Dean S. Venables

Accurate measurement of atmospheric nitrate radicals (NO ₃ ) is crucial for atmospheric chemistry research; however, their high reactivity leads to significant losses in extractive sampling measurements. Increasing the flow rate and decreasing residence time should reduce losses, but some experiments have observed the opposite effect. To clarify the underlying causes for this behavior, we constructed an extractive NO ₃ measurement instrument based on incoherent broad-band cavity enhanced absorption spectroscopy (IBBCEAS). By varying the flow rate, we found that the measured NO ₃ concentration exhibited a non-monotonic variation with the sampling flow rate, initially increasing and then decreasing as flow rate increased. Computational fluid dynamics was used to simulate the flow within the system. The results showed that inertial impaction against the cavity wall opposite the inlet induced complex flow in the inlet zone, leading to additional losses at high flow rates. Based on this effect, we established a ‘dual-zone model’ (comprising a central flow zone and an inlet zone), quantified the competition mechanism between shortening residence time and flow mixing loss, and determined the optimal sampling flow rates for two pipe diameters (1/8″(≈i.d. 1.6 mm): ∼1.85 SLM; 1/4″(≈i.d. 4.35 mm): ∼1.92 SLM). This study provides an optimization for the sampling flow rate of the IBBCEAS extraction sampling system for highly reactive gases and shows that flow rate is an important consideration in minimizing losses and improving accuracy in the system.

Nitrogen dioxide (NO2) is a pollutant emitted primarily by vehicles and is strongly associated with large population centres. NO2 is therefore routinely monitored in cities, but its concentrations in small towns aren’t as well documented, even though some small towns may be traversed by roads with high traffic volumes. This work presents the application of a fast, low-cost cavity-enhanced absorption spectroscopy (CEAS) sensor monitoring of NO2 levels in three small towns in County Cork, Ireland: Macroom, Killeagh and Millstreet. The respective populations of these towns are approximately 4100, 900, and 1800. The spatial and temporal characteristics of NO2 in these towns, and the influence of highly polluting vehicles on local air quality are presented. The potential of the novel CEAS system for characterising NO2 pollution in small population centres is discussed.

Differential Optical Absorption Spectroscopy (DOAS) is a long-standing remote sensing technique used to measure many absorbing trace gases. To attain the necessary sensitivity to weak absorptions from trace gases, conventional DOAS systems normally have kilometre- long path lengths and large telescopes to maximise the collected light. Such systems are expensive and largely limited to research applications. The work describes a low – cost, long path DOAS system with a 1.5 km path length in Cork city, Ireland specifically to measure nitrogen dioxide (NO2) for air quality monitoring. The system consists of a temperature-stabilised 0.8 W blue LED (peak wavelength at 435 nm), 5 cm diameter transmitting and receiving telescopes and a compact spectrometer. A band pass filter (420 nm – 460 nm) is used to eliminate stray light. The system is much smaller than conventional DOAS configurations and is relatively straightforward to assemble. The outlook for using long path DOAS as an affordable approach to monitor the contribution to NO2 to urban air quality is discussed.

Ammonia (NH₃) is the most abundant alkaline gas in the atmosphere and plays a significant role in secondary aerosol formation and nitrogen cycling, influencing regional air quality and ecosystem nitrogen deposition. Agricultural activities are the dominant sources of atmospheric NH₃. However, accurate quantification of NH₃ emissions from animal enclosures remains challenging owing to the high humidity, elevated particulate loading, and wide range of NH₃ concentrations in these settings.In this study, we present an open-path deep-ultraviolet optical absorption spectroscopy system for in situ NH₃ measurements. The instrument measures absorption spectra from 199 to 213 nm, a region in which NH₃ exhibits intense and highly structured absorption features that enable selective and sensitive detection. The entire system is powered by a portable power bank bundle and can operate continuously for over 6 hours. Laboratory characterization experiments demonstrate a detection limit of 74 ppb at a 1 s integration time with an optical path length of 50 cm. Chamber experiments show that the instrument has an excellent dynamic range, exhibiting a robust and linear response to NH₃ from 0.1 to 100 ppm. We demonstrate the application of an early-prototype to monitoring and mapping NH3 in and around animal enclosures. The technique offers new opportunities for improving NH₃ monitoring in animal housing and supports the development of mitigation strategies for agricultural air quality management.

