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BrO measurements over the Eastern North-Atlantic

The aim of the work presented here was to detect BrO in the marine boundary layer over the Eastern North-Atlantic by Multi AXis-Differential Optical Absorption Spectroscopy (MAX-DOAS) of scattered sunlight. With this technique, information about the concentration and the vertical profile of trace gases in the atmosphere can be gained. BrO can be formed in the marine atmosphere by degradation of biogenic organohalogens or by oxidation of bromide in sea salt aerosol. BrO influences the chemistry in marine air in many ways, e.g. since it catalytically destroys ozone, changes the NO2/NO-ratio as well as the OH/HO2-ratio and oxidises DMS. However, the abundance and the significance of BrO in the marine atmosphere is not yet fully understood. We report on data collected during a ship cruise, which took place along the West African Coast in February 2007, within the framework of the Surface Ocean PRocesses in the ANthropocene project (SOPRAN). Tropospheric BrO could be detected during this cruise at peak mixing ratios of (10.2±3.7) ppt at an assumed layer height of 1 km on 18 February 2007. Furthermore, it was found that the mean BrO concentrations increased when cruising close to the African Coast suggesting that at least part of the BrO might have originated from there.
Seasonal variation of aliphatic amines in marine sub-micrometer particles at the Cape Verde islands

Monomethylamine (MA), dimethylamine (DMA) and diethylamine (DEA) were detected at non-negligible concentrations in sub-micrometer particles at the Cap Verde Atmospheric Observatory (CVAO) located on the island of São Vicente in Cape Verde during algal blooms in 2007. The concentrations of these amines in five stage impactor samples ranged from 0–30 pg m−3 for MA, 130–360 pg m−3 for DMA and 5–110 pg m−3 for DEA during the spring bloom in May 2007 and 2–520 pg m−3 for MA, 100–1400 pg m−3 for DMA and 90–760 pg m−3 for DEA during an unexpected winter algal bloom in December 2007. Anomalously high Saharan dust deposition and intensive ocean layer deepening were found at the Atmospheric Observatory and the associated Ocean Observatory during algal bloom periods. The highest amine concentrations in fine particles (impactor stage 2, 0.14–0.42 μm) indicate that amines are likely taken up from the gas phase into the acidic sub-micrometer particles. The contribution of amines to the organic carbon (OC) content ranged from 0.2–2.5% C in the winter months, indicating the importance of this class of compounds to the carbon cycle in the marine environment. Furthermore, aliphatic amines originating from marine biological sources likely contribute significantly to the nitrogen content in the marine atmosphere. The average contribution of the amines to the detected nitrogen species in sub-micrometer particles can be non-negligible, especially in the winter months (0.1% N–1.5% N in the sum of nitrate, ammonium and amines). This indicates that these smaller aliphatic amines can be important for the carbon and the nitrogen cycles in the remote marine environment.
Kinetic double-layer model of aerosol surface chemistry and gas-particle interactions (K2-SURF): Degradation of polycyclic aromatic hydrocarbons exposed to O3, NO2, H2O, OH and NO3

We present a kinetic double-layer surface model (K2-SURF) that describes the degradation of polycyclic aromatic hydrocarbons (PAHs) on aerosol particles exposed to ozone, nitrogen dioxide, water vapor, hydroxyl and nitrate radicals. The model is based on multiple experimental studies of PAH degradation and on the PRA framework (Pöschl-Rudich-Ammann, 2007) for aerosol and cloud surface chemistry and gas-particle interactions. For a wide range of substrates, including solid and liquid organic and inorganic substances (soot, silica, sodium chloride, octanol/decanol, organic acids, etc.), the concentration- and time-dependence of the heterogeneous reaction between PAHs and O3 can be efficiently described with a Langmuir-Hinshelwood-type mechanism. Depending on the substrate material, the Langmuir adsorption constants for O3 vary over three orders of magnitude (Kads,O3 ≈ 10−15–10−13 cm3), and the second-order rate coefficients for the surface layer reaction of O3 with different PAH vary over two orders of magnitude (kSLR,PAH,O3 ≈ 10−18–10−17 cm2 s−1). The available data indicate that the Langmuir adsorption constants for NO2 are similar to those of O3, while those of H2O are several orders of magnitude smaller (Kads,H2O ≈ 10−18–10−17 cm3). The desorption lifetimes and adsorption enthalpies inferred from the Langmuir adsorption constants suggest chemisorption of NO2 and O3 and physisorption of H2O. Note, however, that the exact reaction mechanisms, rate limiting steps and possible intermediates still remain to be resolved (e.g., surface diffusion and formation of O atoms or O3− ions at the surface). The K2-SURF model enables the calculation of ozone uptake coefficients, γO3, and of PAH concentrations in the quasi-static particle surface layer. Competitive adsorption and chemical transformation of the surface (aging) lead to a strong non-linear dependence of γO3 on time and gas phase composition, with different characteristics under dilute atmospheric and concentrated laboratory conditions. Under typical ambient conditions, γO3 of PAH-coated aerosol particles are expected to be in the range of 10−6–10−5. At ambient temperatures, NO2 alone does not efficiently degrade PAHs, but it was found to accelerate the degradation of PAHs exposed to O3. The accelerating effect can be attributed to highly reactive NO3 radicals formed in the gas phase or on the surface. Estimated second-order rate coefficients for O3-NO2 and PAH-NO3 surface layer reactions are in the range of 10−17–10−16 cm2 s−1 and 10−15–10−12 cm2 s−1, respectively. The chemical half-life of PAHs is expected to range from a few minutes on the surface of soot to multiple hours on organic and inorganic solid particles and days on liquid particles. On soot, the degradation of particle-bound PAHs in the atmosphere appears to be dominated by a surface layer reaction with adsorbed ozone. On other substrates, it is likely dominated by gas-surface reactions with OH or NO3 radicals (Eley-Rideal-type mechanism). To our knowledge, K2-SURF is the first atmospheric process model describing multiple types of parallel and sequential surface reactions between multiple gaseous and particle-bound chemical species. It illustrates how the general equations of the PRA framework can be simplified and adapted for specific reaction systems, and we suggest that it may serve as a basis for the development of a general master mechanism of aerosol and cloud surface chemistry.
Corrigendum to "Technical Note: An implementation of the dry removal processes DRY DEPosition and SEDImentation in the Modular Earth Submodel System (MESSy)" published in Atmos. Chem. Phys., 6, 4617–4632, 2006
Large-scale upper tropospheric pollution observed by MIPAS HCN and C2H6 global distributions

