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

Abstract

<p><strong>Abstract.</strong> To reduce the uncertainties in processes driving the vertical distribution of biomass burning pollutants, and thus impacts on regional weather and climate, we present an integrated analysis of vertical profiles of pollutants and meteorological parameters measured on flights during the 2012 South American Biomass Burning Analysis (SAMBBA) field experiment.</p><p> 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 burden was similar from the surface to ~ 1.5 km in the west and ~ 3 km in the east. Black carbon mass loadings were double in the east (1.7 µg cm<sup>−3</sup>) than west (0.85 µg cm<sup>−3</sup>) but aerosol scattering coefficients at 550 nm were similar (~ 120 Mm<sup>−1</sup>), as too were CO 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. Optically thin layers above the mixing layer indicates roles for 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 the spatial distribution and vertical structure of biomass burning smoke is captured.</p><p> Our observations of relatively large concentrations of aerosol aloft and of CO near surface suggest that there is a greater contribution of pollutants from more complete combustion with altitude. Release of appropriate emissions from the initial more flaming and later residual smouldering phases of a fire at appropriate altitudes may be especially important to ensure models can accurately predict aerosol-radiation, aerosol-cloud and air quality impacts.</p>

Twenty-first century droughts have not increasingly exacerbated fire season severity in the Brazilian Amazon

“…We assessed the effect of the anomalously high transport of pyrogenic emissions into the Amazon in 2007 and 2010 on the [CO] trend with a sensitivity analysis. Such emissions may account for up to half of the atmospheric optical depth during the dry season 52,53 , a pattern that also applies to CO 54 . The sensitivity analysis was performed by decreasing the [CO] values in 5% increments whereby we found that a 10% reduction in [CO] for the years 2007 and 2010 was sufficient to yield a significantly decreasing [CO] trend at the 95% confidence level (p value = 0.02), where time explains 40% of the variance in [CO].…”

Isolating Large‐Scale Smoke Impacts on Cloud and Precipitation Processes Over the Amazon With Convection Permitting Resolution

“…AOD measurements were taken from AERONET stations either at Manaus or Alta Floresta, depending on availability and proximity to the flight. The SAMBBA campaign (details in Brito et al., 2014 and Darbyshire et al., 2019) was based at Porto Velho with flights extending over much of the Amazon. In situ measurements from the CDP probe (http://data.ceda.ac.uk/badc/sambba/data) were used to determine flight‐mean values of N d (10th–90th percentile of data above 1 cm −3 ).…”

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

“…However, between January and May, there is systematic 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, ) (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).…”