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Simulations of 222Rn and other short-lived tracers are used to evaluate and intercompare the representations of convective and synoptic processes in 20 global atmospheric transport models. Results show that most established three-dimensional models simulate vertical mixing in the troposphere to within the constraints offered by the observed mean 222Rn concentrations and that subgrid parameterization of convection is essential for this purpose. However, none of the models captures the observed variability of 222Rn concentrations in the upper troposphere, and none reproduces the high 222Rn concentrations measured at 200 hPa over Hawaii. The established three-dimensional models reproduce the frequency and magnitude of high-222Rn episodes observed at Crozet Island in the Indian Ocean, demonstrating that they can resolve the synoptic-scale transport of continental plumes with no significant numerical diffusion. Large differences between models are found in the rates of meridional transport in the upper troposphere (interhemispheric exchange, exchange between tropics and high latitudes). The four two-dimensional models which participated in the intercomparison tend to underestimate the rate of vertical transport from the lower to the upper troposphere but show concentrations of 222Rn in the lower troposphere that are comparable to the zonal mean values in the three-dimensional models.

The ozone budget in the Antarctic region during the stratospheric warming in 2002 is studied, using ozone analyses from the Royal Netherlands Meteorological Institute (KNMI) ozone-transport and assimilation model called TM3DAM. The results show a strong poleward ozone mass flux during this event south of 45[degrees]S between about 20 and 40 hPa, which is about 5 times as large as the ozone flux in 2001 and 2000, and is dominated by eddy transport. Above 10 hPa, there exists a partially compensating equatorward ozone flux, which is dominated by the mean meridional circulation. During this event, not only the ozone column but also the ozone depletion rate in the Antarctic region, computed as a residual from the total ozone tendency and the ozone mass flux into this region, is large. The September-October integrated ozone depletion in 2002 is similar to that in 2000 and larger than that in 2001. Simulations for September 2002 with and without ozone assimilation and parameterized ozone chemistry indicate that the parameterized ozone chemistry alone is able to produce the evolution of the ozone layer in the Antarctic region in agreement with observations. A comparison of the ozone loss directly computed from the model's chemistry parameterization with the residual ozone loss in a simulation with parameterized chemistry but without ozone assimilation shows that the numerical error in the residual ozone loss is small.

The Southern Hemisphere major warming event in September 2002 has led to a breakup of the vortex in the middle and higher stratosphere and to a corresponding splitting of the ozone hole. Daily 3D ozone forecasts, produced at the Royal Netherlands Meteorological Institute (KNMI) with a tracer transport and assimilation model based on the ECMWF dynamical forecasts, provided an accurate prediction of this event a week prior to the actual breakup of the vortex. The ozone forecast model contains parameterizations for gas phase and heterogeneous chemistry. Initial states for the forecast are obtained from the assimilation of near-real-time ozone data from the Global Ozone Monitoring Experiment (GOME) on European Space Agency (ESA) Remote Sensing Satellite-2 (ERS-2). In this paper, the ozone forecasts and analyses are discussed as produced before, during, and after the event. These fields are compared with ground-based Dobson, ozonesonde, and Total Ozone Mapping Spectrometer (TOMS) observations. The total ozone comparisons show that the location of the vortex edge is generally well described by the 5-7-day forecasts in September and October. The GOME assimilation compared with TOMS shows a good correspondence concerning vortex location and ozone features but also reflects clear differences in the average ozone amount between the two retrieval schemes. The assimilation system produces realistic ozone profiles, apart from a systematic underestimation of ozone around 150 hPa inside the vortex in August-October.

Aerosol vertical distributions are important for aerosol radiative forcing assessments and atmospheric remote sensing research. From our perspective, the aerosol layer height (ALH) is one of the major concerns in quantifying aerosol absorption from the ultra-violet aerosol index (UVAI). The UVAI has a global daily record since 1978, whereas a corresponding ALH data set is still limited. In this paper, we attempted to construct such an ALH data set from aerosol extinction profiles provided by the MERRA-2 aerosol reanalysis, meanwhile we evaluated them, together with several satellite ALH products in terms of the UVAI sensitivity to ALH. In the first part of this paper, we derived ALHs from the MERRA-2 aerosol profiles by four definitions. Through the sensitivity studies, we found that the definition of top boundary aerosol layer height (Haert) is more robust to the changes in extinction profile properties than others. The spatial and temporal variation of Haert are also well associated with the major aerosol sources and the atmospheric dynamics. In the second part, we further evaluated the UVAI altitude dependence on the MERRA-2 ALH as well as several satellite ALH. Among all the satellite ALH products in this paper, the correlation between the TROPOMI oxygen (O2) A-band ALH and UVAI, and that between the GOME-2 absorbing aerosol layer height (AAH) and UVAI are in agreement with our a-priori knowledge that the altitude dependence of UVAI increases with aerosol loadings. The correlation between the MERRA-2 Haert and UVAI also matches well with what we found from observational data sets. This implies the top boundary of the aerosol layer derived from MERRA-2 can be an alternative in case there is no observational ALH data available for quantitively aerosol absorption from UVAI and other UVAI-related applications. [ABSTRACT FROM AUTHOR]

