density equal to 4 to 8 kg/km2/day.
Since the year 2000 (following the Council Directive 98/69/EC), gasoline vehicles for the European market have been equipped with an activated carbon canister placed on the vent of the tank. Its purpose is to trap the fuel vapours instead of having them released into the air. The carbon canister has a limited capacity. For the canister to be purged, and the hydrocarbons to go back to the tank, it needs the vehicle to be running. However, it is once the carbon canister gets saturated due to long time parking, or insufficient purging that evaporative VOCs become significant. This likely to be the case in Hong Kong where cars do not run at a sufficient speed to trigger the purge! The size of the carbon canister, the Gasoline Working Capacity of the activated carbon and the purging strategy are key parameters affecting the efficiency of the evaporative emission control system (See Martini et al, 2014).
Accounting for these evaporative VOCs would significantly alter public policies. For instance having cars parked for a longer time in city centers to avoid congestion was not supposed to have any cost in terms of pollution: this would no longer be the case. In addition, the level of evaporative VOCs depends on the age of the cars (what matters is the existing legislation at the car’s building time) and on the size of the tank but in a way that differs from the relationship existing between polluting emissions and tank size or car newness when the car is running. To summarize, it was so far considered that a good car is car that is not running; we now argue that even a car that is not running is a bad car!

Table 1: sources of evaporative VOCs from cars

Source: Yang (2016)

De Gennaro M., Paffumi E., and Martini G., (2016), “Data-driven analysis of the effectiveness of evaporative emissions control systems of passenger cars in real world use condition: Time and spatial mapping”, Atmospheric Environment  129, 277–293.
EPA, (2014a), (accessed in October 2016).
EPA, (2014b) “Evaporative Emissions from On-road Vehicles in MOVES2014”, EPA-420-R-14-014, September 2014.
Martini G., Paffumi E., De Gennaro M. and Mellios G., (2014),  “European type-approval test procedure for evaporative emissions from passenger cars against real-world mobility data from two Italian provinces” Science of the Total Environment 487, 506–520.
Yamada H., (2013), “ Contribution of evaporative emissions from gasoline vehicles toward total VOC emissions in Japan . Science of the Total Environment 449, 143–9.Yang W. (2016) «Evaluation of evaporative VOCS in passengers Vehicles for regional air quality management » Master Dissertation Thesis, City University of Hong Kong. 

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