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To understand diurnal variations in PM_2.5composition and aerosol extract absorption, PM_2.5samples were collected at intervals of 2 hr from 8:00 to 20:00 and 6 hr from 20:00 to 8:00 (the next day) in northern Nanjing, China, during the winter and summer of 2019–2020 and analyzed for bulk components, organic tracers, and light absorption of water and methanol extracts—a proxy measure of brown carbon (BrC). Diurnal patterns of measured species reflected the influences of primary emissions and atmospheric processes. Light absorption coefficients of water (Abs_365,w) and methanol extracts (Abs_365,m) at 365 nm shared a similar diurnal profile peaking at 18:00–20:00, generally following changes in biomass burning tracers. However, Abs_365,w, Abs_365,m, and their normalizations to organic aerosols increased at 14:00–16:00, earlier than that of levoglucosan in the late afternoon, which was attributed to secondarily formed BrC. The methanol extracts showed a less drastic decrease in light absorption at night than the water extracts and elevated absorption efficiency during 2:00–8:00. This is due to the fact that the water‐insoluble OC has a longer lifetime and stronger light absorption than the water‐soluble OC. According to the source apportionment results solved by positive matrix factorization (PMF), biomass burning and secondary formation were the major BrC sources in northern Nanjing, with an average total relative contribution of about 90%. Compared to previous studies, diurnal source cycles were added to the PMF simulations in this work by using time‐resolved speciation data, which avoided misclassification of BrC sources.

 

Abstract. The methanol extraction method was widely applied to isolate organic carbon(OC) from ambient aerosols, followed by measurements of brown carbon (BrC)absorption. However, undissolved OC fractions will lead to underestimatedBrC absorption. In this work, water, methanol (MeOH), MeOH / dichloromethane(MeOH / DCM, 1:1, v/v), MeOH / DCM (1:2, v/v), tetrahydrofuran (THF), andN,N-dimethylformamide (DMF) were tested for extraction efficiencies ofambient OC, and the light absorption of individual solvent extracts wasdetermined. Among the five solvents and solvent mixtures, DMF dissolved thehighest fractions of ambient OC (up to ∼95 %), followed byMeOH and MeOH / DCM mixtures (<90 %), and the DMF extracts hadsignificantly (p<0.05) higher light absorption than other solventextracts. This is because the OC fractions evaporating at highertemperatures (>280∘) are less soluble in MeOH(∼80 %) than in DMF (∼90 %) and containstronger light-absorbing chromophores. Moreover, the light absorption of DMFand MeOH extracts of collocated aerosol samples in Nanjing showed consistenttemporal variations in winter when biomass burning dominated BrC absorption, while the average light absorption of DMF extracts was more than 2 timesgreater than the MeOH extracts in late spring and summer. The average lightabsorption coefficient at 365 nm of DMF extracts was 30.7 % higher (p<0.01) than that of MeOH extracts. Source apportionment resultsindicated that the MeOH solubility of BrC associated with biomass burning,lubricating oil combustion, and coal combustion is similar to their DMFsolubility. The BrC linked with unburned fossil fuels and polymerizationprocesses of aerosol organics was less soluble in MeOH than in DMF, whichwas likely the main reason for the large difference in time series betweenMeOH and DMF extract absorption. These results highlight the importance oftesting different solvents to investigate the structures and lightabsorption of BrC, particularly for the low-volatility fraction potentiallyoriginating from non-combustion sources. 

