Modelling aerosol formation and precursor gases in the boundary layer

Aerosols are common air pollutants that pose serious threats to health. They also impact the Earth’s climate by interacting with solar radiation and altering cloud properties. One important phenomenon associated with the atmospheric aerosol system is new particle formation. This thesis is devoted to study the processes related to new particle formation as well as aerosol precursor gases in the boundary layer, where exchanges between the surface and the atmosphere take place. Model SOSAA, a one-dimensional transport model with detailed chemistry and aerosol microphysics, was constructed, tested and used as the main tool. Variations in the main biogenic volatile organic compounds (BVOCs), OH and H2SO4 across different environment were analysed. The simulated concentrations of main emitted organic compounds in two rural sites agree well with the observations. Due to missing OH reactivity in rural environment, the concentration of OH at Hyytiälä is overestimated. In contrast, the concentration of OH is likely underestimated in an urban environment due to missing production from HONO. H2SO4 production is underestimated at every site studied. New particle formation was studied in two phases: nucleation and growth. Large uncertainties are introduced to nucleation rates when calculating dependencies based on sulphuric acid alone. The oxidation products of monoterpenes by OH are shown not to be a suitable proxy for nucleation at either the urban site in Beijing or the rural site at Hyytiälä in summer. While they mainly contribute to the growth of particles at the rural site in Hyytiälä, the oxidation products of monoterpenes might only have major role in particle growth at Manitou during the evening. The rapid particle growth at Manitou in the daytime is possibly related to the oxidation products of 2-Methyl-3 Buten-2-Ol (MBO) by OH or any compound that has similar concentration and diurnal variation to the oxidation product of MBO by OH. The last model study assessed the climatic impact of aerosols in a hypothetic scenario with high atmospheric methane loading. The impact associated with aerosols and the change in atmospheric composition is shown to be as large as the impact due to methane itself. A NOx mitigation strategy was evaluated by SOSAA as effective to remove methane but the side product of hazardous tropospheric ozone makes the method unfavourable in real life. The process-based model SOSAA developed during this doctoral study is a useful tool for atmospheric research since it provides flexibility in time and space. The results call for further research into the ultimate mechanisms controlling nucleation and particle growth. This thesis also identifies knowledge gaps in the related precursor gases and encourages further studies in fields such as emission inventory of volatile organic compounds, sulphuric acid production, and atmospheric oxidation cycle.Aerosolit voivat heikentää ilmanlaatua, uhata terveyttä ja säädellä ilmastoa. Tässä työssä tarkasteltiin pienhiukkasten muodostumiseen liittyviä prosesseja ja muodostajakaasuja rajakerroksessa, jossa maanpinta ja ilmakehä vuorovaikuttavat. Tämä tehtiin tässä työssä kehitellyllä SOSAA-tietokonemallilla, joka on yksityiskohtaista kemiaa ja aerosolien mikrofysiikkaa sisältävä yksiulotteinen kuljetusmalli. Analyysi käsitteli luonnosta peräisin olevien haihtuvien orgaanisten yhdisteiden (BVOC), OH-radikaalin ja rikkihapon pitoisuusvaihteluita eri ympäristöissä sekä näiden vaikutusta pienhiukkasten muodostumiseen ja kasvuun

Simulating ozone dry deposition at a boreal forest with a multi-layer canopy deposition model

A multi-layer ozone (O-3) dry deposition model has been implemented into SOSAA (a model to Simulate the concentrations of Organic vapours, Sulphuric Acid and Aerosols) to improve the representation of O-3 concentration and flux within and above the forest canopy in the planetary boundary layer. We aim to predict the O-3 uptake by a boreal forest canopy under varying environmental conditions and analyse the influence of different factors on total O-3 uptake by the canopy as well as the vertical distribution of deposition sinks inside the canopy. The newly implemented dry deposition model was validated by an extensive comparison of simulated and observed O-3 turbulent fluxes and concentration profiles within and above the boreal forest canopy at SMEAR II (Station to Measure Ecosystem-Atmosphere Relations II) in Hyytiala, Finland, in August 2010. In this model, the fraction of wet surface on vegetation leaves was parametrised according to the ambient relative humidity (RH). Model results showed that when RH was larger than 70% the O-3 uptake onto wet skin contributed similar to 51% to the total deposition during nighttime and similar to 19% during daytime. The overall contribution of soil uptake was estimated about 36 %. The contribution of sub-canopy deposition below 4.2m was modelled to be similar to 38% of the total O-3 deposition during daytime, which was similar to the contribution reported in previous studies. The chemical contribution to O-3 removal was evaluated directly in the model simulations. According to the simulated averaged diurnal cycle the net chemical production of O-3 compensated up to similar to 4% of dry deposition loss from about 06:00 to 15:00 LT. During nighttime, the net chemical loss of O-3 further enhanced removal by dry deposition by a maximum similar to 9 %. Thus the results indicated an overall relatively small contribution of airborne chemical processes to O-3 removal at this site.Peer reviewe

