Investigation of the wet removal rate of black carbon in East Asia: validation of a below- and in-cloud wet removal scheme in FLEXPART v10.4

Abstract

Abstract. Understanding the global distribution of atmospheric black carbon (BC) is essential to unveil its climatic effect. However, there are still large uncertainties regarding the simulation of BC transport due to inadequate information about the removal process. We accessed the wet removal rate of BC in East Asia based on long-term measurements over the 2010–2016 period at three representative background sites (Baengnyeong and Gosan in South Korea and Noto in Japan). The average wet removal rate, represented by transport efficiency (TE), i.e. the fraction of undeposited BC particles during transport, was estimated as 0.73 in East Asia from 2010 to 2016. According to accumulated precipitation along trajectory, TE was lower in East and North China, where the industrial sector (thin-coated) is dominant; in contrast, that in South Korea and Japan showed higher values due to the transport sector (thick-coated), with emissions mainly from diesel vehicles. By the same token, TE in winter and summer showed the highest and lowest values, respectively, depending on the dominant emission sectors, such as house heating (thick-coated) and industry. The average half-life and e-folding lifetime of BC were 2.8 and 7.1 days, respectively, similar to previous studies, but those values differed according to the geographical location and meteorological conditions of each site. Next, by comparing TE from the FLEXible PARTicle (FLEXPART) Lagrangian transport model (version 10.4), we diagnosed the scavenging coefficients (s−1) of the below- and in-cloud scavenging scheme implemented in FLEXPART. The overall median TE from FLEXPART (0.91) was overestimated compared to the measured value, implying underestimation of wet scavenging coefficients in the model simulation. The median of the below-cloud scavenging coefficient showed a lower value than that calculated from FLEXPART, by a factor of 1.7. On the other hand, the overall median of the FLEXPART in-cloud scavenging coefficients was highly underestimated by 1 order of magnitude compared to the measured value. From the analysis of artificial neural networks, the convective available potential energy, which is well known as an indicator of vertical instability, should be considered in the in-cloud scavenging process to improve the representative regional difference in BC wet scavenging over East Asia. For the first time, this study suggested an effective and straightforward evaluation method for wet scavenging schemes (both below- and in-cloud) by introducing TE along with excluding effects from the inaccurate emission inventories.

Estimation of Surface Concentrations of Black Carbon from Long-Term Measurements at Aeronet Sites over Korea

“…Both Baengnyeong (124.63 • E, 37.97 • N) and Gosan (127.27 • E, 37.34 • N) are representative background sites in Korea and are located in the western region of the Korean Peninsula, with similar longitudes but different latitudes. Thus, these sites are suitable for monitoring pollutant transport from China, North Korea (especially Baengnyeong), and South Korea [12,54]. The Baengnyeong site is one of the intensive measurement stations operated by the Korean National Institute for Environmental Research, and the east and south are subject to local emissions from agricultural sources and small towns [55,56].…”

“…As discussed in the previous section, it could be easily deduced that in-situ BC concentration was increased by low wind speeds (stagnant conditions), low PBLH (low vertical mixing), and low temperatures (high emissions from heating), resulting in accumulated BC at the surface level. The reason for the low correlation with RH is that freshly emitted BC particles have a hydrophobic property, which is less effective with RH [54]. Moreover, the growth factor of more hygroscopic BC-containing particles due to aging is generally lower than that of BC-free particles [102,103].…”