Observed slump of sea land breeze in Brisbane under the effect of aerosols from remote transport during 2019 Australian mega fire events

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.

Airborne observation during KORUS-AQ show aerosol optical depth are more spatially self-consistent than aerosol intensive properties

Aerosol particles can be emitted, transported, removed, or transformed, leading to aerosol variability at scales impacting the climate (days to years and over hundreds of kilometers) or the air quality (hours to days and from meters to hundreds of kilometers). We present the temporal and spatial scales of changes in AOD (Aerosol Optical Depth), and aerosol size (using Angstrom Exponent; AE, and Fine-Mode-Fraction; FMF) over Korea during the 2016 KORUS-AQ (KORea-US Air Quality) atmospheric experiment. We use measurements and retrievals of aerosol optical properties from airborne instruments for remote sensing (4STAR; Spectrometers for Sky-Scanning Sun Tracking Atmospheric Research) and in situ (LARGE; NASA Langley Aerosol Research Group Experiment) on board the NASA DC-8, geostationary satellite (GOCI; Geostationary Ocean Color Imager; Yonsei aerosol retrieval (YAER) version 2) and reanalysis (MERRA-2; Modern-Era Retrospective Analysis for Research and Applications, version 2). Measurements from 4STAR when flying below 500 m, show an average AOD at 501 nm of 0.43 and an average AE of 1.15 with large standard deviation (0.32 and 0.26 for AOD and AE respectively) likely due to mixing of different aerosol types (fine and coarse mode). The majority of AODs due to fine mode aerosol is observed at altitudes lower than 2 km. Even though there are large variations, for 18 out of the 20 flight days, the column AOD measurements by 4STAR along the NASA DC-8 flight trajectories matches the south-Korean regional average derived from GOCI. We also observed that, contrary to prevalent understanding, AE and FMF are more spatially variable than AOD during KORUS-AQ, even when accounting for potential sampling biases by using Monte Carlo resampling. Averaging between measurements and model for the entire KORUS-AQ period, a reduction in correlation by 15 % is 65.0 km for AOD and shorter at 22.7 km for AE. While there are observational and model differences, the predominant factor influencing spatial-temporal homogeneity is the meteorological period. High spatio-temporal variability occur during the dynamic period (25–31 May), and low spatio-temporal variability occur during blocking Rex pattern (01–07 June). The changes in spatial variability scales between AOD and FMF/AE, while inter-related, indicate that microphysical processes that impact mostly the dominant aerosol size, like aerosol particle formation, growth, and coagulation, vary at shorter scales than the aerosol concentration processes that mostly impact AOD, like aerosol emission, transport, and removal.

Aerosol Atmospheric Rivers: Climatology, Event Characteristics, and Detection Algorithm Sensitivities

Leveraging the concept of atmospheric rivers (ARs), a detection technique based on a widely utilized global algorithm to detect ARs (Guan et al., 2018; Guan and Waliser, 2015, 2019) was recently developed to detect aerosol atmospheric rivers (AARs) using the Modern-Era Retrospective analysis for Research and Applications, Version 2 reanalysis (Chakraborty et al., 2021a). The current study further characterizes and quantifies various details of AARs that were not provided in that study, such as AARs’ seasonality, event characteristics, vertical profiles of aerosol mass mixing ratio and wind speed, and the fraction of total annual aerosol transport conducted by AARs. Analysis is also performed to quantify the sensitivity of AAR detection to the criteria and thresholds used by the algorithm. AARs occur more frequently over, and typically extend from, regions with higher aerosol emission. For a number of planetary-scale pathways that exhibit large climatological aerosol transport, AARs contribute 40–80 % to the total annual transport. DU AARs are more frequent in boreal spring, SS AARs are often more frequent during the boreal winter (summer) in the Northern (Southern) Hemisphere, CA AARs are more frequent during dry seasons and often originate from the global rainforests and industrial areas, and SU AARs are present in the Northern Hemisphere during all seasons. For most aerosol types, the mass mixing ratio within AARs is highest near the surface and decreases monotonically with altitude. However, DU and CA AARs over or near the African continent exhibit peaks in their aerosol mixing ratio profiles around 700 hPa. AAR event characteristics are mostly independent of species with mean length, width, and length/width ratio around 4000 km, 600 km, and 8, respectively.

