A Top-Down Perspective on Organic Aerosol: From Satellite to Surface AMS 92nd Annual Meeting January 23, 2012 Colette L. Heald Kateryna Lapina and Bonne Ford

ORGANIC AEROSOL MAKES UP AN IMPORTANT/DOMINANT FRACTION OF OBSERVED AEROSOL Globally makes up 25-75% of total fine aerosol at the surface (ignoring dust here) [Zhang et al., 2007] Sulfate Organics

MODELS UNDERESTIMATE OBSERVED OA Models drastically underestimate SOA from 4 campaigns [Volkamer et al., 2006] ACE-Asia (2001): 3 groups measured high OA off Asia. GEOS-Chem simulation factor of too low [Heald et al., 2005] Obs (Maria et al., 2003) GEOS-Chem Models (from box models to global models) were found to underestimate observed concentrations in the ambient atmosphere. “Anthropogenic” air masses show more aerosol growth than can be explained by the oxidation of aromatics. [de Gouw et al., 2005]

HOW LARGE A MISSING SOURCE OF ORGANIC AEROSOL? Goldstein and Galbally [2007] suggest that SOA source may be anywhere from TgC/yr. Can total aerosol optical depth (AOD) measurements shed any light on the total budget of OA? For comparison, current global model (GEOS-Chem) estimates total ~50 TgC/yr

IF ONLY AEROSOL IN THE ATMOSPHERE WAS OA, WHAT LOADING IS IMPLIED BY SATELLITE AOD? Calculate the “hypothetical” AOD implied by a constant 1  g/sm 3 profile over the land, and see how we need to scale this locally to make up ENTIRE AOD reported by MISR. Inverted OA loading is 3.5 TgC over land. Assume a 6 days lifetime = 215 TgC/yr  extrapolate to include outflow ~430 TgC/yr. (middle of Goldstein & Galbally range) Inverted total MISR AOD: Surface OA concentrations

A MORE REALISTIC POSSIBILITY: REMOVE CONTRIBUTIONS FROM DUST, BC, INORGANICS (assuming all the negative bias in the model is ONLY OA) If remove N. Africa & the Middle (dust), estimate that ~150 TgC/yr source is required to close the MISR-GEOS- Chem* discrepancy. (*Made an assumption about the vertical profile of OA*) DJFJJA MISR GEOS-Chem* MISR- GEOS-Chem* *excluding OA

This is more than THREE TIMES what is currently included in global models…. BUT at the low end of Goldstein & Gallbally [2007] range. HAVE WE REDUCED THE UNCERTAINTY ON THE OA BUDGET? Existing GEOS- Chem sources 140 Our satellite top-down estimate 150 Bottom-up estimate [Goldstein and Galbally, 2007] All units in TgCyr -1 Satellite-based estimate [Heald et al., 2010] AMS surface-based optimization [Spracklen et al., 2011] POA (fixed) SOA (optimized) Also in relatively good agreement with Spracklen et al. [2011] estimate.

MARINE ORGANIC AEROSOL Ocean Surfactant Layer (with Organics) WIND Sea-spray emission [O’Dowd et al., 2004] Under biologically active conditions, OA has been observed to dominate sub-micron aerosol mass. SeaWIFS SPRING (high biological activity)

IS THE OCEAN AN IMPORTANT SOURCE OF OA? Previous estimates range from 2.3 to 75 TgC/yr No marine OAWith marine OA Observations from 5 ship cruises show that marine OA from 2 schemes (based on MODIS / SeaWIFS chlorphyll-a) of ~9 TgC/yr are more than sufficient to reproduce sub-micron OA. [Lapina et al., 2011] OA Emissions Measurements from: J.D. Allan, H. Coe, G. McFiggans, S.R. Zorn, F. Drewnick, T.S. Bates, L.N. Hawkins, L.M. Russell

DIGGING IN DEEPER: 17 AIRCRAFT FIELD CAMPAIGNS * All AMS measurements, except ITCT-2K4 (PILS) and ACE-Asia (filters) Aircraft constraints on the organic aerosol distribution through depth of troposphere in remote, polluted and fire influenced regions. GOAL: investigate vertical profile and compare with one CONSISTENT model. Measurements PIs: Hugh Coe (ITOP, ADRIEX, DABEX, DODO, AMMA, ADIENT, EUCAARI, OP3, VOCALS-UK, TROMPEX), Jose Jimenez, (MILAGRO, IMPEX, ARCTAS), Rodney Weber (ITCT-2K4), Ann Middlebrook (TexAQS), Lynn Russell (ACE-Asia) GEOS-Chem SOA simulation: 2 product model, monoterpenes/sesquiterpenes +OH/O3/NO3 (Griffin et al, 1999), low-NOx isoprene+OH (Kroll et al., 2006), NOx dependent aromatics +OH(Ng et al., 2007)  latest description Henze et al., 2008

OVERALL COMPARISON OF OA SIMULATION (an “update” to Volkamer et al., 2006) Median model underestimate of aircraft observations less than a factor of 5. Key difference from Volkamer et al [2006]: Discrepancy is largest close to source. Range =

CAN WE ATTRIBUTE THE MODEL UNDERESTIMATE? Adding ~100 Tg/yr source of ASOA (as suggested by Spracklen et al., 2011) improves comparison in polluted regions, but leads to too much OA aloft and in remote regions. Higher volatility OA? OA sink?

