Continuous Measurement and Molecular Compositions of Atmospheric Water‐Soluble Brown Carbon in the Nearshore Marine Boundary Layer of Northern China: Secondary Formation and Influencing Factors

Abstract

Understanding the evolution of brown carbon (BrC) in the atmosphere is essential for investigating its climate effects. This study deployed a novel in‐situ BrC continuous observation system to firstly measure water‐soluble BrC absorption (AbsWS‐BrC) in an offshore island over the Bohai Sea in the winter of 2020. The AbsWS‐BrC abundance before the cold wave (BCW) was more than twice higher than that after the cold wave (ACW). This was mainly ascribed to the substantially suppressed formation of secondary WS‐BrC (WS‐BrCsec). Diurnal patterns of AbsWS‐BrC exhibited nighttime peaks, which derived from enhanced primary emissions and strong aqueous‐phase processes. Photochemical processes bleached daytime WS‐BrC during BCW, while daytime peaks of WS‐BrCsec emerged during ACW due to the weakened photobleaching effect. Statistical analysis indicated ambient temperature and relative humidity as well as total oxidized nitrogen (NO2 + NO3−) and reduced nitrogen (NH4+ + NH3) were the dominant factors promoting WS‐BrCsec. Moderate aerosol pH (>2.5) also facilitated the formation of WS‐BrCsec while no obvious dependences of WS‐BrCsec on gas‐particle partitioning of ammonia, O3, and sulfur precursors were found. At the molecular level, BrC chromophores with the identified compositions, carbon oxidation state, O/C, H/C, and absorption spectra were compared between the two periods. Characteristics of water‐soluble CHO‐ and CHON‐BrC were more of secondary origin during BCW, while primary emissions contributed significantly to BrC chromophores during ACW. This study highlights the advantage of high resolution BrC measurement in probing its dynamic evolution and influencing factors.

Tethered balloon system and High-Resolution Mass Spectrometry Reveal Increased Organonitrates Aloft Compared to the Ground Level

“…88 Li et al built upon this observation by showing that increased brown carbon SOA correlated with increases in RH, aerosol water content, and nitrogen precursors (e.g., NO 2 , NO 3 − , and NH x ). 89 Further in line with this narrative are observations from Li et al, who noted an increase in organonitrates (specifically nitrophenols and nitrocatechols) with increased RH and aerosol water content. 90 It is therefore possible that for the captured ambient conditions on April 17, the unique aloft MF (notably the dominance of CHNO compounds) may be related to aqueous phase processing.…”

“…Cheng et al showed that high RH enhanced the formation of brown carbon SOA (often related to biomass burning) in spite of low temperatures . Li et al built upon this observation by showing that increased brown carbon SOA correlated with increases in RH, aerosol water content, and nitrogen precursors (e.g., NO 2 , NO 3 – , and NH x ) . Further in line with this narrative are observations from Li et al, who noted an increase in organonitrates (specifically nitrophenols and nitrocatechols) with increased RH and aerosol water content .…”

Long‐Emission‐Wavelength Humic‐Like Component (L‐HULIS) as a Secondary Source Tracer of Brown Carbon in the Atmosphere

“…This suggests a significant secondary formation of BrC. This Abs BrC,L-HULIS,sec (365) magnitude was lower than that of secondary WS-BrC before the cold wave (5.42 ± 4.84 Mm 1 ), but during the summer campaign comparable to that observed after the cold wave (1.27 ± 0.99 Mm 1 ) (Li, Qin, et al, 2023). During the winter campaign, Abs BrC, L-HULIS,sec (365) was higher in Chengdu, Lanzhou, and Wuhan, but it was the lowest in Shanghai during the summer campaign.…”

“…This suggests that, besides this secondary source, other sources may also contribute to WS-BrC in this winter campaign. Li, Qin, et al (2023) assessed the contribution of secondary WS-BrC using the MRS method on Juehua Island before and after a cold wave. Before the cold wave, secondary WS-BrC was found to contribute 60% of WS-BrC at 365 nm, which decreased to 33% after the cold wave.…”

“…Recently, Wang et al (2019) developed the minimum R-squared (MRS) method to differentiate the light absorption determined by AE33 into secondary BrC and primary BrC in fine particles. The MRS method has also been extended to explore the secondary WS-BrC (Li, Qin, et al, 2023). However, BrC chromophores in the atmosphere rapidly bleach to colorless or produce darker ones due to photolysis (such as UV) or oxidation (such as O 3 , NH 3 , NO x ) (He et al, 2022;Lee et al, 2014;Updyke et al, 2012).…”