It is found that air pollutants are easily confluent and accumulated when the weather in Beijing is controlled by the low-pressure type at surface.
发现当低压类地面天气形势控制时，容易引起污染物在北京地区的汇聚和累积。
A. Surface Closers: BHMA A156.4; rack-and-pinion hydraulic type with adjustable sweep and latch speeds controlled by key-operated valves and forged-steel main arm.
外露的闭门器：BHMAA156.4，齿条和小齿轮和液压式可调闩锁速度，由钥匙阀和锻造钢铁主臂控制。
A. Surface Closers: BHMA A156.4; rack-and-pinion hydraulic type with adjustable sweep and latch speeds controlled by key-operated valves and forged-steel main arm.
外露的闭门器：BHMAA156.4，齿条和小齿轮和液压式可调闩锁速度，由钥匙阀和锻造钢铁主臂控制。
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