Secondary organic aerosol production from local emissions dominates the organic aerosol budget over Seoul, South Korea, during KORUS-AQ

Abstract. Organic aerosol (OA) is an important fraction of submicron aerosols. However,

it is challenging to predict and attribute the specific organic compounds and

sources that lead to observed OA loadings, largely due to contributions from

secondary production. This is especially true for megacities surrounded by

numerous regional sources that create an OA background. Here, we utilize

in situ gas and aerosol observations collected on board the NASA DC-8 during

the NASA–NIER KORUS-AQ (Korea–United States Air Quality) campaign to

investigate the sources and hydrocarbon precursors that led to the secondary

OA (SOA) production observed over Seoul. First, we investigate the

contribution of transported OA to total loadings observed over Seoul by

using observations over the Yellow Sea coupled to FLEXPART Lagrangian

simulations. During KORUS-AQ, the average OA loading advected into Seoul was

∼1–3 µg sm−3. Second, taking this background into

account, the dilution-corrected SOA concentration observed over Seoul was

∼140 µgsm-3ppmv-1 at 0.5 equivalent photochemical

days. This value is at the high end of what has been observed in other

megacities around the world (20–70 µgsm-3ppmv-1 at 0.5

equivalent days). For the average OA concentration observed over Seoul

(13 µg sm−3), it is clear that production of SOA from locally

emitted precursors is the major source in the region. The importance

of local SOA production was supported by the following observations.

(1) FLEXPART source contribution calculations indicate any

hydrocarbons with a lifetime of less than 1 day, which are shown to dominate the

observed SOA production, mainly originate from South Korea. (2) SOA

correlated strongly with other secondary photochemical species, including

short-lived species (formaldehyde, peroxy acetyl nitrate, sum of acyl peroxy

nitrates, dihydroxytoluene, and nitrate aerosol). (3) Results from

an airborne oxidation flow reactor (OFR), flown for the first time, show a

factor of 4.5 increase in potential SOA concentrations over Seoul versus over

the Yellow Sea, a region where background air masses that are advected into

Seoul can be measured. (4) Box model simulations reproduce SOA

observed over Seoul within 11 % on average and suggest that short-lived

hydrocarbons (i.e., xylenes, trimethylbenzenes, and semi-volatile and intermediate-volatility compounds) were the main SOA precursors over Seoul. Toluene

alone contributes 9 % of the modeled SOA over Seoul. Finally, along with

these results, we use the metric ΔOA/ΔCO2 to

examine the amount of OA produced per fuel consumed in a megacity, which

shows less variability across the world than ΔOA∕ΔCO.