ALBERT

Description: Surface ozone is a secondary air pollutant produced during the atmospheric photochemical degradation of emitted volatile organic compounds (VOCs) in the presence of sunlight and nitrogen oxides (NOx). Temperature directly influences ozone production through speeding up the rates of the chemical reactions and increasing the emissions of VOCs, such as isoprene, from vegetation. In this study, we used a box model to examine the non-linear relationship between ozone, NOx and temperature, and compared this to previous observational studies. Under high-NOx conditions, an increase in ozone from 20 to 40 °C of up to 20 ppbv was due to faster reaction rates while increased isoprene emissions added up to a further 11 ppbv of ozone. The increased oxidation rate of emitted VOC with temperature controlled the rate of Ox production, the net influence of peroxy nitrates increased net Ox production per molecule of emitted VOC oxidised. The rate of increase in ozone mixing ratios with temperature from our box model simulations was about half the rate of increase in ozone with temperature observed over central Europe or simulated by a regional chemistry transport model. Modifying the box model setup to approximate stagnant meteorological conditions increased the rate of increase of ozone with temperature as the accumulation of oxidants enhanced ozone production through the increased production of peroxy radicals from the secondary degradation of emitted VOCs. The box model simulations approximating stagnant conditions and the maximal ozone production chemical regime reproduced the 2 ppbv increase in ozone per °C from the observational and regional model data over central Europe. The simulated ozone-temperature relationship was more sensitive to mixing than the choice of chemical mechanism. Our analysis suggests that reductions in NOx emissions would be required to offset the additional ozone production due to an increase in temperature in the future.

Description: Ground-level ozone is a secondary pollutant produced photochemically from reactions of NOx with peroxy radicals produced during VOC degradation. Chemical transport models use simplified representations of this complex gas-phase chemistry to predict O3 levels and inform emission control strategies. Accurate representation of O3 production chemistry is vital for effective predictions. In this study, VOC degradation chemistry in simplified mechanisms is compared to that in the near-explicit MCM mechanism using a boxmodel and by "tagging" all organic degradation products over multi-day runs, thus calculating the Tagged Ozone Production Potential (TOPP) for a selection of VOC representative of urban airmasses. Simplified mechanisms that aggregate VOC degradation products instead of aggregating emitted VOC produce comparable amounts of O3 from VOC degradation to the MCM. First day TOPP values are similar across mechanisms for most VOC, with larger discrepancies arising over the course of the model run. Aromatic and unsaturated aliphatic VOC have largest inter-mechanisms differences on the first day, while alkanes show largest differences on the second day. Simplified mechanisms break down VOC into smaller sized degradation products on the first day faster than the MCM impacting the total amount of O3 produced on subsequent days due to secondary chemistry.

Description: Ground-level ozone is a secondary pollutant produced photochemically from reactions of NOx with peroxy radicals produced during volatile organic compound (VOC) degradation. Chemical transport models use simplified representations of this complex gas-phase chemistry to predict O3 levels and inform emission control strategies. Accurate representation of O3 production chemistry is vital for effective prediction. In this study, VOC degradation chemistry in simplified mechanisms is compared to that in the near-explicit Master Chemical Mechanism (MCM) using a box model and by "tagging" all organic degradation products over multi-day runs, thus calculating the tagged ozone production potential (TOPP) for a selection of VOCs representative of urban air masses. Simplified mechanisms that aggregate VOC degradation products instead of aggregating emitted VOCs produce comparable amounts of O3 from VOC degradation to the MCM. First-day TOPP values are similar across mechanisms for most VOCs, with larger discrepancies arising over the course of the model run. Aromatic and unsaturated aliphatic VOCs have the largest inter-mechanism differences on the first day, while alkanes show largest differences on the second day. Simplified mechanisms break VOCs down into smaller-sized degradation products on the first day faster than the MCM, impacting the total amount of O3 produced on subsequent days due to secondary chemistry.