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The concentration of air negative ions is one of indicators in representative of air quality. The WIMDA series automatic air negative ion observation instruments, which meet the national recommended standards, were developed to satisfy the requirements of air negative ion monitoring. The instrument realizes the automation from data collection to data processing, and can work normally at a relative humidity from 0 to 100%, even under supersaturation condition. The results of operational running show that it is reliable and has high stability, and can reduce the workloads of observers greatly.