Breakout Summary Report
ARM/ASR User and PI Meeting
2 - 6 May 2016
Biomass Burning (BB) Aerosol: Variability in Absorption Efficiency and Radiative Effects
5 May 2016
10:45 AM - 12:45 PM
0
Yan Feng and Arthur Sedlacek, with contribution from Allison Aiken
5 May 2016
10:45 AM - 12:45 PM
0
Yan Feng and Arthur Sedlacek, with contribution from Allison Aiken
Breakout Description
Biomass burning (BB) aerosols are responsible for one of the largest uncertainties in aerosol radiative forcing. This is primarily because the magnitude and sign of BB radiative effects depend strongly on the aerosol absorption, which, in turn, is highly dependent on source types, locations, injection height, aging, and mixing. However, characterization of these variabilities has posed significant challenges for both observations and models. Of particular note is the impact of absorption by elevated BB aerosol layers over clouds. Injection at various altitudes is expected to influence cloud lifecycle and atmospheric dynamics, but has received little attention due to the difficulty of collecting vertically resolved observational data.ARM-sponsored field campaigns that have focused on biomass burning events such as BBOP and the upcoming LASIC, as well as other campaigns that encountered BB emissions, are providing a fundamental and comprehensive data set representative for different environments that will help reduce model uncertainty. With this in mind, this breakout session is designed to provide ASR researchers the opportunity to review BB aerosol absorption data sets from the previous ARM campaigns, synthesize the variability in observed absorption properties, assess knowledge gaps that still exist, and provide a forum for collaboration and exploration of future field studies. We also invite discussions on model representations of BB absorption and direct and indirect radiative effects at different scales and analyses of AERONET and satellite data sets.
Main Discussion
The primary objectives of the biomass burning (BB) breakout session were to provide: 1) ASR researchers a summary of the state of BB aerosol research through presentations from the modeling and observational communities; 2) the opportunity to identify and discuss the knowledge gaps that exist between the models and observations at various scales; and 3) a forum for ARM-ASR researchers to explore potential collaborations on targeted field campaigns and/or laboratory studies with substantive inputs from the modeling community. Towards this end, eight 8 minute presentations covering field observations, laboratory work, and modeling efforts were given, followed by an open forum discussion among all breakout attendees. Sandwiched in between the formal presentations and open forum discussion, a short summary of the ASR Absorbing Aerosol Workshop focused on the BB aerosols. The BB aerosol session was well attended with nearly 40 ARM/ASR meeting attendees.The modeling presentations highlighted the critical need for observational data sets of BB aerosol events. Of particular note is that parameterization development for climate models and the evaluation of particle-resolved aerosol simulations require better characterizations and statistics on BB aerosol absorption efficiency from different sources, the evolution of aerosol optics in aging, and the vertical distribution in the presence of clouds.
Findings from the analysis of the ARM-sponsored BBOP data sets are beginning to reveal new insights into the near-field evolution of BB aerosols, indicating that this data set is both unique and comprehensive. Specific highlights from BBOP presentations include the observation of a close relationship between the O/C ratio and single-scattering albedo changes, linking SOA formation and oxidative aging in the atmosphere to increased absorption in near-UV – implying the potential increase of BrC in aging processes – and the first estimate of the tar ball mass loading fraction in BB plumes. It is believed that BBOP data could be used with models to identified observational needs for future field studies. One BBOP manuscript is currently under review, and two others are in preparation.
In addition to in situ measurements (e.g., BBOP data set), ground-based remote-sensing optical measurements were also discussed in the context of providing column total aerosol absorption and extinction profiles. Such measurements were identified as a critical need in the Arctic, where aerosols are mostly above the boundary layer, and over the southeast Atlantic subtropical stratocumulus and cumulus region, in elucidating the underlying cloud responses to BB aerosols aloft. It is noted that cloud responses to BB aerosols are the main scientific focus of the recently started Layered Atlantic Smoke Interactions with Clouds (LASIC) field campaign. During the open forum discussion, it was suggested that ARM data sets could be used to calibrate AERONET biases and provide additional spatial coverage. The recent deployment of the multi-wavelength HSRL during several field experiments showed promising results in delivering the calibrated aerosol extinction profiles. Key findings are summarized below.
Key Findings
It is clear from the both the presentations and open forum discussions that climate models still underestimate aerosol absorption in major biomass burning regions and exhibit large discrepancies in magnitude and sign of predicted BB aerosol forcing over low clouds and at high latitudes. While particle-resolved aerosol process modeling studies can be used to improve large-scale model parameterizations, they need temporally and spatially resolved observations to evaluate the simulated aerosol mixing state and aging time scales. Therefore, three critical observational needs were identified to improve model performance:(i) Attribution of BB absorption to black carbon (BC), brown carbon (BrC) including primary components (POA) and SOA;
(ii) Evolution of the chemical, optical, and volatility properties of BB aerosol; and
(iii) Spatial distribution of BB aerosol extinction and absorption profiles.