We present global upper tropospheric HCN and C2H6 amounts derived from MIPAS/ENVISAT limb emission spectra. HCN and C2H6 are retrieved in the spectral regions 715.5–782.7 cm−1 and 811.5–835.7 cm−1, respectively. The datasets consist of 54 days between September 2003 and March 2004. This period covers the peak and decline of the southern hemispheric biomass burning period and some months thereafter. HCN is a nearly unambiguous tracer of biomass burning with an assumed tropospheric lifetime of several months. Indeed, the most significant feature in the MIPAS HCN dataset is an upper tropospheric plume of enhanced values caused by southern hemispheric biomass burning, which in September and October 2003 extended from tropical South America over Africa, Australia to the Southern Pacific. The spatial extent of this plume agrees well with the MOPITT CO distribution of September 2003. Further there is good agreement with the shapes and mixing ratios of the southern hemispheric HCN and C2H6 fields measured by the ACE experiment between September and November 2005. The MIPAS HCN plume extended from the lowermost observation height of 8 km up to about 16 km altitude, with maximum values of 500–600 pptv in October 2003. It was still clearly visible in December 2003, but had strongly decreased by March 2004, confirming the assumed tropospheric lifetime. The main sources of C2H6 are production and transmission of fossil fuels, followed by biofuel use and biomass burning. The C2H6 distribution also clearly reflected the southern hemispheric biomass burning plume and its seasonal variation, with maximum amounts of 600–700 pptv. Generally there was good spatial overlap between the southern hemispheric distributions of both pollution tracers, except for the region between Peru and the mid-Pacific. Here C2H6was considerably enhanced, whereas the HCN amounts were low. Backward trajectory calculations suggested that industrial pollution was responsible for the elevated C2H6 concentration in these particular air masses. Except for the Asian monsoon anticyclone in September 2003, there were only comparably small regions of enhanced HCN in the Northern Hemisphere. However, C2H6 showed an equally strong northern hemispheric signal between the equator and low midlatitudes, persisting over the whole observation period. Backward trajectory calculations for air masses from this region also point to industrial sources of this pollution. Generally, C2H6/HCN ratios between 1 and 1.5 indicate biomass burning and ratios larger than 1.5 industrial pollution. However, in March 2004 ratios of up to 2 were also found in some regions of the former southern biomass burning plume.
Hit from both sides: tracking industrial and volcanic plumes in Mexico City with surface measurements and OMI SO2 retrievals during the MILAGRO field campaign

Large sulfur dioxide plumes were measured in the Mexico City Metropolitan Area (MCMA) during the MILAGRO field campaign. This paper seeks to identify the sources of these plumes and the meteorological processes that affect their dispersion in a complex mountain basin. Surface measurements of SO2 and winds are analysed in combination with radar wind profiler data to identify transport directions. Satellite retrievals of vertical SO2 columns from the Ozone Monitoring Instrument (OMI) reveal the dispersion from both the Tula industrial complex and the Popocatepetl volcano. Oversampling the OMI swath data to a fine grid (3 by 3 km) and averaging over the field campaign yielded a high resolution image of the average plume transport. Numerical simulations are used to identify possible transport scenarios. The analysis suggests that both Tula and Popocatepetl contribute to SO2 levels in the MCMA, sometimes on the same day due to strong vertical wind shear. During the field campaign, model estimates suggest that the volcano accounts for about one tenth of the SO2 in the MCMA, with a roughly equal split for the rest between urban sources and the Tula industrial complex. The evaluation of simulations with known sources and pollutants suggests that the combination of observations and meteorological models will be useful in identifying sources and transport processes of other plumes observed during MILAGRO.
Hydration and dehydration at the tropical tropopause

