Investigating sulfate formation processes is important not only for air pollution control but also for understanding the climate system. Although the mechanisms of secondary sulfate production have been widely studied, in situ observational evidence implicating an important role of NO in SO oxidation in the real atmosphere has been rare. In this study, we report two unique cases, from an intensive campaign conducted at the Station for Observing Regional Processes of the Earth System (SORPES) in East China, showing distinctly different mechanisms of sulfate formation by NO and related nitrogen chemistry. The first case occurred in an episode of mineral dust mixed with anthropogenic pollutants and especially high concentrations of NO. It reveals that NO played an important role, not only in surface catalytic reactions of SO but also in dust-induced photochemical heterogeneous reactions of NO, which produced additional sources of OH radicals to promote new particle formation and growth. The second case was caused by aqueous oxidation of S(IV) by NO under foggy/cloudy conditions with high NH concentration. As a by-product, the formed nitrite enhanced HONO formation and further promoted the gas-phase formation of sulfate in the downwind area. This study highlights the effect of NO in enhancing the atmospheric oxidizing capacity and indicates a potentially very important impact of increasing NO on particulate pollution formation and regional climate change in East Asia.

Xie Yuning    丁爱军    Nie Wei    Mao Huiting    Qi Ximeng    Huang Xin    Xu Zheng    Veli matti Kerminen    Tuukka Petäj̈ä T.    Chi    Virkkula A.    Boy    Xue Likun    Guo Jia    Sun    杨修群    Markku Kulmala     Fu Congbin   

Journal of Geophysical Research: Atmospheres

2015

Establishment of the SMEAR Estonia at a hemiboreal mixed deciduous broad-leaved-evergreen needle-leaved forest at Järvselja, South-Eastern Estonia, has strongly enhanced the possibilities for national and international cooperation in the fields of forest ecosystem – atmosphere research and impacts of climatic changes on forest ecosystems, atmospheric trace gases, aerosols and air ions. The station provides a multitude of comprehensive continuously measured data covering key climatic and atmospheric characteristics (state and dynamics of solar radiation, trace gases, aerosols and air ions, meteorological parameters) and forest ecosystem traits (net primary productivity, individual tree growth, gas-exchange characteristics, soil variables). The station follows a multidisciplinary and multiscale approach covering processes in spatial dimensions ranging from nanometres to several hundred square kilometres, being thus able to significantly contribute to worldwide measurement networks and the SMEAR network. Here we present an overview of the station, its data produced and we envision future developments towards sustainable research and development of the large-scale scientific infrastructure SMEAR Estonia.

Noe    Ülo Niinemets    Krasnova Alisa    Krasnov Dmitrii    Motallebi Azimeh    Kängsepp Vivika    Jõgiste Kalev    Hõrrak Urmas    Komsaare Kaupo    Mirme Sander    Vana Marko    Tammet Hannes    Bäck    Timo Vesala    Markku Kulmala     Tuukka Petäj̈ä T.    Kangur Ahto   

Forestry Studies

2015

A satellite-based approach to derive the aerosol direct shortwave (SW) radiative effect (ADRE) was studied in an environment with highly variable aerosol conditions over eastern China from March to October 2009. The method is based on using coincident SW top-of-the-atmosphere (TOA) fluxes from the Clouds and the Earth's Radiant Energy System (CERES) and aerosol optical depths (AODs) from the MODerate Resolution Imaging Spectroradiometer (MODIS) to derive SW clear-sky ADRE. The estimate for the aerosol-free TOA flux (F0,TOA) is obtained by establishing linear regression between CERES SW TOA fluxes and MODIS AODs. A normalization procedure to a fixed solar zenith angle, Earth-Sun distance and atmospheric water vapor content was applied to the CERES fluxes prior to the linear fit against AOD to reduce the flux variation not related to aerosols. In the majority of the cases, the normalization increased positive correlation between observed SW TOA fluxes and AODs, and it decreased RMSE. The key question in the satellite-based approach is the accuracy of the estimated F0,TOA. Comparison with simulated F0,TOA showed that both the satellite method and the model produced qualitatively similar spatial patterns, but absolute values differed. In 58 % of the cases the satellite-based F0,TOA was within ±10 W m−2 of the modeled value (about 7-8 % difference in flux values). Over bright surfaces, the satellite-based method tend to produce lower F0,TOA than the model. The satellite-based clear-sky estimates for median instantaneous and diurnally averaged ADRE over the study area were g'8.8 W m−2 and g'5.1 W m−2, respectively. Over heavily industrialized areas, the cooling at TOA was 2 to more than 3 times the median value, and associated with high AODs (> 0.5). Especially during the summer months, positive ADREs were observed locally over dark surfaces. This was most probably a method artifact related to systematic change of aerosol type, sub-visual cloud contamination or both.

Sundström Anu-Maija    Arola    Kolmonen    Xue    De Leeuw    Markku Kulmala    

Atmospheric Chemistry and Physics

2015

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^-5s^-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.

