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Concentrations of 5 gases (HCl, HNO3, HONO, NH3, SO2) and 8 major inorganic ions in particles (Cl−, NO3−, SO42−, NH4+, Na+, K+, Mg2+, Ca2+) were measured with an online monitor MARGA 2S in two size ranges, Dp <2.5 μm and Dp < 10 μm, in Helsinki, Finland from November 2009 to May 2010. The results were compared with filter sampling, mass concentrations obtained from particle number size distributions, and a conventional SO2 monitor. The MARGA yielded lower concentrations than those analyzed from the filter samples for most ions. Linear regression yielded the following MARGA vs. filter slopes: 0.72 for Cl−, 0.90 for NO3−, 0.85 for SO42−, 0.91 for NH4+ , 0.49 for Na+, 3.0 for Mg2+, and 3.0 for Ca2+ and 0.90 for the MARGA vs. SO2 monitor. For K+ there were not enough data points to calculate a statistically significant linear regression. There were clear seasonal cycles in the concentrations of the nitrogen-containing gases: the median concentrations of HNO3, HONO, and NH3 were 0.09 ppb, 0.37 ppb, and 0.01 ppb in winter, respectively, and 0.15, 0.15, and 0.14 in spring, respectively. The gas-phase fraction of nitrogen decreased roughly with decreasing temperature, so that in the coldest period from January to February the median contribution was 28% but in April to May was 53%. There were also large fractionation variations that temperature alone cannot explain. HONO correlated well with NOx but a large fraction of the HONO-to-NOx ratios were larger than published ratios in a road traffic tunnel, suggesting that a large amount of HONO had other sources than vehicle exhaust. Aerosol acidity was estimated by calculating ion equivalent ratios. The sources of acidic aerosols were studied with trajectory statistics that showed that continental aerosol is mainly neutralized and marine aerosol acidic.

After the heatwave The European heatwave in the summer of 2003 was probably the hottest in the region since AD 1500. Its immediate effects are well documented: the Earth Policy Institute has estimated that it caused at least 35,000 deaths. Now the longer-term effects are beginning to emerge. Based on measurements of ecosystem CO 2 flux, radiation absorption by plants, crop yields and a model simulating the terrestrial biosphere, a multinational team of researchers has found that during July and August 2003, 500 million tonnes of carbon escaped from the forests and fields across Europe as a result of extreme heat and drought. The model results and historical data suggest that this dramatic fall in primary productivity is unprecedented during the past century. If the incidence of extreme droughts like this were to increase, temperate ecosystems could become carbon sources with the potential to accelerate global warming, as has been anticipated for the tropics. Future climate warming is expected to enhance plant growth in temperate ecosystems and to increase carbon sequestration 1 , 2 . But although severe regional heatwaves may become more frequent in a changing climate 3 , 4 , their impact on terrestrial carbon cycling is unclear. Here we report measurements of ecosystem carbon dioxide fluxes, remotely sensed radiation absorbed by plants, and country-level crop yields taken during the European heatwave in 2003. We use a terrestrial biosphere simulation model 5 to assess continental-scale changes in primary productivity during 2003, and their consequences for the net carbon balance. We estimate a 30 per cent reduction in gross primary productivity over Europe, which resulted in a strong anomalous net source of carbon dioxide (0.5 Pg C yr -1 ) to the atmosphere and reversed the effect of four years of net ecosystem carbon sequestration 6 . Our results suggest that productivity reduction in eastern and western Europe can be explained by rainfall deficit and extreme summer heat, respectively. We also find that ecosystem respiration decreased together with gross primary productivity, rather than accelerating with the temperature rise. Model results, corroborated by historical records of crop yields, suggest that such a reduction in Europe's primary productivity is unprecedented during the last century. An increase in future drought events could turn temperate ecosystems into carbon sources, contributing to positive carbon-climate feedbacks already anticipated in the tropics and at high latitudes 1 , 2 .