Long-term Variation of Absorption and Total Aerosol Optical Properties over Typical Provinces of China from Satellite Observations

Increase of atmospheric aerosols has a profound impact on the Earth’s climate. It’s also one of the crucial factors that cuasesd more fequent air pollution events in China. Monthly average Aerosol Optical Depth (AOD) from MODIS and UltraViolet Absorbing aerosol Index (UVAI) from OMI during 2011 to 2019 are used to analyse the trend of absorption and total aerosol optical properties over three typical provinces of China, namely Shandong, Gansu and Guangdong provinces. The results show the average annual AOD of the three provinces are all decreasing while UVAI rises during this period. In addition, the monthly variation of AOD and UVAI are also obviously different over these provinces. In particular, the peak value of AOD appeared in July and the trough appeared in December over Shandong Province. And the peak appeared in April over Gansu Province, but AOD decrease slower then over Shandong Province. And there were two peaks in April and August over Guangdong Province. For UVAI, the peaks over Shandong and Gansu provinces both occur in January, while that over Guangdong Province appears in March. Above mentioned differences in the long-term trend and monthly variation of AOD and UVAI might be closely related to the meteorological conditions and aerosol emission of these three provinces.

Estimation of the anthropogenic aerosol emission effect on the rate of summer warming in Europe

A relationship between aerosol air pollutions and summer air temperatures in Europe was studied. High correlation coefficients between the latitudinal distributions of the zone-averaged trends of the mentioned parameters were found. The potential effects of decrease in the aerosol emission on the cloud optical depth, in the air temperature, and the amount of precipitation in the territory of Europe were estimated on the basis of the obtained regression equations. It was shown that due to the aerosol emission decrease, the average summer temperature in Europe in 2000–2020 could increase by 0.53 °С, which is ~73 % of total summer warming in the region. The empirical estimates obtained in the work were confirmed by the satellite observation data and the numerical calculations of changes in radiation balance components at the top of the atmosphere. It was shown that the radiation emission decrease in the territory of Europe could increase the average radiation balance in Europe in summer months by 2.27 W/m², which is ~65 % of its total change. The increase in the carbon dioxide content in the atmosphere during the same period contributed much less to the observed change in the radiation balance (17.5 %), which supports the hypothesis about the dominant role of aerosols in summer warming in Europe.

The Impact of COVID-19 Lockdowns on Satellite-Observed Aerosol Optical Thickness over the Surrounding Coastal Oceanic Areas of Megacities in the Coastal Zone

Nearly 40 years of aerosol optical thickness (AOT) climate data record (CDR) derived from NOAA operational satellite Advanced Very High Resolution Radiometer (AVHRR) observation over the global oceans is used to study the AOT changes due to the COVID-19 lockdown over the surrounding coastal oceanic areas of 18 megacities in the coast zone (MCCZ). The AOT difference between the annual mean AOT values of 2020 with COVID-19 lockdown and 2019 without the lockdown along with the 2020 AOT annual anomaly are used to effectively identify the AOT changes that are a result of the lockdown. We found that for most of the 18 MCCZ, the COVID-19 lockdowns implemented to contain the spread of the coronavirus resulted in a decrease between 1% and 30% in AOT due to reduced anthropogenic emissions associated with the lockdowns. However, the AOT long-term trend and other aerosol interannual variations due to favorable or unfavorable meteorological conditions may mask AOT changes due to the lockdown effect in some MCCZ. Different seasonal variations of aerosol amount in 2020 relative to 2019 due to other natural aerosol emission sources not influenced by the lockdown, such as dust storms and natural biomass burning and smoke, may also conceal a limited reduction in the annual mean AOT due to the lockdown in MCCZ with relatively loose lockdown. This study indicates that the use of long-term satellite observation is helpful for studying and monitoring the aerosol changes due to the emission reduction associated with the COVID-19 lockdown in the surrounding coastal oceanic areas of MCCZ, which will benefit the future development of the mitigation strategy for air pollution and emissions in megacities.

