Emission Characteristics Of Brown Carbon In Size-segregated Particles Emitted From Typical Ships In China

Brown carbon（Br C）is an important light–absorbing substance in atmosphere,which can strongly absorb light at the short–wave visible band,and has large impact on climate change.Br C emission sources are various,and most of the current studies associated with Br C emissions focus only on biomass and coal combustion.As an important mode of transportation,ships can emit large quantities of various air pollutants,such as particulate matter;therefore,ships emissions are of great concerned.Unfortunately,researches focused on Br C emission from ships are still not available,resulting in a knowledge gap in the understanding of emission characteristics and optical properties of Br C from ship emissions.In this study,six different types of ships in China were selected,including three types of non–cargo ships（engineering ships,research ships,and fishing boats）and three types of cargo ships（river ships,coastal ships,and ocean–going ships）.The emission levels,optical properties,size distributions and influencing factors of Br C from ships were studied.Furthermore,to better illustrate Br C emissions from ships,the pollution levels,optical characteristics,and particle size distribution characteristics of Br C from vehicle sources under different national emission standards（Ⅲ,ⅣandⅤ）were also investigated based on tunnel experiments,and the results were compared with that from ship emissions.The mass concentrations of MSOC and WSOC from different ships varied greatly.Among them,fishing boats of non–cargo ships owed the highest MSOC concentrations with an average of 93.10±72.06,which was 5.23–15.11 times higher that of the other five types of ships.This was mainly owing to ship engines of fishing boats were old and less powerful.Coastal ships of freight ships had the highest WSOC levels,which was 25.48±5.06 mg/m³,2.78–12.35 times higher that of the other ships.By comparing the MSOC and WSOC emissions under different operation conditions,we found that the emission level under idle condition was often higher than that in cruise mode.Abs_365,M and Abs_365,W of BrC per unit PM mass emitted by fishing vessels were277.3 and 95.7 Mm^–1,respectively,2.3–3.8 and 1.1–1.5 times higher than those of other non–cargo ships.However,the Abs_365,M of cargo ships of ocean–going ships and river–going ships were 524 and 328 Mm^–1,and were 448 and 363 Mm^–1 for Abs_365,W,respectively,higher than that of fishing ships.The MAE_365,M and MAE_365,W of Br C from fishing ships were relatively high,of 0.31 and 1.09 m²/g C.respectively,which are0.69–2.21 and 1.70–4.54 times higher than those of the other ships,indicating that the Br C emission from fishing ships had not only high pollution levels,but also strong light–absorption ability.Comparing the optical characteristics of Br C dissolved in different solvents found that Abs₃₆₅ of methanol–soluble Br C were higher than those of water–soluble Br C,partly due to the stronger polarity of methanol and its ability to dissolve more Br C components.However,MAE₃₆₅ of methanol–soluble Br C for ship emission was lower than that of water–soluble Br C.This is possibly due to the weak or non–absorption ability of some chromophore substances that can only dissolve in methanol.In addition,we also found that the light absorption ability of Br C would increase significantly under weak acid environment,and the effects of p H on light Br C absorption ability from ship emissions were influenced by ship fuel types.In contrast to diesel ships,Br C light absorption ability of heavy oil ships increased under strong acid environment.This is possible attributed to heavy metals contained in heavy oil that would complex with aromatic hydrocarbon substances in solution and form Br C under acidic conditions.The mass concentrations of MSOC and WSOC from ships showed bimodal distribution and the distribution characteristics were affected by ship types.The concentration peaks of MSOC and WSOC of different ships were in 0.43–0.65μm of fine particle mode（Dp<2.1μm）,and 4.7–5.8μm of coarse particle mode（Dp>2.1μm）.While the peak concentrations of MSOC from cargo ship emissions in the coarse particle mode varied largely.The major particles MSOCs from non–cargo ships were distributed in fine particle mode（Dp<2.1μm）,which were in coarse particle mode（Dp>2.1μm）of WSOC for both non–cargo ships and cargo ships.Similarly,Abs3_65,Mand Abs_365,W for ship exhausts also showed bimodal distribution,with the major particles were bounded in the size range of 0.43–0.65μm and 4.7–5.8μm,respectively.With the increase of vehicle emission standards,the Abs_365,M,Abs_365,W,MAE_365,M,and MAE_365,W of Br C in per PM mass from vehicle exhaust emission decreased significantly.This is due to that higher vehicle emission standard associated with stricter infrastructure and the innovations of engine technology.Similar to ship emissions,Abs_365,M,Abs_365,W,and MAE₃₆₅ of Br C emissions from vehicles also showed bimodal size distributions.However,there was no obvious trend for MAE_365,W of Br C from vehicles emission with wavelength change.In comparison with ship emissions,vehicle emission owed higher velues of Abs_365,M,Abs_365,W,and MAE₃₆₅ but lower MAE_365,M values.