Michiel van Weele

Three decades of global methane sources and sinks

Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. By reacting with hydroxyl radicals, methane reduces the oxidizing capacity of the atmosphere and generates ozone in the troposphere. Although most sources and sinks of methane have been identified, their relative contributions to atmospheric methane levels are highly uncertain. As such, the factors responsible for the observed stabilization of atmospheric methane levels in the early 2000s, and the renewed rise after 2006, remain unclear. Here, we construct decadal budgets for methane sources and sinks between 1980 and 2010, using a combination of atmospheric measurements and results from chemical transport models, ecosystem models, climate chemistry models and inventories of anthropogenic emissions. The resultant budgets suggest that data-driven approaches and ecosystem models overestimate total natural emissions. We build three contrasting emission scenarios-which differ in fossil fuel and microbial emissions-to explain the decadal variability in atmospheric methane levels detected, here and in previous studies, since 1985. Although uncertainties in emission trends do not allow definitive conclusions to be drawn, we show that the observed stabilization of methane levels between 1999 and 2006 can potentially be explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial emissions. We show that a rise in natural wetland emissions and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006, although the relative contribution of these two sources remains uncertain.

Fresh air in the 21st century

Ozone is an air quality problem today for much of the worlds population. Regions can exceed the ozone air quality standards (AQS) through a combination of local emissions, meteorology favoring pollution episodes, and the clean-air baseline levels of ozone upon which pollution builds. The IPCC 2001 assessment studied a range of global emission scenarios and found that all but one projects increases in global tropospheric ozone during the 21st century. By 2030, near-surface increases over much of the northern hemisphere are estimated to be about 5 ppb (+2 to +7 ppb over the range of scenarios). By 2100 the two more extreme scenarios project baseline ozone increases of >20 ppb, while the other four scenarios give changes of -4 to +10 ppb. Even modest increases in the background abundance of tropospheric ozone might defeat current AQS strategies. The larger increases, however, would gravely threaten both urban and rural air quality over most of the northern hemisphere.

Global Modeling of the Oceanic Source of Organic Aerosols

The global marine organic aerosol budget is investigated by a 3-dimensional chemistry-transport model considering recently proposed parameterisations of the primary marine organic aerosol (POA) and secondary organic aerosol (SOA) formation from the oxidation of marine volatile organic compounds. MODIS and SeaWiFS satellite data of Chlorophyll-a and ECMWF solar incoming radiation, wind speed, and temperature are driving the oceanic emissions in the model. Based on the adopted parameterisations, the SOA and the submicron POA marine sources are evaluated at about 5 Tg (1.5 Tg C ) and 7 to 8 Tg (4 Tg C ), respectively. The computed marine SOA originates from the dimethylsulfide oxidation (78%), the potentially formed dialkyl amine salts (21%), and marine hydrocarbon oxidation (0.1%). Comparison of calculations with observations indicates an additional marine source of soluble organic carbon that could be partially encountered by marine POA chemical ageing.

Implications of variations in photodissociation rates for global tropospheric chemistry

Variations in global photodissociation rates in the troposphere are calculated and the consequences of these variations for tropospheric chemistry are examined. Most of the variations are due to the effects of clouds, surface reflection and stratospheric ozone. Parameterizations are developed and they permit the photodissociation rates to be calculated rapidly as a function of these three factors. The errors introduced by using the parameterization are considered to be acceptable in view of the uncertainty in the data available. The daily-integrated photodissociation rates (DIPRs) of ozone and nitrogen dioxide are presented in detail for the month of January. The DIPR of nitrogen dioxide peaks in the upper Antarctic troposphere due to the combined effects of long daylength and high surface reflection. The DIPR of ozone, on the other hand, is found to attain its global maximum at about 300 hPa in the tropical troposphere. The sensitivity of the DIPRs to the global input data is studied. Perturbed DIPRs are fed into a global chemistry transport model to assess the effects on tropospheric chemistry. The globally averaged lifetime of methane in an atmosphere with clouds is calculated to be 8.8 yr. The removal of all clouds reduces its lifetime with only 2.4%. This fairly small reduction in lifetime, however, is due to the fact that the effects above and below clouds cancel each other out. The DIPR of ozone and therefore the OH production is sensitive to the amount of stratospheric ozone. Whereas a reduction in stratospheric ozone affects predominantly the ozone photodissociation rate, clouds and surface reflection affect all photodissociation rates concurrently.

