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- Blichner, Sara Marie, et al.
(author)
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Reduced effective radiative forcing from cloud-aerosol interactions (ERFaci) with improved treatment of early aerosol growth in an Earth system model
- 2021
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In: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 21:23, s. 17243-17265
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Journal article (peer-reviewed)abstract
- Historically, aerosols of anthropogenic origin have offset some of the warming from increased atmospheric greenhouse gas concentrations. The strength of this negative aerosol forcing, however, is highly uncertain-especially the part originating from cloud-aerosol interactions. An important part of this uncertainty originates from our lack of knowledge about pre-industrial aerosols and how many of these would have acted as cloud condensation nuclei (CCN). In order to simulate CCN concentrations in models, we must adequately model secondary aerosols, including new particle formation (NPF) and early growth, which contributes a large part of atmospheric CCN. In this study, we investigate the effective radiative forcing (ERF) from cloud-aerosol interactions (ERFaci) with an improved treatment of early particle growth, as presented in . We compare the improved scheme to the default scheme, OsloAero, which are both embedded in the atmospheric component of the Norwegian Earth System Model v2 (NorESM2). The improved scheme, OsloAeroSec, includes a sectional scheme that treats the growth of particles from 5-39.6ĝ€¯nm in diameter, which thereafter inputs the particles to the smallest mode in the pre-existing modal aerosol scheme. The default scheme parameterizes the growth of particles from nucleation up to the smallest mode, a process that can take several hours. The explicit treatment of early growth in OsloAeroSec, on the other hand, captures the changes in atmospheric conditions during this growth time in terms of air mass mixing, transport, and condensation and coagulation. We find that the ERFaci with the sectional scheme is-1.16ĝ€¯Wm-2, which is 0.13ĝ€¯Wm-2 weaker compared to the default scheme. This reduction originates from OsloAeroSec producing more particles than the default scheme in pristine, low-aerosol-concentration areas and fewer NPF particles in high-aerosol areas. We find, perhaps surprisingly, that NPF inhibits cloud droplet activation in polluted and/or high-aerosol-concentration regions because the NPF particles increase the condensation sink and reduce the growth of the larger particles which may otherwise activate. This means that in these high-aerosol regions, the model with the lowest NPF-OsloAeroSec-will have the highest cloud droplet activation and thus more reflective clouds. In pristine and/or low-aerosol regions, however, NPF enhances cloud droplet activation because the NPF particles themselves tend to activate. Lastly, we find that sulfate emissions in the present-day simulations increase the hygroscopicity of secondary aerosols compared to pre-industrial simulations. This makes NPF particles more relevant for cloud droplet activation in the present day than the pre-industrial atmosphere because increased hygroscopicity means they can activate at smaller sizes.
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2. |
- Martinsson, Johan, et al.
(author)
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Carbonaceous aerosol source apportionment using the Aethalometer model - evaluation by radiocarbon and levoglucosan analysis at a rural background site in southern Sweden
- 2017
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In: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 17:6, s. 4265-4281
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Journal article (peer-reviewed)abstract
- With the present demand on fast and inexpensive aerosol source apportionment methods, the Aethalometer model was evaluated for a full seasonal cycle (June 2014June 2015) at a rural atmospheric measurement station in southern Sweden by using radiocarbon and levoglucosan measurements. By utilizing differences in absorption of UV and IR, the Aethalometer model apportions carbon mass into wood burning (WB) and fossil fuel combustion (FF) aerosol. In this study, a small modification in the model in conjunction with carbon measurements from thermal-optical analysis allowed apportioned non-light-absorbing biogenic aerosol to vary in time. The absorption differences between WB and FF can be quantified by the absorption angstrom ngstrom exponent (AAE). In this study AAE(WB) was set to 1.81 and AAE(FF) to 1.0. Our observations show that the AAE was elevated during winter (1.36 +/- 0.07) compared to summer (1.12 +/- 0.07). Quantified WB aerosol showed good agreement with levoglucosan concentrations, both in terms of correlation (R-2 = 0 : 70) and in comparison to reference emission inventories. WB aerosol showed strong seasonal variation with high concentrations during winter (0.65 mu gm(-3), 56% of total carbon) and low concentrations during summer (0.07 mu gm(-3), 6% of total carbon). FF aerosol showed less seasonal dependence; however, black carbon (BC) FF showed clear diurnal patterns corresponding to traffic rush hour peaks. The presumed non-light-absorbing biogenic carbonaceous aerosol concentration was high during summer (1.04 mu gm(-3), 72% of total carbon) and low during winter (0.13 mu gm(-3), 8% of total carbon). Aethalometer model results were further compared to radiocarbon and levoglucosan source apportionment results. The comparison showed good agreement for apportioned mass of WB and biogenic carbonaceous aerosol, but discrepancies were found for FF aerosol mass. The Aethalometer model overestimated FF aerosol mass by a factor of 1.3 compared to radiocarbon and levoglucosan source apportionment. A performed sensitivity analysis suggests that this discrepancy can be explained by interference of non-light-absorbing biogenic carbon during winter. In summary, the Aethalometer model offers a costeffective yet robust high-time-resolution source apportionment at rural background stations compared to a radiocarbon and levoglucosan alternative.
