Primary Biological Aerosol Particles in GISS-E2.1 Earth system model

Primary biological aerosol particles (PBAPs) are present globally, contributing to the total observed aerosol loads. Yet, PBAPs likely form a smaller fraction of the total aerosol budget compared to other types of particles such as dust. According to the IPCC AR5 report, the terrestrial emission flux of PBAPs is highly uncertain and was estimated within the range of 50-1000 Tg/yr. Burrows et al. (2009) estimated the global emissions of bacteria-containing particles to range between 0.4 to 1.8 Tg/yr, with a median of 0.74 Tg/yr. However, PBAPs comprise a large fraction of the submicron particles > 0.2 mm in the middle to the upper troposphere and they can be dispersed to distant locations and high altitudes from their source regionss. PBAPs have the potential to play a key role in cloud formation by acting as cloud condensation nuclei (CCN), and ice nucleating particles (INP) active at high sub-zero temperatures, potentially impacting the Earth’s hydrological cycle and climate.

Recent observations suggest that the PBAP concentrations have likely been underestimated in global modeling studies (summarized in Huang et al., 2021). For example, the fragmented biological particles and microbial exudates still cannot be detected with many commonly used techniques and, therefore they were not accounted for in the previous global modeling studies. Other recent studies presented a novel secondary biological aerosol production. Moreover, observations revealed that biological INPs from marine surfaces may be of higher imporatance than what has previously been considered in modeling studies. PBAPs' emission flux is therefore not yet well constrained, and the uncertainty in their emission estimation remains unresolved and requires deeper investigation. Consequently, the climatic impacts and feedbacks of PBAPs remain highly uncertain.

In this study, we build and integrate for the first time a new emission model for PBAPs in the GISS-E2.1 Earth system model in order to calculate the total emission flux of PBAPs from terrestrial and marine surfaces into the atmosphere and estimate their transport and sinks. In this model, we consider different types of PBAPs, i.e., bacteria and fungal spores. For bacteria we used the estimated flux-rates from Burrows et al. (2009) for different ecosystems. In a later step, we will update those values for each ecosystem using recent observations, especially over the marine areas and land ice. For fungal spores, we used the parameterization of Janssen et al. (2021).

We present preliminary results of the emission fluxes, burdens, concentrations, lifetime, and direct radiative forcing due to aerosol-radiation interactions of PBAPs and validate them using previous studies. For example, the lifetime of bacteria of size 1 micron is calculated to equal 3.5 days, which is comparable with the 3.4 days estimated by Burrows et al. (2009). 

 

References

Burrows, S. M. et al., ACP 2009, 9(23),9281, doi: https://doi.org/10.5194/acp-9-9281-2009.

Huang, S. et al., Environment International 2021, 146., doi: https://doi.org/10.1016/j.envint.2020.106197

Janssen et al., ACP 2021, 21(6), 4381., doi: https://doi.org/10.5194/acp-21-4381-2021