P. F. J. van Velthoven

Transport of biomass burning smoke to the upper troposphere by deep convection in the equatorial region

During LBA-CLAIRE-98, we found atmospheric layers with aged biomass smoke at altitudes >10 km over Suriname. CO, CO2, acetonitrile, methyl chloride, hydrocarbons, NO, O3, and aerosols were strongly enhanced in these layers. We estimate that 80-95% of accumulation mode aerosols had been removed during convective transport. Trajectories show that the plumes originated from large fires near the Brazil/Venezuela border during March 1998. This smoke was entrained into deep convection over the northern Amazon, transported out over the Pacific, and then returned to South America by the circulation around a large upper-level anticyclone. Our observations provide evidence for the importance of deep convection in the equatorial region as a mechanism to transport large amounts of pyrogenic pollutants into the upper troposphere. The entrainment of biomass smoke into tropical convective clouds may have significant effects on cloud microphysics and climate dynamics.

Constraining global methane emissions and uptake by ecosystems

Natural methane (CH 4) emissions from wet ecosystems are an important part of today’s global CH 4 budget. Climate affects the exchange of CH 4 between ecosystems and the atmosphere by influencing CH 4 production, oxidation, and transport in the soil. The net CH 4 exchange depends on ecosystem hydrology, soil and vegetation characteristics. Here, the LPJ-WHyMe global dynamical vegetation model is used to simulate global net CH 4 emissions for different ecosystems: northern peatlands (45 –90 N), naturally inundated wetlands (60 ◦ S–45 N), rice agriculture and wet mineral soils. Mineral soils are a potential CH 4 sink, but can also be a source with the direction of the net exchange depending on soil moisture content. The geographical and seasonal distributions are evaluated against multi-dimensional atmospheric inversions for 2003–2005, using two independent four-dimensional variational assimilation systems. The atmospheric inversions are constrained by the atmospheric CH4 observations of the SCIAMACHY satellite instrument and global surface networks. Compared to LPJ-WHyMe the inversions result in a significant reduction in the emissions from northern peatlands and suggest that LPJ-WHyMe maximum annual emissions peak about one month late. The Correspondence to: R. Spahni (spahni@climate.unibe.ch) inversions do not put strong constraints on the division of sources between inundated wetlands and wet mineral soils in the tropics. Based on the inversion results we diagnose model parameters in LPJ-WHyMe and simulate the surface exchange of CH4 over the period 1990–2008. Over the whole period we infer an increase of global ecosystem CH 4 emissions of+1.11 Tg CH4 yr−1, not considering potential additional changes in wetland extent. The increase in simulated CH4 emissions is attributed to enhanced soil respiration resulting from the observed rise in land temperature and in atmospheric carbon dioxide that were used as input. The longterm decline of the atmospheric CH 4 growth rate from 1990 to 2006 cannot be fully explained with the simulated ecosystem emissions. However, these emissions show an increasing trend of+3.62 Tg CH4 yr−1 over 2005–2008 which can partly explain the renewed increase in atmospheric CH 4 conentration during recent years.

Climatologies of NOx and NOy: A comparison of data and models

Abstract Climatologies of tropospheric NOx (NO + NO2) and NOy (total reactive nitrogen: NOx + N03 + 2 × N2O5 + HNO2 + HNO3 + HNO4 + ClONO2 + PAN (peroxyacetylnitrate) + other organic ni trates) have been compiled from data previously published and, in most cases, publicly archived. Emphasis has been on non-urban measurements, including rural and remote ground sites, as well as aircraft data. Although the distribution of data is sparse, a compilation in this manner can begin to provide an understanding of the spatial and temporal distributions of these reactive nitrogen species. The cleanest measurements in the boundary layer are in Alaska, northern Canada and the eastern Pacific, with median NO mixing ratios below 10 pptv, NOx below 50 pptv, and NOy below 300 pptv. The highest NO values (greater than 1 ppbv) were found in eastern North America and Europe, with correspondingly high NOy (∼ 5 ppbv). A significantly narrower range of concentrations is seen in the free troposphere, particularly at 3–6 km, with NO typically about 10 pptv in the boreal summer. NO increases with altitude to ∼ 100 pptv at 9–12 km, whereas NOy does not show a trend with altitude, but varies between 100 and 1000 pptv. Decreasing mixing ratios eastward of the Asian and North American continents are seen in all three species at all altitudes. Model-generated climatologies of NOx and NOy from six chemical transport models are also presented and are compared with observations in the boundary layer and the middle troposphere for summer and winter. These comparisons test our understanding of the chemical and transport processes responsible for these species distributions. Although the model results show differences between them, and disagreement with observations, none are systematically different for all seasons and altitudes. Some of the differences between the observations and model results may likely be attributed to the specific meteorological conditions at the time that measurements were made differing from the model meteorology, which is either climatological flow from GCMs or actual meteorology for an arbitrary year. Differences in emission inventories, and convection and washout schemes in the models will also affect the calculated NOα and NOy distributions.

