Influence of the choice of gas-phase mechanism on predictions of key gaseous pollutants during the AQMEII phase-2 intercomparison

Abstract

h i g h l i g h t sWe intercompared tropospheric gas-phase mechanisms used in AQMEII phase 2. Box model results show O 3 differs by 4 ppbv (5%), NO x 25%, isoprene >100% and HCHO 20%. Key radicals OH and HO 2 differ 40 / 25% between mechanisms, NO 3 by more than 100%. Uncertainty due to gas-phase mechanism choice has to be considered in model simulations. a b s t r a c tThe formulations of tropospheric gas-phase chemistry ("mechanisms") used in the regional-scale chemistry-transport models participating in the Air Quality Modelling Evaluation International Initiative (AQMEII) Phase 2 are intercompared by the means of box model studies. Simulations were conducted under idealized meteorological conditions, and the results are representative of mean boundary layer concentrations. Three sets of meteorological conditions e winter, spring/autumn and summer e were used to capture the annual variability, similar to the 3-D model simulations in AQMEII Phase 2. We also employed the same emissions input data used in the 3-D model intercomparison, and sample from these datasets employing different strategies to evaluate mechanism performance under a realistic range of pollution conditions. Box model simulations using the different mechanisms are conducted with tight constraints on all relevant processes and boundary conditions (photolysis, temperature, entrainment, etc.) to ensure that differences in predicted concentrations of pollutants can be attributed to differences in the formulation of gas-phase chemistry. The results are then compared with each other (but not to measurements), leading to an understanding of mechanism-specific biases compared to the multi-model mean. Our results allow us to quantify the uncertainty in predictions of a given compound in the 3-D simulations introduced by the choice of gas-phase mechanisms, to determine mechanism-specific biases under certain pollution conditions, and to identify (or rule out) the gas-phase mechanism as the cause of an observed discrepancy in 3-D model predictions.Atmospheric Environment 115 (2015) 553e568 We find that the predictions of the median diurnal cycle of O 3 over a set of emission conditions representing a network of station observations is within 4 ppbv (5%) across the different mechanisms. This variability is found to be very similar on both continents. There are considerably larger differences in predicted concentrations of NO x (up to ± 25%), key radicals like OH (40%), HO 2 (25%) and especially NO 3 (>100%). Secondary substances like H 2 O 2 (25%) or HNO 3 (10%), as well as key volatile organic compounds like isoprene (>100%) or CH 2 O (20%) differ substantially as well. Calculation of an indicator of the chemical regime leads to up to 20% of simulations being classified differently by different mechanism, which would lead to different predictions of the most efficient emission reduction strategies.All these differences are despite identical meteorological boundary conditions, photolysis rates, as well as identical biogenic and inorganic anthropogenic emi...

Air quality modelling in the summer over the eastern Mediterranean using WRF-Chem: chemistry and aerosol mechanism intercomparison

“…This indicates considerable uncertainty in the representation of this important part of tropospheric chemistry that also affects aerosol formation. These results are supported by the findings of Knote et al (2014) that also report 3 times higher pan-European averaged NO 3 in the RADM2 mechanism compared to CBMZ in the middle of the night-time chemistry cycle. …”

“…A decrease of the order of 19 % in PM 2.5 concentrations, as a result of setting the dust inflow from the global model to zero, resulted in 10 % increase in O 3 concentrations. The differences in O 3 concentrations are attributed to the different treatment of VOCs as suggested by Knote et al (2014). Hourly correlation coefficients are low for all three mechanisms.…”

“…Since the CM mechanism shows the higher PM 2.5 concentrations and relatively high O 3 concentrations, we conclude that we can rule out the aerosol concentrations and therefore the different aerosol mechanisms, as the responsible factor for the differences in O 3 concentrations between the three simulations. Knote et al (2014) attributed the differences in pollutants concentrations between these mechanisms to the differences in the treatment of VOCs, since the rate constants for the basic O 3 production and loss reactions are similar. Hourly correlation coefficients for O 3 are low (less than 0.30) for all three mechanisms which is comparable to the findings of Mar et al (2016).…”

“…On the other hand, RADM2 showed a negative bias for the whole year, while both mechanisms slightly underestimated nitrogen oxide (NO x ) concentrations. Knote et al (2014) performed box-model intercomparison of several formulations for tropospheric gas-phase chemistry under idealized meteorological conditions in the framework of the second phase of AQMEII. They found significant variabilities in the prediction of gaseous pollutants and key radicals and they highlight that the choice of gas-phase mechanism is a crucial component in modelling studies.…”

Oxidation processes in the Eastern Mediterranean atmosphere: Evidence from the Modelling of HO<sub>x</sub> Measurements over Cyprus

“…Some acid forming reactions are also the starting point of particle formation in the atmosphere e.g. 20 via oxidation of SO2. In sum, oxidation is a means by which major pollutants are processed and removed from the atmosphere, but with concomitant implications to air quality, agriculture, climate and health.…”