Mogensen    Gierens Rosa T.    Crowley    Keronen    Smolander Sampo    Sogachev    Nölscher    Zhou    Markku Kulmala     Tang    Williams    Boy   

Atmospheric Chemistry and Physics

2015

Stabilized Criegee Intermediates (sCIs) have been identified as oxidants of atmospheric trace gases such as SO2, NO2, carboxylic acids or carbonyls. The atmospheric sCI concentrations, and accordingly their importance for trace gas oxidation, are controlled by the rate of the most important loss processes, very likely the unimolecular reactions and the reaction with water vapour (monomer and dimer) ubiquitously present at high concentrations in the troposphere. In this study, the rate coefficients of the unimolecular reaction of the simplest sCI, formaldehyde oxide, CH2OO, and its bimolecular reaction with the water monomer have been experimentally determined at T = (297 ± 1) K and at atmospheric pressure by using a free-jet flow system. CH2OO was produced by the reaction of ozone with C2H4, and CH2OO concentrations were probed indirectly by detecting H2SO4 after titration with SO2. Time-resolved experiments yield a rate coefficient of the unimolecular reaction of k(uni) = (0.19 ± 0.07) s^-1, a value that is supported by quantum-chemical and statistical rate theory calculations as well as by additional measurements performed under CH2OO steady-state conditions. A rate coefficient of k(CH2OO+H2O) = (3.2 ± 1.2) × 10^-16 cm³ molecule^-1 s^-1 has been determined for sufficiently low H2O concentrations (<10¹⁵ molecule cm^-3) that allow separation from the CH2OO reaction with the water dimer. In order to evaluate the accuracy of the experimental approach, the rate coefficients of the reactions with acetaldehyde and acetone were reinvestigated. The obtained rate coefficients k(CH2OO+acetald) = (1.7 ± 0.5) × 10^-12 and k(CH2OO+acetone) = (3.4 ± 0.9) × 10^-13 cm³ molecule^-1 s^-1 are in good agreement with literature data.

Berndt Torsten    Kaethner Ralf    Voigtländer    Stratmann Frank    Pfeifle Mark    Reichle Patrick    Sipilä Mikko    Markku Kulmala     Olzmann Matthias   

Physical Chemistry Chemical Physics

2015

Markku Kulmala    

Nature

2015

The nuclear accidents at Chernobyl and Fukushima released large amounts of Cs radionuclides into the atmosphere which spread over large forest areas. We compared the Cs concentration distribution in different parts of two coniferous forest ecosystems (needle litter, stems and at different depths in the soil) over short and long term periods in Finland and Japan. We also estimated the change in Cs activity concentrations in needle and soil between 1995 and 2013 in Southern Finland based on the back-calculated Cs activity concentrations. We hypothesized that if the Cs activity concentrations measured in 1995 and 2013 showed a similar decline in concentration, the Cs activity concentration in the ecosystem was already stable in 1995. But if not, the Cs activity concentrations were still changing in 2013. Our results showed that the vertical distribution of the Cs fallout in the soil was similar in Hyytiälä and Fukushima. The highest Cs concentrations were observed in the uppermost surface layers of the soil, and they decreased exponentially deeper in the soil. We also observed that Cs activity concentrations estimated from the samples in 1995 and 2013 in Finland showed different behavior in the surface soil layers compared to the deep soil layer. These results suggested that the Cs nuclei were still mobile in the surface soil layers 27 years after the accident. Our results further indicated that, in the aboveground parts of the trees, the Cs concentrations were much closer to steady-state when compared to those of the surface soil layers based on the estimated declining rates of Cs concentration activity in needles which were similar in 1995 and 2013. Despite its mobility and active role in the metabolism of trees, the Cs remains in the structure of the trees for decades, and there is not much exchange of Cs between the heartwood and surface layers of the stem.

Pumpanen    Mizue Ohashi    Endo Izuki    Pertti pepe Hari    Bäck    Markku Kulmala     Nobuhito Ohte   

