Updating the calculation of ammonium particle formation in the Operational Priority Substances (OPS) source-receptor model

In this report we describe the methodology for calculating a new parameterization associated with ammonium particle formation in the Operational Priority Substances (OPS) model. For this purpose, a chemical box model was employed including using the latest reaction and photochemical data
applied in the CB05 chemical mechanism, with particle formation being calculated with the ISOROPPIA II module. By deriving a dependence of the
hourly conversion rate of gaseous NH3 (KNH3) into NH4+ particles in %/hour on the concentration ratios of SO2/NO2 and NH3/NO2 over a range of atmospheric conditions, we have derived three different parameterizations representative of
the night-time, morning/early evening and day-time conversion rates. We find that the rate of particle formation has strong diurnal cycle, as seen in
observations, differing by an order of magnitude between the night-time and day-time due to an associated change in the main chemical precursors. When applying the various parameterizations in OPS we find that, although the introduction of a diurnal dependency in KNH3 causes OPS to exhibit a significant low bias compared to the annually integrated measurements, adopting the new day-time KNH3 parameterization results in an improvement of the performance of OPS when compared to multi-year surface measurements.