Production And Climate Effects Of Natural Aerosols:Observations Of Sea Spray Aerosol And Modeling Of Stratospheric Wildfire Smoke

Climate models use pre-industrial atmosphere as the reference to evaluate the impacts of human activities on the Earth’s radiation balance.Natural aerosols are the key component in the relatively pristine pre-industrial conditions that substantially affect model calculations.Natural aerosols in pristine regions form the baseline used to evaluate the impact of anthropogenic aerosols on climate.However,the abundance and climate effects of natural aerosols remain poorly constrained,especially for natural aerosols over remote areas away from human activities due to limited field measurements and model simulations.Sea spray aerosol(SSA)and stratospheric wildfire smoke significantly contribute to the natural sources of aerosols in the marine boundary layer(MBL)and the stratosphere,respectively.Here(1)we conducted global aircraft measurements over the remote Pacific and Atlantic oceans to investigate the production mechanism of SSA.We show that higher sea surface temperature(SST)enhances the production of SSA over the global remote oceans.(2)We quantified the climate-relevant effective radiative forcing(ERF)of two record-breaking wildfire events from 2017 to 2021 with the Community Earth System Model(CESM).We also found that wildfire smoke can cool the atmosphere 70%-270%more effectively than mass-equivalent volcanic sulfate in the model.The research background of these two natural aerosols and the main conclusions of this study are as follows:(1)SSA is a major component of natural aerosols in the MBL.Despite its importance,the abundance of SSA is poorly constrained.It is generally accepted that wind-driven wave breaking is the principle governing SSA production.This mechanism alone,however,is insufficient to explain the variability of SSA concentration at given wind speed.The role of other parameters,such as SST,remains controversial.This uncertainty arises from limited field measurements,especially over remote oceans.Here we show that higher SST promotes SSA mass generation at a wide range of wind speed levels over the remote Pacific and Atlantic oceans,in addition to demonstrating the wind-driven SSA production mechanism.The results are from a global-scale dataset of airborne SSA measurements at 150-200 m above the ocean surface during the NASA Atmospheric Tomography Mission.Statistical analysis suggests that accounting for SST greatly enhances the predictability of the observed SSA concentration compared to using wind speed alone.Our results support implementing SST into SSA source functions in global models to better understand the atmospheric burdens of SSA and human-induced climate change.(2)Extreme wildfires can directly inject a large amount of smoke into the stratosphere.The radiative forcing(RF)of volcanic sulfate is well quantified.However,the RF of pyrocumulonimbus(pyroCb)smoke with absorbing carbonaceous aerosols has not been considered in climate assessment reports.With the CESM,we studied two record-breaking wildfire events,the 2017 Pacific Northwest Event(PNE)and the 20192020 Australian New Year event(ANY),that perturbed stratospheric chemistry and the earth’s radiation budget.We calculated a global annual-mean ERF of-0.04 ± 0.02 W/m2 and-0.17 ± 0.02 W/m2 at the top of the atmosphere(TOA)for PNE and ANY,respectively.The complexity of longwave RF led to an uncertainty of about 50%in the ERF at the TOA among climate models.We found that modeled ERF at the TOA,200 hPa and the surface from wildfire smoke was 70%-270%more negative than that of mass-equivalent sulfate aerosol,highlighting its important role in the climate radiative budget.