Tore F. Berglen

Comparison of the radiative properties and direct radiative effect of aerosols from a global aerosol model and remote sensing data over ocean

Measurements of C2–C8 non-methane hydrocarbons (NMHCs) have been made in situ at Halley Base, Antarctica (75◦35°S, 26◦19°W) from February 2004 to February 2005 as part of the Chemistry of the Antarctic Boundary Layer and the Interface with Snow (CHABLIS) experiment. The data show long- and short-term variabilities in NMHCs controlled by the seasonal and geographic dependence of emissions and variation in atmospheric removal rates and pathways. Ethane, propane, iso-butane, n-butane and acetylene abundances followed a general OH-dependent sinusoidal seasonal cycle. The yearly averages were 186, 31, 3.2, 4.9 and 19 pptV, respectively, lower than those which were reported in some previous studies. Superimposed on a seasonal cycle was shorter-term variability that could be attributed to both synoptic airmass variability and localized loss processes due to other radical species. Hydrocarbon variability during periods of hour-to-day-long surface O3 depletion in late winter/early spring indicated active halogen atom chemistry estimated to be in the range 1.7 × 103–3.4 × 104 atom cm−3 for Cl and 4.8 × 106–9.6 × 107 atom cm−3 for Br. Longer-term negative deviations from sinusoidal behaviour in the late August were indicative of NMHC reaction with a persistent [Cl] of 2.3×103 atom cm−3.Maximum ethene and propene of 157 and 179 pptV, respectively, were observed in the late February/early March, consistent with increased oceanic biogenic emissions; however, their presence was significant year-round (June–August concentrations of 17.1 ± 18.3 and 7.9 ± 20.0 pptV, respectively).

Uncertainties in the Radiative Forcing Due to Sulfate Aerosols

Radiative transfer calculations based on a new sulfate distribution from a chemistry-transport model simulation have been performed. A wide range of sensitivity experiments have been performed to illustrate the large uncertainty in the radiative forcing due to sulfate aerosols. The most important factors seem to be processes involved in the mixing of sulfate aerosols with other particles and uncertainties in the relative humidities. These factors can explain much of the large range in previous estimates of the radiative forcing due to sulfate aerosols reflected, for example, in the Intergovernmental Panel on Climate Change estimate. Included in this study is a simple subgrid-scale parameterization of relative humidity to investigate a potentially large uncertainty in the radiative forcing due to sulfate aerosol.

The radiative effect of the anthropogenic influence on the stratospheric sulfate aerosol layer

Stratospheric sulfate aerosols have a cooling effect on the Earth’s surface. Sulfur aerosols from large volcanic eruptions are often the dominant source, while non-volcanic background stratospheric sulfate aerosols are supposed to mainly originate from carbonyl sulfide (OCS). Several recent studies indicate, however, that this latter source is too small to account for the observed background stratospheric aerosol concentration. Based on model calculations we suggest that most of the lower stratospheric sulfate aerosol concentration is of anthropogenic origin. We estimate a global mean radiative forcing due to the anthropogenic influence on the stratospheric aerosol layer of −0.05 Wm−2. This represents a new climate forcing mechanism and emphasizes anthropogenic sulfur emission as an important cooling mechanism.

Modeling of the solar radiative impact of biomass burning aerosols during the Dust and Biomass‐burning Experiment (DABEX)

Received 25 January 2008; revised 28 August 2008; accepted 9 September 2008; published 27 November 2008. [1] The radiative forcing associated with biomass burning aerosols has been calculated over West Africa using a chemical transport model. The model simulations focus on the period of JanuaryFebruary 2006 during the Dust and Biomass-burning Experiment (DABEX). All of the main aerosol components for this region are modeled including mineral dust, biomass burning (BB) aerosols, secondary organic carbon associated with BB emissions, and carbonaceous particles from the use of fossil fuel and biofuel. The optical properties of the BB aerosol are specified using aircraft data from DABEX. The modeled aerosol optical depth (AOD) is within 15–20% of data from the few available Aerosol Robotic Network (AERONET) measurement stations. However, the model predicts very high AOD over central Africa, which disagrees somewhat with satellite retrieved AOD from Moderate Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging Spectroradiometer (MISR). This indicates that BB emissions may be too high in central Africa or that very high AOD may be incorrectly screened out of the satellite data. The aerosol single scattering albedo increases with wavelength in our model and in AERONET retrievals, which contrasts with results from a previous biomass burning aerosol campaign. The model gives a strong negative radiative forcing of the BB aerosols at the top of the atmosphere (TOA) in clear-sky conditions over most of the domain, except over the Saharan desert where surface albedos are high. The all-sky TOA radiative forcing is quite inhomogeneous with values varying from 10 to 10 W m 2 . The regional mean TOA radiative forcing is close to zero for the all-sky calculation and around 1.5 W m 2 for the clear-sky calculation. Sensitivity simulations indicate a positive regional mean TOA radiative forcing of up to 3 W m 2 .

Sulphate trends in Europe: are we able to model the recent observed decrease?

Abundance of sulphate in Europe has decreased substantially during the last two decades. In this paper, we investigate these recent trends in sulphate concentrations by applying the Oslo CTM2 model using three different sets of SO2 emission inventories. We perform time slice model simulations with emissions for the years 1985, 1995 and 2000 and compare our results with observations to investigate if there is consistency between measured and modelled sulphate trends. Overall the model reproduces the levels of sulphur and the decreasing sulphate trends reasonably well, although some discrepancies exist. The model shows a strong reduction in the surface concentration of sulphate similar to the observations, although a slightly smaller decrease. Continental and Eastern Europe experience the largest decrease in sulphate from 1985 to 2000; observations give 65 and 63% decrease, respectively, while modelled decreases are from 42 to 58% depending on the inventory. We have also studied to what extent our model results are sensitive and robust. Based on our model simulations we find that the EMEP emissions of the three sets of emission inventories are best to reproduce the trends in sulphate observations.