How do I interpret "emissions of car operated over" in this sentence?

Solution 1:

The paragraph quoted from Stienstra (which equates two hours of exhaust emissions from one Jet Ski with those from a car traveling 100000 miles) does not coincide with reality.

The statement that “All the watercrafts operating in California together produce in 2 hours the same pollution as one car driven 100,000 km” actually is fairly accurate.

Here is the sentence prior to what was quoted from a scientific report:

Emissions of hydrocarbons (HC) and oxides of nitrogen (NOx) from outboard and personal watercraft in 1997 totaled 312 tons per day.

That number of tons per day equals 26 tons per 2 hours, which is close to the approximately 26 tons of CO₂E an average car emits while traveling 100000 kilometers. This figure arises as follows: From the EPA's refs.html, section Miles driven by the average passenger vehicle per year, average car emissions are 4.20 x 10⁻⁴ metric tons CO₂E per mile. For 100000 miles, that's a total of 42 metric tons CO₂E (CO₂ equivalent, which encompasses “carbon dioxide, methane, and nitrous oxide, all expressed as carbon dioxide equivalents”). Emissions per 100000 km is about 5/8 as much, or about 26 tons. (More precisely, 26.098 tons = 42 tons * 100000 miles/160934 km, but even two significant digits is too many.)

Edit: Brian M. Hunt asks,

So is it right to do the math as follows: 312 tons/508,270 personal watercraft + outboards/2 hours being 278 grams per watercraft/outboard per hour?

The figure of 508270 watercraft (161898 personal plus 346372 outboard) is shown in Table 1 on page 2 of the report mentioned earlier, which is a draft proposal summary from the State of California Air Resources Board called Proposed Regulations for Gasoline Spark-Ignition Marine Engines.

The arithmetic shown in Hunt's comment is correct. For example, if you type “312 tons/508,270/2 in grams” into a Google search box, it shows a result of slightly over 278.4 grams, or about 0.0003 short tons (2000-pound tons).

Note that a metric ton is about 1.1 short tons. The EPA refs.html page mentioned above shows its data using metric tons (1000-kg tons), and I'd supposed the Air Resources Board report did so too, because metric units are used in its Figures 1 and 2 and Tables 2 and 3. If so, average emissions per watercraft per hour would be about 307 grams per hour rather than 278. If not, my earlier figure of 26.098 [metric] tons would need to be recast as 28.77 short tons, which still is quite comparable to 26 tons.