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Inverse modelling-based reconstruction of the Chernobyl source term available for long-range transport

The reconstruction of the Chernobyl accident source term has been previously carried out using core inventories, but also back and forth confrontations between model simulations and activity concentration or deposited activity measurements. The approach presented in this paper is based on inverse modelling techniques. It relies both on the activity concentration measurements and on the adjoint of a chemistry-transport model. The location of the release is assumed to be known, and one is looking for a source term available for long-range transport that depends both on time and altitude. The method relies on the maximum entropy on the mean principle and exploits source positivity. The inversion results are mainly sensitive to two tuning parameters, a mass scale and the scale of the prior errors in the inversion. To overcome this hardship, we resort to the statistical L-curve method to estimate balanced values for these two parameters. Once this is done, many of the retrieved features of the source are robust within a reasonable range of parameter values. Our results favour the acknowledged three-step scenario, with a strong initial release (26 to 27 April), followed by a weak emission period of four days (28 April–1 May) and again a release, longer but less intense than the initial one (2 May–6 May). The retrieved quantities of iodine-131, caesium-134 and caesium-137 that have been released are in good agreement with the latest reported estimations. Yet, a stronger apportionment of the total released activity is ascribed to the first period and less to the third one. Finer chronological details are obtained, such as a sequence of eruptive episodes in the first two days, likely related to the modulation of the boundary layer diurnal cycle. In addition, the first two-day release surges are found to have effectively reached an altitude up to the top of the domain (5000 m).
Technical Note: A new SIze REsolved Aerosol Model (SIREAM)

We briefly present in this short paper a new SIze REsolved Aerosol Model (SIREAM) which simulates the evolution of atmospheric aerosol by solving the General Dynamic Equation (GDE). SIREAM segregates the aerosol size distribution into sections and solves the GDE by splitting coagulation and condensation/evaporation-nucleation. A quasi-stationary sectional approach is used to describe the size distribution change due to condensation/evaporation, and a hybrid equilibrium/dynamical mass-transfer method has been developed to lower the computational burden. SIREAM uses the same physical parameterizations as those used in the Modal Aerosol Model, MAM Sartelet et al. (2006). It is hosted in the modeling system Polyphemus Mallet et al., 2007, but can be linked to any other three-dimensional Chemistry-Transport Model.
Understanding the formation of biogenic secondary organic aerosol from α-pinene in smog chamber studies: role of organic peroxy radicals

This study focusses on the description of the nucleation process observed during the ozone reaction of the biogenic monoterpene α-pinene in smog chambers. Therefore, a detailed aerosol dynamics model (UHMA) was extended by a tropospheric chemistry module and a detailed description of the first steps of organic nucleation. We assume secondary ozonides to act as nucleation initiating molecules, which are subsequently activated by reactions with organic peroxy radicals (RO2). With this set-up the observed particle size distributions of an exemplary experiment in Valencia was reproduced, when only the long-lived organic compounds like carboxylic acids and carbonyl compounds are detected by the available aerosol size distribution instruments. Our results indicate that fragile or reactive species might get destroyed because of weak bond breakage during the size classification. This assumption would imply a serious detection problem in nucleation studies to be solved.
Daytime tropospheric loss of hexanal and trans-2-hexenal: OH kinetics and UV photolysis

The ultraviolet (λ=250–370 nm) photolysis and the OH-initiated oxidation of hexanal and trans-2-hexenal, which are relevant atmospheric processes, have been investigated at room temperature and as a function of temperature (T=263–353 K), respectively. This kinetic study as a function of temperature is reported here for the first time. Absolute absorption cross sections (σλ) were obtained using a recently built system operating in the UV region. The obtained σλ allowed the estimation of the photolysis rates (J) across the troposphere. Kinetic measurements of the gas-phase reaction of hydroxyl radicals (OH) with hexanal and trans-2-hexenal were performed by using the laser pulsed photolysis/laser-induced fluorescence technique. Rate coefficients kOH for both aldehydes were determined at temperatures between 263 and 353 K at 50 Torr in helium or argon bath gases. The temperature dependence of kOH for both aldehydes was found to be slightly negative. The tropospheric lifetime of hexanal and trans-2-hexenal due to the chemical removal by OH radicals has been estimated across the troposphere. The loss rate due to the OH chemical removal was compared with the estimated photolysis rates. Our results show that OH-reaction is the main loss process for these aldehydes in the troposphere, although photolysis is not negligible for hexanal.
Improved ozone profile retrievals from GOME data with degradation correction in reflectance

