Abstract. The 2019 Australian mega fires were unprecedented
considering their intensity and consistency. There has been much research
on the environmental and ecological effects of these mega fires, most of
which focused on the effect of huge aerosol loadings and the ecological
devastation. Sea land breeze (SLB) is a regional thermodynamic circulation
closely related to coastal pollution dispersion, yet few have looked into how
it is influenced by different types of aerosols transported from either
nearby or remote areas. Mega fires provide an optimal scenario of large
aerosol emissions. Near the coastal site of Brisbane Archerfield during
January 2020, when mega fires were the strongest, reanalysis data from
Modern-Era Retrospective analysis for Research and Applications version 2
(MERRA-2) showed that mega fires did release huge amounts of aerosols,
making aerosol optical depth (AOD) of total aerosols, black carbon (BC) and
organic carbon (OC) approximately 240 %, 425 % and 630 % of the averages
in other non-fire years. Using 20 years' wind observations of hourly time
resolution from a global observation network managed by the National Oceanic and
Atmospheric Administration (NOAA), we found that the SLB day number during
that month was only 4, accounting for 33.3 % of the multi-years'
average. The land wind (LW) speed and sea wind (SW) speed also decreased by
22.3 % and 14.8 % compared with their averages respectively.
Surprisingly, fire spot and fire radiative power (FRP) analysis showed that
heating effects and aerosol emission of the nearby fire spots were not the main
causes of the local SLB anomaly, while the remote transport of aerosols from the
fire centre was mainly responsible for the decrease of SW, which was
partially offset by the heating effect of nearby fire spots and the warming
effect of long-range transported BC and CO2. The large-scale cooling
effect of aerosols on sea surface temperature (SST) and the burst of BC
contributed to the slump of LW. The remote transport of total aerosols was
mainly caused by free diffusion, while the large-scale wind field played a
secondary role at 500 m. The large-scale wind field played a more important role
in aerosol transport at 3 km than at 500 m, especially for the gathered
smoke, but free diffusion remained the major contributor. The decrease of
SLB speed boosted the local accumulation of aerosols, thus making SLB
speed decrease further, forming a positive feedback mechanism.