请给出全国范围内空气质量最差的十个城市，并按照排名顺序列出，请使用json格式回答。
user
```json
{
  "worst_air_quality_cities": [
    {"rank": 1, "city": "石家庄"},
    {"rank": 2, "city": "邯郸"},
    {"rank": 3, "city": "保定"},
    {"rank": 4, "city": "唐山"},
    {"rank": 5, "city": "衡水"},
    {"rank": 6, "city": "邢台"},
    {"rank": 7, "city": "廊坊"},
    {"rank": 8, "city": "郑州"},
    {"rank": 9, "city": "西安"},
    {"rank": 10, "city": "济南"}
  ]
}
```
assistant