We present a simple method to perform degradation correction to Global Ozone Monitoring Experiment (GOME) reflectance spectra by comparing the average reflectance for 60° N–60° S with that at the beginning of GOME observations (July–December 1995) after removing the dependences on solar zenith angle and seasonal variation. The results indicate positive biases of up to ~15–25% in the wavelength range 289–370 nm during 2000–2002; the degradation also exhibits significant dependence on wavelength and viewing zenith angle. These results are consistent with previous studies using radiative transfer models and ozone observations. The degradation causes retrieval biases of up to ~3% (10 DU, 1 DU=2.69×1016 molecules cm−2), 30% (10 DU), 10%, and 40% in total column ozone, tropospheric column ozone, stratospheric ozone and tropospheric ozone, respectively, from our GOME ozone profile retrieval algorithm. In addition, retrieval biases due to degradation vary significantly with latitude. The application of this degradation correction improves the retrievals relative to Dobson and ozonesonde measurements at Hohenpeißenberg station during 2000–2003 and improves the spatiotemporal consistency of retrieval quality during 1996–2003. However, because this method assumes that the deseasonalized globally-averaged reflectance does not change much with time, retrievals with this correction may be inadequate for trend analysis. In addition, it does not correct for instrument biases that have occurred since launch.
The T1-T2 study: evolution of aerosol properties downwind of Mexico City

As part of a major atmospheric chemistry and aerosol field program carried out in March 2006, a study was conducted in the area to the north and northeast of Mexico City to investigate the evolution of aerosols and their associated optical properties in the first few hours after their emission. The focus of the T1-T2 aerosol study was to investigate changes in the specific absorption αABS (absorption per unit mass, with unit of m2 g−1) of black carbon as it aged and became coated with compounds such as sulfate and organic carbon, evolving from an external to an internal mixture. Such evolution has been reported in previous studies. The T1 site was located just to the north of the Mexico City metropolitan area; the T2 site was situated approximately 35 km farther to the northeast. Nephelometers, particle soot absorption photometers, photoacoustic absorption spectrometers, and organic and elemental carbon analyzers were used to measure the optical properties of the aerosols and the carbon concentrations at each of the sites. Radar wind profilers and radiosonde systems helped to characterize the meteorology and to identify periods when transport from Mexico City over T1 and T2 occurred. Organic and elemental carbon concentrations at T1 showed diurnal cycles reflecting the nocturnal and early morning buildup from nearby sources, while concentrations at T2 appeared to be more affected by transport from Mexico City. Specific absorption during transport periods was lower than during other times, consistent with the likelihood of fresher emissions being found when the winds blew from Mexico City over T1 and T2. The specific absorption at T2 was larger than at T1, which is also consistent with the expectation of more aged particles with encapsulated black carbon being found at the more distant location. In situ measurements of single scattering albedo with an aircraft and a ground station showed general agreement with column-averaged values derived from rotating shadowband radiometer data, although some differences were found that may be related to boundary-layer evolution.
Geochemical perspectives from a new aerosol chemical mass closure

The aerosol chemical mass closure is revisited and a simple and inexpensive methodology is proposed. This methodology relies on data obtained for aerosol mass, and concentration of the major ions and the two main carbon components, the organic carbon (OC) and the black carbon (BC). Atmospheric particles are separated into coarse (AD>2 μm) and fine (AD<2 μm) fractions and are treated separately. For the coarse fraction the carbonaceous component is minor and assumption is made for the conversion factor k of OC-to-POM (Particulate Organic Matter) which is fixed to the value of 1.8 accounting for secondary species. The coarse soluble calcium is shown to display a correlation (regression coefficient f, y axis intercept b) with the missing mass. Conversely, the fine fraction is dominated by organic species and assumption is made for dust which is assumed to have the same f factor as the coarse mode dust. The fine mode mass obtained from chemical analyses is then adjusted to the actual weighed mass by tuning the k conversion factor. The k coefficient is kept different in the two modes due to the expected different origins of the organic particles. Using the f and k coefficient obtained from the data set, the mass closure is reached for each individual sample with an undetermined fraction less than 10%. The procedure has been applied to different urban and peri-urban environments in Europe and in Beijing and its efficiency and uncertainties on f and k values are discussed. The f and k coefficients are shown to offer consistent geochemical indications on aerosol origin and transformations. f allows to retrieve dust mass and its value accounting for Ca abundance in dust at the site of investigation may serve as an indicator of dust origin and aerosol interactions with anthropogenic acids. f values were found to vary in the 0.08–0.12 range in European urban areas, and a broader range in Beijing (0.01–0.16). As expected, k appears to be a relevant proxy for particle origin and ageing and varies in the 1.4–1.8 range. For Beijing, k exhibits high values of about 1.7 in winter and summer. Winter values suggest that fresh coal aerosol might be responsible for such a high k value, which was not taken into account in previous works.
Systematic analysis of interannual and seasonal variations of model-simulated tropospheric NO2 in Asia and comparison with GOME-satellite data

