Characterisation of regional ambient biomass burning organic aerosol mixing ratios and their evolution with ageing

M.D. Jolleys*1, H. Coe1, G. McFiggans1, G. Capes1, J.D. Allan1,2, J. Crosier1,2, P.I. Williams1,2, G. Allen1, K.N. Bower1,

J. L. Jimenez3, L.M. Russell4, M. Grutter5, D. Baumgardner5

1 Centre for Atmospheric Science, University of Manchester, UK 2 National Centre for Atmospheric Science, University of Manchester, UK

3 Cooperative Institute of Research in the Environmental Sciences and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, USA 4 Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, USA

5 Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City, Mexico

Introduction Biomass burning (BB) represents a significant source of organic aerosol (OA) on a global scale, forming an important influence on climate through perturbations to Earth's solar radiation balance. Around 90% of total global primary organic aerosol (POA) is derived from wildfires, prescribed burns and biofuel combustion which contribute towards the overall BB emissions source (1). The physical and chemical properties of biomass burning organic aerosol (BBOA) and the geographical distribution of fires act to optimize the potential for radiative forcing. However, the impacts of BBOA on global climate remain highly uncertain. A critical element of this uncertainty relates to the evolution of aerosol loadings within the atmosphere and the role of secondary organic aerosol (SOA) formation, which can act to both increase overall loadings and change optical properties. While SOA formation leading to significant additional OA mass has been widely observed in anthropogenic, urban emissions, its net contributions in ageing biomass burning plumes remain unclear. Emission ratios of OA expressed relative to a co-emitted reference species such as CO represent a key method of determining changes in OA concentration with age, such that:- Previous assessments of ambient emissions have identified both increasing (2, 3) and decreasing (4) OA/ΔCO with ageing, while other studies suggest little or no net change in OA/ΔCO during ageing regardless of physical and chemical changes to OA (5, 6).

References (1) Bond, T. C.; et al. Journal of Geophysical Research - Atmospheres 2004, 109, (D14). (2) De Gouw, J.; Jimenez, J. L. Environmental Science and Technology 2009, 43, (20), 7614-7618. (3) DeCarlo, P. F.; et al. Atmospheric Chemistry and Physics 2008, 8, (14), 4027-4048. (4) Akagi, S. K.; et al. Atmospheric Chemistry and Physics 2012, 12, (3), 1397-1421. (5) Capes, G.; et al. Journal of Geophysical Research - Atmospheres 2008, 113, (D20). (6) Cubison, M. J.; et al. Atmospheric Chemistry and Physics 2011, 11, (23), 12049-12064. (7) DeCarlo, P. F.; et al. Atmospheric Chemistry and Physics 2010, 10, (12), 5257-5280. Background image: http://free-images.gatag.net

Conclusions

• No evidence of significant net SOA formation in biomass burning emissions from four ambient datasets.

• Contrasts in average OA/ΔCO values between different regions – possible result of changes in fuel type/combustion conditions.

• OA/ΔCO consistently lower for aged OA compared to fresh OA, but magnitude of difference changes between regions.

• OA increasingly oxidized for the aged fraction (higher f44) – suggests any increase in OA mass is exceeded by OA loss.

• Variability in OA/ΔCO close to source consistently greater than any changes occurring with ageing – likely to be a greater influence on atmospheric burden.

300

250

200

150

100

50

0

OA

co

nce

ntr

atio

n (

ug m

-3)

50004000300020001000

Excess CO concentration (ug m-3

)

DABEXSlope = 0.064 ± 0.001Intercept = -5.239 ± 0.398

ACTIVESlope = 0.286 ± 0.008Intercept = 1.308 ± 1.062

MILAGROSlope = 0.049 ± 0.001Intercept = -0.213 ± 0.103

ITOPSlope = 0.016 ± 0.001Intercept = 0.686 ± 0.233

ACTIVE MILAGRO DABEX ITOP

Campaigns A series of four ambient biomass burning datasets were included within this analysis, with the aim of characterising OA emissions and transformations across a number of different regions :- ACTIVE (Aerosol and Chemical Transport in Tropical Convection):- November 2005 and February 2006 – Darwin, northern Australia Measurements by NERC Dornier-228 of active fires around the Tiwi islands to the north and more aged, transported emissions from agricultural fires to the east in Queensland. DABEX (Dust and Biomass Experiment):- February 2006 – West Africa Fresh biomass burning emissions from widespread fires to the south over Benin and Nigeria sampled by the FAAM BAe-146 aircraft. Layers of lofted aged biomass burning aerosol also encountered further to the west and downwind from the source region. ITOP (Intercontinental Transport of Ozone and Precursors):- July and August 2004 - Azores/North Atlantic Highly aged plumes from North American boreal forest fires sampled by the BAe-146 around the Azores, following transportation across the North Atlantic. MILAGRO (Megacities Initiative: Local and Global Research Observations):- March 2006 - Mexico Ground site at Altzomoni near the Paso de Cortez to the southeast of Mexico City strongly influenced by biomass burning, with contributions from both local mountain forest fires and savanna fires to the southeast of the city region.

