Contribution of regional aerosol nucleation to low-level CCN in an Amazonian deep convective environment: Results from a regionally nested global model
Abstract. Global model studies and observations have shown that downward transport of aerosol nucleated in the free troposphere is a major source of cloud condensation nuclei (CCN) to the global boundary layer. In Amazonia, observations show that this downward transport can occur during strong convective activity. However, it is not clear from these studies over what spatial scale this cycle of aerosol formation and downward supply of CCN is occurring. Here, we aim to quantify the extent to which the supply of aerosol to the Amazonian boundary layer is generated from nucleation within a 1000 km regional domain or from aerosol produced further afield, and the effectiveness of the transport by deep convection. We run the atmosphere-only configuration of the HadGEM3 climate model incorporating a 440 km × 1080 km regional domain over Amazonia with 4 km resolution. Simulations were performed over several diurnal cycles of convection. Below 1 km altitude in the regional domain, our results show that nucleation within the regional domain accounts for only 1.8 % of all Aitken and accumulation mode aerosol particles, whereas nucleation that occurred outside the domain (in the global model) accounts for 81 %. The remaining aerosol is primary in origin. Above 10 km, the regional-domain nucleation accounts for up to 64 % of Aitken and accumulation mode aerosol, but over several days very few particles nucleated above 10 km in the regional domain are transported into the boundary layer within the domain, and in fact very little air is mixed that far down. Rather, particles transported downwards into the boundary layer originated from outside the regional domain and entered the domain at lower altitudes. Our model results show that CCN entering the Amazonian boundary layer are transported downwards gradually over multiple convective cycles on scales much larger than 1000 km. Therefore, on a 1000 km scale in the model (approximately one-third the size of Amazonia), trace gas emission, new particle formation, transport and CCN production do not form a "closed loop" regulated by the biosphere. Rather, on this scale, long-range transport of aerosol is a much more important factor controlling CCN in the boundary layer.
Xuemei Wang et al.
Xuemei Wang et al.
Xuemei Wang et al.
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Contribution of regional aerosol nucleation to low-level CCN in an Amazonian deep convective environment: Results from a regionally nested global model, by Xuemei Wang et al.
This well-designed study aims to investigate the new particle formation related to the deep convection transport and the horizontal advection from neighbors’ regions by using the HadGEM3 climate model nest with a regional domain over Amazonia. Combining high resolution with resolved convection with the GLOMAP-mode aerosol scheme and the global model with raw resolution allows for testing different hypotheses. The findings are interesting and corroborate empirical data from the ACRIDICON-CHUVA experiment. Furthermore, results allow discriminating of the regional/deep convection and the long-range transport contribution to the aerosol profile in Amazonas.
I would recommend the authors revise the new literature on this subject; for instance, the three recent studies from Bardakov et al. on Tellus, JGR, and James (https://doi.org/10.1080/16000889.2021.1979856, https://doi.org/10.1029/2022JD037265, and https://doi.org/10.1029/2019MS001931). These studies, using large eddy simulation, were able to present and quantify the convective transport, chemical reactions, and new particle formation in detail. As these models with 100 m resolution are much more appropriate to describe the updrafts and downdrafts, these studies provide a detailed description of the aerosol-deep convection interaction. I recommend authors read/refer to these studies and consider what is new in the present study. Another comment is related to the deep convection described by the regional model; convection is too deep, deeper than observed. The typical height of deep convective cloud in this region, at this season, is around 9 km (observed by radar – rain drops) and 14 km of cloud top (lidar). Therefore, 20 km, as shown in the figures, is out of the reality of the convective system. It is higher than tropopause (around 16 km). In addition, the looping simulations show the accumulation and Aitkens mode moving westerly, so above tropopause flow. How do these particles penetrate the stratosphere? Are these features real? The conceptual model presented in the conclusions shows the maximum height as 12 km, well below the layer shown in the results.
The regional model shows an increase of around 100% in the nucleation particle concentration in the lower levels. This is not well discussed in the manuscript. From where do these particles come? Are these particles formed in the boundary layer by monoterpenes oxidation?
I am curious to know the concentrations of Monoterpenes and Ozone employed in the simulations and how they compare with the data measured at ATTO. Finally, the simulations BioOxEmCCS used in the main simulations showed a peak well above the measured in ACRIDICON-CHUVA (12 km, against 14 km) and with much less middle levels concentration (around twice). What is the effect of these differences in the results?