It is interesting to note that these observational needs align well with the knowledge gaps identified in the ASR-sponsored Absorbing Aerosol Workshop (January, 2016) despite the broader spectrum of research interests and expertise present in the BB breakout session.
Based on the observational needs, three actionable items were identified during this breakout:
(1) Attribution of BC and BrC absorption. It was suggested that absorption measurements need to be extended into the near-UV (BrC absorption) and IR wavelengths (dust influences). While ARM has some capabilities for UV bands (e.g., UVSAS, MFRSR, and Aethalometer) these measurements are not part of the ARM “baseline” data set nor do they directly measure the absorption coefficient. Compared with direct measurements, inference of absorption coefficient from the difference between extinction and scattering coefficients may introduce large uncertainties in retrievals at low-absorption conditions. It was also recognized that unification of these multiple wavelength absorption measurements in UV and visible from different instruments is needed for better cross-comparison and to bridge the gaps to models (i.e., inference of the refractive indices). Finally, it was also recognized that dust might influence the BB absorption in the super-micron size and have distinct absorption signals in the IR. AERI is the only ARM instrument that provides optical retrievals in the IR, but its application and performance on elimination of dust influence on the BB absorption in the visible likely needs further investigations. For example, in situ iron content (e.g., hematite) could also be used to identify the presence of mineral dust.
(2) Aerosol Evolution. It is clear that BB aerosols undergo significant evolution both in the near and far fields. Controlled evolution studies focusing on aerosol chemical and optical properties through targeted laboratory studies supported by ASR are being conducted on SOA chemistry from BB volatile organic compounds (VOCs) and are planned on tar balls. Each set of studies is expected to contribute to our understanding of how chemical transformations of BB aerosols influence aerosol absorption. As presented in the BBOP-centric talks, significant changes in several aerosol properties are observed in the first few hours following emission. Based on these findings, it was suggested that modelers could use the BBOP data to evaluate model performance on spatial distribution, and evolution in aerosol composition, size distribution, and optical properties within a couple of hours from sources. Tar balls were cited, in particular, as one particle type for which little is known regarding its formation and evolution. The need for additional focused laboratory studies and field campaigns was also identified to help constrain BB VOCs types and amounts that lead to absorbing SOA formation, photochemical “coloring” (increase absorption) and “bleaching” (decrease absorption) reactions, and attribution of absorption by primary and secondary organic aerosols;
(3) Vertical distribution of BB aerosol and aerosol absorption. Similar to the conclusions drawn by the AA workshop participants, the vertical distribution of BB aerosols is considered a high priority and cited as a critical need by researchers. While vertically resolved remote sensing such as HSRL or MPL are part of the ARM instrument suite, these instruments retrieve extinction, not absorption, profiles. Other approaches are limited and subject to uncertainties, relying on inference from the combination of surface and column retrievals such as the AOS and MFRSR. Improvements on those retrieval methods were also suggested, including the use of more wavelengths, aerosol size distribution and species data – available from other ancillary instruments and some of the improvement can be adopted from the AERONET retrieval method. However, the success of these approaches in the weak signal limit may be limited by supplementary data such as surface albedo measurements. It was noted that NOAA is pursuing the development of a “mini” photoacoustic instrument that could be deployed on a UAV (unmanned aerial vehicle) that, if successful, could be very useful in providing in situ profiles of aerosol absorption. Meanwhile, SP2 could provide BC vertical profiles such as in the recent ACME-V and should be considered for more deployment in future field studies. Finally, it was recognized that the horizontal or spatial distribution of BB aerosol physico-chemical properties is also important, especially in the first few hours of the plumes.
Issues
Detailed understanding of the tar ball (TB) formation mechanism is lacking. What drives the transformation of OA to TBs? What role does photochemistry play in this transformation? It was recognized that a nighttime field measurements of BB emissions could provide crucial insights into the TB formation mechanism.Unification of observational wavelength of absorption in UV and visible from different instruments in order to facilitate better cross-comparison and to bridge models calculations.
Needs
It was agreed that closer communications between the modeling and observational/experimental communities is needed.Extension of wavelength-resolved aerosol absorption measurements to include near-UV (< 400 nm) and near-IR (>800 nm).
Cross-comparison of ARM/ASR aerosol optical measurements with AERONET data.
Laboratory studies targeting SOA formation related to aerosol absorption, photochemical coloring. and bleaching.
Field experiments targeting attribution of primary and secondary BrC in both the near and far fields.
Vertically resolved measurements and retrievals of aerosol extinction and absorption.
Decisions
n/aFuture Plans
The open forum discussion pointed out the upcoming NOAA-led FIREx campaign as building upon what BBOP started. Aerodyne ASR scientists will participate in this field campaign. Interest was expressed about the value of a targeted DOE BBOP-2 field campaign.A session on “Cloud adjustments to the presence of shortwave-absorbing aerosols and large-scale feedbacks” is being convened at the 2017 AMS annual meeting in January, 2017, focusing on BB aerosol absorption and impact on clouds.