Tethered-balloon measurements of actinic flux in a cloud-capped marine boundary layer

As part of the Atlantic Stratocumulus Experiment (ASTEX), more than 34 tethered-balloon soundings were carried out on Santa Maria Island, Azores. The main purpose of the soundings was to measure vertical profiles of actinic flux under different meteorological conditions. In addition, vertical profiles of temperature, relative humidity, and wind speed and direction were measured. The actinic flux was measured with a photoelectrical detector with a spectral response that ranges from 330 nm to 390 nm. It is in this region where the photodissociation of nitrogen dioxide, the main precursor of ozone in the troposphere, takes place. Several vertical profiles of actinic flux under clear and cloudy conditions are presented. These profiles show the different behavior of the actinic flux below, in, and above clouds. These observations are complemented with detailed measurements of cloud characteristics; relevant cloud properties for this study are the cloud-base and cloud-top heights and the cloud optical depth. A slightly increasing tendency for the actinic flux in the whole atmospheric boundary layer was observed under clear sky conditions. Under the presence of clouds and compared with clear sky values measured at the same solar zenith angle and at the surface, lower values were observed below cloud (ratio ranging from 0.39 to 0.92) and higher at the top of the cloud (ratio ranging from 1.68 to 2.32). In the cloud an increase in the values of actinic flux with height was measured. The actinic flux measurements are evaluated against values obtained from a multilayer delta-Eddington model. An excellent agreement is found for the soundings made under total overcast conditions.

An empirical model to predict the UV-index based on solar zenith angles and total ozone

The clear sky UV-index is expressed as a function of two predictable quantities: the solar zenith angle and total ozone. This function is derived by fitting the measurements of total ozone and the UV-index obtained from two instruments, one in the mid-latitudes and one in the tropics. The shape of the function was chosen so that it represents the essentials of the underlying physics. This new function gives good results for all solar zenith angles between 0° and 90° and a wide range of total ozone values. Copyright

The effect of stratospheric sulfur from Mount Pinatubo on tropospheric oxidizing capacity and methane

The eruption of Mount Pinatubo in 1991 injected a large amount of SO2 into the stratosphere, which formed sulfate aerosols. Increased scattering and absorption of UV radiation by the enhanced stratospheric SO2 and aerosols decreased the amount of UV radiation reaching the troposphere, causing changes in tropospheric photochemistry. These changes affected the oxidizing capacity of the atmosphere and the removal rate of CH4 in the years following the eruption. We use the three-dimensional chemistry transport model TM5 coupled to the aerosol microphysics module M7 to simulate the evolution of SO2 and sulfate aerosols from the Pinatubo eruption. Their effect on tropospheric photolysis frequencies and concentrations of OH and CH4 is quantified for the first time. We find that UV attenuation by stratospheric sulfur decreased the photolysis frequencies of both ozone and NO2 by about 2% globally, decreasing global OH concentrations by a similar amount in the first 2 years after the eruption. SO2 absorption mainly affects OH primary production by ozone photolysis, while aerosol scattering also alters OH recycling. The effect of stratospheric sulfur on global OH and CH4 is dominated by the effect of aerosol extinction, while SO2 absorption contributes by 12.5% to the overall effect in the first year after the eruption. The reduction in OH concentrations causes an increase in the CH4 growth rate of 4 and 2 ppb/yr in the first and second years after the eruption, respectively, contributing 11 Tg to the 27 Tg observed CH4 burden change in late 1991 and early 1992.

