Atmospheric Potential Gradient Measurements from a Rooftop in Bangkok

Atmospheric potential gradient measurements at 1 Hz (averaged to 1 minute samples) were taken on the roof of a 6th floor building, approximately 100 m from a busy toll road in Lak Si, northern Bangkok. The measurement period covered the transition from hot season into rainy season. The heavy monsoon rains and heavy traffic can both be significant disruptors of the fair weather atmospheric potential gradient. Heavy traffic increased potential gradient due to aerosol loading, while rain storms increased the magnitude, but also changed the sign of the field due to charge processes within clouds. Increasing relative humidity led to a decrease in potential gradient, implying a lack of hygroscopic growth. Cross correlations of aerosol number in different size fractions showed that aerosols with a D50 value of 155 nm had the strongest correlation. A strong diurnal signal is present in frequency domain analysis of potential gradient.