| Organic aerosol(OA)is an important component of atmospheric particulate matter.A group of colored OA compounds termed brown carbon(Br C)is tends to absorb more solar radiation in the near-ultraviolet and visible spectral ranges,and has the potential to affect the regional air quality,atmospheric photochemistry,and Earth's energy budget.Asian dust can serve as an important vector for long-range transport of organic matter and microorganisms,especially for alpine and other remote regions.A comprehensive field campaign was carried out along the transport pathway of Asian dust during the spring of 2016,including a desert site(Erenhot),a rural site(Zhangbei),and an urban site(Jinan),in northern China.Light-absorbing and fluorescence properties,molecular characteristics,radiative forcing,and potential sources of Br C were investigated based on atmospheric total suspended particulate(TSP)samples using ultraviolet–visible absorption spectrophotometry,fluorescence excitation–emission matrices(EEM),and high-performance liquid chromatograph and high-resolution mass spectrometry(HPLC-HRMS).The potential sources and evolution of atmospheric Br C were analyzed by combining parallel factor analysis(PARAFAC)and other source analysis methods,with the aim of providing useful information on the climatic and ecological effects of different types of Br C in northern China.The average concentration of water-soluble organic carbon(WSOC)was 2.32±0.50?g m-3 at Erenhot,4.89±2.27?g m-3 at Zhangbei,and 8.40±2.39?g m-3 at Jinan.During the sampling period,and the WSOC concentrations of the dust samples from Erenhot(3.91±0.96?g m-3)were almost twice those of non-dust periods,demonstrating that dust deposition is an important source of organic matter in such remote region.Samples from Zhangbei had higher mass absorption efficiency at 365nm(MAE365,1.32±0.34 m~2 g-1)than those from Jinan(1.00±0.23 m~2 g-1)and Erenhot(0.84±0.30 m~2 g-1).Erenhot suffered frequent and severe dust storms but was less affected by anthropogenic activities;Zhangbei was influenced by local biomass burning and anthropogenic activities,while Jinan was affected by heavy vehicle exhaust.The simplistic radiative forcing models were applied to estimate the fractions of solar energy absorbed by WS-Br C relative to black carbon and the forcing efficiency of Br C.The results indicate that the light-absorbing effect of Br C should not be ignored in radiative forcing models,especially at shorter wavelengths.EEM spectroscopy combined with PARAFAC analysis identified four independent fluorescent components,including two humic-like and two protein-like substances.The less-and highly-oxygenated humic-like substances(LO-HULIS and HO-HULIS,respectively),which together account for?64%of total fluorescence in the highly polluted urban Jinan site.During dust events,the relative contribution of HO-HULIS increased significantly,due mainly to high molecular weight and highly-oxygenated HULIS compounds from desert regions.The most likely source of tryptophan-like fluorophores are biomass burning-derived phenolic compounds,which represented a considerable proportion(?45%)of total fluorescence at the rural Zhangbei site.We speculate that the susceptible phenolic-like components are transformed into humic-like fluorophores through atmospheric aging processes,providing an additional pathway for the formation of atmospheric HULIS.Tyrosine-like fluorophores are associated with biogenic substances(e.g.,microbes and plant debris)from desert regions,which contributed more(?24%)in Erenhot samples.Compared to other samples,those affected by combustion sources at Zhangbei site associated with relatively higher mass absorption efficiency,fluorescence quantum yield,weaker polarity but lower average molecular mass and degree of aging;while the dust samples of Erenhot associated with relatively lower mass absorption efficiency and fluorescence quantum yield but higher polarity,molecular mass and degree of aging.The HULIS-A(HULIS eluted by acetonitrile)and acetonitrile-soluble organic matter(ASOM)fractions were extracted from the samples.Molecular characteristics of HULIS-A and ASOM were analyzed by an HPLC-HRMS system.Generally,211–1904different formulas were identified from the samples collected in clear day of these three sites.The number of formulas identified in OA from remote areas(Erenhot)was lower than those form rural(Zhangbei)and urban(Jinan)areas.The degree of unsaturation and aromaticity of OA from clean days in urban areas were higher than the other sites,which was relevant to biogenic/anthropogenic secondary organic aerosol,nitroaromatic compounds,organosulfates and nitrooxy-organosulfates.These compounds may be derived from atmospheric photooxidation reactions of long-chain aliphatic hydrocarbons and aromatic compounds from motor vehicle exhausts and other combustion sources.The oxidation degree of OA for polluted days of Zhangbei and Jinan was higher than those for clear days,while the unsaturation and aromaticity were lower.The very high sulfate and nitrate concentrations on the polluted day of Jinan promoted the transformation of CHO and CHON compounds into sulfur-containing compounds.There were 1375 and 1200 formulas identified in the HULIS-A fraction of dust samples from Erenhot and Zhangbei,respectively,of which 612 formulas were shared.The average molecular weights of compounds detected in dust samples from Erenhot and Zhangbei were 316 and 303 Da,respectively.The higher proportion of protein-like compounds in dust samples may associated with the increase in the proportion of tyrosine-like fluorophore and can affect downwind areas after long-range transport.The percentage of lignin-like compounds was the highest in both samples,indicating that dust-sourced WSOC may be influenced by biogenic organic compounds,such as long-chain hydroxy fatty acids,glycosides,flavonoids,steroid compounds and their derivatives,and can affect OA chemical compositions in downwind areas after long-range transport. |
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