PAN
Emily Fischer has updated the PAN simulation. The code has been merged with v9.02.h at this point, and a publication (Fischer et al., 2013) has been submitted to ACP.
Updates to Chemistry
1. I replaced the isoprene chemical mechanism with the Paulot scheme. This scheme has already been implemented into the standard code. Here are some bullets on the effect of the addition of nighttime chemistry:
- Nighttime reactions of NO3 increase PAN formation
- The difference in monthly mean simulated PAN is largest in regions with the largest PAN production (Eastern U.S. and East Asia)
- The addition of nighttime chemistry increases monthly mean surface PAN mixing ratios in these regions by ~50 pptv in spring and summer.
- This does propagate to higher altitudes, but the changes are very modest. Springtime PAN mixing ratios increase broadly over the Arctic by 5-10 pptv (~5%). The impact in summer is more heterogeneous at higher altitudes, but the overall increase is on the order of ~2% over most of the Arctic.
- These changes also DO impact ozone.
- This chemistry increases ozone over the Arctic during spring. Surface ozone increases by ~0.5 ppbv, and the impact is larger aloft (0.75 ppbv at 4 km)
2. I added nighttime chemistry from reactions of organic peroxy radicals with NO3 following Stone et al. (2013). This may not be incorporated into the standard chemistry, and needs to be discussed by the GEOS-Chem Steering Committee.
3. I updated the rate coefficients for the reactions of HO2 with the >C2 peroxy radicals to Equation (iv) in Saunders et al. (2003). These changes have been incorporated into the standard code.
4. I added several new NMVOCs. The extended mechanism includes ethanol, benzene, toluene and ethylbenzene (lumped), xylenes and trimethyl benzenes(lumped), and monoterpenes (lumped). Hydroxyacetone and methylglyoxal, which are species in past versions, are treated as tracers in this simulation. Hydroxyacetone has a 1-2 day lifetime. Methylglyoxal is treated as a tracer so that it can be emitted from biomass burning plumes and so I could track PAN production via this pathway.
- The ethanol code was provided by Dylan Millet.
- The inclusion and treatment of aromatics was motivated by Liu et al. (2010). I calculated the associated yield of methylglyoxal using recommended values for the individual aromatic species(toluene, o-xylene, m-xylene, p-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene) from Nishino et al. (2010) and the observed mean aromatic speciation for Chinese cities from Barletta et al. (2006). Thus the treatment is particular to the limited observations of aromatic speciation in China. It would be good to determine how different these ratios would be for the US, Europe or other developed/developing regions.
- We adopted the treatment of monoterpene oxidation from theRACM2 chemical mechanism (Goliff et al., 2013), lumping terpenes with one double bond (alpha-pinene, beta-pinene, sabinene and delta-3-carene) into one proxy. Unlike Ito et al. (2007), hydroxyacetone is not a product of terpene oxidation in the revised RACM2 mechanism used here. So I expect that the mechanism used here will make less PAN than that in Ito et al., 2007. This code was provided by Jingqiu Mao.
- I also added emissions of acetaldehyde from the ocean. This code was also provided by Dylan Millet.
5. I increased the deposition flux of PAN by assuming PAN reactivity with surfaces is more similar to O3 than NO2. (I changed F0 from 0.1 to 1.0 in drydep_mod.F. F0 is a ‘reactivity factor for oxidation of biological substances’)
- 0.1 is used for slightly reactive species (NO2, HNO2)
- 1 is used for highly reactive species (O3, H2O2, and recently OVOCs)
This change increases the global annual PAN deposition from 6.5 Tg/year to 10.7 Tg/year. This is a change of 0.47 Tg N/year. During the springtime PAN maximum, faster deposition decreased PAN at the surface by 15 – 20% (30 – 50 pptv) throughout the northern hemisphere. At higher altitudes, there is also a hemispheric impact, with PAN decreases of ~10% (20 – 40 pptv). In summer the surface decrease in PAN is most notable over the Arctic, but here it is on the order og 25%. At 4 km, the PAN decrease is about 10%, on the order of 5 – 10 pptv. These changes do propagate to ozone. The faster deposition causes a springtime decrease of ozone of almost a ppb at the surface and in the lower free troposphere. In summer, the changes to ozone are most pronounced for the Arctic, but still 0.5 ppbv over the eastern U.S. The limited evidence from the literature suggests faster PAN deposition than previously assumed. In our model this means f0=1 is probably better than f0=0.1, though a different deposition model would probably be ideal in the longer term.