Using the 1-D atmospheric chemistry transport model SOSAA, we have investigated the atmospheric reactivity of a boreal forest ecosystem during the HUMPPA-COPEC-10 campaign (summer 2010, at SMEAR II in southern Finland). For the very first time, we present vertically resolved model simulations of the NO3 and O3 reactivity (R) together with the modelled and measured reactivity of OH. We find that OH is the most reactive oxidant (R∼3 s-1) followed by NO3 (R∼0.07 s-1) and O3 (R∼2×10-5 s-1). The missing OH reactivity was found to be large in accordance with measurements (∼65 %) as would be expected from the chemical subset described in the model. The accounted OH radical sinks were inorganic compounds (∼41 %, mainly due to reaction with CO), emitted monoterpenes (∼14 %) and oxidised biogenic volatile organic compounds (∼44 %). The missing reactivity is expected to be due to unknown biogenic volatile organic compounds and their photoproducts, indicating that the true main sink of OH is not expected to be inorganic compounds. The NO3 radical was found to react mainly with primary emitted monoterpenes (∼60 %) and inorganic compounds (∼37 %, including NO2). NO2 is, however, only a temporary sink of NO3 under the conditions of the campaign (with typical temperatures of 20–25 ∘C) and does not affect the NO3 concentration. We discuss the difference between instantaneous and steady-state reactivity and present the first boreal forest steady-state lifetime of NO3 (113 s). O3 almost exclusively reacts with inorganic compounds (∼91 %, mainly NO, but also NO2 during night) and less with primary emitted sesquiterpenes (∼6 %) and monoterpenes (∼3 %). When considering the concentration of the oxidants investigated, we find that OH is the oxidant that is capable of removing organic compounds at a faster rate during daytime, whereas NO3 can remove organic molecules at a faster rate during night-time. O3 competes with OH and NO3 during a short period of time in the early morning (around 5 a.m. local time) and in the evening (around 7–8 p.m.). As part of this study, we developed a simple empirical parameterisation for conversion of measured spectral irradiance into actinic flux. Further, the meteorological conditions were evaluated using radiosonde observations and ground-based measurements. The overall vertical structure of the boundary layer is discussed, together with validation of the surface energy balance and turbulent fluxes. The sensible heat and momentum fluxes above the canopy were on average overestimated, while the latent heat flux was underestimated.

Atmospheric concentrations of nitrous acid (HONO), one of the major precursors of the hydroxyl radical (OH) in the troposphere, significantly exceed the values predicted by the assumption of a photostationary state (PSS) during daytime. Therefore, additional sources of HONO were intensively investigated in the last decades. This study presents budget calculations of HONO based on simultaneous measurements of all relevant species, including HONO and OH at two different measurement heights, i.e. 1 m above the ground and about 2 to 3 m above the canopy (24 m above the ground), conducted in a boreal forest environment. We observed mean HONO concentrations of about 6.5 × 108 molecules cm−3 (26 ppt) during daytime, more than 20 times higher than expected from the PSS of 0.2 × 108 molecules cm−3 (1 ppt). To close the budgets at both heights, a strong additional source term during daytime is required. This unidentified source is at its maximum at noon (up to 1.1 × 106 molecules cm−3 s−1, 160 ppt h−1) and in general up to 2.3 times stronger above the canopy than close to the ground. The insignificance of known gas phase reactions and other processes like dry deposition or advection compared to the photolytic decomposition of HONO at this measurement site was an ideal prerequisite to study possible correlations of this unknown term to proposed HONO sources. But neither the proposed emissions from soils nor the proposed photolysis of adsorbed HNO3 contributed substantially to the unknown source. However, the unknown source was found to be perfectly correlated to the unbalanced photolytic loss of HONO.

Measurements of OH and HO2 radicals were conducted in a pine-dominated forest in southern Finland during the HUMPPA-COPEC-2010 (Hyytiälä United Measurements of Photochemistry and Particles in Air – Comprehensive Organic Precursor Emission and Concentration study) field campaign in summer 2010. Simultaneous side-by-side measurements of hydroxyl radicals were conducted with two instruments using chemical ionization mass spectrometry (CIMS) and laser-induced fluorescence (LIF), indicating small systematic disagreement, OHLIF / OHCIMS = (1.31 ± 0.14). Subsequently, the LIF instrument was moved to the top of a 20 m tower, just above the canopy, to investigate the radical chemistry at the ecosystem–atmosphere interface. Comprehensive measurements including observations of many volatile organic compounds (VOCs) and the total OH reactivity were conducted and analysed using steady-state calculations as well as an observationally constrained box model. Production rates of OH calculated from measured OH precursors are consistent with those derived from the steady-state assumption and measured total OH loss under conditions of moderate OH reactivity. The primary photolytic sources of OH contribute up to one-third to the total OH production. OH recycling, which occurs mainly by HO2 reacting with NO and O3, dominates the total hydroxyl radical production in this boreal forest. Box model simulations agree with measurements for hydroxyl radicals (OHmod. / OHobs. = 1.00 ± 0.16), while HO2 mixing ratios are significantly under-predicted (HO2mod. / HO2obs. = 0.3 ± 0.2), and simulated OH reactivity does not match the observed OH reactivity. The simultaneous under-prediction of HO2 and OH reactivity in periods in which OH concentrations were simulated realistically suggests that the missing OH reactivity is an unaccounted-for source of HO2. Detailed analysis of the HOx production, loss, and recycling pathways suggests that in periods of high total OH reactivity there are additional recycling processes forming OH directly, not via reaction of HO2 with NO or O3, or unaccounted-for primary HOx sources. Under conditions of moderate observed OH reactivity and high actinic flux, an additional RO2 source of approximately 1 × 106 molec cm−3 s−1 would be required to close the radical budget. Nevertheless, a major fraction of the OH recycling occurs via the reaction of HO2 with NO and O3 in this terpene-dominated environment.