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4. |
- Martinsson, Johan, et al.
(author)
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Evaluation of delta C-13 in Carbonaceous Aerosol Source Apportionment at a Rural Measurement Site
- 2017
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In: Aerosol and Air Quality Research. - : Taiwan Association for Aerosol Research. - 1680-8584 .- 2071-1409. ; 17:8, s. 2081-2094
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Journal article (peer-reviewed)abstract
- The stable isotope of carbon, C-13, has been used in several studies for source characterization of carbonaceous aerosol since there are specific signatures for different sources. In rural areas, the influence of different sources is complex and the application of delta C-13 for source characterization of the total carbonaceous aerosol (TC) can therefore be difficult, especially the separation between biomass burning and biogenic sources. We measured delta C-13 from 25 filter samples collected during one year at a rural background site in southern Sweden. Throughout the year, the measured delta C-13 showed low variability (-26.73 to -25.64%). We found that the measured delta C-13 did not correlate with other commonly used source apportionment tracers (C-14, levoglucosan). delta C-13 values showed lower variability during the cold months compared to the summer, and this narrowing of the delta C-13 values together with elevated levoglucosan concentrations may indicate contribution from sources with lower delta C-13 variation, such as biomass or fossil fuel combustion. Comparison of two Monte Carlo based source apportionment models showed no significant difference in results when delta C-13 was incorporated in the model. The insignificant change of redistributed fraction of carbon between the sources was mainly a consequence of relatively narrow range of delta C-13 values and was complicated by an unaccounted kinetic isotopic effect and overlapping delta C-13 end-member values for biomass burning and biogenic sources.
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5. |
- Martinsson, Johan, et al.
(author)
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Exploring sources of biogenic secondary organic aerosol compounds using chemical analysis and the FLEXPART model
- 2017
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In: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 17:18, s. 11025-11040
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Journal article (peer-reviewed)abstract
- Molecular tracers in secondary organic aerosols (SOAs) can provide information on origin of SOA, as well as regional scale processes involved in their formation. In this study 9 carboxylic acids, 11 organosulfates (OSs) and 2 nitrooxy organosulfates (NOSs) were determined in daily aerosol particle filter samples from Vavihill measurement station in southern Sweden during June and July 2012. Several of the observed compounds are photo-oxidation products from biogenic volatile organic compounds (BVOCs). Highest average mass concentrations were observed for carboxylic acids derived from fatty acids and monoterpenes (12. 3 ± 15. 6 and 13. 8 ± 11. 6 ng mg-3, respectively). The FLEXPART model was used to link nine specific surface types to single measured compounds. It was found that the surface category sea and ocean was dominating the air mass exposure (56 %) but contributed to low mass concentration of observed chemical compounds. A principal component (PC) analysis identified four components, where the one with highest explanatory power (49 %) displayed clear impact of coniferous forest on measured mass concentration of a majority of the compounds. The three remaining PCs were more difficult to interpret, although azelaic, suberic, and pimelic acid were closely related to each other but not to any clear surface category. Hence, future studies should aim to deduce the biogenic sources and surface category of these compounds. This study bridges micro-level chemical speciation to air mass surface exposure at the macro level.
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