I don't see why Chinese government should be ashamed by what it just said.
Chinese government certainly should be ashamed that it failed to cope with Chinese severe air pollution, but it doesn't mean it has no right to stop foreigners from monitoring air quality in China.
We all know that air quality should be no concern for a foreign embassy, we also know what they are really up to by doing what they are doing.
If they are that keen on improving Chinese air quality, share some of their best emission-reducing technology with China at a discounted price, that alone will do much more change than measuring PM2.5 index.