The Comparisons of Real-time Ammonia Adsorption Measurement in Varying Inlet Tubes and the Different Ammonia Measurement Methods in the Atmosphere

Abstract

Ammonia (NH₃) is an important, albeit sticky, precursor for producing secondary inorganic aerosols (SIA), especially in the form of ammonium nitrate (NH₄NO₃) and ammonium sulfate ((NH₄)₂SO₄). To reduce SIAs, many researchers have attempted to measure the concentration of ambient NH₃ using real-time or passive methods. However, NH₃ is a highly sticky gas and is therefore difficult to measure using real-time methods without incurring losses during measurement. In this study, four different tubing materials, semi seamless tubes, perfluoroalkoxy (PFA), polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF), were used to ascertain the adsorption of NH₃ in inlets using real-time instruments. Without heating sample tubes and at 0% relative humidity (RH), this study shows that PTFE had the least adsorption(i.e., 0% at 1 and 2m of sample tube), and semi-seamless tubes had the highest adsorption (i.e., 27.5% at 1 m of sample tube). To calculate the adsorption of NH₃ under ambient conditions, at various inlet lengths, the RH of NH₃ was varied from 20% to 80%, which showed that shorter inlets and higher RH lower NH₃ adsorption at inlets (i.e., 1.74 ppb m⁻¹ at 80% RH and 7.48 ppb m⁻¹ at 20% RH). Additionally, inlet heating was effective in reducing the adsorption of NH₃ as the RH decreased. Applying the inlet system (i.e., 2 m of PTFE tube with heating) showed excellent correlation (slope: 0.995 and coefficient: 0.992) between two different real-time measurements while measuring ambient air.