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Biogenic volatile organic compounds (BVOCs; e.g. terpenes) are highly reactive compounds typically present at sub-parts-per-billion mole fractions in the air. Due to this, their measurements are challenging and they may suffer losses during sampling, storage and analyses. Even though online measurements of BVOCs are becoming more common, the use of sorbent tubes is expected to continue because they offer greater spatial coverage compared to online measurements, and no infrastructure (e.g. electricity, housing/shelter with stable temperature and humidity, sampling lines) is required for sampling. In this study, the performance of an offline technique for the measurement of BVOCs based on sorbent tube sampling was evaluated. Tested compounds included eight monoterpenes, five sesquiterpenes and five oxygenated BVOCs, which are generally either directly emitted (1,8-cineol, linalool, bornyl acetate) or oxidation products (nopinone and 4-acetyl-1-methylcyclohexene). Two sorbent materials (Tenax TA and Carbopack B) and four tube materials (stainless steel (SS), SilcoNert 1000, glass and glass-coated SS) were used. The laboratory evaluations determined the storage stability, breakthrough volumes, suitable tube materials, recovery from ozone scrubbers and particulate filters, and sampling efficiency. In addition, an intercomparison between two laboratories was conducted. No multibed configurations were tested. Of the sorbent materials Tenax TA showed acceptable results for these BVOCs, while with Carbopack B losses and increases in some compounds were detected. Studied compounds were found to be stable in Tenax TA tubes for at least 1 month at - 20 and at + 20 ∘ C. Breakthrough tests indicated that α - and β -pinene have clearly lower breakthrough volumes in the Tenax TA tubes used (4–7 and 8–26 L, respectively) than other terpenes (> 160 L). SS, SilcoNert 1000 and glass were all shown to be suitable tube materials. Results from Tenax TA sorbent tube sampling agreed with online sampling for most compounds. Heated SS tubes, sodium thiosulfate filters and KI/Cu traps were found to be suitable ozone scrubbers for the studied BVOCs. Tested particle filters had a greater impact on limonene (relative difference < + 7 %) than on α - and β -pinene (relative difference ±2 %). The laboratory intercomparison of α - and β -pinene measurements showed that in general, measured values by the two laboratories were in good agreement with Tenax TA. [ABSTRACT FROM AUTHOR]

Production and use of many synthetic halogenated trace gases are regulated internationally due to their contribution to stratospheric ozone depletion or climate change. In many applications they have been replaced by shorter-lived compounds, which have become measurable in the atmosphere as emissions increased. Non-target monitoring of trace gases rather than targeted measurements of well-known substances is needed to keep up with such changes in the atmospheric composition. We regularly deploy gas chromatography (GC) coupled to time-of-flight mass spectrometry (TOF-MS) for analysis of flask air samples and in situ measurements at the Taunus Observatory, a site in central Germany. TOF-MS acquires data over a continuous mass range that enables a retrospective analysis of the dataset, which can be considered a type of digital air archive. This archive can be used if new substances come into use and their mass spectrometric fingerprint is identified. However, quantifying new replacement halocarbons can be challenging, as mole fractions are generally low, requiring high measurement precision and low detection limits. In addition, calibration can be demanding, as calibration gases may not contain sufficiently high amounts of newly measured substances or the amounts in the calibration gas may have not been quantified. This paper presents an indirect data evaluation approach for TOF-MS data, where the calibration is linked to another compound which could be quantified in the calibration gas. We also present an approach to evaluate the quality of the indirect calibration method, select periods of stable instrument performance and determine well suited reference compounds. The method is applied to three short-lived synthetic halocarbons: HFO-1234yf, HFO-1234ze(E), and HCFO-1233zd(E). They represent replacements for longer-lived hydrofluorocarbons (HFCs) and exhibit increasing mole fractions in the atmosphere. The indirectly calibrated results are compared to directly calibrated measurements using data from TOF-MS canister sample analysis and TOF-MS in situ measurements, which are available for some periods of our dataset. The application of the indirect calibration method on several test cases can result in uncertainties of around 6 % to 11 %. For hydro(chloro-)fluoroolefines (denoted H(C)FOs), uncertainties up to 23 % are achieved. The indirectly calculated mole fractions of the investigated H(C)FOs at Taunus Observatory range between measured mole fractions at urban Dübendorf and Jungfraujoch stations in Switzerland. [ABSTRACT FROM AUTHOR]