Aerosol emission rates from playing wind instruments – Implications for COVID-19 transmission during music performance

BackgroundThe pandemic of COVID-19 led to exceeding restrictions especially in public life and music business. Airborne transmission of SARS-CoV-2 demands for risk assessment also in wind playing situations. Previous studies focused on short-range transmission, whereas long-range transmission has not been assessed so far.Methods and findingsWe measured resulting aerosol concentrations in a hermetically closed cabin of 20 m3 in an operating theatre from 20 minutes standardized wind instrument playing (19 flute, 11 oboe, 1 clarinet, 1 trumpet players). Based on the data, we calculated total aerosol emission rates showing uniform distribution for both instrument groups (flute, oboe). Aerosol emission from wind instruments playing ranged from 7 ± 327 particles/second (P/s) up to 2583 ± 236 P/s, average rate ± standard deviation. The analysis of the aerosol particle size distribution showed that about 70 − 80% of emitted particles had a size ≤ 0.4 µm and thus being alveolar. Masking the bell with a surgical mask did not reduce aerosol emission. Aerosol emission rates were higher from wind instruments playing than from speaking and breathing. Differences between instrumental groups could not be found, but high interindividual variance as expressed by uniform distribution of aerosol emission rates.ConclusionsOur findings indicate that aerosol emission depends on physiological factors and playing techniques rather than on the type of instrument, in contrast to some previous studies. Based on our results, we present risk calculations for long-range transmission of COVID-19 for three typical woodwind playing situations.

Hemisphere-asymmetric tropical cyclones response to anthropogenic aerosol forcing

How anthropogenic forcing could change tropical cyclones (TCs) is a keen societal concern owing to its significant socio-economic impacts. However, a global picture of the anthropogenic aerosol effect on TCs has not yet emerged. Here we show that anthropogenic aerosol emission can reduce northern hemisphere (NH) TCs but increase southern hemisphere (SH) TCs primarily through altering vertical wind shear and mid-tropospheric upward motion in the TC formation zones. These circulation changes are driven by anthropogenic aerosol-induced NH-cooler-than-SH and NH-increased versus SH-decreased meridional (equator to mid-latitudes) temperature gradients. The cooler NH produces a low-level southward cross-equatorial transport of moist static energy, weakening the NH ascent in the TC formation zones; meanwhile, the increased meridional temperature gradients strengthen vertical wind shear, reducing NH TC genesis. The opposite is true for the SH. The results may help to constrain the models’ uncertainty in the future TC projection. Reduction of anthropogenic aerosol emission may increase the NH TCs threat.

Nonlinear response of Asian summer monsoon precipitation to emission reductions in South and East Asia

Anthropogenic aerosols over South and East Asia currently have a stronger impact on the Asian Summer Monsoon (ASM) than greenhouse gas emissions, yet projected aerosol emission changes in these regions are subject to considerable uncertainties such as timescale, location, emission type, and even the sign of the change. We use a circulation/climate model with idealised aerosol distributions to demonstrate that the sum of ASM responses to aerosol emission reductions in each region is very different to the response to simultaneous reductions in both regions, implying the ASM response to aerosol emissions reductions is highly nonlinear. The phenomenon is independent of whether aerosols are scattering or absorbing, and results from interaction of induced atmospheric circulation changes. The nonlinearity from interactions between aerosol forcing from different regions represents a new source of uncertainty in projections of ASM changes over the next 30-40 years, and may limit the utility of country-dependent aerosol trajectories when considering their Asia-wide effects, though we recommend further work to establish whether the nonlinearity is buffered by other drivers. To understand likely changes in the ASM due to aerosol reductions, countries will need to accurately take account of emissions reductions from across the wider region, rather than approximating them using simple scenarios and emulators. The nonlinearity in the response to forcing therefore presents a regional public goods issue for countries affected by the ASM, as the costs and benefits of aerosol emissions reductions are not internalised; in fact, forcings from different countries such as India and China work jointly to determine outcomes across the region.