Jason A. Lowe

Marine aerosol, sea-salt, and the marine sulphur cycle: a short review

A short review of the marine aerosol size distribution and the contribution of sea-salt to this distribution is presented. The potential role of sea salt in the marine boundary layer sulphur cycle is highlighted.

Climate change scenarios for global impacts studies

We describe a set of global climate change scenarios that have been used in a series of studies investigating the global impacts of climate change on several environmental systems and resources — ecosystems, food security, water resources, malaria and coastal flooding. These scenarios derive from modelling experiments completed by the Hadley Centre over the last four years using successive versions of their coupled ocean–atmosphere global climate model. The scenarios benefit from ensemble simulations (made using HadCM2) and from an un-flux-corrected experiment (made using HadCM3), but consider only the effects of increasing greenhouse gas concentrations. The effects of associated changes in sulphate aerosol concentrations are not considered. The scenarios are presented for three future time periods — 30-year means centred on the 2020s, the 2050s and the 2080s — and are expressed with respect to the mean 1961–1990 climate. A global land observed climatology at 0.5° latitude/longitude resolution is used to describe current climate. Other scenario variables — atmospheric CO2 concentrations, global-mean sea-level rise and non-climatic assumptions relating to population and economy — are also provided. We discuss the limitations of the created scenarios and in particular draw attention to sources of uncertainty that we have not fully sampled.

Simulated and observed decadal variability in ocean heat content

Previous analyses by Levitus et al. [2000] (“Levitus”) of ocean temperature data have shown that ocean heat content has increased over the last fifty years with substantial temporal variability superimposed. The HadCM3 coupled atmosphere–ocean general circulation model (AOGCM) simulates the Levitus trend if both natural and anthropogenic forcings are included. In the relatively well-observed northern hemisphere upper ocean, HadCM3 has similar temporal variability to Levitus but, like other AOGCMs, it has generally less variability than Levitus for the world ocean. We analyse the causes of this discrepancy, which could result from deficiencies in either the model or the observational dataset. A substantial contribution to the Levitus variability comes from a strong maximum around 500 m depth, absent in HadCM3. We demonstrate a possibly large sensitivity to the method of filling in the observational dataset outside the well-observed region, and advocate caution in using it to assess AOGCM heat content changes.

Coupling sea-salt and sulphate interactions and its impact on cloud droplet concentration predictions

A parameterisation of internal mixing between sulphate and sea-salt aerosol is developed to determine the available externally mixed sulphate cloud condensation nuclei (CCN) population. This parameterisation is then combined with a multi-component aerosol-cloud parameterisation to predict cloud droplet concentration incorporating the physical competition between sea-salt and sulphate nuclei in the cloud nucleation processes. The results of the combined parameterisation indicate a significantly reduced role, compared to previous estimates, for sulphate in cloud droplet nucleation, and consequently, in indirect radiative forcing. However, the results also imply that cloud droplet concentration, and consequently, cloud albedo, has a greater susceptibility to change resulting from further anthropogenic SO2 emissions.

Predictions of global and regional sea‐level rise using AOGCMs with and without flux adjustment

Future global and regional sea-level changes have been calculated using two versions (HadCM2 and HadCM3) of the Hadley Centre coupled atmosphere-ocean general circulation model forced by the IS92a scenario for emissions of greenhouse gases. HadCM3 is a newly developed model which does not require flux adjustment to maintain a stable climatology. Global-average sea-level rise from 1990 to 2100 is predicted to be 0.48 m in HadCM2 and 0.44 m in HadCMS, 60% resulting from thermal expansion of sea-water and the rest from loss of mass of glaciers and small ice-caps. Sea-level rise is smaller in HadCM3 principally because the radiative forcing is slightly less, giving reduced ocean heat uptake and thermal expansion. However, the heat penetrates less deeply in HadCMS; consequently the surface warming is nearly the same. There is marked geographical variation of sea-level change, which is generally similar in the two models; local values range between zero and twice the global average.

Observations and modelling of aerosol growth in marine stratocumulus*case study

Airborne measurements of the growth of the marine accumulation mode after multiple cycles through stratocumulus cloud are presented. The nss-sulphate cloud residual mode was log-normal in spectral shape and it’s mode radius was observed to progressively increase in size from 0.78 to 0.94 μm over 155 min of air parcel evolution through the cloudy marine boundary layer. The primary reason for this observed growth was thought to result from aqueous phase oxidation of SO2 to aerosol sulphate in activated cloud drops. An aqueous phase aerosol–cloud-chemistry model was used to simulate this case study of aerosol growth and was able to closely reproduce the observed growth. The model simulations illustrate that aqueous phase oxidation of SO2 in cloud droplets was able to provide enough additional sulphate mass to increase the size of activated aerosol. During a typical cloud cycle simulation, ≈4.6 nmoles kg-1air (0.44 μg m-3) of sulphate mass was produced with ≈70% of sulphate production occurring in cloud droplets activated upon sea-salt nuclei and ≈30% occurring upon nss-sulphate nuclei, even though sea-salt nuclei contributed less than 15% to the activated droplet population. The high fraction of nss-sulphate mass internally mixed with sea-salt aerosol suggests that aqueous phase oxidation of SO2 in cloud droplets activated upon sea-salt nuclei is the dominant nss-sulphate formation mechanism and that sea-salt aerosol provides the primary chemical sink for SO2 in the cloudy marine boundary layer.

