Ammonia Monitoring using various Techniques in Environmental Applications

Introduction

Ammonia emissions are of particular interest due to a variety of environmental impacts. As the primary atmospheric base, ammonia plays a crucial role in determining the acid-neutralizing capacity of tropospheric air masses [1]. Airborne sulfuric and nitric acid can react with gaseous NH₃ to form fine particulates, such as ammonium sulfate or ammonium nitrate. Fine aerosol is of special concern because small particles are able to penetrate deeply into the lung and may lead to adverse health effects [2]. As compared to gas-phase NH₃, secondary aerosols have a much slower removal rate in the atmosphere and, thus, a higher chance for long-range transport [3, 4]. Dry and wet deposition of gaseous ammonia and ammonium aerosol contributes to both acidification and eutrophication, which may cause damage to sensitive ecosystems such as forests, heathlands and bogs [3]. In Switzerland, the deposition of nitrogen (from N_Ox and NH₃) is above the critical loads for forests in about 90% of the forest land [5], and long-term measurements of NO₂ and NH₃ fluxes have shown that rates of NH₃ deposition typically exceed those of NO₂, sometimes by an order of magnitude [6].

The major sources for atmospheric NH₃ are agricultural activities and animal feedlot operations, followed by biomass burning (including forest fires) and to a lesser extent fossil fuel combustion [7]. Industrial sources, although minor with respect to global considerations, may be very important locally [8]. The contribution of road traffic to the non-agricultural ammonia emissions has been considered to be rather small. However, recent studies indicate that NH3 emission rates from automobiles may be higher than previously estimated, and that traffic might be an important source of ammonia in urban areas [9].

Ammonia measurements are challenging for a number of reasons. While NH₃ is a gas with a boiling point of -33.4 °C, it readily reacts with acids to form ammonium aerosols. In industrial processes this form can dominate and behave like small particulates, which has large implications on sampling and measurement strategies, as well as on legal regulations. The absorption of ammonia - and to some extend of ammonium aerosols - in diluted acid, followed by analysis of NH₄ ⁺ is the classical approach, which has been published in various countries for both emission [10] and ambient air [11] measurements. Continuous emission monitoring systems (CEMS) can be based on a variety of analytical concepts: chemiluminescence, UV-absorption, NDIR, or FTIR to name the most important ones [12]. However, reliable calibration is challenging because NH₃ reference gases tend to be unstable, and sampling is difficult due to the strong tendency of ammonia to adsorb to surfaces such as sampling line and measurement cell [13-15].

The following paper is based on three case studies that include a wide variety of sampling and measurement strategies in the field of ammonia measurements. We will start with the concepts of the reference method which is included in the Swiss regulations for measurements at stationary sources. Further examples are a large method validation study at a highway tunnel, and ambient air measurements at very low concentratinos using a novel instrument based on photoaccoustic laser spectroscopy. The last example also illustrates how excellent ambient air measurement data might be used to validate and complement classical emission data.