Abstract. Gas–particle partitioning of water-soluble organic compounds plays a significant role in influencing the formation, transport, and lifetime oforganic aerosols in the atmosphere, but is poorly characterized. In this work, gas- and particle-phase concentrations of isoprene oxidation products(C5-alkene triols and 2-methylterols), levoglucosan, and sugar polyols were measured simultaneously at a suburban site of the western Yangtze RiverDelta in east China. All target polyols were primarily distributed into the particle phase (85.9 %–99.8 %). Given the uncertainties inmeasurements and vapor pressure predictions, a dependence of particle-phase fractions on vapor pressures cannot be determined. To explore the impactof aerosol liquid water on gas–particle partitioning of polyol tracers, three partitioning schemes (Cases 1–3) were proposed based onequilibriums of gas vs. organic and aqueous phases in aerosols. If particulate organic matter (OM) is presumed as the only absorbing phase(Case 1), the measurement-based absorptive partitioning coefficients (Kp,OMm) of isoprene oxidation products and levoglucosan were more than 10 times greater than predicted values (Kp,OMt). The agreement betweenKp,OMm and Kp,OMt was substantially improved when solubility in a separate aqueous phase wasincluded, whenever water-soluble and water-insoluble OM partitioned into separate (Case 2) or single (Case 3) liquid phases,suggesting that the partitioning of polyol tracers into the aqueous phase in aerosols should not be ignored. The measurement-based effective Henry'slaw coefficients (KH,em) of polyol tracers were orders of magnitude higher than their predicted values in pure water(KH,wt). Due to the moderate correlations between log⁡(KH,em/KH,wt) andmolality of sulfate ions, the gap between KH,em and KH,wt of polyol tracers could not be fullyparameterized by the equation defining “salting-in” effects and might be ascribed to mechanisms of reactive uptake, aqueous phase reaction,“like-dissolves-like” principle, etc. These study results also partly reveal the discrepancy between observation and modeling of organicaerosols. 

Abstract. We present a comprehensive simulation of tropospheric chlorinewithin the GEOS-Chem global 3-D model of oxidant–aerosol–halogen atmosphericchemistry. The simulation includes explicit accounting of chloridemobilization from sea salt aerosol by acid displacement of HCl and by otherheterogeneous processes. Additional small sources of tropospheric chlorine(combustion, organochlorines, transport from stratosphere) are also included.Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl2, BrCl, ICl,HOCl, ClNO3, ClNO2, and minor species, is produced by theHCl+OH reaction and by heterogeneous conversion of sea salt aerosolchloride to BrCl, ClNO2, Cl2, and ICl. The modelsuccessfully simulates the observed mixing ratios of HCl in marine air(highest at northern midlatitudes) and the associated HNO3decrease from acid displacement. It captures the high ClNO2 mixingratios observed in continental surface air at night and attributes thechlorine to HCl volatilized from sea salt aerosol and transported inlandfollowing uptake by fine aerosol. The model successfully simulates thevertical profiles of HCl measured from aircraft, where enhancements in thecontinental boundary layer can again be largely explained by transport inlandof the marine source. It does not reproduce the boundary layer Cl2mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in thedaytime, low at night); the model is too high at night, which could be due touncertainty in the rate of the ClNO2+Cl- reaction, but we haveno explanation for the high observed Cl2 in daytime. The globalmean tropospheric concentration of Cl atoms in the model is 620 cm−3and contributes 1.0 % of the global oxidation of methane, 20 % ofethane, 14 % of propane, and 4 % of methanol. Chlorine chemistryincreases global mean tropospheric BrO by 85 %, mainly through theHOBr+Cl- reaction, and decreases global burdens of troposphericozone by 7 % and OH by 3 % through the associated bromine radicalchemistry. ClNO2 chemistry drives increases in ozone of up to8 ppb over polluted continents in winter. 

The sensitivity of the Arctic precipitation phases (solid and liquid) to the forcings from greenhouse gases (GHGs) and aerosols over 2016–2080 was investigated by using the Community Earth System Model Version 1. Results show that the warming caused by the two forcings results in an increasing trend in total precipitation and a solid‐to‐liquid precipitation transition in the Arctic. Under RCP8.5 scenario, the increased rate of Arctic mean precipitation with global warming forced by aerosol reduction (7.7%/°C) is twice greater than that by increased GHG emission (3.5%/°C). The sensitivity of rainfall to precipitation ratio (RPR) to various forcings is much higher than that of total precipitation in the Arctic. The increased rate of RPR due to global aerosol forcing (8.4%/°C) is approximately 3 times that due to GHG forcing (2.9%/°C) in the Arctic, the differences even larger over Greenland and the eastern Arctic Ocean, resulting in more rainfall in these areas.