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Traffic and personal care products have effects on air quality

PTR-TOF VOC monitoring sites track Innsbruck's urban environment

The air quality in urban areas is affected by the abundance of trace gases like ozone as well as fine particulate matter. The monitoring and control of concentrations and the corresponding emission sources is of vital importance for the health of all citizens and the environment.

Non-methane volatile organic compounds (NMVOC) contribute significantly to the formation of potential hazardous air pollutants like ozone and secondary aerosol particles. In urban areas, NMVOC like benzene, toluene and xylene (BTX) can typically be found as these are associated with combustion related processes like traffic.  

While solvents have historically been recognized as a significant source of NMVOC (e.g. Theloke, 2007), new measurements based on Proton-Transfer-Reaction Mass Spectrometry (PTR-MS) technology (e.g. Karl et al., 2018; Coggon et al., 2021) revealed that the use of personal care products like deodorants, chemical solvents from paintings, adhesives and others contribute significantly to the emission of so-called volatile care products (VCP). Siloxanes for example are associated with personal care products. PTR-MS is a measurement technique that allows the real-time, quantitative, and simultaneous detection of a broad range of trace VOC and VCP.

Rapid monitoring of NMVOC fluxes and concentrations reveal the complexity of NMVOC distributions in urban areas. Here, we look at the observations from two measurement sites in the city of Innsbruck. The Innsbruck Atmospheric Observatory (IAO) of the University of Innsbruck (Karl et al., 2020) is in the center of Innsbruck. The second measurement site is located at the headquarters of IONICON in the business district to the east of Innsbruck. Here, we operate our newest generation PTR-TOFMS instruments.

Figure 1 shows the diurnal cycle of the sum of siloxane fluxes, which are widely used in cosmetics (e.g., as softening, smoothing, and moistening agents) and detergents, in comparison to benzene. The diurnal cycle of mixing ratios and emission fluxes are anticorrelated, highlighting the influence of meteorology on the distribution of ambient mixing ratios. Most anthropogenic NMVOC accumulate during the night due to a shallow planetary boundary layer. As emissions increase during the day, the mixing height also increases, leading to an overall decrease of daytime concentrations. Nevertheless, Eddy Covariance flux measurements detect slight emission enhancements of siloxanes during the morning and afternoon rush hours (Figure 1 A), while benzene emission fluxes (Figure 1 D) increase throughout the day, peaking in the afternoon.

IONICON PTR-TOF instruments below the flux tower of the University of Innsbruck. Image credit: Thomas Karl
Figure 1: The diurnal cycle of (A) the sum of siloxane fluxes and (B) their mixing ratios, (C) the ratio of siloxane to benzene flux and (D) the benzene flux from measurements at the IAO.

How much do mixing ratios of benzene and siloxanes vary across the city of Innsbruck?

Figure 2 compares the distribution of concentrations for benzene and the sum of siloxanes that were monitored at both measurement sites, the IAO and at IONICON (IBK-East). It appears that concentrations of benzene and local sources for siloxanes are distributed quite similarly over the city, exhibiting mixing ratios in the order of 3-200 pptV. Real-time detection of NMVOC at the pptV level is essential to investigate these classes of compounds in urban air.

Figure 2: Comparison of mixing ratios for benzene and siloxanes between the two sites.

How to keep a clear view of the complexity of sources and processes in urban air?

With PTR-TOF MS several hundred to thousand compounds can be detected in urban air. Non-negative matrix factorization (NNMF) is a method for reducing this amount of data by grouping the data into so-called factors of similar characteristics and is used for e.g. source apportionment. NNMF is incorporated into the IONICON Data Analyzer (IDA) allowing fast and quantitative factorization of data giving researchers more time for interpretation of the results. Figure 3 shows mass spectra of exemplary factors for BTX and siloxanes. The BTX factor mainly comprises aromatic hydrocarbons. The VCP factor consists of a number of D-siloxanes as well as of a series of pure hydrocarbons.

Learn more about the use of PTR-MS in environmental applications, VOC monitoring in mobile PTR-TOF monitoring labs or have a look at current IONICON PTR-TOF trace VOC analyzers.

Figure 3: Exemplary factors that could be determined for BTX and VCP in IBK-East using Non-negative matrix factorization (NNMF) with the new IONICON Data Analyzer (IDA).