When using biomass or waste as a feedstock for power production, alkalis are a major concern because of their negative effects on equipment. To investigate the release behavior of alkalis and the effectiveness of countermeasures, it is indispensable to quantify alkali emissions with a high time resolution (less than 1 min). This paper presents a newly developed alkali detector based on the principle of surface ionization. The detector includes a number of improvements compared to previous designs that enable a reproducible measurement of alkalis in heavily tar- and particle-laden product gases. Our redesigned alkali detector has demonstrated high sensitivity and improved characteristics for making measurements of tar- and particle-laden process gases. Using an ultrasonic nebulizer as a source for alkali aerosols, we could conduct a calibration of the alkali detector over 4 orders of magnitude. In combination with a dilution setup and a sampling lance, we could take online measurements of gasifier producer gas with a high degree of reproducibility and a high time resolution of 1 s. The measurements of product gas have proven the feasibility of using the detector for field measurements.