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2012 .

(4 publications)

J. Browse, K. S. Carslaw, S. R. Arnold, K. Pringle, and O. Boucher. The scavenging processes controlling the seasonal cycle in Arctic sulphate and black carbon aerosol. Atmospheric Chemistry & Physics, 12:6775-6798, August 2012. [ bib | DOI | ADS link ]

The seasonal cycle in Arctic aerosol is typified by high concentrations of large aged anthropogenic particles transported from lower latitudes in the late Arctic winter and early spring followed by a sharp transition to low concentrations of locally sourced smaller particles in the summer. However, multi-model assessments show that many models fail to simulate a realistic cycle. Here, we use a global aerosol microphysics model (GLOMAP) and surface-level aerosol observations to understand how wet scavenging processes control the seasonal variation in Arctic black carbon (BC) and sulphate aerosol. We show that the transition from high wintertime concentrations to low concentrations in the summer is controlled by the transition from ice-phase cloud scavenging to the much more efficient warm cloud scavenging in the late spring troposphere. This seasonal cycle is amplified further by the appearance of warm drizzling cloud in the late spring and summer boundary layer. Implementing these processes in GLOMAP greatly improves the agreement between the model and observations at the three Arctic ground-stations Alert, Barrow and Zeppelin Mountain on Svalbard. The SO4 model-observation correlation coefficient (R) increases from: -0.33 to 0.71 at Alert (82.5deg N), from -0.16 to 0.70 at Point Barrow (71.0deg N) and from -0.42 to 0.40 at Zeppelin Mountain (78deg N). The BC model-observation correlation coefficient increases from -0.68 to 0.72 at Alert and from -0.42 to 0.44 at Barrow. Observations at three marginal Arctic sites (Janiskoski, Oulanka and Karasjok) indicate a far weaker aerosol seasonal cycle, which we show is consistent with the much smaller seasonal change in the frequency of ice clouds compared to higher latitude sites. Our results suggest that the seasonal cycle in Arctic aerosol is driven by temperature-dependent scavenging processes that may be susceptible to modification in a future climate.

S. Verma, O. Boucher, M. Shekar Reddy, H. C. Upadhyaya, P. Le Van, F. S. Binkowski, and O. P. Sharma. Tropospheric distribution of sulphate aerosols mass and number concentration during INDOEX-IFP and its transport over the Indian Ocean: a GCM study. Atmospheric Chemistry & Physics, 12:6185-6196, July 2012. [ bib | DOI | ADS link ]

The sulphate aerosols mass and number concentration during the Indian Ocean Experiment (INDOEX) Intensive Field Phase-1999 (INDOEX-IFP) has been simulated using an interactive chemistry GCM. The model considers an interactive scheme for feedback from chemistry to meteorology with internally resolving microphysical properties of aerosols. In particular, the interactive scheme has the ability to predict both particle mass and number concentration for the Aitken and accumulation modes as prognostic variables. <BR /><BR /> On the basis of size distribution retrieved from the observations made along the cruise route during IFP-1999, the model successfully simulates the order of magnitude of aerosol number concentration. The results show the southward migration of minimum concentrations, which follows ITCZ (Inter Tropical Convergence Zone) migration. Sulphate surface concentration during INDOEX-IFP at Kaashidhoo (73.46deg E, 4.96deg N) gives an agreement within a factor of 2 to 3. The measured aerosol optical depth (AOD) from all aerosol species at KCO was 0.37 0.11 while the model simulated sulphate AOD ranged from 0.05 to 0.11. As sulphate constitutes 29% of the observed AOD, the model predicted values of sulphate AOD are hence fairly close to the measured values. The model thus has capability to predict the vertically integrated column sulphate burden. Furthermore, the model results indicate that Indian contribution to the estimated sulphate burden over India is more than 60% with values upto 40% over the Arabian Sea.

O. Boucher, P. R. Halloran, E. J. Burke, M. Doutriaux-Boucher, C. D. Jones, J. Lowe, M. A. Ringer, E. Robertson, and P. Wu. Reversibility in an Earth System model in response to CO2 concentration changes. Environmental Research Letters, 7(2):024013, June 2012. [ bib | DOI | ADS link ]

We use the HadGEM2-ES Earth System model to examine the degree of reversibility of a wide range of components of the Earth System under idealized climate change scenarios where the atmospheric CO2 concentration is gradually increased to four times the pre-industrial level and then reduced at a similar rate from several points along this trajectory. While some modelled quantities respond almost immediately to the atmospheric CO2 concentrations, others exhibit a time lag relative to the change in CO2. Most quantities also exhibit a lag relative to the global-mean surface temperature change, which can be described as a hysteresis behaviour. The most surprising responses are from low-level clouds and ocean stratification in the Southern Ocean, which both exhibit hysteresis on timescales longer than expected. We see no evidence of critical thresholds in these simulations, although some of the hysteresis phenomena become more apparent above 2 × CO2 or 3 × CO2. Our findings have implications for the parametrization of climate impacts in integrated assessment and simple climate models and for future climate studies of geoengineering scenarios.

N. Huneeus, F. Chevallier, and O. Boucher. Estimating aerosol emissions by assimilating observed aerosol optical depth in a global aerosol model. Atmospheric Chemistry & Physics, 12:4585-4606, May 2012. [ bib | DOI | ADS link ]

This study estimates the emission fluxes of a range of aerosol species and one aerosol precursor at the global scale. These fluxes are estimated by assimilating daily total and fine mode aerosol optical depth (AOD) at 550 nm from the Moderate Resolution Imaging Spectroradiometer (MODIS) into a global aerosol model of intermediate complexity. Monthly emissions are fitted homogenously for each species over a set of predefined regions. The performance of the assimilation is evaluated by comparing the AOD after assimilation against the MODIS observations and against independent observations. The system is effective in forcing the model towards the observations, for both total and fine mode AOD. Significant improvements for the root mean square error and correlation coefficient against both the assimilated and independent datasets are observed as well as a significant decrease in the mean bias against the assimilated observations. These improvements are larger over land than over ocean. The impact of the assimilation of fine mode AOD over ocean demonstrates potential for further improvement by including fine mode AOD observations over continents. The Angström exponent is also improved in African, European and dusty stations. The estimated emission flux for black carbon is 15 Tg yr-1, 119 Tg yr-1 for particulate organic matter, 17 Pg yr-1 for sea salt, 83 TgS yr-1 for SO2 and 1383 Tg yr-1 for desert dust. They represent a difference of +45 %, +40 %, +26 %, +13 % and -39 % respectively, with respect to the a priori values. The initial errors attributed to the emission fluxes are reduced for all estimated species.

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