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Formation of ozone and organic aerosol in continental atmospheres depends onwhether isoprene emitted by vegetation is oxidized by the high-NOx pathway(where peroxy radicals react with NO) or by low-NOx pathways (where peroxyradicals react by alternate channels, mostly with HO2). We used mixedlayer observations from the SEAC4RS aircraft campaign over the SoutheastUS to test the ability of the GEOS-Chem chemical transport model at differentgrid resolutions (0.25°  ×  0.3125°,2°  ×  2.5°, 4°  ×  5°) tosimulate this chemistry under high-isoprene, variable-NOx conditions.Observations of isoprene and NOx over the Southeast US show a negativecorrelation, reflecting the spatial segregation of emissions; this negativecorrelation is captured in the model at0.25°  ×  0.3125° resolution but not at coarserresolutions. As a result, less isoprene oxidation takes place by thehigh-NOx pathway in the model at 0.25°  ×  0.3125°resolution (54 %) than at coarser resolution (59 %). The cumulativeprobability distribution functions (CDFs) of NOx, isoprene, and ozoneconcentrations show little difference across model resolutions and goodagreement with observations, while formaldehyde is overestimated at coarseresolution because excessive isoprene oxidation takes place by thehigh-NOx pathway with high formaldehyde yield. The good agreement ofsimulated and observed concentration variances implies that smaller-scalenon-linearities (urban and power plant plumes) are not important on theregional scale. Correlations of simulated vs. observed concentrations do notimprove with grid resolution because finer modes of variability areintrinsically more difficult to capture. Higher model resolution leads todecreased conversion of NOx to organic nitrates and increased conversionto nitric acid, with total reactive nitrogen oxides (NOy) changing littleacross model resolutions. Model concentrations in the lower free troposphereare also insensitive to grid resolution. The overall low sensitivity ofmodeled concentrations to grid resolution implies that coarse resolution isadequate when modeling continental boundary layer chemistry for globalapplications

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