The smelter industry in Kola Peninsula is the largest source of anthropogenic SO2 in the Arctic part of Europe and one of the largest within the Arctic domain. Due to socio-economic changes in Russia, the emissions have been decreasing especially since the late 1990s resulting in decreased SO2 concentrations close to Kola in eastern Lapland, Finland. At the same time, the frequency of new particle formation days has been decreasing distinctively at SMEAR I station in eastern Lapland, especially during spring and autumn. We show that sulfur species, namely sulfur dioxide and sulfuric acid, have an important role in both new particle formation and subsequent growth and that the decrease in new particle formation days is a result of the reduction of sulfur emissions originating from Kola Peninsula. In addition to sulfur species, there are many other quantities, such as formation rate of aerosol particles, condensation sink and nucleation mode particle number concentration, which are related to the number of observed new particle formation (NPF) days and need to be addressed when linking sulfur emissions and NPF. We show that while most of these quantities exhibit statistically significant trends, the reduction in Kola sulfur emissions is the most obvious reason for the rapid decline in NPF days. Sulfuric acid explains approximately 20–50% of the aerosol condensational growth observed at SMEAR I, and there is a large seasonal variation with highest values obtained during spring and autumn. We found that (i) particles form earlier after sunrise during late winter and early spring due to high concentrations of SO2 and H2SO4; (ii) several events occurred during the absence of light, and they were connected to higher than average concentrations of SO2; and (iii) high SO2 concentrations could advance the onset of nucleation by several hours. Moreover, air masses coming over Kola Peninsula seemed to favour new particle formation.

In the analyses of VOC fluxes measured above plant canopies, one usually assumes the flux above canopy to equal the exchange at the surface. Thus one assumes the chemical degradation to be much slower than the turbulent transport. We used a stochastic Lagrangian transport model in which the chemical degradation was described as first order decay in order to study the effect of the chemical degradation on above canopy fluxes of chemically reactive species. With the model we explored the sensitivity of the ratio of the above canopy flux to the surface emission on several parameters such as chemical lifetime of the compound, friction velocity, stability, and canopy density. Our results show that friction velocity and chemical lifetime affected the loss during transport the most. The canopy density had a significant effect if the chemically reactive compound was emitted from the forest floor. We used the results of the simulations together with oxidant data measured during HUMPPA-COPEC-2010 campaign at a Scots pine site to estimate the effect of the chemistry on fluxes of three typical biogenic VOCs, isoprene, α-pinene, and β-caryophyllene. Of these, the chemical degradation had a major effect on the fluxes of the most reactive species β-caryophyllene, while the fluxes of α-pinene were affected during nighttime. For these two compounds representing the mono- and sesquiterpenes groups, the effect of chemical degradation had also a significant diurnal cycle with the highest chemical loss at night. The different day and night time loss terms need to be accounted for, when measured fluxes of reactive compounds are used to reveal relations between primary emission and environmental parameters.

The problem of providing dispersion models with meteorological information from general atmospheric models used, for example, for weather forecasting is considered. As part of a generalized meteorological-to-dispersion model interface, a noniterative scheme diagnosing the surface layer characteristics from wind, temperature, and humidity profiles was developed. The scheme verification included long-term comparison with data of meteorological masts at Cabauw, the Netherlands, and Hyytiälä, Finland. The algorithm compatibility and consistency with the High-Resolution Limited-Area Model (HIRLAM) was also checked, as this model is routinely used as a meteorological driver for the Air Quality and Emergency Modeling System (SILAM). The comparison with Cabauw mast data showed a good quantitative agreement between observed and diagnosed heat and momentum fluxes: the temporal correlation coefficient was ∼0.8, bias was less than 10% of the absolute flux levels, regression slope deviated from unity for less than 20% with the intercept being less than 10% of the absolute flux values, and so on. In the case of complex surface features (Hyytiälä mast in forest) the scheme proved to be robust with large deviations appearing only if the input profile data were taken outside the constant-flux layer. Comparison with the HIRLAM model showed qualitatively good agreement but also highlighted several differences between the goals, standards, and methodologies of meteorological and dispersion models. The scheme was implemented in SILAM, which served as the development platform.