We present direct measurements of the summertime total reactivity of NO3 towards organic trace gases, k NO3OTM, at a rural mountain site (988ma.s.l.) in southern Germany in 2017. The diel cycle of k NO3OTM was strongly influenced by local meteorology with high reactivity observed during the day (values of up to 0.3 s-1) and values close to the detection limit (0.005 s-1) at night when the measurement site was in the residual layer and free troposphere. Daytime values of k NO3OTM were sufficiently large that the loss of NO3 due to reaction with organic trace gases competed with its photolysis and reaction with NO. Within experimental uncertainty, monoterpenes and isoprene accounted for all of the measured NO3 reactivity. Averaged over the daylight hours, more than 25%of NO3 was removed via reaction with biogenic volatile organic compounds (BVOCs), implying a significant daytime loss of NOx and the formation of organic nitrates due to NO3 chemistry. Ambient NO3 concentrations were measured on one night and were comparable to those derived from a stationary-state calculation using measured values of k NO3OTM. We present and compare the first simultaneous, direct reactivity measurements for the NO3 and OH radicals. The decoupling of the measurement site from ground-level emissions resulted in lower reactivity at night for both radicals, though the correlation between OH and NO3 reactivity was weak as would be anticipated given their divergent trends in rate constants with many organic trace gases. [ABSTRACT FROM AUTHOR]

An operational chemical ionization mass spectrometer (CIMS) for hydroxyl radical (OH) and sulfuric acid (H2SO4) concentration measurements was adapted to include observations of OH reactivity, which is the inverse of OH lifetime, for long-term monitoring at the Global Atmosphere Watch (GAW) site Hohenpeissenberg (MOHp), Germany. OH measurement using CIMS is achieved by reacting OH with SO2, leading to the production of H2SO4, which is then detected. The adaptation for OH reactivity consists of the implementation of a second SO2 injection, at a fixed point further down flow in the sample tube to detect the OH decay caused by reactions with OH reactants present in the sample. The method can measure OH reactivity from less than 1 to 40 s1 with the upper limit due to the fixed positioning of the second SO2 injection. To determine OH reactivity from OH concentration measurements, the reaction time between the two titration zones and OH wall losses in the sample tube need to be determined accurately through OH reactivity calibration. Potential measurement artefacts as a result of HOx recycling in the presence of NO have to be considered. Therefore, NO contamination from gases used in instrument operation must be minimized and ambient NO must be measured concurrently to determine the measurement error. This CIMS system is shown here to perform very well for OH reactivity below 15 s1 and NO concentrations below 4 ppb, both values that are rarely exceeded at the MOHp site. Thus when deployed in suitable chemical environments, this method can provide valuable continuous long-term measurements of OH reactivity. The characterization utilizes results from chamber, laboratory and modelling studies and includes the discussion and quantification of sources of uncertainties. [ABSTRACT FROM AUTHOR]

Air quality observations are performed globally to monitor the status of the atmosphere and its level of pollution and to assess mitigation strategies. Regulations of air quality monitoring programmes in various countries demand high-precision measurements for harmful substances often at low trace concentrations. These requirements can only be achieved by using high-quality calibration gases including high-purity zero gas. For volatile organic compound (VOC) observations, zero gas is defined as being hydrocarbon-free and can be, for example, purified air, nitrogen or helium. It is essential for the characterisation of the measurement devices and procedures, for instrument operation as well as for calibrations. Two commercial and one self-built gas purifiers were tested for their VOC removal efficiency following a standardised procedure. The tested gas purifiers included one adsorption cartridge with an inorganic media and two types of metal catalysts. A large range of VOCs were investigated, including the most abundant species typically measured at air monitoring stations. Both catalysts were able to remove a large range of VOCs whilst the tested adsorption cartridge was not suitable to remove light compounds up to C4. Memory effects occurred for the adsorption cartridge when exposed to higher concentration. This study emphasises the importance of explicitly examining a gas purifier for its intended application before applying it in the field. [ABSTRACT FROM AUTHOR]