Modification of a conventional photolytic converter for improving aircraft measurements of NO2_{2} via chemiluminescence

Nitrogen oxides (NO$_{x}$≡NO+NO$_{2}$) are centrally involved in the photochemical processes taking place in the Earth\u27s atmosphere. Measurements of NO$_{2}$, particularly in remote areas where concentrations are of the order of parts per trillion by volume (pptv), are still a challenge and subject to extensive research. In this study, we present NO$_{2}$ measurements via photolysis–chemiluminescence during the research aircraft campaign CAFE Africa (Chemistry of the Atmosphere – Field Experiment in Africa) 2018 around Cabo Verde and the results of laboratory experiments to characterize the photolytic converter used. We find the NO$_{2}$ reservoir species MPN (methyl peroxy nitrate) to produce the only relevant thermal interference in the converter under the operating conditions during CAFE Africa. We identify a memory effect within the conventional photolytic converter (type 1) associated with high NO concentrations and rapidly changing water vapor concentrations, accompanying changes in altitude during aircraft measurements, which is due to the porous structure of the converter material. As a result, NO$_{2}$ artifacts, which are amplified by low conversion efficiencies, and a varying instrumental background adversely affect the NO$_{2}$ measurements. We test and characterize an alternative photolytic converter (type 2) made from quartz glass, which improves the reliability of NO$_{2}$ measurements in laboratory and field studies

Review of Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy (IBBCEAS) for Gas Sensing

Incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) is of importance for gas detection in environmental monitoring. This review summarizes the unique properties, development and recent progress of the IBBCEAS technique. Principle of IBBCEAS for gas sensing is described, and the development of IBBCEAS from the perspective of system structure is elaborated, including light source, cavity and detection scheme. Performances of the reported IBBCEAS sensor system in laboratory and field measurements are reported. Potential applications of this technique are discussed

Measurement of Nitrous Acid Production from Aerosol Surfaces using Photo-fragmentation Laser Induced Fluorescence

The hydroxyl radical (OH) is the primary oxidant in the atmosphere with its concentration determining the lifetime of many species and its reaction with volatile organic compounds leading to the production of secondary organic aerosols and tropospheric ozone. It is therefore important that its sources are well understood. Nitrous acid (HONO) is an important source of OH, building up overnight and is photolysed to form OH in the morning. HONO is also present during the day at much lower concentrations. Models however are currently under predicting these daytime concentrations, indicating a missing source of HONO. With HONO being a dominant source of OH in polluted environments it is important that its concentration is accurately modelled in order to predict a correct OH production rate. In order to identify and study these missing sources and determine their atmospheric relevance, a photo-fragmentation laser induced fluorescence (PF-LIF) instrument has been built and coupled to an aerosol flow tube to provide a fast and sensitive measurement of HONO. Two aerosol types, previously proposed as possible HONO sources, have been investigated. Illuminated TiO2, was found to generate HONO in the presence of NO2 and the reactive uptake coefficient was calculated for a range of NO2 concentrations, peaking at 2.5×10-4 for 30 ppb NO2. Investigation of ammonium and sodium nitrate aerosols showed no measureable HONO production. Results from the TiO2 experiment were included in a box model for Beijing where, assuming all the observed aerosol surface area was pure TiO2, the HONO contribution from this aerosol source was modelled to be roughly 10% of the modelled concentration at midday. This model was not able to replicate the measured HONO data, however, indicating the need for other sources of HONO. As part of the atmospheric chemistry of amines project measurements of OH and HO2 were carried out at the EUPHORE chamber using LIF. The measured OH and OH calculated from the amine decay did not agree indicating either a possible interference from the amine oxidation products in the LIF system or in homogeneous air at the LIF and amine sampling ports. The measured HO2 in the dark was observed to have a lifetime exceeding 2 hours. This slow HO2 decay could not be reproduced by a kinetic model which predicted a HO2 lifetime of seconds suggesting an unknown a source of HO2 in the dark