Zero offset in computer-recorded analog data in measuring aerosol scattering coefficient: a “threshold” or a “blank”?

When measuring the aerosol scattering coefficient (ASC) with an integrating nephelometer, the instrumental “zero” value is established by passing filtered, particle-free air (zero air) through the instrument and setting the instrumental analog signal to zero. Ideally, the computer logged analog value of the zero air should also be zero. However, more commonly a non-zero analog value (zero offset) is obtained. This zero offset is normally regarded as a “blank” and the value is subtracted from the data values. During a one-month field experiment where we measured ASC, we consistently obtained a zero offset that ranged from 4 to 25 mV (equivalent to ASC values of 0.8 to 5×10-6 m-1). In this paper, we demonstrate that the zero offset we observed is in reality a threshold to the recorded analog data and that it should not be subtracted from the data record. Treating the zero offset as a blank could lead to an error in ASC data of as high as 21%. The zero offset observed in our experiment was not produced by the nephelometer. Rather, it was generated during the transmission of the analog signal from the instrument to an analog-to-digital converter. Thus, our results should serve as a caution for those who perform ASC or other measurements using analog data transmission.