Modeling sensitivities of BVOCs to different versions of MEGAN emission schemes in WRF-Chem (v3.6) and its impacts over eastern China
Authors: Zhang Mingshuai , Zhao Chun , Yang Yuhan , Du Qiuyan , Shen Yonglin , Lin Shengfu , Gu Dasa , Su Wenjing , Liu Cheng 
Published: October 2021

Abstract. Biogenic volatile organic compounds (BVOCs) simulated by current air quality
and climate models still have large uncertainties, which can influence
atmospheric chemistry and secondary pollutant formation. These modeling
sensitivities are primarily due to two sources. One originates from
different treatments in the physical and chemical processes associated with
the emission rates of BVOCs. The other is errors in the specification of
vegetation types and their distribution over a specific region. In this
study, the version of the Weather Research and Forecasting model coupled with
Chemistry (WRF-Chem) updated by the University of Science and Technology of
China (USTC version of WRF-Chem) from the public WRF-Chem(v3.6) is used. The
modeling results over eastern China with different versions (v1.0, v2.0, v3.0)
of the Model of Emissions of Gases and Aerosols from Nature (MEGAN) in WRF-Chem
are examined or documented. Sensitivity experiments with these three
versions of MEGAN and two vegetation datasets are conducted to investigate
the difference of three MEGAN versions in modeling BVOCs and its dependence
on the vegetation distributions. The experiments are also conducted for
spring (April) and summer (July) to examine the seasonality of the modeling
results. The results indicate that MEGAN v3.0 simulates the largest amount of
biogenic isoprene emissions over eastern China. The different performance among
MEGAN versions is primarily due to their different treatments of applying
emission factors and vegetation types. In particular, the results highlight
the importance of considering the sub-grid vegetation fraction in estimating
BVOC emissions over eastern China, which has a large area of urbanization. Among all
activity factors, the temperature-dependent factor dominates the seasonal change
of activity factor in all three versions of MEGAN, while the different
response to the leaf area index (LAI) change determines the difference among
the three versions in seasonal variation of BVOC emissions. The simulated
surface ozone concentration due to BVOCs can be significantly different
(ranging from 1 to more than 10 ppbv in some regions) among the
experiments with three versions of MEGAN, which is mainly due to their
impacts on surface VOCs and NOx concentrations. Theoretically MEGAN v3.0 that
is coupled with the land surface scheme and considers the sub-grid
vegetation effect should overcome previous versions of MEGAN in WRF-Chem.
However, considering uncertainties of retrievals and anthropogenic emissions
over eastern China, it is still difficult to apply satellite retrievals of
formaldehyde and/or limited sparse in situ observations to constrain the
uncertain parameters or functions in BVOC emission schemes and their
impacts on photochemistry and ozone production. More accurate vegetation
distribution and measurements of biogenic emission fluxes and species
concentrations are still needed to better evaluate and optimize models.