Showing papers by "Jim Haywood published in 2014"

Ground-based aerosol characterization during the South American Biomass Burning Analysis (SAMBBA) field experiment

Abstract: . This paper investigates the physical and chemical characteristics of aerosols at ground level at a site heavily impacted by biomass burning. The site is located near Porto Velho, Rondonia, in the southwestern part of the Brazilian Amazon rainforest, and was selected for the deployment of a large suite of instruments, among them an Aerosol Chemical Speciation Monitor. Our measurements were made during the South American Biomass Burning Analysis (SAMBBA) field experiment, which consisted of a combination of aircraft and ground-based measurements over Brazil, aimed to investigate the impacts of biomass burning emissions on climate, air quality, and numerical weather prediction over South America. The campaign took place during the dry season and the transition to the wet season in September/October 2012. During most of the campaign, the site was impacted by regional biomass burning pollution (average CO mixing ratio of 0.6 ppm), occasionally superimposed by intense (up to 2 ppm of CO), freshly emitted biomass burning plumes. Aerosol number concentrations ranged from ~1000 cm−3 to peaks of up to 35 000 cm−3 (during biomass burning (BB) events, corresponding to an average submicron mass mean concentrations of 13.7 μg m−3 and peak concentrations close to 100 μg m−3. Organic aerosol strongly dominated the submicron non-refractory composition, with an average concentration of 11.4 μg m−3. The inorganic species, NH4, SO4, NO3, and Cl, were observed, on average, at concentrations of 0.44, 0.34, 0.19, and 0.01 μg m−3, respectively. Equivalent black carbon (BCe) ranged from 0.2 to 5.5 μg m−3, with an average concentration of 1.3 μg m−3. During BB peaks, organics accounted for over 90% of total mass (submicron non-refractory plus BCe), among the highest values described in the literature. We examined the ageing of biomass burning organic aerosol (BBOA) using the changes in the H : C and O : C ratios, and found that throughout most of the aerosol processing (O : C ≅ 0.25 to O : C ≅ 0.6), no remarkable change is observed in the H : C ratio (~1.35). Such a result contrasts strongly with previous observations of chemical ageing of both urban and Amazonian biogenic aerosols. At higher levels of processing (O : C > 0.6), the H : C ratio changes with a H : C / O : C slope of −0.5, possibly due to the development of a combination of BB (H : C / O : C slope = 0) and biogenic (H : C /O :C slope =−1) organic aerosol (OA). An analysis of the ΔOA /ΔCO mass ratios yields very little enhancement in the OA loading with atmospheric processing, consistent with previous observations. These results indicate that negligible secondary organic aerosol (SOA) formation occurs throughout the observed BB plume processing, or that SOA formation is almost entirely balanced by OA volatilization. Positive matrix factorization (PMF) of the organic aerosol spectra resulted in three factors: fresh BBOA, aged BBOA, and low-volatility oxygenated organic aerosol (LV-OOA). Analysis of the diurnal patterns and correlation with external markers indicates that during the first part of the campaign, OA concentrations are impacted by local fire plumes with some chemical processing occurring in the near-surface layer. During the second part of the campaign, long-range transport of BB plumes above the surface layer, as well as potential SOAs formed aloft, dominates OA concentrations at our ground-based sampling site. This manuscript describes the first ground-based deployment of the aerosol mass spectrometry at a site heavily impacted by biomass burning in the Amazon region, allowing a deeper understanding of aerosol life cycle in this important ecosystem.

Aerosol direct radiative effect of smoke over clouds over the southeast Atlantic Ocean from 2006 to 2009

Abstract: The aerosol direct radiative effect (DRE) of African smoke was analyzed in cloud scenes over the southeast Atlantic Ocean, using Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) satellite observations and Hadley Centre Global Environmental Model version 2 (HadGEM2) climate model simulations. The observed mean DRE was about 30–35 W m−2 in August and September 2006–2009. In some years, short episodes of high-aerosol DRE can be observed, due to high-aerosol loadings, while in other years the loadings are lower but more prolonged. Climate models that use evenly distributed monthly averaged emission fields will not reproduce these high-aerosol loadings. Furthermore, the simulated monthly mean aerosol DRE in HadGEM2 is only about 6 W m−2 in August. The difference with SCIAMACHY mean observations can be partly explained by an underestimation of the aerosol absorption Angstrom exponent in the ultraviolet. However, the subsequent increase of aerosol DRE simulation by about 20% is not enough to explain the observed discrepancy between simulations and observations.

Boundary tracking using a suboptimal sliding mode algorithm

Abstract: In this paper, a sliding mode control algorithm is proposed that allows a single, sensor enabled agent to navigate along the boundary of a contaminated region. The proposed control algorithm only requires knowledge of the local measurement of the scalar field. No gradient information is required for the spatial phenomenon and this distinguishes this work from much of the existing literature. The efficacy of the proposed approach is demonstrated using synthetic volcanic eruption dispersion data made available from a complex Lagrangian model.

A method to represent subgrid-scale updraft velocity in kilometer-scale models: Implication for aerosol activation

Abstract: Updraft velocities strongly control the activation of aerosol particles or that component that act as cloud condensation nuclei (CCN). For kilometer-scale models, vertical motions are partially resolved but the subgrid-scale (SGS) contribution needs to be parametrized or constrained to properly represent the activation of CCNs. This study presents a method to estimate the missing SGS (or unresolved) contribution to vertical velocity variability in models with horizontal grid sizes up to ∼2 km. A framework based on Large Eddy Simulations (LES) and high-resolution aircraft observations of stratocumulus and shallow cumulus clouds has been developed and applied to output from the United Kingdom Met Office Unified Model (UM) operating at kilometer-scale resolutions in numerical weather prediction configuration. For a stratocumulus deck simulation, we show that the UM 1 km model underestimates significantly the variability of updraft velocity with an averaged cloud base standard deviation between 0.04 and 0.05 m s−1 compared to LES and aircraft estimates of 0.38 and 0.54 m s−1, respectively. Once the SGS variability is considered, the UM corrected averages are between 0.34 and 0.44 m s−1. Off-line calculations of CCN-activated fraction using an activation scheme have been performed to illustrate the implication of including the SGS vertical velocity. It suggests increased CCN-activated fraction from 0.52 to 0.89 (respectively, 0.10 to 0.54) for a clean (respectively, polluted) aerosol environment for simulations with a 1 km horizontal grid size. Our results highlight the importance of representing the SGS vertical velocity in kilometer-scale simulations of aerosol-cloud interactions.