Observation of the Effect of Gait-induced Functional Electrical Stimulation on Stroke Patients with Foot Drop

Objective: To explore the effects of functional electrical stimulation and functional mid frequency electrical stimulation on lower limb function and balance function in stroke patients. Methods: 20 cases of stroke patients with foot drop after admission were randomly divided into the observation group and the control group, 10 cases in each group. On the basis of the two groups of patients, the observation group used the gait induced functional electrical stimulation to stimulate the peroneal nerve and the pretibial muscle in the observation group. The control group used the computer medium frequency functional electrical stimulation to stimulate the peroneal nerve and the anterior tibial muscle for 2 weeks. Before and after treatment, the lower extremity simple Fugl-Meyer scale (FMA), the Berg balance scale (BBS) and the improved Ashworth scale were evaluated respectively, and the comparative analysis was carried out in the group and between the groups. Results: After 2 weeks of treatment, the scores of FMA and BBS in the two groups were significantly higher than those before the treatment (P < 0.05), and the scores of FMA and BBS in the observation group were higher than those in the control group (P < 0.05), and the flexor muscle tension of the ankle plantar flexor muscle of the observed group was lower than that of the control group (P < 0.05). Conclusions: Exercise therapy combined with gait induced functional electrical stimulation or computer intermediate frequency functional electrical stimulation can significantly improve lower limb function and balance function in patients with ptosis, and the therapeutic effect of functional electrical stimulation combined with gait is better.

SOSAA — a new model to simulate the concentrations of organic vapours, sulphuric acid and aerosols inside the ABL — Part 2: Aerosol dynamics and one case study at a boreal forest site

Natural and anthropogenic aerosols may have a great impact on climate as they directly interact with solar radiation and indirectly affect the Earth’s radiation balance and precipitation by modifying clouds. In order to quantify the direct and indirect effects, it is essential to understand the complex processes that connect aerosol particles to cloud droplets. Modern measurement techniques are able to detect particle sizes down to 1 nm in diameter, from ground to the stratosphere. However, the data are not sufficient in order to fully understand the processes. Here we demonstrate how the newly developed one-dimensional column model SOSAA was used to investigate the complex processes of aerosols at a boreal forest site for a six-month period during the spring and summer of 2010. Two nucleation mechanisms (kinetic and organic) were tested in this study, and both mechanisms produced a good prediction of the particle number concentrations in spring. However, overestimation of the particle number concentration in summer by the organic mechanism suggests that the OH oxidation products from monoterpenes may not be the essential compounds in atmospheric nucleation. In general, SOSAA was correct in predicting new particle formation events for 35% of the time and partly correct for 45% of the time.Peer reviewe

Global distribution of Asian, Middle Eastern, and North African dust simulated by CESM1/CARMA