Nitrogen dioxide (NO2) is a major urban air pollutant, but current low-cost sensors for NO2 based on electrochemical cells have important drawbacks, including modest accuracy and susceptibility to temperature, humidity, and chemical interferences. These sensors are also too slow for mobile monitoring and for measuring the large and rapid fluctuations of NO2 in the transport microenvironment. These are important monitoring approaches and settings for NO2 because vehicles are the dominant source of NO2 in cities. Here we present our work in adapting cavity-enhanced absorption spectroscopy (CEAS) to develop fast (< 5 s), sensitive (±1 ppb), and portable sensors for NO2 at lower cost.We characterise sensor performance in laboratory intercomparisons, and present adaptations to different platforms (vehicles, bicycles, and backpacks). Case studies are presented of mobile and stationary monitoring of transport emissions in Cork city and in smaller towns in Ireland. These measurements show the disproportionate impact of a small number of highly polluting vehicles. A perspective on the challenges and prospects for this approach is discussed.

Naphthalene, a representative semi-volatile organic compound (SVOC), poses significant challenges for optical quantification due to surface adsorption and the difficulty of generating stable standards. To address these issues, a deep-ultraviolet Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy (IBBCEAS) system (mirror reflectivity >99.25%, 245–275 nm) was developed. The system achieved a detection limit of 1.3×10-7 cm-1 A dual-mode sampling strategy was employed to validate quantitative performance. First, steady-state saturation transpiration measurements yielded naphthalene absorption cross sections that agreed excellently with high-resolution literature data which validated the system. Second, an Integrated Absorption Spectroscopy (IAS) strategy was implemented to quantify transient pulse injections. The method demonstrated a strict linearity (R2≈0.99) between the flow integrated optical signal and injected mass. Crucially, the physical validity of the IAS framework was rigorously supported by varying the carrier gas flow rate: a slope ratio of ≈2.0 was observed when the flow rate was halved, consistent with the principle of mass conservation. This work demonstrates that the IBBCEAS-IAS framework effectively mitigates quantification errors caused by adsorption desorption hysteresis. Furthermore, the proven capability for measuring transient pulses highlights the method's potential for coupling with preconcentration desorption techniques (e.g., adsorbent tubes or cold traps) to accurately quantify the total concentration of sticky SVOCs.

This study presents a generalized, high-precision measurement system based on Integrated Absorption Spectroscopy (IAS) for determining gas-phase absorption cross sections of low-volatility organic compounds (LVOCs), particularly semi-volatile organic compounds (SVOCs) in the atmosphere. Accurate cross sections and their temperature dependence are essential for modeling atmospheric and high-temperature processes. We coupled a temperature-controlled inlet and cell (473 K) with a nitrogen carrier gas to measure the cross sections of 2-nitrophenol (2-NP) and naphthalene from 250 to 400 nm. At 473 K, peak cross sections for 2-NP were 2.31 × 10⁻¹⁷ cm²/molecule at 260 nm and 1.16 × 10⁻¹⁷ cm²/molecule at 335 nm. For naphthalene, values between 258 and 280 nm decreased from 1.62 × 10⁻¹⁷ to 1.28 × 10⁻¹⁷ cm²/molecule. Thermally induced spectral broadening and reduced peak cross sections align with thermodynamic theory. These high-temperature data resolve discrepancies among low-temperature datasets. For example, our maximum cross section for 2-NP (300–400 nm) is 29% lower than that reported by Chen et al. (293 K), whereas the value from Sangwan and Zhu (295 K) is 86.8% lower than Chen’s, supporting the higher reliability of Chen’s data. The IAS method thus offers a robust approach for quantifying absorption cross sections under atmospherically relevant conditions.