FRAGMENTATION LOSS OF ORGANICS FunctionalizationFragmentation (break C-C bonds) OH + Oxidation By OH 5% 95% Simple sensitivity tests in GEOS-Chem: assuming that 5% of reacted organics are fragmented (LOSS) assume that 95% are functionalized (NO CHANGE) Testing 2 types of fragmentation loss: 1.Oxidation of gas-phase organics (faster, k OH =2x cm 3 /molecules/s) 2.Heterogeneous oxidation of SOA (slower, k OH =1x cm 3 /molecules/s) [Molina et al., 2004; Kwan et al., 2006; Kroll et al., 2007; Chan et al., 2007; Kroll et al., 2009] Increase volatility Decrease volatility

IMPACT OF FRAGMENTATION ON SIMULATED SOA BUDGET (GEOS-Chem: 2008 simulation) GASPARTICLE Gas-Phase Fragmentation -47% Heterogeneous Fragmentation = slow leak -15% Fragmentation of gas-phase organics efficiently “prevents” SOA formation. Is most efficient for more volatile organics. Fragmentation from heterogeneous oxidation is much slower but is still a potentially important sink of SOA. Gas-Phase: -47% (likely upper limit) Heterogeneous: -15%

Effect of gas-phase fragmentation sink is comparable to increasing volatility away from source (via enthalpy of vaporization). Adding ~100 Tg/yr of ASOA and a gas fragmentation sink brings model simulation to within 1  g/m 3 of observed concentrations in 15 of 17 campaigns. Model may need SOURCES and SINKS. ASOAx30 +heterogeneous fragmentation sink ASOAx30 +gas-phase fragmentation sink ASOAx30 +  H=25 kJ/mol (increase volatility) ~equivalent IMPACT OF FRAGMENTATION SINK & CHANGING VOLATILITY ON COMPARISONS WITH FIELD DATA [Heald et al., 2011]

IS AEROSOL CONTRIBUTING TO CLIMATE TRENDS IN SOUTHEASTERN US? [Portmann et al., 2009] Summertime trends Data: Global Historical Climate Network Daily (GHCND) “Although clearly speculative, increasing biogenic secondary organic aerosol/cloud effects linked to forest regrowth and/or interactions with anthropogenic pollution is one possibility that is qualitatively consistent, not only with the spatial structure, but also with the seasonality of the correlation of the unusual negative temperature trends with precipitation found in the southeastern United States.”

SEASONAL AEROSOL MAXIMUM IN THE SOUTHEASTERN US Annual Mean AOD (MISR) Summer-Winter AOD (MISR) Summer-Winter AOD (MODIS)AOD at Walker Branch MODIS MISR [Goldstein et al., 2009] Seasonal maximum in AOD consistent with biogenic emissions (implication: biogenic SOA)

IS THIS CONSISTENT WITH OUR UNDERSTANDING OF AEROSOL IN THE REGION? NO! Model does not reproduce the Southeastern maximum. Spring (MAM) Summer (JJA)

BUT, IT’S ALSO INCONSISTENT WITH SURFACE DATA Surface data does not indicate that organic aerosol dominates at surface. Also see stronger seasonal cycle in Northeast. Possibly aerosol aloft? [Malm et al., 2003] Preliminary Comparison of Vertical Profile in the SE US Observed Seasonality of Surface PM Bonne Ford (CSU)

Acknowledgments: Satellite OA: David Ridley, Easan Drury, Sonia Kreidenweis Marine OA: Dominick Spracklen, Steve Arnold, James Allan, Hugh Coe and Gordon McFiggans, Soeren Zorn, Frank Drewnick, Tim Bates, Lelia Hawkins, Lynn Russell, Sasha Smirnov, Colin O’Dowd, Andy Hind 17 Aircraft Campaigns: Hugh Coe, Jose Jimenez, Rodney Weber, Ann Middlebrook, Roya Bahreini, Lynn Russell, Matthew Jolleys, May Fu, James Allan, Keith Bower, Gerard Capes, Jonathan Crosier, Will Morgan, Niall Robinson, Paul Williams, Mike Cubison, Pete DeCarlo, Ed Dunlea and MISR, MODIS & CALIOP retrieval teams