Abstract
AbstractWe implement a new isoprene oxidation mechanism in a global 3‐D chemical transport model (GEOS‐Chem). Model results are evaluated with observations for ozone, isoprene oxidation products, and related species from the International Consortium for Atmospheric Research on Transport and Transformation aircraft campaign over the eastern United States in summer 2004. The model achieves an unbiased simulation of ozone in the boundary layer and the free troposphere, reflecting canceling effects from recent model updates for isoprene chemistry, bromine chemistry, and HO2 loss to aerosols. Simulation of the ozone‐CO correlation is improved relative to previous versions of the model, and this is attributed to a lower and reversible yield of isoprene nitrates, increasing the ozone production efficiency per unit of nitrogen oxides (NOx ≡ NO + NO2). The model successfully reproduces the observed concentrations of organic nitrates (∑ANs) and their correlations with HCHO and ozone. ∑ANs in the model is principally composed of secondary isoprene nitrates, including a major contribution from nighttime isoprene oxidation. The correlations of ∑ANs with HCHO and ozone then provide sensitive tests of isoprene chemistry and argue in particular against a fast isomerization channel for isoprene peroxy radicals. ∑ANs can provide an important reservoir for exporting NOx from the U.S. boundary layer. We find that the dependence of surface ozone on isoprene emission is positive throughout the U.S., even if NOx emissions are reduced by a factor of 4. Previous models showed negative dependences that we attribute to erroneous titration of OH by isoprene.
Highlights
[3] Daytime oxidation of isoprene is initialized by its reaction with OH, leading to the production of peroxy radicals (ISOPO2)
Several mechanisms have been proposed to explain this discrepancy, including OH regeneration via oxidation of epoxydiols formed from the oxidation of ISOPOOH based on laboratory studies [Paulot et al, 2009b] and fast isomerization of ISOPO2 based on theoretical studies [Peeters and Müller, 2010; Peeters et al, 2009]
Hudman et al [2007] found that the lightning NOx source inferred from the ICARTT data was much larger than expected, and Hudman et al [2008] found the need for a 60% reduction in CO emissions relative to the National Emission Inventory (NEI 99) from the U.S Environmental Protection Agency (EPA)
Summary
[2] Isoprene (2-methyl-1,3-butadiene), the most important nonmethane volatile organic compound (NMVOC) emitted. 3. Simulation of Ozone and Related Species [21] Here we use the ICARTT observations to evaluate our new isoprene oxidation mechanism implemented in GEOS-Chem. Several objective improvements have been made to GEOS-Chem that have had significant consequences for the ozone simulation (http:// www.geos-chem.org): (1) decrease in isoprene nitrate yield (from 18% to 11.7%) and partial recycling of NOx (section 2.2), (2) inclusion of tropospheric bromine chemistry [Parrella et al, 2012], (3) heterogeneous loss of HOx radicals [Mao et al, 2013], and (4) correction of the diurnal cycle of NOx emissions by shifting emissions peaks by 6 h to reflect the proper timing of local transportation. A large fraction of secondary nitrates in our model, such as ETHLN and MACRN (Figure 3), can degrade in a matter of hours returning NOx, with little contribution to ∑ANs
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