Journal of Environmental Radioactivity

2015

Atmospheric HSO/HO nucleation influencing effects have been studied in the flow tube IfT-LFT (Institute for Tropospheric Research-Laminar Flow Tube) at 293 ± 0.5 K and a pressure of 1 bar using synthetic air as the carrier gas. The presence of a possible background amine concentration in the order of 10-10 molecule cm throughout the experiments has to be taken into account. In a first set of investigations, ozonolysis of olefins (tetramethylethylene, 1-methyl-cyclohexene, α-pinene and limonene) for close to atmospheric concentrations, served as the source of OH radicals and possibly other oxidants initiating HSO formation starting from SO. The oxidant generation is inevitably associated with the formation of organic oxidation products arising from the parent olefins. These products (first generation mainly) showed no clear effect on the number of nucleated particles within a wide range of experimental conditions for HSO concentrations higher than ∼10 molecule cm. Also the early growth process of the nucleated particles was not significantly influenced by the organic oxidation products in line with the expected growth by organic products using literature data. An additional, HSO-independent process of particle (nano-CN) formation was observed in the case of α-pinene and limonene ozonolysis for HSO concentrations smaller than ∼10 molecule cm. Furthermore, the findings confirm the appearance of an additional oxidant for SO beside OH radicals, very likely stabilized Criegee Intermediates (sCI). A second set of experiments has been performed in the presence of added amines in the concentrations range of a few 10-10 molecule cm applying photolytic OH radical generation for HSO production without addition of other organics. All amines showed significant nucleation enhancement with increasing efficiency in the order pyridine < aniline < dimethylamine < trimethylamine. This result supports the idea of H SO cluster stabilization by amines due to strong H SO↔amine interactions. On the other hand, this study indicates that for organic oxidation products (in presence of the possible amine background as stated) a distinct HSO/H O nucleation enhancement can be due to increased H SO formation caused by additional organic oxidant production (sCI) rather than by stabilization of HSO clusters due to HSO↔organics interactions. © Author(s) 2014.

Berndt Torsten    Sipilä Mikko    Stratmann Frank    Tuukka Petäj̈ä T.    Joonas Vanhanen    Mikkilä Jyri    Patokoski Johanna    Taipale Risto    Mauldin    Markku Kulmala    

Atmospheric Chemistry and Physics

2014

This work concentrates on the simultaneous mobility and mass measurement of negative ions generated by the ionizing radiation in a Am aerosol charger in N2 (5.0), a 1:1-mixture of N2 and synthetic air, pure synthetic air (5.0), and filtered laboratory air at ∼30% relative humidity. Therefore, a high-resolution mobility analyzer (UDMA) and an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF) were operated in series. Experiments with N2 as carrier gas showed a dominating signal at an electrical mobility of 2.09 cm/Vs with 90% of the ions being nitrate based. The ion composition was altered after a baking-out to a spectrum with three strong mobility-peaks at Z1 = 2.34 cm/Vs, Z2 = 1.42 cm/Vs, Z3 = 1.08 cm/Vs and a higher diversity of ions in the corresponding mass spectra. The carrier gas was gradually changed from N2 (5.0) to a 1:1-mixture of N2 with synthetic air and pure synthetic air (5.0), having only a minor effect on the overall pattern of the ion spectrum. Using room air leads to a domination of the nitrate based ions. The mobility-dependent transmission efficiency of the UDMA was modeled using an empirical, laminar diffusion deposition model. The data were further compared to an empirical mass-mobility relationship to evaluate the fragmentation of the ion clusters in the inlet of the mass spectrometer. This study suggests that the nitrate ion, NO3 , is found to be the dominant ion species produced in an aerosol charger, and that it may be mostly responsible for the charging of aerosol particles in negative polarity. Copyright © 2014 American Association for Aerosol Research.

Steiner Gerhard    Jokinen    Heikki Junninen    Sipilä Mikko    Tuukka Petäj̈ä T.    Douglas Worsnop    Reischl Georg P.    Markku Kulmala    

Aerosol Science and Technology

2014

The importance of gas-phase products from alkene ozonolysis other than OH radicals, most likely stabilized Criegee Intermediates (sCI), for the process of atmospheric SO oxidation to HSO has been recently discovered. Subjects of this work are investigations on HSO formation as a function of water vapour content (RH=2-65%) and temperature (278-343K) starting from the ozonolysis of trans-2-butene and 2,3-dimethyl-2-butene (TME). HSO production other than via the OH radical reaction was attributed to the reaction of SO with sCI, i.e. acetaldehyde oxide arising from trans-2-butene ozonolysis and acetone oxide from TME. Measurements have been conducted in an atmospheric pressure flow tube using NO-CI-APi-TOF mass spectrometry for HSO detection. The sCI yields derived from HSO measurements at 293K were 0.49±0.22 for acetaldehyde oxide and 0.45±0.20 for acetone oxide. Our findings indicate a HSO yield from sCI+SO of unity or close to unity. The deduced rate coefficient ratio for the reaction of sCI with HO and SO, k(sCI+HO)/k(sCI+SO), was found to be strongly dependent on the structure of the Criegee Intermediate, for acetaldehyde oxide at 293K: (8.8±0.4)·10 (syn- and anti-conformer in total) and for acetone oxide: <4·10. HSO formation from sCI was pushed back with rising temperature in both reaction systems most probably due to an enhancement of sCI decomposition. The ratio k(dec)/k(sCI+SO) increased by a factor of 34 (acetone oxide) increasing the temperature from 278 to 343K. In the case of acetaldehyde oxide the temperature effect is less pronounced. The relevance of atmospheric HSO formation via sCI+SO is discussed in view of its dependence on the structure of the Criegee Intermediate. © 2014 Elsevier Ltd.

Berndt Torsten    Jokinen    Sipilä Mikko    Mauldin    Hartmut Herrmann    Stratmann Frank    Heikki Junninen    Markku Kulmala    

Atmospheric Environment

2014