Development and application of a simultaneous measurement method for gaseous ammonia and particulate ammonium in ambient air

Abstract

Ammonia gas is one of the precursors contributing to the formation of secondary particulate ammonium via reactions with atmospheric acids, such as sulfuric and/or nitric acids, which are present in ambient air. In this study, a new instrument that is suitable for measuring ammonia gas and fine particulate ammonium (PM2.5 NH4+) concentrations simultaneously under ambient conditions was developed. A wetted frit sampler was connected in the back of a counter-current flow tube (CCFT) sampler, and the NH3 gas and PM2.5 NH4+ samples were collected by CCFT and wetted frit samplers, respectively. An air sample was drawn through the samplers at a flow rate of 1.0 dm3 min–1 and an absorption water flow rate of 120 mm3 min–1. Then, the ammonium that formed in the absorption solution was detected by the indophenol method using a continuous flow analysis system. The estimated detection limits were 43 and 49 ng m–3 for ammonia gas and PM2.5 NH4+, respectively. Notably, the ammonia gas was collected on the CCFT sampler with a collection efficiency of 98.5%, but most of the PM2.5 NH4+ passed through it and was captured on the wetted frit sampler with a collection efficiency of approximately 100%. The present method was applied to measure NH3 gas and PM2.5 NH4+ at two urban sites: Osaka, Japan and Ho Chi Minh City, Vietnam. It was found that the simultaneous measurement method performed very well and that the measured concentrations were comparable with those obtained with the annular denuder method.