Why now, after 15 months from the start of the accident? Because the Tokyo Metropolitan government under Governor Shintaro Ishihara is finally forced to do so because 9 spots in Mizumoto Park in Katsushika-ku (one of the eastern Special Wards with higher levels of radiation than the rest of Tokyo) exceeded even their very lax (and near-impossible to be applied) standard of decontamination.
What's that lax standard? They will decontaminate only if the air radiation level at 1 meter off the ground at a particular spot exceeds the air radiation levels of the surrounding area by more than 1 microsievert/hour.
As recently as on June 12, the Tokyo Metropolitan government issued a statement saying they will not decontaminate locations in Mizumoto Park precisely because of the standard just mentioned above. Their June 12 press release says (my summary):
Japan Communist Party of the Tokyo Metropolitan Assembly announced the result of their measurement in the park on June 11, and one location allegedly measured 1.10 microsievert/hour.
Japan Communist Party notified the Ministry of Education and Science, and the Ministry asked that we investigate, including the radiation levels in the surrounding area.
So we did, using Hitachi-Aloka Medical's scintillation survey meter TCS-172B. We sent out someone from the Environment Bureau in the evening. The result was 0.99 microsievert/hour at 1 meter off the ground, and the surrounding areas measured 0.18 microsievert/hour. So, it is below the standard for decontamination set by the Ministry of Education and Science, which specifies the air radiation to be more than 1 microsievert/hour higher than that of the surrounding area.
So our conclusion is that there is no need for decontamination.
Never mind that Goshi Hosono's Ministry of the Environment sets the decontamination standard at 0.23 microsievert/hour air radiation. Tokyo is sticking by the Ministry of Education. By insisting on the Ministry of Education's standard, Shintaro Ishihara's government may have been hoping to avoid doing any "decontamination" in their city, which is by the way one of the finalists in the selection of the host city for 2020 Olympics.
But on June 25, the Metropolitan government had to measure the park again after citizens armed with survey meters alerted them again, according to ANN (TV Asahi) News. Now, they are grudgingly admitting that for some unknown reason the radiation levels at 9 spots in the park are higher by more than 1 microsievert/hour than the surrounding areas, with the highest at 1.22 microsievert/hour.
That should be the highest official measurement of air radiation in Tokyo, at 1 meter. Before the Fukushima accident, or for that matter, up to 3:59AM on March 15, 2011, the official radiation measurement in Shinjuku was 0.0384 microsievert/hour. (Go to the page at the Tokyo Metropolitan Institute of Public Health for more data.)
Tokyo Metropolitan TV (Tokyo MX) has a more detailed report, though. According to Tokyo MX, the locations were exactly what the Japan Communist Party of Tokyo Metropolitan Assembly had reported on June 11, but on June 25 the measurement was done by the Construction Bureau of the Tokyo Metropolitan government.
(Some very lax reporting by TV Asahi. Not surprising.)