The effect of stabilising atmospheric carbon dioxide concentrations on global and regional climate change

The effect on climate of stabilising atmospheric carbon dioxide concentrations at 550ppm and 750ppm is investigated using a coupled ocean-atmosphere model and compared with the response to a baseline case (1% per year increase in carbon dioxide concentrations beyond 1990). Changes in other well-mixed greenhouse gases are not considered (although these are expected to increase in the future), so in practical terms the simulated changes in climate correspond to lower levels of carbon dioxide stabilisation. The global-mean warming between 1990 and 2100 is reduced by 40% and 55% respectively, in close agreement with estimates using energy balance models. Sea-level rise up to 2100 is also reduced, but in the longer stabilisation runs, unlike temperature, sea-level continues to rise throughout the simulations with little reduction of the rate of rise. The patterns of temperature and precipitation change are largely unchanged except that the southern hemisphere warms relative to the northern hemisphere. Changes over five subcontinental regions are considered in more detail. All of the regions, for all of the simulations, show a statistically significant warming by 2100. The reduction in annual-mean warming resulting from stabilisation is also significant by 2100. The seasonal changes in precipitation are significant by 2100 in the baseline simulation but the significance of differences in precipitation between the baseline and stabilisation simulations depends on location and season.

The effect of clouds on aerosol growth in the rural atmosphere

Measurements of accumulation mode aerosol in the atmospheric boundary layer under cloudy and cloud-free conditions, and in the lower free troposphere under cloud-free conditions, were conducted over the rural northwest of England. Normalised size distributions in the cloud-free . . boundary layer CFBL and the cloud-free free troposphere CFFT exhibited almost identical spectral similarities with both size distributions possessing a concentration peak mode-radius of f 0.05 mm or less. By comparison, aerosol distributions observed in cloudy air exhibited a distinctive log-normal distribution with mode-radii occurring at f 0.1 mm concomitant with a local minimum at f 0.05 mm. The consistent and noticeable difference in spectral features observed between cloudy and cloud-free conditions suggest that a greater amount of gas-to-par- ticle conversion occurs on cloudy days, presumably through in-cloud aqueous phase oxidation processes, leading to larger sized accumulation mode particles. Apart from the distinct difference between cloudy and cloud-free aerosol spectra on cloudy days, aerosol concentration and mass were observed to be significantly enhanced above that of the ambient background in the vicinity of clouds. Volatility analysis during one case of cloud processing indicated an increase in the relative contribution of aerosol mass volatile at temperatures characteristic of sulphuric acid, along .

New particle formation in the marine environment

Publisher Summary The source of condensation nuclei (CN ) in the marine environment and their evolution into CCN (Cloud Condensation Nuclei) are the focus of this chapter. CN are formed through homogeneous nucleation of H2SO4, H2O, and possibly NH3 vapor. The surface area of existing aerosol in the marine boundary layer is thought to provide a sufficient condensation sink for H2SO4 vapor, and thus, inhibits the vapor pressure required for homogeneous nucleation from being reached. Some cases of new-particle-formation, however, are observed under conditions of very low existing aerosol surface area. The timescales for freshly formed CN (r<5 nm) to grow into CCN (r<50 nm) under typical marine boundary layer conditions are thought to exceed the lifetime of marine CCN. It is postulated that the free troposphere is the most likely location for CN and CCN formation, as the tropospheric environmental conditions promote both CN formation and their growth into CCN because of longer residence timescales. This chapter presents observations of CN formation and decay from Antarctica and at a coastal site on the North East Atlantic.

Physicochemical properties of atmospheric aerosol at South UIST

Four field campaigns over the period November 1993 to August 1994 were undertaken on the island of South Uist, off the northwest coast of Scotland as part of the BMCAPE project. Measurements were made of the concentration and chemical composition of aerosol particles, utilising a variety of instruments. Sulphur and nitrogen gas species were also measured throughout these campaigns, together with appropriate meteorological parameters. A variety of air mass types were encountered during the campaigns and the relationship between the physical and chemical aerosol properties are discussed in terms of air mass histories and season factors.