Dust aerosols affect the radiative and energy balance at local and global scales by scattering and absorbing sunlight and infrared light. A previous study suggests that dust size distribution is one of the major sources of uncertainty in modeling the dust global distribution. Climate models overestimate the fine dust (≤5 µm) by an order of magnitude, while underestimates of the coarse dust (≥5 µm) range between 0.5 to 1.5 orders of magnitude compared with the global observations. Here we improved the simulated size distribution of dust aerosol using a sectional aerosol model (Community Aerosol and Radiation Model for Atmospheres) coupled with the Community Earth System Model (CESM1/CARMA). Simulated dust mass size distributions peak at around 2–3 µm in diameter and increase by 4 orders of magnitude from 0.1 to 2 µm. Our model demonstrates that North African, Middle Eastern, and Asian dust accounts for ∼ 59.7 %, 12.5 %, and 13.3 % of the global annual mean dust emissions, with the remaining 14.5 % originating from scattered smaller dust sources. The model dust vertical distributions are validated against the NASA Atmospheric Tomography (ATom) field campaign datasets. Both simulations and ATom in situ measurements during the ATom field campaign suggest that dust mass concentrations over the remote ocean drop by 2 to 3 orders of magnitude from the surface to the upper troposphere (200 hPa). Our model suggests that Asian dust contributes to more than 40 % of annual mean dust mass abundances in the global upper troposphere and lower stratosphere (UTLS). The model suggests that Asian dust dominates the dust mass budget in the UTLS of the Asian summer monsoon (ASM) region, with a relative contribution 1–2 orders of magnitude higher than the dust originating from the North African and Middle Eastern deserts.</p

Aerosol dynamics within and above forest in relation to turbulent transport and dry deposition

A 1-D atmospheric boundary layer (ABL) model coupled with a detailed atmospheric chemistry and aerosol dynamical model, the model SOSAA, was used to predict the ABL and detailed aerosol population (characterized by the number size distribution) time evolution. The model was applied over a period of 10 days in May 2013 to a pine forest site in southern Finland. The period was characterized by frequent new particle formation events and simultaneous intensive aerosol transformation. The aim of the study was to analyze and quantify the role of aerosol and ABL dynamics in the vertical transport of aerosols. It was of particular interest to what extent the fluxes above the canopy deviate from the particle dry deposition on the canopy foliage due to the above-mentioned processes. The model simulations revealed that the particle concentration change due to aerosol dynamics frequently exceeded the effect of particle deposition by even an order of magnitude or more. The impact was, however, strongly dependent on particle size and time. In spite of the fact that the timescale of turbulent transfer inside the canopy is much smaller than the timescales of aerosol dynamics and dry deposition, leading us to assume well-mixed properties of air, the fluxes at the canopy top frequently deviated from deposition inside the forest. This was due to transformation of aerosol concentration throughout the ABL and resulting complicated pattern of vertical transport. Therefore we argue that the comparison of timescales of aerosol dynamics and deposition defined for the processes below the flux measurement level do not unambiguously describe the importance of aerosol dynamics for vertical transport above the canopy. We conclude that under dynamical conditions reported in the current study the micrometeorological particle flux measurements can significantly deviate from the dry deposition into the canopy. The deviation can be systematic for certain size ranges so that the time-averaged particle fluxes can be also biased with respect to deposition sink.Peer reviewe

Mitigation Impact of Different Harvest Scenarios of Finnish Forests That Account for Albedo, Aerosols, and Trade-Offs of Carbon Sequestration and Avoided Emissions

The pressure to increase forest and land carbon stocks simultaneously with increasing forest based biomass harvest for energy and materials emphasizes the need for dedicated analyses of impacts and possible trade-offs between these different mitigation options including also forest related biophysical factors, surface albedo and the formation of biogenic Secondary Organic Aerosols (SOA). We analyzed the change in global radiative forcing (RF) due to changes in these climatic agents as affected by the change in state of Finnish forests under increased or decreased harvest scenarios from a baseline. We also included avoided emissions due to wood material and energy substitution. Increasing harvests from baseline (65% of Current Annual Increment) decreased the total carbon sink (carbon in trees, soil and harvested wood products) at least for 50 years. When we coupled this change in carbon with other biosphere responses, surface albedo and aerosols, decreasing harvests from the baseline produced the largest cooling effect during 50 years. Accounting also for the avoided emissions due to increased wood use, the RF responses of the two lowest harvest scenarios were within uncertainty range. Our results show that the effects of forest management on SOA formation should be included in the analyses trying to deduce the net climate impact of forest use. The inclusion of the rarely considered SOA effects enforces the view that the lower the harvest, the more climatic cooling boreal forests provide. These results should act as a caution mark for policy makers who are emphasizing the increased utilization of forest biomass for short-living products and bioenergy as an efficient measure to mitigate climate change.Peer reviewe