Systematic analyses of interannual and seasonal variations of tropospheric NO2 vertical column densities (VCDs) based on GOME satellite data and the regional scale chemical transport model (CTM), Community Multi-scale Air Quality (CMAQ), are presented for the atmosphere over eastern Asia between 1996 and June 2003. A newly developed year-by-year emission inventory (REAS) was used in CMAQ. The horizontal distribution of annual averaged GOME NO2 VCDs generally agrees well with the CMAQ results. However, CMAQ/REAS results underestimate the GOME retrievals with factors of 2–4 over polluted industrial regions such as Central East China (CEC), a major part of Korea, Hong Kong, and central and western Japan. The most probable reasons for the underestimation typically over the CEC are accuracy of the basic energy statistic data, emission factors, and socio-economic data used for construction of emission inventory. For the Japan region, GOME and CMAQ NO2 data show reasonable agreement with respect to interannual variation and show no clear increasing trend. For CEC, GOME and CMAQ NO2 data indicate a very rapid increasing trend from 2000. Analyses of the seasonal cycle of NO2 VCDs show that GOME data have larger dips than CMAQ NO2 during February–April and September–November. Sensitivity experiments with fixed emission intensity reveal that the detection of emission trends from satellite in fall or winter has a larger error caused by the variability of meteorology. Examination during summer time and annual averaged NO2 VCDs are robust with respect to variability of meteorology and are therefore more suitable for analyses of emission trends. Analysis of recent trends of annual emissions in China shows that the increasing trends of 1996–1998 and 2000–2002 for GOME and CMAQ/REAS show good agreement, but the rate of increase by GOME is approximately 10–11% yr−1 after 2000; it is slightly steeper than CMAQ/REAS (8–9% yr−1). The greatest difference was apparent between the years 1998 and 2000: CMAQ/REAS only shows a few percentage points of increase, whereas GOME gives a greater than 8% yr−1 increase. The exact reason remains unclear, but the most likely explanation is that the emission trend based on the Chinese emission related statistics underestimates the rapid growth of emissions.
Estimation of a "radiatively correct" black carbon specific absorption during the Mexico City Metropolitan Area (MCMA) 2003 field campaign

During the Mexico City Metropolitan Area (MCMA) field campaign of 2003, measurements of the shortwave radiation field allowed the inference of the black carbon (BC) specific absorption, αλ, defined as the monochromatic absorption cross section per unit mass (with units of m2/g). The averaged values of αλ derived from the method here are either 8.9 m2/g or 8.2 m2/g at 500 nm, depending upon the physical and optical parameters assumed for BC. These results are reasonably consistent with those of Schuster et al. (2005), 9.5 m2/g, and Baumgartner et al. (2002), 7.0 m2/g, both measured at 550 nm. The αλ values reported in this paper should only be considered effective, "radiatively correct" values because when used in radiative transfer calculations the calculated irradiances match the measured irradiances at 500 nm. The specific absorption so defined can assume a wide range of values, depending upon: (1) the assumptions made prior to the retrieval (e.g., shell/core aerosol configuration), and (2) values chosen for BC density and refractive index. The range of possible values is large, corresponding to a "worst case" uncertainty of about ±70%, assuming that all errors are additive and of the same sign so that no error cancellation occurs.
The global impact of supersaturation in a coupled chemistry-climate model

Ice supersaturation is important for understanding condensation in the upper troposphere. Many general circulation models however do not permit supersaturation. In this study, a coupled chemistry climate model, the Whole Atmosphere Community Climate Model (WACCM), is modified to include supersaturation for the ice phase. Rather than a study of a detailed parameterization of supersaturation, the study is intended as a sensitivity experiment, to understand the potential impact of supersaturation, and of expected changes to stratospheric water vapor, on climate and chemistry. High clouds decrease and water vapor in the stratosphere increases at a similar rate to the prescribed supersaturation (20% supersaturation increases water vapor by nearly 20%). The stratospheric Brewer-Dobson circulation slows at high southern latitudes, consistent with slight changes in temperature likely induced by changes to cloud radiative forcing. The cloud changes also cause an increase in the seasonal cycle of near tropopause temperatures, increasing them in boreal summer over boreal winter. There are also impacts on chemistry, with small increases in ozone in the tropical lower stratosphere driven by enhanced production. The radiative impact of changing water vapor is dominated by the reduction in cloud forcing associated with fewer clouds (~+0.6 Wm−2) with a small component likely from the radiative effect (greenhouse trapping) of the extra water vapor (~+0.2 Wm−2), consistent with previous work. Representing supersaturation is thus important, and changes to supersaturation resulting from changes in aerosol loading for example, might have a modest impact on global radiative forcing, mostly through changes to clouds. There is no evidence of a strong impact of water vapor on tropical tropopause temperatures.