I was trying to answer a question on skeptics.stackexchange.com, where we found this paragraph in a scientific report:

This compares to 1,374 tons per day statewide for passenger cars. While reductions from gasoline SI marine engines are not included in California’s SIP as a planned measure for meeting attainment goals, the impact of outboard and personal watercraft engines on the state’s total emissions inventory makes control of this category of emissions necessary. Table 1 lists the relative contribution of outboard and personal watercraft exhaust emissions in California in tons per day. As a comparison to other sources, the exhaust emissions from two hours of personal watercraft operation is equivalent to the emissions from a 1998 passenger car operated over 100,000 miles.

The sentence in bold created a lot of confusion, since Stienstra (1998) wrote an article, cited the report and paraphrased it like this:

The state Air Resources Board is also considering taking statewide action against personal watercraft, after a staff report said that two hours of exhaust emissions from a Jet Ski is equivalent to the emissions created by driving a 1998 automobile 100,000 miles.

While users in skeptics.stackexchange.com interpreted as:

Two hours of use of a jet-ski produces the same pollution as two hours of driving a 1998 car with 100,000 miles on the clock. (And not driving a 100,000 miles with that car).

Another user here found another interpretation:

All the watercrafts operating in California together produce in 2 hours the same pollution as one car driven 100,000 km.

Is Stienstra (1998)'s interpretation the intended meaning and why? Could we be 100% sure?

Stienstra, T. (1998, Jul 08). U.S. park service seeks ban on jet skis 950 square miles of bay area waters would be affected. San Francisco Examiner. Retrieved from http://search.proquest.com/docview/270486538?accountid=8555

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    "Driving a car 100,000 miles" seems to me to be the clear interpretation, and used specifically to emphasize the pollution produced by the watercraft. I see no justification for the "driving a car with a lot of miles on it". – Dan Bron Sep 4 '14 at 14:08
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    Stienstra's interpretation is the intended meaning. As to which interpretation is more convenient (I suppose you mean to your cause?) is really neither here nor there. You can be sure by making some calculations using some of the numbers in the tables provided in the report. – Jim Sep 4 '14 at 14:08
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    @DanBron, but it seems illogical that a jet ski produce as much pollution in an hour only as a car does in 100,000km (probably thousands of hours driving). – George Chalhoub Sep 4 '14 at 14:19
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    arb.ca.gov/emfac for some data. @georgechalhoub- I think that's the point of the report. PWC emissions are outrageous and something needs to be done about it. – Jim Sep 4 '14 at 14:21
  • @georgechalhoub Another more likely possibility just occurred to me: that the report means that all the watercraft operating in California together produce in 2 hours the same pollution as one car driven 100,000 km. Then Stienstra's mistake could be to interpret that as meaning that one jetski operated for 2 hours produces that amount of pollution. I think this is a better fit both with the logic of the situation and the language of the report. – Senex Sep 4 '14 at 14:27

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.

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