High-resolution water measurements from three tropical airborne missions in Northern Australia, Southern Brazil and West Africa in different seasons are analysed to study the transport and transformation of water in the tropical tropopause layer (TTL) and its impact on the stratosphere. The mean profiles are quite different according to the season and location of the campaigns, with lowest mixing ratios below 2 ppmv at the cold point tropopause during the Australian mission in November/December and high TTL mixing ratios during the African measurements in August. We present backward trajectory calculations considering freeze-drying of the air to the minimum saturation mixing ratio and initialised with climatological satellite data. This trajectory-based reconstruction of water agrees well with the observed H2O average profiles and therefore demonstrates that the water vapour set point in the TTL is primarily determined by the Lagrangian saturation history. Deep convection was found to moisten the TTL, in several events even above the cold point up to 420 K potential temperatures. However, our study does not provide evidence for a larger impact of these highly-localised events on global scales.
A comprehensive evaluation of seasonal simulations of ozone in the northeastern US during summers of 2001–2005

Regional air quality simulations were conducted for summers 2001–2005 in the eastern US and subjected to extensive evaluation using various ground and airborne measurements. A brief climate evaluation focused on transport by comparing modeled dominant map types with ones from reanalysis. Reasonable agreement was found for their frequency of occurrence and distinctness of circulation patterns. The two most frequent map types from reanalysis were the Bermuda High (22%) and passage of a Canadian cold frontal over the northeastern US (20%). The model captured their frequency of occurrence at 25% and 18% respectively. The simulated five average distributions of 1-h ozone (O3) daily maxima using the Community Multiscale Air Quality (CMAQ) modeling system reproduced salient features in observations. This suggests that the ability of the regional climate model to depict transport processes accurately is critical for reasonable simulations of surface O3. Comparison of mean bias, root mean square error, and index of agreement for CMAQ summer surface 8-h O3 daily maxima and observations showed –0.6±14 nmol/mol, 14 nmol/mol, and 71% respectively. CMAQ performed best in moderately polluted conditions and less satisfactorily in highly polluted ones. This highlights the common problem of overestimating/underestimating lower/higher modeled O3 levels. Diagnostic analysis suggested that significant overestimation of inland nighttime low O3 mixing ratios may be attributed to underestimates of nitric oxide (NO) emissions at night. The absence of the second daily peak in simulations for the Appledore Island marine site possibly resulted from coarse grid resolution misrepresentation of land surface type. Comparison with shipboard measurements suggested that CMAQ has an inherent problem of underpredicting O3 levels in continental outflow. Modeled O3 vertical profiles exhibited a lack of structure indicating that key processes missing from CMAQ, such as lightning produced NO and stratospheric intrusions, are important for accurate upper tropospheric representations.
Study of successive contrasting monsoons (2001–2002) in terms of aerosol variability over a tropical station Pune, India

The present study suggests that aerosols play a major role in cloud formation and affect significantly the precipitation over a regional scale. The study reveals that there is a high variability of aerosol index during a bad monsoon year 2002, indicating an extension of cycle to more than 100 days from a normal 50 day cycle of absorbing and non-absorbing aerosols over a tropical urban station Pune. Pre-monsoon of 2002 shows a high loading of coarse-mode aerosols (absorbing dust aerosols) which indicate vertical and horizontal temperature variations in turn affecting the seasonal rainfall at a regional scale. Cloud formation highly depends on aerosol concentration, but the activation process is not monotonic. The surface meteorological features help to initiate the cloud process. The surface temperatures were high during the pre-monsoon of 2002 leading to increase of aerosol optical depth as compared to 2001. The effect of surface wind speed, though, complicated to understand, results in low values in 2002 with high aerosol optical depth and vice-versa in 2001.
High resolution modeling of CO2 over Europe: implications for representation errors of satellite retrievals

Satellite retrievals for column CO2 with better spatial and temporal sampling are expected to improve the current surface flux estimates of CO2 via inverse techniques. However, the spatial scale mismatch between remotely sensed CO2 and current generation inverse models can induce representation errors, which can cause systematic biases in flux estimates. This study is focused on estimating these representation errors associated with utilization of satellite measurements in global models with a horizontal resolution of about 1 degree or less. For this we used simulated CO2 from the high resolution modeling framework WRF-VPRM, which links CO2 fluxes from a diagnostic biosphere model to a weather forecasting model at 10×10 km2 horizontal resolution. Sub-grid variability of column averaged CO2, i.e. the variability not resolved by global models, reached up to 1.2 ppm with a median value of 0.4 ppm. Statistical analysis of the simulation results indicate that orography plays an important role. Using sub-grid variability of orography and CO2 fluxes as well as resolved mixing ratio of CO2, a linear model can be formulated that could explain about 50% of the spatial patterns in the systematic (bias or correlated error) component of representation error in column and near-surface CO2 during day- and night-times. These findings give hints for a parameterization of representation error which would allow for the representation error to taken into account in inverse models or data assimilation systems.