Observation of upper tropospheric sulfur dioxide- and acetone-pollution: Potential implications for hydroxyl radicaland aerosol formation

Aircraft-based measurements of sulfur dioxide, acetone, carbon dioxide, and condensation nuclei (CN) were made over the north-eastern Atlantic at upper tropospheric altitudes, around 9000 m. On October 14, 1993, strong SO2- and acetone-pollution (both up to 3 ppbv) were observed, which were accompanied by a CO2-enhancement of up to 6 ppmv, and large CN-concentrations of up to about 1500 cm−3 (for radii ≥ 6 nm). CN, excess CO2, and to a lesser degree also acetone, were positively correlated with SO2. Air mass trajectory analyses indicate, that most of the air masses encountered by our aircraft originated from the polluted planetary boundary layer of the North-Eastern U. S. approximately 4–5 days prior to our measurements, and that polluted boundary layer air experienced fast vertical transport to the upper troposphere as well as horizontal transport across the Atlantic. From our data we conclude, that in the polluted air mass around 9000 m altitude HOx-formation, photochemical SO2-conversion to gaseous H2SO4, and eventually also CN-formation by homogeneous bimolecular (H2SO4-H2O) nucleation may have taken place with enhanced efficiency.

The 13C, 14C, and 18O isotopic composition of CO, CH4, and CO2 in the higher southern latitudes lower stratosphere

Large air samples were collected in the lower stratosphere (10–12 km) from 43° to 85°S in June 1993, using a special compressor system. For the important trace gases CO, CH4 and CO2, concentration and isotopic analyses were carried out and significant correlations were discovered. The 14CO isotope is considerably in excess of tropospheric levels with very high values from 40 to 120 14CO molecules/cm3 STP (corresponding to 12,500 percent modern carbon, at 30 ppbv), and is negatively correlated with CO. The linear relationship is used to estimate OH to be 2.9×105 cm−3. The 18O/16O ratios for CO are the lowest ever measured and reflect the inverse kinetic isotope effect in the oxidation of CO by OH. The 13C/12C ratios for CO are not much different from tropospheric values and confirm that fractionation is small but also that the in situ contribution from CH4 oxidation is minor. For CH4 a correlation between δ13C and concentration exists from which a fractionation factor for the sink reaction (k12/k13) of about 1.012 is calculated, well in excess of results from laboratory experiments for OH +CH4. The most plausible explanation presently is the removal of approximately 9% of CH4 by Cl atoms, which, as laboratory experiments have just confirmed, induces a very large fractionation. We also reveal a linear correlation between 14CO and 14CO2, precursor and product. Finally, an analysis of potential vorticity shows a structure that seems to give an overall agreement with the trace gas variations.

Distributions of NO, NO x , and NO y in the upper troposphere and lower stratosphere between 28° and 61°N during POLINAT 2

During the Pollution From Aircraft Emissions in the North Atlantic Flight Corridor 2 (POLINAT 2) field campaign the distribution of NO, NOx , and NOy in the upper troposphere and lowermost stratosphere over the eastern North Atlantic was measured using the Deutsches Zentrum fur Luft- und Raumfahrt research aircraft Falcon. Based from Shannon Airport in Ireland, 14 flights were carried out between September 19 and October 25, 1997. The measurements were performed in and out of the North Atlantic flight corridor covering latitudes between 28°N and 61°N. A marked latitudinal gradient in NO, NOx , and NOy , the sum of all reactive nitrogen compounds, was observed. Mean NO volume mixing ratios in the upper troposphere increased from about 50 parts per trillion by volume (pptv) at 28°N to about 180 pptv at 59°N. A similar latitude dependence was also found for NO x and NO y . In the northern part of the POLINAT 2 measuring area, NO and NO x volume mixing ratios increased significantly with increasing altitude withmaximum values around the tropopause, while in the southern part of the measuring area no strong altitude gradient was observed. NO and NOx did not show a substantial gradient across the tropopause. NO/NOy and NOy /O3 ratios showed maximum values of about 0.30 ppbv/ppbv and 0.012 ppbv/ppbv, respectively, around the tropopause. The POLINAT 2 observations suggest that aircraft emissions are an important source of NOx and NOy in the region studied. Also, the present measurements contribute to the data set obtained in the North Atlantic flight corridor during the last few years and help to establish a NOx climatology around the tropopause for this region.

