Evaluation of isoprene nitrate chemistry in detailed chemical mechanisms

Abstract

Abstract. Isoprene nitrates are important chemical species in the atmosphere which contribute to the chemical cycles that form ozone and secondary organic aerosol (SOA) with implications for climate and air quality. Accurate chemical mechanisms are important for the prediction of the atmospheric chemistry of species such as isoprene nitrates in chemical models. In recent years, studies into the chemistry of isoprene nitrates have resulted in the development of a range of mechanisms available for use in the simulation of atmospheric isoprene oxidation. This work uses a 0-D chemical box model to assess the ability of three chemically detailed mechanisms to predict the observed diurnal profiles of four groups of isoprene-derived nitrates in the summertime in the Chinese megacity of Beijing. An analysis of modelled C5H9NO5 isomers, including isoprene hydroperoxy nitrate (IPN) species, highlights the significant contribution of non-IPN species to the C5H9NO5 measurement, including the potentially large contribution of nitrooxy hydroxyepoxide (INHE). The changing isomer distribution of isoprene hydroxy nitrates (IHNs) derived from OH-initiated and NO3-initiated chemistry is discussed, as is the importance of up-to-date alkoxy radical chemistry for the accurate prediction of isoprene carbonyl nitrate (ICN) formation. All mechanisms under-predicted C4H7NO5 as predominately formed from the major isoprene oxidation products, methyl vinyl ketone (MVK) and methacrolein (MACR). This work explores the current capability of existing chemical mechanisms to accurately represent isoprene nitrate chemistry in urban areas significantly impacted by anthropogenic and biogenic chemical interactions. It suggests considerations to be taken when investigating isoprene nitrates in ambient scenarios, investigates the potential impact of varying isomer distributions on iodide chemical ionisation mass spectrometry (I−-CIMS) calibrations, and makes some proposals for the future development of isoprene mechanisms.

Daytime Isoprene Nitrates Under Changing NOx and O3

“…This requirement for very high O3 and low NOx means that we should expect very low concentrations of daytime INHE and INPE under typical urban conditions. This is consistent with modelling of Beijing which showed that while INHE may comprise a large fraction of night-time C5H9NO5 compounds, the daytime contribution is very small (Mayhew et al, 2022).…”

“…Model Validation As a test of the ability of the steady-state models to represent conditions present under ambient scenarios, the model results were compared to measurements collected in the summer of 2017 in Beijing. (Shi et al, 2019;Hamilton et al, 2021;Whalley et al, 2021;Reeves et al, 2021;Newland et al, 2021;Mayhew et al, 2022) The NOx mixing ratios measured in the afternoon periods in Beijing ranged between 5 ppb and 20 ppb, and O3 mixing ratios ranged from around 60 ppb to 140 ppb.…”

“…It is also important to note that the absolute concentrations of ICN predicted to form in these models are very low (the peaks in concentration corresponding to mixing ratios of just over 1 ppt), due to low production rates, which is consistent with low daytime ICN mixing ratios previously identified in Beijing. (Reeves et al, 2021;Mayhew et al, 2022) Recent work has highlighted species with the formula C4H7NO5 as potentially major oxidation products of isoprene. (Tsiligiannis et al, 2022) Consistent with previous modelling studies (Mayhew et al, 2022), these models largely form C4H7NO5 from OH-initiated oxidation, meaning the concentration isopleth is similar to that of OH and IHN.…”

“…(Reeves et al, 2021;Mayhew et al, 2022) Recent work has highlighted species with the formula C4H7NO5 as potentially major oxidation products of isoprene. (Tsiligiannis et al, 2022) Consistent with previous modelling studies (Mayhew et al, 2022), these models largely form C4H7NO5 from OH-initiated oxidation, meaning the concentration isopleth is similar to that of OH and IHN. In contrast, INCE shows reasonably high concentrations under high-O3 mixing ratios at a range of NOx mixing ratios.…”

The Important Contribution of Secondary Formation and Biomass Burning to Oxidized Organic Nitrogen (OON) in a Polluted Urban Area: Insights from In Situ FIGAERO-CIMS Measurements

“…The fraction of C4-5N group in gOON enhances slightly during daytime (Fig. S8e), which might be due to strong photochemical formation of isoprene-nitrates (Fisher et al, 2016;Reeves et al, 2020;Mayhew et al, 2022;Hamilton et al, 2021). The C8-10N fraction in total pOON slightly enhances during nighttime (Fig.…”