Predicting the Climatic impacts of Primary Biological Aerosol Particles; Sensitivity study using GISS-E2.1 Earth system model

The present study attempts to investigate the climatic impacts of PBAPs using the GISS-E2.1 Earth system model with the newly built PBAP emission model recently introduced to GISS-E2.1 to calculate the terrestrial and marine fluxes of PBAPs and estimate their transport and sinks. The current version of the PBAPs emission model accounts for different tracers of PBAPs including bacteria, fungal spores, and marine PBAPs (MPBAPs). The new PBAP tracers are allowed to interact with the radiation and to affect the liquid cloud droplet number concentration (CDNC).

Primary biological aerosol particles (PBAPs) can play a key role in cloud formation and phase regionally and locally by acting as cloud condensation nuclei (CCN), and ice nucleating particles (INP) at high sub-zero temperatures. Earlier studies suggested that the climatic impacts of PBAPs are negligible due to their globally small contribution to the total observed aerosol loads compared to other aerosols such as dust. However, PBAPs emissions are not yet well constrained. According to the IPCC AR5 report, the terrestrial emission flux of PBAPs was estimated in the range of 50-1000 Tg/yr, while AR6 neither updated the earlier estimates nor mentioned PBAPs at all. Recent observations proposed that the PBAPs' concentrations have likely been underestimated in earlier modelling studies. This suggests that PBAPs emission together with their climatic impacts and feedback remain highly uncertain, and thus, require a deeper investigation.

The study involves several scenarios where the emission fluxes of PBAPs were varied and different climatic diagnostics including, precipitation, cloud parameters, and direct and indirect radiative forcing resulting from these runs were compared with the ones from the control run of GISS-E2.1 with no PBAPs. We further investigated whether these differences were statistically significant. In this context, we present the results of the impact of changing the PBAPs' number fluxes on emission mass fluxes, burdens, number and mass concentrations, and atmospheric lifetime. For bacteria and when using the best estimate of number fluxes for bacteria cell diameter of 1 mm, the global average of emission, burden, and atmospheric lifetime was estimated to be 0.79 Tg/yr, 7.5 Gg, and 3.5 days, respectively. Those values are comparable with what has been reported by Burrows et al. (2009). For fungal spores, we estimated 2.55 Tg/yr, 19.5 Gg, and 2.8 days, which were comparable with Janssen et al. (2021). We further found that PBAPs have an overall negative/cooling direct forcing (NDF), however, the global average of the NDF was an order of magnitude smaller than the NDF of other aerosols, e.g., seasalt and OC. Nevertheless, regionally, the higher the emission of PBAPs (over vegetated surfaces), the cooling can be evidenced, which cannot be negligible (values up to ~ -0.4 W/m²). Moreover, adding PBAPs contributed to more global negative/cooling indirect forcing (NIF) (-28.8 w/m²) than the NIF from the control run (-27.1 W/m²).   

 

References

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

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

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