Assessment of variability in urban HONO using MAX-DOAS measurements in Central London

In polluted cities with large sources of NO_x, rapid photolysis of nitrous acid (HONO) may be a major daytime source of the main atmospheric oxidant, OH. Current understanding of urban HONO is problematic. Its abundance is generally underestimated by models, and urban and rural networks. Campaigns with in situ instruments routinely identify a mystery midday HONO source. These measurements do not directly measure HONO and offer no insight into its vertical distribution. We use remotely sensed daytime differential slant column density (dSCD) measurements of HONO from a Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument which was installed on a 60 m rooftop during summer 2022 on the Bloomsbury University College London (UCL) campus. Modelled (GEOS-Chem) and surface network measurements of air quality and meteorology are used to characterise conditions conducive to HONO detection. To be detected, dSCDs must exceed the detection limit, which we define as 2 times the root mean square of the fit of residuals divided by the maximum absorption cross section of HONO. We find that MAX-DOAS HONO dSCDs at elevation angles measuring the lowest layers of the atmosphere are only ever above the instrument detection limit in winter mornings (8:30 am–12:00 pm local time), suggesting substantial nighttime accumulation of HONO. This early morning HONO decreases rapidly (within 3-4 hours) to below detection after sunrise. Consistent characteristics of these mornings include cloud-free, cold (< 0°C) and calm conditions (wind speeds < 2.5 m s^-1), a shallow boundary layer (< 100 m), substantial surface ozone depletion (< 10 µg m^-3), and relatively large contribution of NO to total NO_x (NO/NO_x mass ratio ³ 0.3). Work is underway to further characterise urban HONO using 190 m measurements of NO_x from the Central London BT Tower observatory and assess the reaction kinetics balancing HONO loss and formation. New knowledge of HONO gained from MAX-DOAS measurements will then be used to evaluate best understanding of urban HONO as simulated with the GEOS-Chem model.