Ambient UVB Dose and Sun Enjoyment Are Important Predictors of Vitamin D Status in an Older Population

Background: UVB-induced skin synthesis is considered the key source of vitamin D, yet exposure to UVB is poorly accounted for in epidemiological studies.Objectives: The aim of this study was to examine the association of serum 25-hydroxyvitamin D [25(OH)D] concentration with accurately measured ambient UVB dose, sun enjoyment, supplements, and other factors.Methods: An all-Irish cohort of community-dwelling participants aged >60 y [median age: 73; 67% female; median 25(OH)D: 54.5 nmol/L] was used. Participants from this large, cross-sectional study completed a questionnaire to provide information on demographic factors and lifestyle (including supplement use and sun enjoyment). The Tropospheric Emission Monitoring Internet Service database was used to extract the daily ambient UVB dose at wavelengths that could induce vitamin D synthesis (D-UVB) over Ireland (latitude: 51°N-55°N). Blood sampling occurred throughout the year. Ambient exposure at the place of residence was calculated for each participant individually. Associations between determinants and serum 25(OH)D concentration were examined in a multivariate model. Random forest analysis was used to establish prediction models of vitamin D deficiency, and area under the curve (AUC) is shown.Results: In total, 5138 individuals were included. Median D-UVB was 63 mJ/cm2, which varied between seasons and latitudes, despite the small latitude differential. Vitamin D supplementation (β = 27.7; P < 10 × 10-10), D-UVB (β = 1.58 per 1000 mJ/cm2; P < 10 × 10-10), and sun enjoyment (β = 6.6; P < 0.001) were strongly positively associated with serum 25(OH)D. Those who avoided sunshine were largely at risk of deficiency (<40 nmol/L), whereas those who enjoyed sunshine tended to be vitamin D sufficient (≥50 nmol/L). D-UVB and sun enjoyment improved prediction of deficiency in non-supplement-taking individuals; the overall AUC improved by 3.5%.Conclusion: D-UVB and sun enjoyment are important predictors of vitamin D status, even in this elderly population at northern latitudes. Accurate estimation of ambient UVB can help to further clarify the role of other determinants of vitamin D status and inform sunshine recommendation guidelines.

Space-based surface UV monitoring for Europe using SCIAMACHY and MSG

In order to characterize the solar UV radiation reaching the Earths surface it is monitored from space by means of (i) the clear-sky UV index at local solar noon, which is most relevant for operational UV forecasting, and (ii) the daily UV dose including cloud shielding effects, which is most relevant for long-term UV monitoring and assessments of health risks and biological UV effects. Optimal space- based surface UV monitoring combines information from platforms in different orbits. Space-based total ozone column products from polar orbiting platforms can be used adequately for UV monitoring because the diurnal variability in the total ozone column is limited. However, cloud cover and cloud optical thickness typically vary significantly on time scales of minutes to hours, especially over land in relation to convective activity. Because diurnal variations in cloud amount and cloud optical thickness impact dramatically on the daily-integrated UV radiation levels transmitted to the Earths surface, the time variations in (key) cloud parameters over the day need to be captured by observations. Sampling of the diurnal variations in clouds is most efficiently done from geostationary platforms. Here we demonstrate examples of calculations of the clear- sky UV index and the UV daily dose for erythema over Europe based on assimilated total ozone column data derived from observations by GOME aboard ERS-2 and its successor SCIAMACHY aboard ENVISAT, in combination with cloud information retrieved from MVIRI aboard Meteosat-7 and its successor SEVIRI aboard MSG (Meteosat-8). Some first validations with ground-based surface spectral UV data are presented.

Data Quality and Validation of Satellite Measurements of Tropospheric Composition

Validation is the essential part of satellite remote sensing, since the retrieved data must be fit-for-purpose and their significance quantified, whether they are for scientific research or environmental monitoring. Data are validated by comparing satellite data sets with those obtained from ground-based, balloon and airborne instrumentation, or from instruments on other satellites, or with the output of models; all can be fraught with sampling difficulties and comparability. Chapter 7 discusses these problems in some detail and indicates the quality assurance that is used in the field. The possibilities of optimising retrieval algorithms are dealt with, as well the problem of instrument degradation over time. The differing needs for data on trace gases and cloud and aerosol data are mentioned, as are the use of correlative methods. The chapter concludes with requirements for future measurements and possible validation strategies.