A comparison of dry and wet season aerosol number fluxes over the Amazon rain forest

Atmospheric Chemistry and Physics (Impact Factor: 4.88). 01/2009; 10(6). DOI: 10.5194/acpd-9-26881-2009
Source: DOAJ


Vertical number fluxes of aerosol particles and vertical fluxes of CO2 were measured with the eddy covariance method at the top of a 53 m high tower in the Amazon rain forest as part of the LBA (The Large Scale Biosphere Atmosphere Experiment in Amazonia) experiment. The observed aerosol number fluxes included particles with sizes down to 10 nm in diameter. The measurements were carried out during the wet and dry season in 2008. In this study focus is on the dry season aerosol fluxes, with significant influence from biomass burning, and these are compared with aerosol fluxes measured during the wet season. The primary goal is to quantify the dry deposition sink and to investigate whether particle deposition velocities change when going from the clean wet season into the more polluted dry season. Furthermore, it is tested whether the rain forest is always a net sink of particles in terms of number concentrations, or if particle emission from the surface under certain circumstances may dominate over the dry deposition sink. The particle deposition velocity vd increased linearly with increasing friction velocity in both seasons and the relations are described by vdd=(2.7 u* −0.2)×10−3 (dry season) and vdw=2.5 u*×10−3 (wet season), where u* is the friction velocity. The fact that the two relations are very similar to each other indicates that the seasonal change in aerosol number size distribution is not enough for causing any significant change in deposition velocity. In general, particle deposition velocities in this study are low compared to studies over boreal forests. The reason is probably domination of accumulation mode particles in the Amazon boundary layer, both in the dry and wet season, and low wind speeds in the tropics compared to the midlatitudes. Net particle deposition fluxes prevailed in daytime in both seasons and the deposition flux was considerably larger in the dry season due to the much higher dry season particle concentration. In the dry season, nocturnal particle fluxes behaved very similar to the nocturnal CO2 fluxes. Throughout the night, the measured particle flux at the top of the tower was close to zero, but early in the morning there was an upward particle flux peak that is not likely a result of entrainment or local pollution. It is possible that these morning upward particle fluxes are associated with emission of natural biogenic particles from the rain forest. Emitted particles may be stored within the canopy during stable conditions at nighttime, similarly to CO2, and being released from the canopy when conditions become more turbulent in the morning.

