Tropospheric ozone is generated by the photochemical oxidation of volatile organic compounds (VOCs) in the presence of nitrogen oxides (NO_x, comprising NO and NO₂) [1]. The Republic of Korea (South Korea), and especially the Seoul megacity, has a longstanding issue with elevated ozone levels ^[1][2][3][4][1,2,3,4]. Background ozone levels are approximately 80 parts per billion by volume (ppbV) ^[4][5][4,5]. The large collaborative Korea-United States Air Quality campaign (KORUS-AQ) ^[4][5][4,5] provides one example of a major multinational study that has sought to provide measurements that improve understanding of the cause of the elevated ozone levels. In addition to local ozone production, this study pointed to the wider regional problem of elevated ozone, an issue that more recent publications are continuing to investigate (see, for example, ^[1][6][1,6]).

The effective measurement of the VOCs that contribute both to ozone formation and secondary organic aerosols (SOAs) is important. The KORUS-AQ report [4] identified that, in addition to automobile emissions, fugitive solvent emissions are a significant contributor to elevated VOC levels. In South Korea, the conventional measurement of photochemical smog precursors follows the United States Environmental Protection Agency’s Photochemical Assessment Monitoring Stations (PAMS) approach and typically uses thermal desorption-gas chromatography-flame ionization detection (TD-GC-FID) for the analysis of over 50 hydrocarbons and several oxygenates [7]. There are 18 PAMS stations collecting precursor data in South Korea [8], and it is made available publicly in monthly air quality reporting [9]. Kang et al. [10] have recently described a two-year TD-GC-FID study with hour-by-hour monitoring from a single PAMS site in central Seoul. However, although this approach is widely deployed, it is only suited to the determination of ambient (or fenceline) concentrations of ozone precursors at fixed locations.

Gaining a better understanding of fugitive emissions from industry, especially in industrial parks with many similar businesses in close proximity, requires mobile analytical tools that acquire data in real-time because events are frequently transient. TD-GC-FID is not suitable for mobile measurement due to the long analysis time (tens of minutes) which would average several locations into one measurement. As early as 2016, this need was identified by the Korean Ministry for the Environment, and in 2017 a detailed report on the evaluation was published [11]. In this study, selected ion flow tube mass spectrometry (SIFT-MS) was the primary technology evaluated for VOC measurement, and this more broadly than ozone precursors. Subsequently, SIFT-MS instrumentation has been increasingly utilized as a complementary tool to the existing regulatory measurements, because it provides mobile, real-time analysis. The move to more extensive monitoring has been driven by increased regulation [12]. Although SIFT-MS instruments do not conduct regulatory analysis, they enable more rapid detection of pollution incidents and identification of the responsible emission source, enabling sampling to be triggered for regulatory analysis. Hence, they support improved air quality management.

Conventional environmental VOC analysis primarily utilizes GC analysis ^[13][15] (with various options for sample introduction and detection, depending on the regulatory method). The most fundamental component of the technique is the chromatographic separation which is usually optimized to resolve all analytes as a function of time (Figure 1). A typical GC analysis will take tens of minutes to complete. Furthermore, to achieve low or sub-ppbV detection limits, significant volumes of air may need to be collected and concentrated prior to analysis, further delaying the reporting of analytical results. In the TD-GC-FID approach used for ozone precursor monitoring, pre-concentration is achieved using adsorption onto a suitable adsorbent material, followed by rapid heating to focus a concentrated sample into the GC injector during the sample injection (Figure 1) ^[14][16]. Both sample preconcentration and the gas chromatography column can discriminate against certain chemical functionalities and hence the suitability of configuration is an important consideration in GC method development.

In contrast, SIFT-MS is a direct-injection mass spectrometry (DIMS) technique in which air is introduced, analyzed, and reported continuously (Figure 1). The reporting rate depends on the analytical method but is typically from 5 to 120 s for multiple analytes at sub-ppbV concentrations.

The SIFT-MS technique has been described in detail elsewhere ^[15][16][14,17]. Briefly, continuous sample ionization is achieved by using ultra-soft chemical ionization that efficiently ionizes a very broad range of VOCs but does not ionize the bulk constituents of air. A microwave discharge through air is used to generate the reagent ions, with up to eight available (H₃O⁺, NO⁺, O₂^+•, O^−•, OH⁻, O₂^−•, NO₂⁻, and NO₃⁻) on commercial SIFT-MS instruments ^[17][18]. Rapid switching of reagent ions provides high specificity because the multiple reaction mechanisms give independent measurements of each analyte, while the elimination of chromatographic separation means that it is straightforward to analyze VOCs of diverse chemical functionalities in a single procedure ^[18][19][20][13,19,20]. Instrument detection limits in the part-per-trillion by volume (pptV) range are typically achieved for 1-s ion dwell times for direct analysis of air with no preconcentration or drying required ^[21][22][21,22].

The literature reviewed below utilized the Voice200ultra model SIFT-MS instruments (Syft Technologies Limited, Christchurch, New Zealand) with positive reagent ion sources operating on nitrogen carrier gas ^[15][23][24][14,23,24]. Since the instruments are being used for quantitative analysis of pollutants with reference to regulatory limits, calibration has been utilized widely in contrast to library-based quantitation ^[18][13]. A similar approach has been used in the field-based environmental monitoring conducted by Shaw and co-workers ^{[22][25][26][27]}[22,25,26,27]. Calibrations are routinely linear [28] in real-time monitoring applications, as observed previously [21].

The South Korean environmental applications illustrate the tremendous flexibility of the SIFT-MS technique for volatile compound monitoring, both in terms of sample delivery and instrument deployment. Table 1 summarizes the instrument configurations and installation types used across the applications as discussed below. The widespread field deployment of SIFT-MS instruments is noteworthy in the South Korean adoption of SIFT-MS: 11 are in air monitoring sheds, 39 are used primarily for monitoring VOCs while the mobile laboratory is in motion, and one is on board the ship Isabu. Together with field-based research conducted in China ^[25][29][25,29], the UK ^[22][27][22,27], and Vietnam ^[26][30][26,30], this demonstrates the robustness of the quadrupole mass filter (QMF)-based SIFT-MS instrumentation. Coupled with real-time data visualization and internet connectivity, field-based systems enable real-time decisions to be made ^[31][32][31,32].

Finally, a comment on the preferential adoption of SIFT-MS over the related proton transfer reaction mass spectrometry (PTR-MS) technique ^[42][43][42,43], as utilized in the KORUS-AQ study ^[4][5][4,5], is necessary. PTR-MS has been widely utilized in academic atmospheric research ^[44][45][46][44,45,46], especially its time-of-flight (TOF) variant [45], whereas SIFT-MS has been utilized to a limited extent ^[15][18][13,14]. In South Korea, the need, driven by increased regulation [12], has been for mobile measurements of VOCs. The availability of an existing SIFT-MS mobile laboratory solution was a key reason for its preferential adoption together with the ability of SIFT-MS to identify and target VOCs quantitatively immediately on site. Moreover, in SIFT-MS instruments, the use of QMFs for mass-selective detection, rather than the TOF systems utilized in most modern PTR-MS instruments [43], provides greater robustness for analysis on-the-move and operation that is less sensitive to temperature.