Sensitivity to grid resolution in the ability of a chemical transport model to simulate observed oxidant chemistry under high-isoprene conditions

Abstract

Abstract. Formation of ozone and organic aerosol in continental atmospheres depends on whether isoprene emitted by vegetation is oxidized by the high-NO x pathway (where peroxy radicals react with NO) or by low-NO x pathways (where peroxy radicals react by alternate channels, mostly with HO 2 ). We used mixed layer observations from the SEAC 4 RS aircraft campaign over the Southeast US to test the ability of the GEOS-Chem chemical transport model at different grid resolutions (0.25to simulate this chemistry under high-isoprene, variable-NO x conditions. Observations of isoprene and NO x over the Southeast US show a negative correlation, reflecting the spatial segregation of emissions; this negative correlation is captured in the model at 0.25 • × 0.3125 • resolution but not at coarser resolutions. As a result, less isoprene oxidation takes place by the high-NO x pathway in the model at 0.25 • × 0.3125 • resolution (54 %) than at coarser resolution (59 %). The cumulative probability distribution functions (CDFs) of NO x , isoprene, and ozone concentrations show little difference across model resolutions and good agreement with observations, while formaldehyde is overestimated at coarse resolution because excessive isoprene oxidation takes place by the high-NO x pathway with high formaldehyde yield. The good agreement of simulated and observed concentration variances implies that smaller-scale non-linearities (urban and power plant plumes) are not important on the regional scale. Correlations of simulated vs. observed concentrations do not improve with grid resolution because finer modes of variability are intrinsically more difficult to capture. Higher model resolution leads to decreased conversion of NO x to organic nitrates and increased conversion to nitric acid, with total reactive nitrogen oxides (NO y ) changing little across model resolutions. Model concentrations in the lower free troposphere are also insensitive to grid resolution. The overall low sensitivity of modeled concentrations to grid resolution implies that coarse resolution is adequate when modelPublished by Copernicus Publications on behalf of the European Geosciences Union. K. Yu et al.: Sensitivity to grid resolution under high-isoprene conditionsing continental boundary layer chemistry for global applications.

High-resolution inversion of methane emissions in the Southeast US using SEAC<sup>4</sup>RS aircraft observations of atmospheric methane: anthropogenic and wetland sources

“…We use a nested version of GEOS-Chem as described by Kim et al (2015) with 0.25 • × 0.3125 • horizontal resolution over the North America window and adjacent oceans (9.75-60 • N, 130-60 • W), driven by GEOS-FP assimilated meteorological data from the NASA Global Modeling and Assimilation Office (GMAO). The same version of the GEOS-Chem has been applied to simulation of other chemical observations from the SEAC 4 RS campaign (Kim et al, 2015;Fisher et al, 2016;Marais et al, 2016;Travis et al, 2016;Zhu et al, 2016;Yu et al, 2016;Chan Miller et al, 2017). The boundary conditions for the nested-grid simulation are from a 4 • × 5 • global simulation by using methane emissions optimized with three years of GOSAT satellite data.…”

Glyoxal yield from isoprene oxidation and relation to formaldehyde: chemical mechanism, constraints from SENEX aircraft observations, and interpretation of OMI satellite data

“…Yu et al (2016) pointed out that isoprene and NO x emissions in the southeast US are spatially segregated and show that the 0.25 • × 0.3125 • resolution of GEOS-Chem is adequate for separating the populations of high-and low-NO x conditions for isoprene oxidation. Figure 1 shows the CHOCHO formation pathways from isoprene oxidation by OH (the main isoprene sink) as implemented in this work.…”

Surface ozone and its precursors at Summit, Greenland: comparison between observations and model simulations

“…First, the coarse model resolution (e.g., 4 × 5 horizontal resolution) could artificially smear the intense emission sources throughout the entire grid cell (e.g., over urban regions), leading to underestimates of downwind concentrations for species like O 3 and O 3 precursors (Jang et al, 1995;Yu et al, 2016). Second, ventilation in the lower atmosphere could be better resolved by a finer model resolution, leading to more efficient vertical advection (Wang et al, 2004;Chen et al, 2009;Yu et al, 2016). However, on average, the monthly mean model-simulated PAN mixing ratios were still underestimated by 20 % during this period compared with observations.…”