Ground-based aerosol characterization during the South American Biomass Burning Analysis (SAMBBA) field experiment

摘要

This paper investigates the physical and chemical\udcharacteristics of aerosols at ground level at a site\udheavily impacted by biomass burning. The site is located\udnear Porto Velho, Rondônia, in the southwestern part of the\udBrazilian Amazon rainforest, and was selected for the deployment\udof a large suite of instruments, among them an\udAerosol Chemical Speciation Monitor. Our measurements\udwere made during the South American Biomass Burning\udAnalysis (SAMBBA) field experiment, which consisted of\uda combination of aircraft and ground-based measurements\udover Brazil, aimed to investigate the impacts of biomass\udburning emissions on climate, air quality, and numerical\udweather prediction over South America. The campaign took\udplace during the dry season and the transition to the wet season\udin September/October 2012.\udDuring most of the campaign, the site was impacted by\udregional biomass burning pollution (average CO mixing ratio\udof 0.6 ppm), occasionally superimposed by intense (up\udto 2 ppm of CO), freshly emitted biomass burning plumes.\udAerosol number concentrations ranged from ∼ 1000 cm−3\udto\udpeaks of up to 35 000 cm−3\ud(during biomass burning (BB)\udevents, corresponding to an average submicron mass mean\udconcentrations of 13.7 µg m−3\udand peak concentrations close\udto 100 µg m−3\ud. Organic aerosol strongly dominated the submicron\udnon-refractory composition, with an average concentration\udof 11.4 µg m−3\ud. The inorganic species, NH4, SO4,\udNO3, and Cl, were observed, on average, at concentrations\udof 0.44, 0.34, 0.19, and 0.01 µg m−3\ud, respectively. Equivalent\udblack carbon (BCe) ranged from 0.2 to 5.5 µg m−3\ud,\udwith an average concentration of 1.3 µg m−3\ud. During BB\udpeaks, organics accounted for over 90 % of total mass (submicron\udnon-refractory plus BCe), among the highest values\uddescribed in the literature.\udWe examined the ageing of biomass burning organic\udaerosol (BBOA) using the changes in the H : C and O : C\udratios, and found that throughout most of the aerosol processing\ud(O : C ∼= 0.25 to O : C ∼= 0.6), no remarkable change\udis observed in the H : C ratio (∼ 1.35). Such a result contrasts\udstrongly with previous observations of chemical ageing\udof both urban and Amazonian biogenic aerosols. At higher\udlevels of processing (O : C > 0.6), the H : C ratio changes\udwith a H : C/O : C slope of −0.5, possibly due to the development\udof a combination of BB (H : C/O : C slope =\ud0) and biogenic (H : C/O : C slope = −1) organic aerosol\ud(OA). An analysis of the 1OA/1CO mass ratios yields\udvery little enhancement in the OA loading with atmospheric\udprocessing, consistent with previous observations. These\udresults indicate that negligible secondary organic aerosol\ud(SOA) formation occurs throughout the observed BB plume\udPublished by Copernicus Publications on behalf of the European Geosciences Union.\ud12070 J. Brito et al.: Ground-based aerosol characterization during SAMBBA\udprocessing, or that SOA formation is almost entirely balanced\udby OA volatilization.\udPositive matrix factorization (PMF) of the organic aerosol\udspectra resulted in three factors: fresh BBOA, aged BBOA,\udand low-volatility oxygenated organic aerosol (LV-OOA).\udAnalysis of the diurnal patterns and correlation with external\udmarkers indicates that during the first part of the campaign,\udOA concentrations are impacted by local fire plumes\udwith some chemical processing occurring in the near-surface\udlayer. During the second part of the campaign, long-range\udtransport of BB plumes above the surface layer, as well as\udpotential SOAs formed aloft, dominates OA concentrations\udat our ground-based sampling site.\udThis manuscript describes the first ground-based deployment\udof the aerosol mass spectrometry at a site heavily impacted\udby biomass burning in the Amazon region, allowing\uda deeper understanding of aerosol life cycle in this important\udecosystem.