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.