The Influence of biogenic emissions on tropospheric ozone over Equatorial Africa during 2006

1. The Influence of biogenic emissions on tropospheric ozone over Equatorial Africa during 2006 J.E. Williams, R. Scheele, P.F.J. van Velthoven, J-P. Cammas, V. Thouret and C. Lacy-Galeux

2. Objectives of the study • A climatological biogenic emission inventory is available as part of the RETRO emission database – how is it different than previous inventories ?? • How does this modify the tropical troposphere in a global CTM compared to the most common inventories adopted by the modelling community?? • How influential is the emission of NO from soils towards the composition of the African troposphere in a CTM?? • What about the effect of the additional biogenic volatile organic compounds??. • How do biogenic emissions from Africa influence the oxidative capacity of the global troposphere??

4. Biogenic NO emissions Regions: (_) Saharan (40-20°N), (_) Sahel (10-20°N), (_) Guinea (0-10°N), and (_) southern Africa(40-0°S) Yienger and Levy (1995) vs Lathiére et al (2006)

5. Isoprene Emissions (incl monoterp) Regions: (_) Saharan (40-20°N), (_) Sahel (10-20°N), (_) Guinea (0-10°N), and (_) southern Africa (40-0°S) Guenther et al (1995) vs Lathiere et al (2006)

6. Volatile Organic Compounds Regions: (_) Saharan (40-20°N), (_) Sahel (10-20°N), (_) Guinea (0-10°N), and (_) southern Africa (40-0°S) Additional VOC’s : CH3COOH, HCHO and CH3CHO Different inventories: Acetone, C2H5OH

7. Percentage differences in annual global fluxes for Africa Fraction = LATH/POET Longitudinal differences not shown but emission regions do shift with season

8. Sensitivity studies • 34 layers,3° x 2°, ECMWF 6 hour re-analysis • Lightning NOx, biomass burning and anthropogenic emissions kept equal throughout. • Base run using POET inventory for global biogenics w/ RETRO Anthro/GFEDv2 (POET). • Sensitivity run using Lathiere et al (2006) biogenics w/ RETRO Anthro/GFEDv2 (LATH). • Sensitivity run: LATH but no SOIL NO in Africa (NOSOIL). • Sensitivity run: LATH but no BVOC,s in Africa (NOBIO). (CO,C2H4,C2H6,C3H8,C3H6,C2H5OH,CH3CHO, CH3COOH, acetone and HCHO) • 2D cross-section between 12-21E : 3hrly output of selected trace species averaged into seasonal means.

9. Chemical Mechanism • Modified CBM4 mechanism w/ latest reaction data (64 reactions, 16 photochemical reactions, 4 aqueous phase reactions) – lumped mechanism NO + HO2 NO2 + OH (60.9%) NO + CH3O2 HCHO + HO2 + NO2 (19.4%) NO + C2O3  HCHO + XO2 + HO2 + NO2 (3%) NO + XO2  NO2 (16.5%) NO + O3  NO2 (night time conversion) NO2 + hv  O3 OH + NO2  HNO3 NO2 + C2O3 ↔ PAN (main source Isoprene Ox. Via aldehydes) NO2 + XO2N ORGNTR (main source Isoprene Oxidation) Minor radical reservoirs : HNO4, N2O5

10. LATH-POET/LATH • [O3] shows regionalsimilarities with [NO]↑ • (c.f. DJF – s.Africa). • [Isoprene] signature when [NO]↑ is ~ equal. • (c.f. MAM – s.Africa) • In the absence of strong [NO]↑ less [O3] in LATH.

11. LATH-POET/LATH • [PAN] shows regional similarities with [Isoprene]↑ • (10S-10N). • Main source of CH3C(O)O2 • Low in Sahara. • Weak [PAN] on [NO] ↑ BB NOx + [Isoprene]

12. LATH-POET/LATH • [HNO3] shows regional similarities with [O3] • O3 + h (+H2O)  2OH • OH + NO2  HNO3 • 5% More N deposition as HNO3 adds to decreases in [O3] for the background

13. LATH-POET/LATH • [OH] shows regional similarities with [O3] • O3 + h (+H2O)  2OH • Weaker oxidation capacity in Lath POET LATH

14. LATH-NOSOIL/LATH • Soil NOx responsible for >40% [O3] near the source regions. • [O3] through the • entire column – not • limited to lower tropo.

15. LATH-NOSOIL/LATH [OH] shows regional similarities with [O3] O3 + h (+H2O)  2OH Background [OH] decreases - weaker oxidation capacity

16. LATH-NOSOIL/LATH [HNO3] shows regional similarities with [O3] O3 + h (+H2O)  2OH Double effect of less [NO2] and less [O3]

18. (_) POET, (_) LATH, (_) NOSOIL, (_) OBS

19. MOZAIC profiles from Windhoek (22.5°S, 17.5°E) • LATH agrees better with MOZAIC profiles • Largest diff • shown by NOSOIL run • Effect of soil NOx propagates up the column High Soil NOx in DJF (_) POET, (_) LATH, (_) NOSOIL, (_) OBS

20. MOZAIC in-flight data over Africa  Soil NOx High [O3] over biomass burning regions is not captured (_) POET, (_) LATH, (_) NOSOIL, (_) OBS

21. French and German Falcons + UK BAe146 No simulation captures the high [O3] in measurements Lack of variation in soil NO emission between measurements Low [O3] captured ! Analysis software Courtesy B Josse

22. TM4 generally over-estimates surface [O3] in remote regions by 50-100% Suggests over production near strong BB sources which is transported over measurement sites (_) POET, (_) LATH, (_) NOSOIL, (_) OBS

23. Both POET and LATH overpredict [NO2] For some stations soil NO emissions account for nearly 100% of the resident [NO2]. (_) POET, (_) LATH, (_) NOSOIL, (_) OBS

24. Generally too [HNO3] is too low for many months [O3] and [NO2] both too high. OH + NO2 HNO3 Why [HNO3] is low ?? Missing source or exaggerated sink?? (_) POET, (_) LATH, (_) NOSOIL, (_) OBS

25. Conclusions • The seasonality, distribution and annual fluxes for biogenic species are quite different for distinct regions of Africa between the two emission inventories. • TM4 becomes less chemically active when adopting the new biogenic inventory which increases the lifetime of dominant trace gases such as CO. • Soil NOx influences [O3] throughout the troposphere over Africa when the effect is integrated over time. • Comparisons with measurements suggest more monthly variability should be introduced into the Lathiére Climatology (weekly?). • Production of [O3] due to soil NO emission in Africa reduces the global atmospheric lifetime of CH4 and CO by 3% and 4%, respectively. To be submitted to AMMA ACP Special Issue