Characterizing the Aging of Biomass Burning Organic Aerosol by Use of Mixing Ratios: A Meta-analysis of Four Regions

Abstract

Characteristic organic aerosol (OA) emission ratios (ERs) and normalized excess mixing ratios (NEMRs) for biomass burning (BB) events have been calculated from ambient measurements recorded during four field campaigns. Normalized OA mass concentrations measured using Aerodyne Research Inc. quadrupole aerosol mass spectrometers (Q-AMS) reveal a systematic variation in average values between different geographical regions. For each region, a consistent, characteristic ratio is seemingly established when measurements are collated from plumes of all ages and origins. However, there is evidence of strong regional and local-scale variability between separate measurement periods throughout the tropical, subtropical, and boreal environments studied. ERs close to source typically exceed NEMRs in the far-field, despite apparent compositional change and increasing oxidation with age. The absence of any significant downwind mass enhancement suggests no regional net source of secondary organic aerosol (SOA) from atmospheric aging of BB sources, in contrast with the substantial levels of net SOA formation associated with urban sources. A consistent trend of moderately reduced ΔOA/ΔCO ratios with aging indicates a small net loss of OA, likely as a result of the evaporation of organic material from initial fire emissions. Variability in ERs close to source is shown to substantially exceed the magnitude of any changes between fresh and aged OA, emphasizing the importance of fuel and combustion conditions in determining OA loadings from biomass burning.

Properties and evolution of biomass burning organic aerosol from Canadian boreal forest fires

“…Average ratios for individual flights sampling aged emissions range from 0.056 ± 0.003 (B624) to 0.114 ± 0.003 (B622), giving an overall range of 0.058. The level of average OA / CO for fresh emissions from boreal forest fires during BORTAS falls between the upper extent derived from the eucalypt forests of northern Australia during the Aerosol and Chemical Transport in Tropical Convection (ACTIVE) campaign (0.329), and lower ratios from several other campaigns where OA enhancements were comparatively reduced (0.019-0.065; Jolleys et al, 2012). Average OA / CO from aged plumes during BORTAS was again within the range identified from previous field observations, although with closer proximity to ratios from the lower extent of the observed range, including aged boreal forest fire plumes sampled during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign .…”

“…However, under ambient conditions the importance of SOA addition relative to primary (POA) emissions is more disputable. Despite widespread evidence for the increasing oxygenation of BBOA with aging (Capes et al, 2008;DeCarlo et al, 2008;Cubison et al, 2011;Jolleys et al, 2012), net mass enhancements are not observed consistently. The underlying causes of this variable SOA contribution, including the implications of initial OA composition, also remain ambiguous and require further refinement.…”

“…Data were screened in order to isolate emissions with a biomass burning influence, resulting in a total number of 26 valid plume interceptions (3 fresh and 23 aged) across the 5 flights. Screening was performed using the guidelines proposed by Capes et al (2008) and Jolleys et al (2012) based upon minimum CO (the excess CO concentration above background levels) and absolute number concentrations. Respective thresholds of 20 ppb and 2000 cm −3 were applied for CO and number concentration.…”

“…The extent of variability amongst aged emissions during ACTIVE also exceeded that observed during BOR-TAS, with flights throughout the former campaign sampling plumes from fires in a number of different source regions. However, analysis of ERs from vegetation fires performed under laboratory conditions during the second Fire Lab At Missoula Experiment (FLAME II) also revealed extensive variability in OA / CO directly at source, even amongst single plant species (Jolleys et al, 2014). The single source region from which BORTAS plumes originated could therefore still be expected to give rise to significantly contrasting OA / CO, while the effects of atmospheric processing during transportation provide further perturbation of initial ERs.…”

“…Several fundamental aspects of the BBOA lifecycle remain poorly characterised (Hallquist et al, 2009), including the conditions and processes controlling formation and the effects of transformations occurring during aging, such as gas-particle partitioning of low volatility organic compounds following photo-oxidation, heterogeneous reactions with existing OA and losses through dilution-based evaporation or volatilisation (Reid et al, 2005;Grieshop et al, 2009;Hennigan et al, 2011). Variability at source has been shown to be extensive, in response to changes in both fuel properties and combustion conditions (McMeeking et al, 2009;Jolleys et al, 2012Jolleys et al, , 2014. The importance of secondary organic aerosol (SOA) in aging plumes is also particularly unclear.…”