Abstract
During late 2019 and early 2020, Australia experienced one of the most active bushfire seasons that advected large emissions over the adjacent ocean. Herein, we present a comprehensive research on mixed atmospheric aerosol particulate pollution emitted by wildfires in the atmosphere and the ocean. Based on a wide range of physical and biochemical data, including the Aerosol Robotic Network, multi-satellite observations, and Argo floats, we investigated the spatio-temporal variations and mixed compositions of aerosol particles, deposition in the coastal waters of eastern Australia and the South Pacific Ocean, and biogeochemical responses in the water column. Four types of wildfire-derived mixed particles were classified by using the optical properties of aerosols into four types, including the background aerosols, mineral dust, wildfire smoke particles, and residual smoke. The coarse particles accounted for more than 60% of the mineral dust on 22 November 2019 in the Tasman Sea; afterwards, during the wildfire smoke episode from December 2019 to January 2020, the particles affected large areas of the atmosphere such as eastern Australia, the South Pacific Ocean, and South America. The maximum value of the aerosol optical depth reached 2.74, and the proportion of fine particles accounted for 98.9% in the smoke episode. Mineral dust and smoke particles from the fire emissions changed the particle composition in the surface ocean. Particle deposition accounted for increases in chlorophyll-a concentration (Chla) standardized anomaly up to maximum of 23.3 with a lag time of less than 8 days. In the vertical direction, float observations showed the impact of exogenous particles on the water column could up to 64.7 m deep, resulting in Chla of 1.85 mg/m3. The high Chla lasted for a minimum period of two months until it returned to normal level.
Highlights
During late 2019 and early 2020, Australia experienced one of the most active bushfire seasons that advected large emissions over the adjacent ocean
Bioavailable Fe derived from direct emissions from combustion sources and processes by anthropogenic atmospheric pollutants in mineral dust play a vital role in ocean fertilisation over various surface ocean areas, where primary production and nitrogen fixation are limited by Fe scarcity[9,27,28]
We found four types of aerosols according to the dominant particles based on aerosol optical depth (AOD) and angstrom exponent (AE) observations: (1) background aerosols before 22 November 2019; (2) dust particulate pollution on 22 November 2019; (3) wildfire smoke from 8 December 2019 to 13 January 2020; and (4) residual subsidence after 17 January 2020. AOD440 < 0.1 and AE ~ 1 indicated the local background aerosols before the air particulate pollution event. AOD440 > 0.15 and AE rapidly decreasing to less than 0.5 suggested a dust episode (Fig. 4a,b)
Summary
During late 2019 and early 2020, Australia experienced one of the most active bushfire seasons that advected large emissions over the adjacent ocean. The coarse particles accounted for more than 60% of the mineral dust on 22 November 2019 in the Tasman Sea; afterwards, during the wildfire smoke episode from December 2019 to January 2020, the particles affected large areas of the atmosphere such as eastern Australia, the South Pacific Ocean, and South America. Compared to mineral dust particles, the results of a number of studies pointed toward an elevated solubility of iron in aerosols produced by the combustion of fossil fuels and biomass, which can potentially provide more bioavailable Fe to the surface o cean[9,22,23,24]. Bioavailable Fe derived from direct emissions from combustion sources and processes by anthropogenic atmospheric pollutants in mineral dust play a vital role in ocean fertilisation over various surface ocean areas, where primary production and nitrogen fixation are limited by Fe scarcity[9,27,28]. Symbols and abbreviations δ ΔX Δσθ α (443), β (443) AE AERONET AOD440 aph_443 Argo bbp_443 bbp_700 BGC-Argo Chla dV(r)/dlnr GAMSSA GHRSST GIOP MLD MODIS NASA OMI r rVF SDA SOCCOM SST TROPOMI UVAI VC VF
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