Atmospheric Trace Gas (NO2 and O3) Variability in South Korean Coastal Waters, and Implications for Remote Sensing of Coastal Ocean Color Dynamics

Abstract

Coastal environments are highly dynamic, and are characterized by short-term, local-scale variability in atmospheric and oceanic processes. Yet, high-frequency measurements of atmospheric composition, and particularly nitrogen dioxide (NO2) and ozone (O3) dynamics, are scarce over the ocean, introducing uncertainties in satellite retrievals of coastal ocean biogeochemistry and ecology. Combining measurements from different platforms, the Korea-US Ocean Color and Air Quality field campaign provided a unique opportunity to capture, for the first time, the strong spatial dynamics and diurnal variability in total column (TC) NO2 and O3 over the coastal waters of South Korea. Measurements were conducted using a shipboard Pandora Spectrometer Instrument specifically designed to collect accurate, high-frequency observations from a research vessel, and were combined with ground-based observations at coastal land sites, synoptic satellite imagery, and air-mass trajectory simulations to assess source contributions to atmospheric pollution over the coastal ocean. TCO3 showed only small (<20%) variability that was driven primarily by larger-scale meteorological processes captured successfully in the relatively coarse satellite imagery from Aura-OMI. In contrast, TCNO2 over the ocean varied by more than an order of magnitude (0.07–0.92 DU), mostly affected by urban emissions and highly dynamic air mass transport pathways. Diurnal patterns varied widely across the ocean domain, with TCNO2 in the coastal area of Geoje and offshore Seoul varying by more than 0.6 DU and 0.4 DU, respectively, over a period of less than 3 h. On a polar orbit, Aura-OMI is not capable of detecting these short-term changes in TCNO2. If unaccounted for in atmospheric correction retrievals of ocean color, the observed variability in TCNO2 would be misinterpreted as a change in ocean remote sensing reflectance, Rrs, by more than 80% and 40% at 412 and 443 nm, respectively, introducing a significant false variability in retrievals of coastal ocean ecological processes from space.

Comparison of Near‐Surface NO2 Pollution With Pandora Total Column NO2 During the Korea‐United States Ocean Color (KORUS OC) Campaign

“…Generally, lower in situ and TC NO 2 amounts were recorded on the Onnuri during KORUS-OC than landbased sites during KORUS-AQ Tzortziou et al, 2018). We observed average PSI TC NO 2 values near 0.20 DU, with the highest values occurring as the ship sailed on the Yellow Sea near the coast off Seoul on 1 June .…”

“…The initial (0900 KST) jump in surface NO 2 might have been associated with a pulse of air enriched in CO and NO 2 that mixed down over the ship (O 3 dropped from ~40 to 35 ppbv coincident with the NO 2 and CO increases). Tzortziou et al () examined the 1 June PSI TC NO 2 on the Onnuri in detail, comparing the shipboard readings to the 1 June time series from the PSI No. 40 in Seoul at Yonsei University.…”

“…At 1200 local time, however, the air parcels reaching Onnuri were coming mostly from the city of Seoul, coinciding with the peak in TC NO 2 observed over the ocean and similar to our HYPSLIT forward trajectories. Note that the Seoul PSI 40 TC NO 2 averaged 1.0 (±0.64) DU over the period 16 May to 15 June 2016 (Tzortziou et al, ). OMI TC NO 2 averaged 0.74 DU over the same period, but there was a gap in OMI data from 29 May to 11 June 2016.…”

“…This case shows both surface and PSI TC NO 2 responding simultaneously to a polluted air mass from Seoul, but TC NO 2 measurements are offset in time from surface NO 2 measurements. The initial (0900 KST) jump in surface NO 2 might have been associated with a pulse of air enriched in CO and NO 2 that mixed down over (Figure 8d in Tzortziou et al, 2018). There were two additional peaks in the PSI 40 TC NO 2 time series on 1 June, both reaching 1.2 DU over Seoul between 1400 and 1800 KST.…”

“…There were two additional peaks in the PSI 40 TC NO 2 time series on 1 June, both reaching 1.2 DU over Seoul between 1400 and 1800 KST. Tzortziou et al (2018) also ran HYSPLIT back trajectories from Onnuri locations, starting at four times on 1 June between 1000 and 1600 KST. Trajectories initiated over the Onnuri location before 1000 local time, when TC NO 2 over the ocean was <0.25 DU, indicated transport of air masses from a region north of the city of Seoul where TC NO 2 is typically relatively low ( Figure 10 in Tzortziou et al, 2018).…”