Contact: matthew.jolleys@postgrad.manchester.ac.uk

Top: Location of fires sampled during ACTIVE, with flight tracks for periods where fresh plumes were intercepted. Bottom: Location of MILAGRO site and fires in surrounding area, with back trajectories showing air mass origins.

Above, top to bottom: Organic aerosol mass concentration versus excess CO from all campaigns, for all data and segregated fresh and aged OA fractions, respectively. Coefficients are for linear regressions with uncertainties of ± 1σ. Average OA/ΔCO is substantially elevated for ACTIVE above average values from all other campaigns.

Below left: Schematic diagram showing changes in OA/ΔCO with photochemical age in emissions from biomass burning and urban sources. Markers on y-axis represent source emission ratios from chamber studies/literature sources. Figure adapted from DeCarlo et al. (7).

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

f 44

30x10-3252015105

f60

Fresh OA Aged OA

1.0

0.8

0.6

0.4

0.2

0.0

O

3 /C

O

250

200

150

100

50

OA

co

nce

ntr

atio

n (

µg

m-3

)

12001000800600400200

Excess CO concentration (µg m-3

)

0.5

0.4

0.3

0.2

0.1

0.0

f44

ACTIVE - Fresh

200

150

100

50

OA

co

nce

ntr

atio

n (

µg

m-3

)

50004000300020001000

Excess CO concentration (µg m-3

)

MILAGRO - Fresh

200

150

100

50

OA

concentr

ation (

µg m

-3)

12001000800600400200

Excess CO concentration (µg m-3

)

DABEX - Fresh

30

20

10

OA

co

nce

ntr

atio

n (

µg

m-3

)

180160140120100806040

Excess CO concentration (µg m-3

)

0.5

0.4

0.3

0.2

0.1

0.0

f44

ACTIVE - Aged20

15

10

5

OA

co

nce

ntr

atio

n (

µg

m-3

)

500400300200100

Excess CO concentration (µg m-3

)

MILAGRO - Aged

20

15

10

5

0

OA

concentr

ation (

µg m

-3)

35030025020015010050

Excess CO concentration (µg m-3

)

DABEX - Aged

Below: OA vs ΔCO for fresh and aged fractions during ACTIVE, MILAGRO and DABEX, coloured by f44. There is a trend of increasing f44 for aged OA in all instances, indicating OA becomes more oxidised with ageing, with an increased abundance of m/z 44 from the CO2

+ ion associated with oxygenated compounds such as organic acids. Data from DABEX are subject to considerable noise given operation of the Q-AMS at higher altitudes.

Above: f44 vs f60 by ΔO3/ΔCO for fresh and aged OA during MILAGRO. Below right: OA/ΔCO probability density functions for ACTIVE, MILAGRO and DABEX, showing variable distributions between flights.

Results OA/ΔCO variability: Data from four studies show high level of variability in regional average OA/ΔCO, ranging from a maximum of 0.286 (ACTIVE) to 0.016 (ITOP) (A). Considerable variability is also observed between separate measurement periods within campaigns (G). Variability at source is consistent with that observed in chamber studies (H) Changes with ageing: There is no evidence to support a consistent increase in average OA/ΔCO due to the formation of SOA in any locations. Segregating data into fresh and aged OA fractions based on major OA peaks, where coinciding with peaks in ΔCO and number concentration and a reduction in ΔO3/ΔCO, shows OA/ΔCO to decrease with ageing (B-C). Compositional changes: Greater prevalence of m/z 44 mass fragment relative to total OA (f44) indicates OA becomes increasingly oxidised with age (D-E), suggesting OA loss through evaporation and deposition have a greater influence on OA/ΔCO. Progression of f44 and f60 (indicative of fresh BBOA) (F) is consistent with that observed by Cubision et al. (6).

A

B

C

D

E

F

G

H