J. M. Sigler

Cloud modulation of surface solar irradiance at a pasture site in southern Brazil

Broken cloud fields create mosaic radiative landscapes with interchanging cloud-shaded and sunlit areas. While clouds attenuate solar radiation incident on cloud-shaded areas, sunlit ground surfaces may actually receive more irradiance than under a clear sky due to light scattering and reflection from neighboring clouds. In this paper, we studied these two opposite but closely related aspects of cloud modulation of surface solar irradiance at a pasture site in southern Brazil. We analyzed a high-resolution time series of surface measurements obtained during the 1999 wet season. Surface solar irradiance frequently (more than 20% of the time) exceeded clear-sky levels and occasionally surpassed the extraterrestrial radiation. Clouds created a bimodal frequency distribution of surface solar irradiance, producing an average of approximately 50 and 14% for attenuation and enhancement, respectively, as compared to corresponding clear-sky level irradiance. The average duration of enhancement periods was about 1/3 of the average duration of attenuation periods. On the daily basis, cloud-induced enhancement contributed an average of 4% of the daily solar input to the surface and compensated for more than 10% of the attenuation due to the presence of clouds. Through spectral analysis, two temporal regimes were shown to modulate the surface irradiance by clouds. One was a convective/mesoscale of tens of minutes to hours and the other was a turbulent scale of several minutes corresponding to the classical Kolmogorov f−5/3 power law.

Ozone dynamics and deposition processes at a deforested site in the Amazon basin.

Abstract The goal of this study is to investigate the impact of deforestation on ozone dynamics and deposition in the Brazilian Amazon basin. This goal is accomplished through i) analyses of ozone levels and deposition rates at a deforested site during the rainy season; and ii) comparisons of these data with similar information derived at a forest. At the pasture site maximum ozone mixing ratios reach 20 parts per billion on a volume basis (ppbv) but about 6 ppbv prevail over the forest. Maximum ozone deposition velocities for pastures can reach 0.7 cm s–1, which is about threefold lower than values derived for forests. Combining ozone abundance and deposition velocities, pasture maximum ozone fluxes reach ∼0.2 µg (ozone) m–2 s–1. This flux represents approximately 70% of the deposition rates measured over the forest. Hence, this study suggests that conversion of rainforests to pastures could lead to a net reduction (30%) in the ozone sink in the Amazon.

Análise da variabilidade diária da precipitação em área de pastagem para a época chuvosa de 1999 - projeto TRMM/LBA

As part of the investigations of the experiment TRMM/LBA in Amazonia, measurements of the rain were obtained in a pasture site. In this work precipitation distribution were analyzed during the period from January 10 to February 28, 1999 (wet season). The monthly totals of precipitation for the period were 322.3 and 468.6 mm for January and February, respectively. The analyses of the frequency distribution for the events lower than 1 mm.h-1 and higher than 1, 3, 5, 10, 15 and 20 mm.h-1 showed different time distribution. The events lower than 1 mm.h-1 presented a more regular temporary distribution, even representing only 3% of the total volume of precipitation (27 mm). The events with precipitation above 1, 3, 5 mm.h-1 presented a similar pattern among them, and the most frequent cases ocurred during late afternoon. Those events represented about 97, 91 and 86% of the total of the precipitation, respectively. Finally, the events above 10, 15 and 20 mm.h-1 represented 73, 68 and 62% of the total of the precipitation respectively, and the largest relative frequencies concentrated among 2 up to 4 HL and 15 up to 17 HL. The rainfall observed can be classified in 2 distint system: local convection and mesoscale convective system, which provoke peaks at 16 LT and 4 LT, respectively.

Reply to comment by Ted Michaels on “Recent trends in anthropogenic mercury emission in the northeast United States”: Waste-to-energy's low mercury emissions

[1] Sigler and Lee [2006] (hereinafter referred to as SL06) combined measurements of mercury (Hg) and a combustion tracer at a background site to estimate regional Hg emission rates in the northeast United States from 1999/2000 to 2003/2004. Significant interannual variation in regional Hg emission was observed. Inventory calculations of Hg flux from the regional electric power sector suggested that the power sector strongly influences annual variation in Hg emission in the northeast but may not account for as large of a percentage of the total atmospheric Hg flux as expected given previous inventory estimates. [2] We thank Michaels [2007] (hereinafter referred to as M07) for comments on SL06. M07 cited substantial reductions in Hg emissions from municipal waste combustors (MWC) nationwide since 1990 and a clear ability of MWC to achieve federal emissions standards and suggested improper speculation by SL06 as to the role of MWC in Hg emissions in the northeast. Here we briefly address several of the important issues raised by M07. [3] We stress that SL06 did not claim that MWC cannot or do not test significantly lower than U.S. emission standards as defined by the Clean Air Act. More importantly, SL06 did not speculate that ‘‘high mercury emission levels in the northeast might be attributable’’ to MWC and failure to achieve standards, as suggested by M07, or to the performance of any other source category. SL06 cited multiple sources which indicate significant reductions in Hg emissions from MWC in several northeastern states since the late 1990s and were led to specifically investigate inventory Hg flux from the electric power sector because the substantial decline in MWC emissions due to legislative impact would leave power plants as by far the most important Hg emission category in the northeast [U.S. Environmental Protection Agency (EPA), 1997]. Rather than qualifying or searching for reasons behind high emission rates, SL06 sought to understand the observed interannual variation in Hg emissions determined from atmospheric measurements and gain insight into the relative contributions of different sources to total emissions in the northeast. [4] SL06 posited several potential contributing factors for the observation that power sector emissions were unable to account for as high a percentage of total Hg emissions in the northeast as expected. Among them (SL06, paragraph 38) was the possibility that ‘‘emissions from municipal and medical waste combustion emissions have not been reduced to the level targeted by EPA in 1997 and still contribute significantly to the regional Hg emission rate.’’ While not intended to suggest that MWC are responsible for ‘‘high mercury emission levels’’ in the northeast or are unable to test below federal standards, this contention could be misleading and deserves clarification. [5] As M07 noted, U.S. Hg emissions from large MWC declined significantly (95%) between 1990 and 2000 because of Maximum Achievable Control Technologies (MACT) compliance [EPA, 2002]. According to EPA [1997], New Source Performance Standards (NSPS) adopted in 1995 sought to reduce U.S. Hg emissions from municipal and medical waste combustion by at least an additional 90% over 1995 levels by 2000. Using inventory estimates presented by EPA [1997] as a baseline, we therefore expected MWC emissions to have declined from roughly one third of total Hg emission (1995 inventory) in the source region considered by SL06 (New England, MidAtlantic, Maryland, and Delaware) to on the order of 5% during the observation period (1999/2000–2003/2004), assuming no changes in emission among other major sources. This is clearly not the case, despite the dramatic reductions in bulk Hg emissions from MWC that have been achieved during the past decade. For example, recent inventory data suggest that MWC may have contributed approximately 21% of total Hg emissions in the northeast (not including Pennsylvania, Maryland, and Delaware) as recently as 2002, roughly the midpoint of the observation period of SL06, despite an 86% reduction in total MWC emissions in the same region since 1998 [Northeast States for Coordinated Air Use Management (NESCAUM), 2005]. We also note that total Hg emissions from MWC in New England, New York, and New Jersey were approximately 8.5 Mg in 1995 [EPA, 1997]. If a 90% reduction were JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D13314, doi:10.1029/2006JD008069, 2007 Click Here for Full Article