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    • "Dry deposition on temperate areas of eastern North America, for example, amount to 48 % of nitrate deposition, and 18 % of ammonium deposition (Bowen and Valiela 2001). Although some conclude that dry deposition is low in the tropics (Ahlm et al. 2010), measured ratios of dry to wet deposition fluxes in tropical latitudes range widely, from 0.11 to 1 (Clark et al. 1998; Baker et al. 2007; Boy et al. 2008; Wullaert et al. 2010), and dry deposition could be larger for rainforests (Hofhansl et al. 2010). Dry deposition should be a function of deforestation, since forests furnish greater area for dry deposition that grasslands, but we lacked data with which to assess such differences, so we used the one value for all watersheds. "
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    ABSTRACT: A series of eight watersheds on the Pacific coast of Panama where conversion of mature lowland wet forest to pastures by artisanal burning provided watershed-scale experimental units with a wide range of forest cover (23, 29, 47, 56, 66, 73, 73, 91, and 92 %). We used these watersheds as a landscape-scale experiment to assess effects of degree of deforestation on within-watershed retention and hydrological export of atmospheric inputs of nutrients. Retention was estimated by comparing rainfall nutrient concentrations (volume-weighted to allow for evapotranspiration) to concentrations in freshwater reaches of receiving streams. Retention of rain-derived nutrients in these Panama watersheds averaged 77, 85, 80, and 62 % for nitrate, ammonium, dissolved organic N, and phosphate, respectively. Retention of rain-derived inorganic nitrogen, however, depended on watershed cover: retention of nitrate and ammonium in pasture-dominated watersheds was 95 and 98 %, while fully forested watersheds retained 65 and 80 % of atmospheric nitrate and ammonium inputs. Watershed forest cover did not affect retention of dissolved organic nitrogen and phosphate. Exports from more forested watersheds yielded DIN/P near 16, while pasture-dominated watersheds exported N/P near 2. The differences in magnitude of exports and ratios suggest that deforestation in these Panamanian forests results in exports that affect growth of plants and algae in the receiving stream and estuarine ecosystems. Watershed retention of dissolved inorganic nitrogen calculated from wet plus dry atmospheric deposition varied from 90 % in pasture- to 65 % in forest-dominated watersheds, respectively. Discharges of DIN to receiving waters from the watersheds therefore rose from 10 % of atmospheric inputs for pasture-dominated watersheds, to about 35 % of atmospheric inputs for fully forested watersheds. These results from watersheds with no agriculture or urbanization, but different conversion of forest to pasture by burning, show significant, deforestation-dependent retention within tropical watersheds, but also ecologically significant, and deforestation-dependent, exports that are biologically significant because of the paucity of nutrients in receiving tropical stream and coastal waters.
    Full-text · Article · Oct 2013 · Biogeochemistry
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    • "However, the use of turbulence parameters from a different year and season may be reasonable in light of previous micrometeorology research at the K34 tower (<5 km from the TT34 tower). A study that overlapped with AMAZE 2008 demonstrated that both the 2008 wet and dry seasons have similar average daytime values of friction velocities above the canopy (∼0.35 m s −1 in the wet season and ∼0.40 m s −1 in the dry season [Ahlm et al., 2010, Figure 2i]). Another earlier study at the K34 tower which continuously measured the friction velocity above the canopy between July 1999 and September 2000 (and therefore spanned both dry and wet seasons) observed comparable average daytime friction velocities of ∼0.40 m s −1 [Araujo et al., 2002, Figure 5b]. "
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    ABSTRACT: Through rapid reactions with ozone, which can initiate the formation of secondary organic aerosols, the emission of sesquiterpenes from vegetation in Amazonia may have significant impacts on tropospheric chemistry and climate. Little is known, however, about sesquiterpene emissions, transport, and chemistry within plant canopies owing to analytical difficulties stemming from very low ambient concentrations, high reactivities, and sampling losses. Here, we present ambient sesquiterpene concentration measurements obtained during the 2010 dry season within and above a primary tropical forest canopy in Amazonia. We show that by peaking at night instead of during the day, and near the ground instead of within the canopy, sesquiterpene concentrations followed a pattern different from that of monoterpenes, suggesting that unlike monoterpene emissions, which are mainly light dependent, sesquiterpene emissions are mainly temperature dependent. In addition, we observed that sesquiterpene concentrations were inversely related with ozone (with respect to time of day and vertical concentration), suggesting that ambient concentrations are highly sensitive to ozone. These conclusions are supported by experiments in a tropical rain forest mesocosm, where little atmospheric oxidation occurs and sesquiterpene and monoterpene concentrations followed similar diurnal patterns. We estimate that the daytime dry season ozone flux of -0.6 to -1.5 nmol m-2 s-1 due to in-canopy sesquiterpene reactivity could account for 7%-28% of the net ozone flux. Our study provides experimental evidence that a large fraction of total plant sesquiterpene emissions (46%-61% by mass) undergo within-canopy ozonolysis, which may benefit plants by reducing ozone uptake and its associated oxidative damage.
    Full-text · Article · Oct 2011 · Journal of Geophysical Research Atmospheres
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    • "That was an indication that the source of primary biogenic aerosol emission, in terms of number concentrations, may be low when considering the total size range of particles. Also in the dry season, deposition fluxes dominated most of the time (Ahlm et al., 2010). However, upward particle fluxes often appeared in the morning. "
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    ABSTRACT: Size-resolved vertical aerosol number fluxes of particles in the diameter range 0.25–2.5 µm were measured with the eddy covariance method from a 53 m high tower over the Amazon rain forest, 60 km NNW of Manaus, Brazil. This study focuses on data measured during the relatively clean wet season, but a shorter measurement period from the more polluted dry season is used as a comparison. Size-resolved net particle fluxes of the five lowest size bins, representing 0.25–0.45 µm in diameter, were in general dominated by deposition in more or less all wind sectors in the wet season. This is an indication that the source of pri-mary biogenic aerosol particles may be small in this particle size range. Transfer velocities within this particle size range were observed to increase linearly with increasing friction velocity and increasing particle diameter. In the diameter range 0.5–2.5 µm, vertical particle fluxes were highly dependent on wind direction. In wind sectors where anthropogenic influence was low, net upward fluxes were observed. However, in wind sectors associated with higher anthropogenic influence, deposition fluxes dominated. The net upward fluxes were interpreted as a result of primary biogenic aerosol emission, but deposition of anthropogenic particles seems to have masked this emission in wind sec-tors with higher anthropogenic influence. The net emission fluxes were at maximum in the afternoon when the mixed layer is well developed, and were best correlated with hori-zontal wind speed according to the equation log 10 F = 0.48 · U + 2.21 where F is the net emission number flux of 0.5–2.5 µm parti-cles [m −2 s −1 ] and U is the horizontal wind speed [ms −1 ] at the top of the tower.
    Full-text · Article · Nov 2010 · Atmospheric Chemistry and Physics
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