Identification of extratropical two‐way troposphere‐stratosphere mixing based on CARIBIC measurements of O3, CO, and ultrafine particles

Simultaneous measurements of O3, CO, and ultrafine aerosol particles (UFP), conducted on board of a Boeing 767-ER passenger aircraft flying from Sri Lanka to Germany (project CARIBIC), are used to study two-way cross-tropopause mixing near a subtropical tropopause fold. On the equatorward side of the fold, downward mixing of stratospheric air into the upper troposphere is identified by enhanced concentrations of O3 and 14CO. Very high UFP number concentrations of up to 1.5×104 cm−3 (STP) were encountered inside the poleward half of the fold. This accumulation of small particles is explained by recent extensive aerosol nucleation, most likely triggered by the mixing of stratospheric air with tropospheric air injected into the fold. Further, nine particle formation events were observed outside the fold which are attributed to isolated cells of deep convection and to rising air parcels under cyclonic conditions that mix with surrounding air. In the upper troposphere O3 and CO were found to be correlated with high ΔO3/ΔCO ratios of 0.6 to 1.5. In the fold the correlation was strongly negative with ΔO3/ΔCO; = −3.5; but the high CO mixing ratios of 100 ppb at O3 mixing ratios of 250 ppb point to earlier injection of tropospheric air, in agreement with the UFP measurements.

Tropospheric O3 distribution over the Indian Ocean during spring 1995 evaluated with a chemistry-climate model

An analysis of tropospheric O3 over the Indian Ocean during spring 1995 is presented based on O3 soundings and results from the European Centre Hamburg (ECHAM) chemistry-general circulation model. The ECHAM model is nudged toward actual meteorology using European Centre for Medium-Range Weather Forecasts analyses, to enable a direct comparison between model results and in situ observations. The model reproduces observed CO levels in different air mass categories. The model also reproduces the general tendencies and the diurnal variation in the observed surface pressure, although the amplitude of the diurnal variation in the amplitude is underestimated. The model simulates the general O3 tendencies as seen in the sonde observations. Tropospheric O3 profiles were characterized by low surface concentrations (<10 ppbv), midtropospheric maxima (60–100 ppbv, at 700–250 hPa) and upper tropospheric minima (<20 ppbv, at 250–100 hPa). Large-scale upper tropospheric O3 minima were caused by connective transport of O3-depleted boundary layer air in the intertropical convergence zone (ITCZ). Similarly, an upper tropospheric O3 minimum was caused by Cyclone Marlene south of the ITCZ. The midtropospheric O3 maxima were caused by transport of polluted African air. The ECHAM model appears to overestimate surface O3 levels and does not reproduce the diurnal variations very well. This could be related to unaccounted multiphase O3 destruction mechanisms involving low level clouds and aerosols, and missing halogen chemistry.

Improvement and Evaluation of the Parameterisation of Nitrogen Oxide Production by Lightning

Abstract In order to describe the production of nitrogen oxides (NO x ) by lightning in chemistry-transport models the spatial and temporal distribution of this NO x source needs to be specified. Various meteorological model parameters can be used as a proxy for specifying this distribution. In order to determine the most suitable parameter as a proxy for lightning, we have correlated different meteorological quantities from the ECMWF model, such as convective precipitation and cloud top height, with ground-based lightning observations made during the EULINOX project. Convective precipitation gave the best correlation for summer conditions over Europe. Using the established relationship between convective precipitation and lightning intensity, we have tested different vertical distributions of the NO x lightning source in the global chemistry-transport model TM3. The model simulated NO fields have been compared with the NO observations made during the 1998 EULINOX and the 1997 POLINAT/SONEX campaigns. We found that a prescribed lightning NO x profile gave the best agreement with the observations. The observations covered various conditions: typical background situations, inside thunderstorms, and the outflow of thunderstorms of various ages. The model underestimated the NO concentrations in cases of fresh lightning NO x , likely due to its insufficient spatial and temporal resolution, but under most other circumstances the new parameterisation performed reasonably well, and is a clear improvement over the previously used parameterisation based on cloud top heights.

Influence of stratosphere‐troposphere exchange on tropospheric ozone over the tropical Indian Ocean during the winter monsoon

Ozone (O 3 ) and relative humidity (RH) soundings, launched over the Indian Ocean during the 1998 winter monsoon (February-March), were analyzed. In the marine boundary layer (MBL), O 3 mixing ratios were relatively low (10-20 ppbv) except close to the Indian subcontinent (40-50 ppbv) where profiles were strongly influenced by pollution. Sometimes, relatively low O 3 levels were observed in the upper troposphere. These were associated with deep convection in regions where MBL O 3 levels were also low. In the midtroposphere (300-500 hPa), O 3 maxima (60-90 ppbv) were often found with low RH. A remarkable new finding of this study is that in more than a third of the profiles, laminae with very high O 3 mixing ratios (up to 120 ppbv) were observed just below the tropical tropopause (between 100 and 200 hPa). Back trajectory analyses showed that these layers originated in the vicinity of the subtropical jet stream (STJ). We hypothesize that stratosphere-troposphere exchange (STE) near the subtropical jet by either shear-induced differential advection or clear-air turbulence (CAT) caused the midtropospheric maxima (STE followed by descent) and the upper tropospheric laminae. Another new finding is that stratospheric intrusions were not only found near the STJ but also deep within the tropics. Given the thickness of the midtroposphere intrusions (typically 3-5 km) and the very high O 3 mixing ratios of the upper tropospheric laminae, it seems that STE plays an important role in the tropical tropospheric O 3 budget, at least over the Indian Ocean during the winter monsoon.