A prescribed fire experiment was conducted on 26 June 2009 in Hyytiälä, Finland, to study aerosol and trace gas emissions from prescribed fires of slash fuels and the effects of fire on soil properties in a controlled environment. A 0.8 ha forest near the SMEAR II measurement station (Station for Measuring Ecosystem-Atmosphere Relations) was cut clear; some tree trunks, all tree tops and branches were left on the ground and burned. The amount of burned organic material was ~46.8 tons (i.e., ~60 tons ha−1). The flaming phase lasted 2 h 15 min, the smoldering phase 3 h. Measurements were conducted on the ground with both fixed and mobile instrumentation, and in the air from a research aircraft. In the middle of the burning area, CO2 concentration peaked around 2000–3000 ppm above the baseline, and peak vertical flow velocities were ~9 m s−1, as measured with a 10 Hz 3-D sonic anemometer placed within the burn area. In the mobile measurements the peak particle number concentrations were approximately 1–2 × 106 cm−3 in the plume at a distance of 100–200 m from the burn area. On the ground at the SMEAR II station the geometric mean diameter of the mode with the highest concentration was 80 ± 1 nm during the flaming phase and in the middle of the smoldering phase, but, at the end of the smoldering phase, the largest mode was 122 nm. In the volume size distributions, geometric mean diameter of the largest volume mode was 153 nm during the flaming phase and 300 nm during the smoldering phase. The lowest single-scattering albedo of the ground-level measurements was 0.7 in the flaming-phase plume and ~0.9 in the smoldering phase. Elevated concentrations of several volatile organic compounds (VOC) (including acetonitrile, a biomass burning marker) were observed in the smoke plume at ground level. Measurements at the forest floor (i.e., a richly organic layer of soil and debris, characteristic of forested land) showed that VOC fluxes were generally low and consisted mainly of monoterpenes, and VOC flux peaked after the burning. After one year, the fluxes had nearly stabilized close to the level before the burning. The clear-cutting and burning of slash increased the total long-term CO2 release from the soil, and altered the physical, chemical and biological properties of the soil, such as increased the available nitrogen contents of the soil, which in turn, affected the long-term fluxes of greenhouse gases.

The performance of the miniature Versatile Aerosol Concentration Enrichment System (m-VACES; Geller et al., 2005) was investigated in laboratory and field studies using online instruments. Laboratory tests focused on the behavior of monodisperse ammonium sulfate (AS) or dioctyl sebacate (DOS) particles in the m-VACES measured with the aerodynamic particle sizer (APS) and scanning mobility particle sizer (SMPS). The ambient measurements were conducted at an urban site in Helsinki, Finland, where the operation of the m-VACES was explored in conjunction with a Soot Particle Aerosol Mass Spectrometer (SP-AMS) in addition to the SMPS. In laboratory tests, the growth of particles in water vapor produced a stable droplet size distribution independent of the original particle size. However, when the droplets were dried with the goal of measuring the original size distribution, a shift to larger particles was observed for small particle sizes (up to ~ 200 nm in mobility diameter). That growth was probably caused by water-soluble organic compounds absorbed on the water droplets from the gas phase, but not evaporated in the drying phase. In ambient measurements, a similar enrichment was observed for nitrate and sulfate in the m-VACES whereas the presence of acidic ambient particles affected the enrichment of ammonium. Gaseous ammonia was likely to be absorbed on acidic particles in the m-VACES, neutralizing the aerosol. For organics, the enrichment efficiency was comparable with sulfate and nitrate but a small positive artifact for hydrocarbons and nitrogen-containing organic compounds was noticed. Ambient and concentrated organic aerosol (OA) was analyzed further with positive matrix factorization (PMF). A three-factor solution was chosen for both of the data sets but the factors were slightly different for the ambient and concentrated OA, however, the data set used for the PMF analysis was limited in size (3 days) and therefore had substantial uncertainty. Overall, the operation of the m-VACES was not found to lead to any severe sampling artifacts. The effect of acidity could be an issue in locations where the aerosol is acidic, however, in those cases the use of a denuder (which was not used in this study) is recommended. Further ambient tests are needed for the characterization of the m-VACES as the time period for the ambient measurements was only 5 days in this study. Especially for OA additional tests are important as the chemical properties of organics can differ widely depending on time and location.