Effective operating procedures for advanced aerosol instruments at high-altitude observatories: A technical note

Advanced aerosol measurements at high-altitude observatories are scientifically valuable but operationally demanding, and the established standard procedures, such as those of WMO/GAW and ACTRIS, were developed largely for near sea level stations. This technical note sets out operating procedures for advanced aerosol instruments at high-altitude sites, with an emphasis on tropical observatories, and is intended to complement rather than restate those guidelines. Its organizing theme is the reduced ambient pressure that defines such sites: at 1600 m the pressure is about 840 hPa, roughly 17% below the sea level value. We describe how this single factor lowers pump suction and volumetric sample flow, increases the gas mean free path and hence the slip correction, the particle diffusion coefficient and ultrafine particle losses, and alters the internal operation of condensation particle counters, mobility spectrometers, aerodynamic and optical sizers, cloud condensation nuclei counters, aerosol mass spectrometers, and absorption photometers. We then treat the sampling system from inlet to instrument, including size cut off, whole air and interstitial inlets for in cloud and fog conditions, sample drying below 40% relative humidity, leak testing, and active flow control, followed by correction of the measured concentrations to standard temperature and pressure. Site selection, infrastructure, and meteorological instrumentation suited to harsh mountain conditions are also addressed. Procedures are illustrated for the NABHA Laboratory at Munnar, India. Residual instrument specific uncertainties at reduced pressure are identified as a direction for future work.