“Over the past 10 years, through our enormous hard work, Beijing’s air pollution has visibly improved,” Mr. Du said in a statement. “This is an indisputable fact.”  He added, “We have patience for that small group of people who don’t understand, and we believe that, as time passes, and in the face of facts, they will eventually understand.”  Mr. Andrews said he based his study, which has not undergone peer review, on official government statistics. He spent more than a year in Beijing as a Princeton in Asia fellow at the National Resources Defense Council, or N.R.D.C., a nonprofit environmental group. But he said his study was independent of his association with the council.  With roughly seven months remaining before Beijing plays host to the Olympics, air quality has become a major concern. Chinese officials have pledged that pollution will not be a problem during the Olympics and are discussing contingency plans that include possible factory shutdowns or traffic restrictions, if necessary.  But Beijing officials also have pointed to improved Blue Sky air quality ratings as proof that the city is fulfilling its commitment to clean itself up for the Olympics. The Blue Sky program, used in 84 major cities across China, measures sulfur dioxide, nitrogen dioxide and airborne particulate matter known as PM10.  Pollutants are measured on a scale of 1 to 500, with 500 considered the worst score. Any rating below 101 meets national air quality standards and is considered a “Blue Sky day.” The city’s overall daily rating is determined by whichever of the three pollutants has the highest rating for a 24-hour period. In Beijing, PM10 is almost always the worst pollutant.  In 1998, Beijing recorded only 100 Blue Sky days. In 2007, that number had risen to 246, a fact hailed last week in China’s state media.  But Mr. Andrews said the recent improvement was largely the result of changing the formula in 2006. He said the city’s daily pollution rating was based on readings from a subset of the more than two dozen monitoring stations around the city. Mr. Andrews said readings from seven stations were used from 1998 to 2005.  But in 2006, the city dropped two stations located near areas with high traffic and replaced them with three other stations in quieter areas. The impact on the Blue Sky ratings was drastic; Mr. Andrews found that using the original seven stations would have meant 38 fewer Blue Sky days in 2006, and 55 fewer last year.  This would have meant that Beijing had fewer days meeting national air quality standards last year than in 2002, when there were 203.  Mr. Andrews also found that ratings began to change after officials set targets for every monitoring station in the city. He said this political imperative coincided with a rising number of days that rated just below the break point of 101 to qualify as a Blue Sky day.  A version of this article appears in print on , on Page A3 of the New York edition with the headline: Consultant Questions Beijing’s Claim of Cleaner Air. Order Reprints| Today's Paper|Subscribe
The new study found flaws in Beijing’s “Blue Sky” system of air-quality monitoring and noted that the city changed its formula for measuring pollution in 2006, using less polluted locales than before.