Beijing - China's battle against a persistent air pollution crisis, which all but shut down a city of 11 million this week, is being hampered by tough weather conditions, an environmental official said on Tuesday.Air quality in cities is of increasing concern to China's stability-obsessed leaders, anxious to douse potential unrest as a more affluent urban population turns against a growth-at-all-costs economic model that has besmirched much of the country's air, water and soil.The government has announced many plans to fight pollution over the years but has made little apparent progress, especially in the country's north and northeast.Harbin, a frigid northeastern city of 11 million people, virtually ground to a halt on Monday when a pollution index showed airborne contaminants at around 50 times the levels recommended by the World Health Organisation."This severe smog, first of all, is caused by climate conditions," Fang Li, the deputy chief of Beijing's municipal environmental protection bureau, told reporters."Right now, total pollution emissions have exceeded environmental capacity," he told a news conference on new smog emergency measures in the capital.Fang blamed thick fog for the pollution still blanketing Harbin and large parts of the northeast."Harbin, coincidentally, on its first day of winter heating encountered thick fog - these two things are closely linked," he said."If you just equate heavy pollution with winter heating, then the whole winter would be like this. That's not possible".Collective central heatingCollective central heating, activated on a date set by the government, provides heat to 65% of Harbin, figures quoted last year in the state media show. Much of that heat comes from burning coal.Beijing's central heating normally comes on in mid-November.Beijing, sometimes derided as "Greyjing" or "Beige-jing" by English-speaking residents, suffered its own smog emergency last winter, when the pollution index reached 45 times the recommended level one particularly bad day in January.Smoke from factories and heating plants, winds from the Gobi Desert and fumes from millions of vehicles can combine to blanket the city in a pungent shroud for days.If three days of hazardous and near hazardous pollution are forecast this winter, Fang said, the capital will temporarily halt construction, factory production, outdoor barbecues and fireworks.If hazardous pollution is expected for three consecutive days, school will be cancelled and temporary curbs imposed on driving, he added.