NitroMAC: An instrument for the measurement of HONO and intercomparison with a long-path absorption photometer

A boron dipyrromethene (BODIPY) based probe for selective passive sampling of atmospheric nitrous acid (HONO) indoors

People spend up to 90% of their time indoors, and yet our understanding of indoor air quality and the chemical processes driving it are poorly understood, despite levels of key pollutants typically being higher indoors compared to outdoors. Nitrous acid (HONO) is a species that drives these indoor chemical processes, with potentially detrimental health effects. In this work, a BODIPY-based probe was synthesized with the aim of developing the first selective passive sampler for atmospheric HONO. Our probe and its products are easily detected by UV-Vis spectroscopy with molar extinct coefficients of 37 863 and 33 787 M^-1 cm^-1, respectively, and a detection limit of 14.8 ng mL^-1. When protonated, the probe fluoresces with a quantum yield of 33%, which is turned off upon reaction. The synthesized BODIPY probe was characterized using NMR and UV-Vis spectroscopy. Products were characterized by UV-Vis and ultra high-resolution mass spectrometry. The reaction kinetics of the probe with nitrite was studied using UV-Vis spectroscopy, which had a pseudo-first-order rate of k = 7.7 × 10^-4 s^-1. The rapid reaction makes this probe suitable for targeted ambient sampling of HONO. This was investigated through a proof-of-concept experiment with gaseous HONO produced by a custom high-purity calibration source delivering the sample to the BODIPY probe in an acidic aqueous solution in clean air and a real indoor air matrix. The probe showed quantitative uptake of HONO in both cases to form the same products observed from reaction with nitrite, with no indication of interferences from ambient NO or NO₂. The chemical and physical characteristics of the probe therefore make it ideal for use in passive samplers for selective sampling of HONO from the atmosphere.

Attenuation Coefficients of Tubular Conduits for Liquid Phase Absorbance Measurement: Shot Noise Limited Optimum Path Length

We trace the history of liquid core waveguides (LCWs, also called liquid core optical fibers) and the role Teflon AF (TAF) has played in their development. We show that, in any shot noise limited situation, the optimum signal-to-noise ratio (S/N) occurs at a path length of 1/α_a{ln[1 + 2(α_a/α_b)]}, approximately 2/α_b under most conditions, α_a and α_b being the light attenuation coefficient due to the analyte and the background, respectively. The analysis shows that LCW length should be selected depending on the applicable α_b value. An overly long LCW may exhibit a lower signal-to-noise ratio. Water-filled TAF-clad fused-silica (FS) tubes show the lowest attenuation across the wavelength range. Nevertheless, except at λ ≥ 600 nm, the observed α_b values far exceed those reported for pure water: it appears that both impurities in the water and waveguide losses are involved. In examining the attenuation in various water-filled tubes, we find that the transmission of air-surrounded FS tubes is second only to TAF-clad FS tubes and is better than that of TAF tubes or externally mirrored FS tubes. Surprisingly, except for a window centered at ∼250 nm, light transmission in a water-filled poly(tetrafluoroethylene) (PTFE) tube is worse than in poly(ether ether ketone) (PEEK) tubing. Light transmission in PTFE tubes improves with increasing wall thickness.

Cloud Processing of Secondary Organic Aerosol from Isoprene and Methacrolein Photooxidation

Aerosol-cloud interaction contributes to the largest uncertainties in the estimation and interpretation of the Earth's changing energy budget. The present study explores experimentally the impacts of water condensation-evaporation events, mimicking processes occurring in atmospheric clouds, on the molecular composition of secondary organic aerosol (SOA) from the photooxidation of methacrolein. A range of on- and off-line mass spectrometry techniques were used to obtain a detailed chemical characterization of SOA formed in control experiments in dry conditions, in triphasic experiments simulating gas-particle-cloud droplet interactions (starting from dry conditions and from 60% relative humidity (RH)), and in bulk aqueous-phase experiments. We observed that cloud events trigger fast SOA formation accompanied by evaporative losses. These evaporative losses decreased SOA concentration in the simulation chamber by 25-32% upon RH increase, while aqueous SOA was found to be metastable and slowly evaporated after cloud dissipation. In the simulation chamber, SOA composition measured with a high-resolution time-of-flight aerosol mass spectrometer, did not change during cloud events compared with high RH conditions (RH > 80%). In all experiments, off-line mass spectrometry techniques emphasize the critical role of 2-methylglyceric acid as a major product of isoprene chemistry, as an important contributor to the total SOA mass (15-20%) and as a key building block of oligomers found in the particulate phase. Interestingly, the comparison between the series of oligomers obtained from experiments performed under different conditions show a markedly different reactivity. In particular, long reaction times at high RH seem to create the conditions for aqueous-phase processing to occur in a more efficient manner than during two relatively short cloud events.

Validation of in situ Measurements of Atmospheric Nitrous Acid Using Incoherent Broadband Cavity-enhanced Absorption Spectroscopy

Incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) is a useful technique for measuring trace gaseous species in the atmosphere. Recently, IBBCEAS was used to measure concentrations of nitrous acid (HONO) in the troposphere to resolve controversies related to its formation and loss. Here, measurements of HONO and a mixture of HONO and NO₂ using IBBCEAS were validated by comparing them with those obtained with a NOx analyzer. Good agreement was found between these methods, given their respective experimental uncertainties. The detection limit of our IBBCEAS instrument was 0.2 ppbv, with a signal-to-noise ratio of 1, and a 5-min integration time.

Identification of the major HOx radical pathways in an indoor air environment

OH and HO₂ profiles measured in a real environment have been compared to the results of the INCA-Indoor model to improve our understanding of indoor chemistry. Significant levels of both radicals have been measured and their profiles display similar diurnal behavior, reaching peak concentrations during direct sunlight (up to 1.6×10⁶ and 4.0×10⁷  cm^-3 for OH and HO₂ , respectively). Concentrations of O₃ , NO_x , volatile organic compounds (VOCs), HONO, and photolysis frequencies were constrained to the observed values. The HO_x profiles are well simulated in terms of variation for both species (Pearson's coefficients: p_OH =0.55, p_HO2 =0.76) and concentration for OH (mean normalized bias error: MNBE_OH =-30%), HO₂ concentration being always underestimated (MNBE_HO2 =-62%). Production and loss pathways analysis confirmed HONO photolysis role as an OH precursor (here up to 50% of the production rate). HO₂ formation is linked to OH-initiated VOC oxidation. A sensitivity analysis was conducted by varying HONO, VOCs, and NO concentrations. OH, HO₂ , and formaldehyde concentrations increase with HONO concentrations; OH and formaldehyde concentrations are weakly dependent on NO, whereas HO₂ concentrations are strongly reduced with increasing NO. Increasing VOC concentrations decreases OH by consumption and enhances HO₂ and formaldehyde.

Assessment of indoor HONO formation mechanisms based on in situ measurements and modeling

The photolysis of HONO has been found to be the oxidation driver through OH formation in the indoor air measurement campaign SURFin, an extensive campaign carried out in July 2012 in a classroom in Marseille. In this study, the INCA-Indoor model is used to evaluate different HONO formation mechanisms that have been used previously in indoor air quality models. In order to avoid biases in the results due to the uncertainty in rate constants, those parameters were adjusted to fit one representative day of the SURFin campaign. Then, the mechanisms have been tested with the optimized parameters against other experiments carried out during the SURFin campaign. Based on the observations and these findings, we propose a new mechanism incorporating sorption of NO₂ onto surfaces with possible saturation of these surfaces. This mechanism is able to better reproduce the experimental profiles over a large range of conditions.