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

Abstract

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.

Towards a satellite formaldehyde – in situ hybrid estimate for organic aerosol abundance

“…6.3) were not located outside of the SE US. Similar vertical profile shapes of OA and submicron particles were also observed in a campaign outside the US over South Korea (Nault et al, 2018). Although OA accounted for ∼ 40 % of the total submicron particles, the shape of OA and total submicron particles' vertical profiles were nearly identical.…”

“…This also indicates that SOAs are enhanced near the source regions statistically. Nault et al (2018) also showed the production of HCHO and SOA are similar and plateau around 0.5-1 photochemical days. So, in the near field of emissions and chemistry, the production of these two species is similar; however, outside the near field of emissions and rapid chemistry, the long lifetime of OA vs. the steady state of HCHO would start controlling the slopes and correlations.…”

“…During KORUS-AQ, the high OA to HCHO air masses also had high acetonitrile. By the time we sampled, most organic aerosols were secondary (Nault et al, 2018). This indicates that the formation rates of OA and HCHO from different emission sources contribute to the different slopes of OA-HCHO.…”

“…Moreover, although the lifetime of HCHO (1-3 h) is shorter than OA (1 week), HCHO continues to form from slower-reacting VOCs, as well as from the oxidation of latergeneration products. Observations across megacities around the world show that OA formation in polluted/urban areas happens over about 1 day (e.g., DeCarlo et al, 2010;Hodzic and Jimenez, 2011;Hayes et al, 2013Hayes et al, , 2015, and HCHO is also significantly formed over this timescale (Nault et al, 2018). In addition, Veefkind et al (2011) found that satellite AOD correlated with HCHO over the summertime SE US, BB regions, and southeast Asian industrialized regions.…”

Atmospheric Acetaldehyde: Importance of Air‐Sea Exchange and a Missing Source in the Remote Troposphere

“…Equations and are both purely observationally constrained and therefore are not affected by model uncertainties. HR‐AMS‐measured PM 1 OA mass concentration (DeCarlo et al, ; Nault et al, ; Schroder et al, ) is used in equation , and total aerosol surface area (nucleation, Aitken, accumulation modes) calculated from AMP measurements (Brock et al, ; Kupc et al, ; Williamson et al, ) is used in equation . Figure shows the combined results from both ATom‐1 and ATom‐2, excluding measurements below 3 km (where oceanic emissions play an important role).…”