The purpose of this study is to demonstrate that S-5P/TROPOMI aerosol layer height (ALH) observations can be used to improve the single scattering albedo (SSA) retrieval from ultraviolet satellite observations. We take the Thomas Fire in southern California on 12 December 2017 as a case study. In the first part of this paper, we apply conventional radiative transfer simulations to retrieve the SSA. With forward simulations constrained by TROPOMI ALH, we can determine the uncertainty in SSA due to the assumed spectral dependence of refractive indices between two wavelengths of the near-ultraviolet absorbing aerosol index (UVAI). A significant gap in the retrieved SSA (0.24) between radiative transfer simulations with grey and colored aerosols implies that inappropriate spectral dependences may cause severe misinterpretations of aerosol absorption. In the second part of this paper, we propose a data-driven method to quantify aerosol absorption from long-term measurements of UVAI, the aerosol optical depth (AOD) and ALH using support vector regression (SVR). We present the potential of TROPOMI ALH in this new method. The SVR predicted SSA (0.96±0.01) outperforms that predicted by radiative transfer simulations (0.90±0.05), considering the AERONET SSA measurement is 0.96 and assuming that the aerosol absorption should be homogeneous within the plume (i.e. small SSA standard deviation). We thus believe that the upcoming TROPOMI ALH product can make it feasible to quantify aerosol absorption via data-driven methods, which would play an important role in constructing a long-term global SSA data set. [ABSTRACT FROM AUTHOR]

Mercury was measured onboard the IAGOS-CARIBIC passenger aircraft since May 2005 until February 2016 during nearly monthly sequences of mostly four intercontinental flights from Germany to destinations in North and South America, Africa, and South and East Asia. Most of these mercury data were obtained using an internal default signal integration procedure of the Tekran instrument but since April 2014 more precise and accurate data were obtained using post-flight manual integration of the instrument raw signal. In this paper we use the latter data. Elevated upper tropospheric total mercury (TM) concentrations due to large scale biomass burning were observed in the upper troposphere (UT) at the equator and southern latitudes during the flights to Latin America and South Africa in boreal autumn (SON) and boreal winter (DJF). TM concentrations in the lowermost stratosphere (LMS) decrease with altitude above the thermal tropopause but the gradient is less steep than reported before. Seasonal variation of the vertical TM distribution in the UT and LMS is similar to that of other trace gases with surface sources and stratospheric sinks. Using speciation experiments, we show that nearly identical TM and gaseous elementary mercury (GEM) concentrations exist at and below the tropopause. Above the thermal tropopause GEM concentrations are almost always smaller than those of TM and the TM – GEM (i.e. Hg2+) difference increases up to ~ 40 % of TM at ~ 2 km and more above the thermal tropopause. Correlations with N2O as a reference tracer suggest stratospheric lifetimes of 72 ± 37 and 74 ± 27 yr for TM and GEM, respectively, comparable to the stratospheric lifetime of COS. This coincidence, combined with pieces of evidence from us and other researchers, corroborates the hypothesis that Hg2+ formed by oxidation in the stratosphere attaches to sulfate particles formed mainly by oxidation of COS and is removed with them from the stratosphere by air mass exchange, gravitational sedimentation, and cloud scavenging processes. [ABSTRACT FROM AUTHOR]

The absorbing aerosol index (AAI) based on the near Ultra-Violet (near-UV) remote sensing techniques is a qualitative parameter that allows to retrieve aerosol optical properties with confidence. In the first part of this study, a series of AAI sensitivity analysis is presented exclusively on biomass burning aerosols. Later on, this study applies a radiative transfer model (DISAMAR) to simulate the AAI measured by the Ozone Monitoring Instrument (OMI) and to derive the aerosol single scattering albedo (ω0). The inputs for the radiative transfer calculations are satellite measurement geometry and surface conditions from OMI, aerosol optical thickness (τ) from the MODerate-resolution Imaging Spectroradiometer (MODIS), and aerosol micro-physical parameters from the AErosol RObotic NETwork (AERONET), respectively. This approach is applied to the Chile wildfires for the period from 26 to 30 January 2017, when the OMI observed AAI of this event reached its peak. The Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP) failed to capture the evolution of the smoke plume, therefore the aerosol profile is parameterized. The simulated plume ascends to an altitude of 4.5–4.9 km, which is in good agreement with measurements. Due to the relatively small data size of this case, an outlier detection criterion has to be applied. The results show that the AAI simulated by DISAMAR is consistent with observations. The correlation coefficients are over 0.85. The retrieved mean ω0 at 550 nm is approximately 0.84, slightly smaller than the value of 0.90 measured independently by the AERONET instrument. The relative distance between the AERONET site and the plume, the assumption of homogeneous and static plume properties, the lack of the aerosol profile information, and the uncertainties in observations are primarily responsible for this discrepancy. Except for the observational errors, the impact of remaining error sources on ω0 retrieval is difficult to quantify. [ABSTRACT FROM AUTHOR]