Brown carbon in the continental troposphere: sources, evolution and radiative impacts Evolution of Brown Carbon in Wildfire Plumes -Submitted to GRL- Rodney Weber, Haviland Forrister, Jiumeng Liu, Athanasios Nenes, Eric Scheuer, Jack Dibb Luke Ziemba, Kenneth L. Thornhill, Bruce Anderson, Glenn Diskin Anne Perring, Joshua Schwarz Pedro Campuzano-Jost, Jose-Luis Jimenez Georgia Tech, University of New Hampshire, NASA Langley, NOAA, University of Colorado

BrC Background – Biomass burning emissions projected to increase. – Contribution likely underestimated, (tracers lost). – Biomass burning is a major source of BrC. – BrC is ubiquitous (if use a direct measurement). – BrC makes non-negligible contribution to TOA radiative forcing (DC3 results, Liu et al, GRL 2014) – Studies suggest BrC can be predicted from BC/OA emissions (Saleh et al., Nature GeoSci 2014), application to models. – Recent chamber experiments show BrC is not stable (bleaches) (Lee ES&T 2014, Zhao ACPD 2015). Goal: Investigate stability of BrC in ambient smoke plumes. Focus on Rim 1 and Rim 2 fires, uniquely large  t.

Measuring BrC in Aerosol Extracts UV-Vis Light source Spectrophotometer detector Sample inlet Sample outlet 2.5 m wave guide Dissolved Species Why measure BrC in solutions ? Isolate BrC from other light absorbers (BC, mineral dust) Long path wave guide  high sensitivity  Insights in to BrC sources, chem., evolution… X Loss of aerosol optical effects (particle size, mixing state) Filter SEAC4RS Approach: BrC = water then methanol extract (series), Here focus on total (water+methanol) light absorption at 365 nm Abs(365nm)  measuring chromophore “concentrations” UNH filters, 300 to 600 nm

SEAC4RS Biomass Burning Events: Rim 1 and 2 Wildfires Tracked over large region

Investigating Plume Evolution All data merged to filter sampling times. Account for dilution with Normalized Excess Mixing Ratios wrt CO (  BrC/  CO), … Plume transport time estimated from HYSPLIT back trajectories. Close to fire, contributions from different fires? As expected: Not much change in BC relative to CO with plume age. Now look at BrC Time from Idaho fires Time from Yosemite fires 1 st test with BC

BrC Evolution in Rim Fires: BrC Decreases with Plume Age relative to CO Most of RIM fire biomass- burning BrC is not stable. Half-life approx. 9 to 15 hours. Cause: bleaching, BrC is semi- volatile … ? Transport time assuming two separate fires All times back to Yosemite fire ½ life ~ 9 hrs from exp fit ½ life ~ 15 hrs

BrC Evolution in Rim Fires: Decreases during periods of sun exposure Photo-bleaching ? Transport time assuming two separate fires All times back to Yosemite fire

Trends in AAE, BC coating thickness,  OA/  CO, O:C Ratio and f60 AAE also decreases, somewhat similar trend as BrC. BC coating has no trend  BrC is driving AAE not optics of shell/cores Different trend in BrC loss and OA BrC chromophores are small mass fraction of OA BrC and O:C ratio have opposite, but similar trends BrC and f60 similar trends

Trends in  BrC/  CO vs AAE and f60 for Rim and other fires

BrC decreases with smoke age relative to CO, but some fraction remains Transport time assuming two separate fires BrC: in plume / bkg) ~ 6 Some fraction of the BrC is stable; a possible source of the background tropospheric BrC ?

DC3: Observed Throughout “Background” Free BC BrC Total Inferred from PSAP Altitude, km BrC/BC Observed BrC is ~ 20% of direct radiative forcing Liu et al, GRL Surface albedo, gases, altitude-resolved aerosols, T, … BC Only -25 W/m 2 BC + BrC -19 W/m 2 TOA Instantaneous Forcing 13 km 0 km Radiative transfer model: SBDART Is this stable smoke BrC?

Results 1.BrC prevalent in biomass burning plumes, but most is unstable with half-life of 9 – 15 hrs (bleaching, semi-volatile BrC loss, …?). 2.“Fresh” smoke BrC is dynamic, but a stable fraction exists and may be source of observed ubiquitous background BrC that is optically important. 3.Can’t predict BrC solely on emissions. Changes with aging appear complex. Smoke has multiple environmental impacts. 1.Climate (BrC) 2.Visibility (BrC) 3.Linked to adverse health impacts (BrC is well correlated with measures of aerosol toxicity, Verma and Weber et al., 2014, 2015).