Long-term sub second-response monitoring of gaseous ammonia in ambient air by positive inhaling ion mobility spectrometry

Highlights

• Ion mobility spectrometry was developed to monitor gaseous ammonia in the ambient air in a long-term measurement.

• The quantitative analysis frequency has achieved 10 Hz with a data averaging of 10 times.

• The limit of detection of sub-ppb level was obtained.

• The affect of ambient humidity was greatly reduced using drift temperature of over 150 °C.

Abstract

A real-time dynamic measurements of ammonia (NH₃) is crucial for understanding the atmospheric nucleation process. A novel method was developed for on line monitoring at the sub-second time scale for the gaseous ammonia in ambient air for months, based on a positive inhaling ion mobility spectrometry (IMS) with a ⁶³Ni ion source. The selective detection of NH₃ was achieved using a high resolution IMS with an optimization of the drift tube temperature above 150 °C. This method improved the peak-to-peak resolution significantly, thus avoided the interferences of the adjacent peaks to the quantitative analysis of NH₃. The time resolution of the IMS was less than 0.1 s at a data averaging of 10 times. The limit of detection (LOD) achieved at sub-ppb level while a linear response of peak intensity versus concentration of NH₃ in the range of 10–60 ppb and 60–400 ppb were obtained. The relative standard deviations (RSD), the confidence level and the errors were 1.06%, 95% and ± 0.21 ppb by measuring 100 ppb NH₃ for 100 times. The effect of ambient humidity could be greatly reduced by using the drift temperature of over 150 °C. At last, the application of measuring the NH₃ concentration evolutions of Dalian city was performed from June 19 to December 3 in 2015. The results illustrated a potential method of using IMS for a real-time measuring atmospheric NH₃ at an unprecedented accuracy and sensitivity with long-term stability.

Introduction

Ammonia (NH₃) plays an important role in atmospheric nucleation. For example, atmospherically relevant ammonia mixing ratio of 100 parts per trillion by volume would increase the nucleation rate of sulphuric acid particles more than 100–1000-fold [1], [2]. Moreover, through the atmospheric deposition process, large quantities of NH₃ get into soil and water, which may lead to eutrophication and acidification that have adverse impact on biological diversity [3], [4]. Due to these negative impacts, advanced apparatus are in need for monitoring NH₃ concentration in atmospheric environment. In particular, it would be desirable to develop NH₃ concentration measurement instruments enabling continuous operation on line with fast response time.

Many methods of measuring NH₃ concentration have been developed in recent years. For a long time, the off-line methods have been widely used to measure NH₃ in air, for example, bulk denuder method sampling gaseous NH₃ out of the air steam [5]. However, these off-line methods suffered from many disadvantages, such as time-consuming, labor-intensive, sample contamination, and low temporal resolution. After decades of research effort, absorption spectroscopic methods have become more viable due to their sensitivity and practical nature for the NH₃ measurement [6], [7]. One of such new on-line approach could determine the NH₃ concentration by absorption spectrophotometry method. The limit of detection (LOD) has achieved ppt-level by analyzing the resultant of NH₃ transferring to the liquid phase [8]. Chemical ionization mass spectrometry (CIMS) method has also been adopted for NH₃ monitoring [9], [10].

In recent years, some inter-comparisons of NH₃ measuring methods have been reported. Von Bobrutzki et al. used eleven instruments to measure the atmospheric NH₃ in Southern Scotland, such as optical techniques, wet-chemistry systems, photo-acoustic spectrometers and mass spectrometric apparatus, to assess the performance and characterize the response times of the instruments [11]. These instruments exhibit excellent performances in monitoring NH₃. However, the high cost, capital investment or running or both of these methods, prevents them from being applied in large-scaled air quality monitoring program. On the other hand, several gas sensor based methods were developed to monitor NH₃ concentrations. However, the minimum response time was from seconds to ten minutes with a LOD of several ppb, which is sensitive to monitor source emission NH₃ [12], [13].

Due to fast changing condition for the atmospheric environment, such as the airflow transport consists of differently sized eddies, as well as many relevant pollutant concentrations, the required pollutant concentration monitoring time scale should be as short as 0.1 s. However, few monitoring instruments can meet such a short response time requirement.

It is well known that ion mobility spectrometry (IMS) has many advantages over the analytical methods mentioned above, such as fast response, high sensitivity, low cost, robustness, and simple operation. IMS technology has been applied in detection of anaesthetic [14], [15], [16], [17], narcotic drugs [18], explosives [19], [20] and chemical warfare agents [21], [22]. Currently, there are a few reports about IMS technology for detecting NH₃ gas concentrations in the air and from the human breath [23], [24], [25], [26], [27]. Compared with the atmospheric environment, the temperature, humidity and NH₃ concentration of human breath have smaller changes. Due to the intermittent operation in short time, it is not very suited for atmospheric environment measurements in a long-term. Therefore, an exhaustive and accurate quantification of the NH₃ concentration measurement for the atmospheric environment is needed. Firstly, ambient concentrations range of NH₃ is very wide, from several ppb in remote areas [28] to several ppm levels near feeding stock facilities [13], [29], [30], [31]. Obtaining the NH₃ concentration in such a large range accurately is a big challenge. Secondly, as polar compound, NH₃ is easy to stick or interact with the surface materials of the path way, which cause slow response and affect NH₃ quantification [7], [11]. Thirdly, as one of the major components of air, moisture may cause interference response to different instruments [13]. Finally, human activity is one of the most important NH₃ emission sources [32], which may cause sample contamination.

In this study, a high resolution IMS with ⁶³Ni ion source was constructed and optimized for on line measuring ambient NH₃ with sub-second measurement cycle in the air. The operating conditions of instrument were optimized to detect NH₃ selectively. The better LODs and larger quantitative measurement range for NH₃ were obtained. Then the effect of relative humidity (RH) in the drift gas and sampling environment was studied. In the final section, the method was applied to on-line monitor the atmospheric NH₃ in Dalian city.