The vertical distribution of biomass burning pollution over tropical South America from aircraft in situ measurements during SAMBBA

Abstract

Abstract. We examine processes driving the vertical distribution of biomass burning pollution following an integrated analysis of over 200 pollutant and meteorological profiles measured in situ during the South AMerican Biomass Burning Analysis (SAMBBA) field experiment. This study will aid future work examining the impact of biomass burning on weather, climate and air quality. During the dry season there were significant contrasts in the composition and vertical distribution of haze between western and eastern regions of tropical South America. Owing to an active or residual convective mixing layer, the aerosol abundance was similar from the surface to ∼1.5 km in the west and ∼3 km in the east. Black carbon mass loadings were double as much in the east (1.7 µg m−3) than the west (0.85 µg m−3), but aerosol scattering coefficients at 550 nm were similar (∼120 Mm−1), as too were CO near-surface concentrations (310–340 ppb). We attribute these contrasts to the more flaming combustion of Cerrado fires in the east and more smouldering combustion of deforestation and pasture fires in the west. Horizontal wind shear was important in inhibiting mixed layer growth and plume rise, in addition to advecting pollutants from the Cerrado regions into the remote tropical forest of central Amazonia. Thin layers above the mixing layer indicate the roles of both plume injection and shallow moist convection in delivering pollution to the lower free troposphere. However, detrainment of large smoke plumes into the upper free troposphere was very infrequently observed. Our results reiterate that thermodynamics control the pollutant vertical distribution and thus point to the need for correct model representation so that the spatial distribution and vertical structure of biomass burning smoke is captured. We observed an increase of aerosol abundance relative to CO with altitude both in the background haze and plume enhancement ratios. It is unlikely associated with thermodynamic partitioning, aerosol deposition or local non-fire sources. We speculate it may be linked to long-range transport from West Africa or fire combustion efficiency coupled to plume injection height. Further enquiry is required to explain the phenomenon and explore impacts on regional climate and air quality.

Influx of African biomass burning aerosol during the Amazonian dry season through layered transatlantic transport of black carbon-rich smoke

“…The high EnRBC in African 685 plumes can be attributed to more flaming combustion in the comparatively dry grassland, savanna, and shrubland vegetation in contrast to more smoldering combustion of Amazonian deforestation fires in the moist tropical forests. For comparison, Darbyshire et al (2019) reported from the SAMBBA aircraft campaign over the southern Amazon basin, EnRBC of 3 ng m -3 ppb -1 in the west associated with more smoldering combustion of pasture and forested areas, in contrast to EnRBC of 12 ng m -3 ppb -1 in eastern part influ-690 enced by Cerrado fires. Note that differences between ground-based (this section) and aircraft EnRBC (section 3.1) presented in this paper are due to removal processes, combustion phase and, possibly, to the use of different measurement techniques.…”

“…The elevated cCO = 150 ± 30 ppb along with the high MrBC indicates that the UPL air masses originated from BB emissions. Moreover, the ratio between these two co-emitted species can be used as tracer for the origin and age of BB plumes (Darbyshire et al, 2019;Guyon et al, 2005;Saturno et al, 2018b). The aged UPL is characterized by a higher rBC enhancement ratio, EnRBC = 12.4 ± 0.2 ng m -3 ppb -1 compared to fresh Amazonian BB, with EnRBC of 7.8 ± 0.1 ng/m -3 ppb -1 (Fig.…”

Biomass burning aerosol over the Amazon: analysis of aircraft, surface and satellite observations using a global aerosol model

“…Monoterpenes are oxidised to form a product that condenses irreversibly in the particle phase (Scott et al, 2014). Size-resolved emissions of mineral dust are prescribed from daily varying emissions fluxes provided for AEROCOM (Dentener et al, 2006).…”

“…Marenco et al (2016) analysed lidar data during the SAMBBA campaign and found that the mean height of aerosol layers was 2.0 ± 0.4 km, suggesting that the majority of the aerosol is injected into the boundary layer. Fire emissions in GLOMAP are distributed vertically over six ecosystem-dependent altitudes between the surface and 6 km according to Dentener et al (2006). Over Brazil ∼ 53 % of emissions were injected below 500 m elevation, ∼ 30 % between 500 and 1000 m elevation, and ∼ 17 % between 1000 and 3000 m elevation.…”

Substantial Increases in Eastern Amazon and Cerrado Biomass Burning‐Sourced Tropospheric Ozone

“…Open circles represent extreme drought/anomalous fire years and are not included in trend analysis.10.1029/2019GL084143systematic positive bias of~5 ppbv (although this is within the observational variability). Comparisons with aircraft observations from the South AMerican Biomass Burning Analysis campaign(Darbyshire et al, 2019) (September-October 2012) show the model successfully reproduces the boundary layer vertical ozone profile between 30 and 45 ppbv, with a slight positive bias of 2-3 ppbv. These observations and comparison are also consistent with ozonesondes from Natal(2007)(2008).…”