Modelling the Multiphase Chemistry of Biomass Burning Compounds with CAPRAM-BBM1.0

Biomass burning (BB) is an increasingly important contributor to air pollution on global, regional and local scales affecting air quality, public health and climate. Anhydrosugars (e.g., levoglucosan) and methoxyphenols (guaiacol, vanillin, etc.) are key tracer compounds emitted through biomass burning. Once emitted, they can undergo complex multiphase chemical processing in tropospheric aerosol particles and fog/cloud droplets. Their multiphase chemistry contributes to the formation and modification of the secondary organic aerosol (SOA) composition. However, the chemical multiphase processing of levoglucosan and methoxyphenols is not yet well understood and investigated by atmospheric chemistry models. A detailed multiphase oxidation mechanism has not been developed so far.
The present work aimed at a better understanding of the multiphase chemistry of BB tracers, such as levoglucosan and vanillin, by detailed process model studies with a new developed CAPRAM biomass burning module (CAPRAM-BBM1.0). This module was developed based on the kinetic data from measurements in our lab at TROPOS-ACD [1,2] and other literature studies as well as evaluated estimation methods [3]. CAPRAM-BBM1.0 includes 2881 processes (10 phase transfers and 2871 aqueous-phase reactions) and was coupled with the multiphase chemistry mechanism MCMv3.3.1/CAPRAM4.0 [3,4] and the extended CAPRAM aromatics module (CAPRAM-AM1.0) [5,6]. Afterwards, CAPRAM-BBM1.0 was applied in a multiphase chemistry process model for a winter/spring residential wood burning scenario in Europe [7,8].
The model results show that levoglucosan and vanillin are effectively oxidized under cloud conditions leading to concentration reductions of 75%/40% and 97%/94% after the third model day under spring/winter conditions. The chemistry of BB tracers contributes to the formation of BB-SOA and affects also the aqueous-phase budgets of key radical oxidants such as OH and NO3. Aqueous-phase oxidation of BB compounds contributes significantly to the aqSOA formation and aging. For example, a 38% higher organic mass is modelled for the spring case when CAPRAM-BBM1.0 is coupled to the core mechanism. Particularly, the formation of functionalized mono- and dicarboxylic acids is enhanced by a factor of 6.5 and 1.2 in the spring cases when chemistry of BB tracers is considered. Detailed chemical rate analyses show that the daytime oxidation by OH acts as the most important sink for BB tracers. Furthermore, the simulations reveal that in-cloud oxidations represent the main loss for methoxyphenols but their importance strongly depends on the respective Henry’s Law solubilities of the phenolic compounds. All in all, the present studies illustrated the potential role of the chemistry of BB compounds for the formation and processing of SOA.

[1] Hoffmann, D. et al. (2010), Environmental Science & Technology, 44(2), 694-699.,
[2] He, L. et al. (2019), The Journal of Physical Chemistry A, 123(36), 7828-7838.
[3] Bräuer P. et al. (2019), Atmospheric Chemistry and Physics, 19, 9209–9239.
[4] MCM, http://mcm.york.ac.uk/home.htt.
[5] Hoffmann, E. H. et al. (2018), Physical Chemistry Chemical Physics, 20(16), 10960-10977.
[6] Hoffmann, E. H. et al. (2019), ACS Earth and Space Chemistry, 3, 2452–2471.
[7] Poulain, L. et al. (2011), Atmospheric Chemistry and Physics, 11(24), 12697-12713.
[8] Wolke, R. et al. (2005), Atmospheric Environment, 39(23), 4375-4388