On the microphysical processing of aged combustion aerosols impacting warm rain microphysics over Asian megacities

Abstract

This paper delineates the process of activation of soot or black carbon particles into cloud droplets over Chennai, India (an Asian megacity). Archaic wood-fired cooking stove emissions from crowded shantytown settlements dispense copious amounts of carbonaceous particles into the city's atmosphere during morning and evening cooking periods. Traffic emissions also add considerably to the city's particulate matter loads. At other times, when these sources are reduced (e.g. during non-cooking periods), a pall of residual pollution hovers above the city. This study assesses this problem definitively through process modelling studies undertaken during a typical pre-monsoonal month (October) when the levels of pollution are very high. The novelty of this paper includes a clear quantification of the percentage of activated nuclei resulting from these sources using a sophisticated cloud chemical parcel model. The results show that 85% of the soot and black carbon particles are activated into very small low-lying cloud droplets within Chennai's boundary layer. We further show for the first time that the presence of these particles results in a slowing down of the auto-conversion process (cloud water to rain droplets). This study could serve as a pointer for other Asian Cities with a sizeable shantytown population in India and elsewhere.

Quick Predictions of Onset Times and Rain Amounts from Monsoon Showers over Urban Built Environments

“…The dry aerosol profile (solid lines) over the city is shown in Figure 6, where the spectra comprise background sulphate (sourced from the oxidation of dimethyl sulfide (DMS) as well as from overland pollutants) and sea salt particles (film and jet modes sourced from the Bay of Bengal), along with partially soluble soot and black carbon particles mainly sourced from a heavy traffic flow in the city, termed the 'transient' mode [31,33,38,39]. From Figure 6, we find that the transient black carbon mode is sandwiched between the sulphate and the salt modes.…”

“…It is clear that the former amount agrees much more favourably with what is predicted from a full WRF-LES run (~0.6 gm −3 ). In what we show next, we use the predicted rain intensities in ArcGIS to model drainage flow over North Chennai slums, a vulnerable area with a population of ~145,301 residing in an area spanning 4.28 km 2 [33,48] without access to appropriate drainage. To aggravate matters, the area in question is a low-lying area.…”

“…The extreme precipitation event caused incessant flooding in several low-lying regions because of an improper drainage system [51]. These areas include Madhavaram, Tondiarpet, Tiruvottiyur, Manali and Royapuram (see Figure 9) and account for nearly 34% of the total slum population in the city [33]. The total geographic area and slum population for each region are, respectively, (7.2 × 10 5 m 2 , 21,968), ( 1…”

“…We assessed these properties with the use a sophisticated parcel model [30][31]. A Lagrangian chemical parcel model (CPM) was used that enables bin-wise growth of the dry aerosol particles into size-resolved cloud droplets [31][32][33]. The model was initialised with temperature, pressure, updraft speed and relative humidity values below the cloud base (see Table 1).…”

Countering tropical cyclonic storm impacts on low-rise houses

“…Tropical storms over urban areas are fed with a cocktail of internally mixed aerosol particles -1) oceanic sea salt particles and 2) ammonium sulphate, mixed with carbonaceous particles. This heterogeneous mixture profoundly affects the cloud nucleation process, subsequently modulating the precipitation amounts (Gumber et al, 2020). This results in thermodynamical perturbations and differential heating, affecting wind speeds.…”