Development of the MEGAN3 BVOC Emission Model for Use with the SILAM Chemical Transport Model

This project has aimed to investigate and propose improvements to the methods used in the System for Integrated ModeLing of Atmospheric coMposition (SILAM) model for simulating biogenic volatile organic compound (BVOC) emissions. The goal is to study an option in SILAM to use the Model for Emission of Gases and Aerosols in Nature, Version 3 (MEGAN3) as an alternative to SILAM’s existing BVOC calculation algorithm, which is a more simplified approach. SILAM is an atmospheric chemical transport, dispersion, and deposition modelling system owned and continuously developed by the Finnish Meteorological Institute (FMI). The model’s most well-known use is in forecasting air quality in Europe and southeast Asia. Although traffic and other urban emissions are important when modelling air quality, accurate modelling of biogenic emissions is also very important when developing a comprehensive, high-quality regional and sub-regional scale model. One of the motivations of this project is that if BVOC emission simulation in SILAM were improved, the improvements would be passed into subsequent atmospheric chemistry algorithms which form the molecules responsible to produce secondary organic aerosols (SOA). SOA have significant impacts on local and regional weather, climate, and air quality. The development in this project will therefore offer the potential for future improvement of air quality forecasting in the SILAM model. Because SILAM requires meteorological forecast as input boundary conditions, this study used output generated by the Environment-High Resolution Limited Area Model (Enviro-HIRLAM), developed by the HIRLAM Consortium in collaboration with universities in Denmark, Finland, the Baltic States, Ukraine, Russia, Turkey, Kazakhstan, and Spain. Enviro-HIRLAM includes multiple aerosol modes, which account for the effects of aerosols in the meteorological forecast. Running SILAM with and without the aerosol effects included in the Enviro-HIRLAM meteorological output showed that aerosols likely caused a minor decrease in BVOC emission rate. This project has focused on the boreal forest of Hyytiälä, southern Finland, the site of the Station for Measuring Ecosystem-Atmosphere Relations - II (SMEAR-II, 61.847°N, 24.294°E) during a one day trial on July 14, 2010. After performing a test run over the Hyytiälä region in July 2010 for analysis, it was found that SILAM significantly underestimates BVOC emission rates of both isoprene and monoterpene, likely because of an oversimplified approach used in the model. The current approach in SILAM, called ‘Guenther Modified’, uses only a few equations from MEGAN and can be classified as a strongly simplified MEGAN version, with selected assumptions. It references a land cover classification map and lookup table, taking into account only three parameters (air temperature, month, and solar radiation) when performing the calculations. It does not take into account several other important parameters, which affect the BVOC emission rates. Based on qualitative analysis, this appears to be a simplified but limited approach. Therefore, based on these findings, the next step to improve SILAM simulations is to propose a full implementation of MEGAN as a replacement to the current logic in SILAM, which is to use land classification and a lookup table for BVOC emission estimates. MEGAN, which is a much more comprehensive model for simulating BVOC emissions from terrestrial ecosystems. MEGAN includes additional input parameters, such as Leaf Area Index (LAI), relative humidity, CO2 concentration, land cover, soil moisture, soil type, and canopy height. Furthermore, this study found that in the future, simulations involving BVOCs could also potentially be improved in SILAM by adding modern schemes for chemical reactions and SOA formation in future development of SILAM. After gaining in-depth understanding of the strengths and limitations of BVOC in the SILAM model, as practical result, some recommendations for improvements to the model are proposed

Positive feedback mechanism between biogenic volatile organic compounds and the methane lifetime in future climates

A multitude of biogeochemical feedback mechanisms govern the climate sensitivity of Earth in response to radiation balance perturbations. One feedback mechanism, which remained missing from most current Earth System Models applied to predict future climate change in IPCC AR6, is the impact of higher temperatures on the emissions of biogenic volatile organic compounds (BVOCs), and their subsequent effects on the hydroxyl radical (OH) concentrations. OH, in turn, is the main sink term for many gaseous compounds including methane, which is the second most important human-influenced greenhouse gas in terms of climate forcing. In this study, we investigate the impact of this feedback mechanism by applying two models, a one-dimensional chemistry-transport model, and a global chemistry-transport model. The results indicate that in a 6 K temperature increase scenario, the BVOC-OH-CH4 feedback increases the lifetime of methane by 11.4% locally over the boreal region when the temperature rise only affects chemical reaction rates, and not both, chemistry and BVOC emissions. This would lead to a local increase in radiative forcing through methane (Delta RFCH4) of approximately 0.013 Wm(-2) per year, which is 2.1% of the current Delta RFCH4. In the whole Northern hemisphere, we predict an increase in the concentration of methane by 0.024% per year comparing simulations with temperature increase only in the chemistry or temperature increase in chemistry and BVOC emissions. This equals approximately 7% of the annual growth rate of methane during the years 2008-2017 (6.6 +/- 0.3 ppb yr-1) and leads to an Delta RFCH4 of 1.9 mWm(-2) per year.Peer reviewe

A modelling study of OH, NO3 and H2SO4 in 2007– 2018 at SMEAR II, Finland : analysis of long-term trends

Major atmospheric oxidants (OH, O3 and NO3) dominate the atmospheric oxidation capacity, while H2SO4 is considered as a main driver for new particle formation. Although numerous studies have investigated the long-term trend of ozone in Europe, the trends of OH, NO3 and H2SO4 at specific sites are to a large extent unknown. The one-dimensional model SOSAA has been applied in several studies at the SMEAR II station and has been validated by measurements in several projects. Here, we applied the SOSAA model for the years 2007–2018 to simulate the atmospheric chemical components, especially the atmospheric oxidants OH and NO3, as well as H2SO4 at SMEAR II. The simulations were evaluated with observations from several shorter and longer campaigns at SMEAR II. Our results show that daily OH increased by 2.39% per year and NO3 decreased by 3.41% per year, with different trends of these oxidants during day and night. On the contrary, daytime sulfuric acid concentrations decreased by 2.78% per year, which correlated with the observed decreasing concentration of newly formed particles in the size range of 3– 25 nm with 1.4% per year at SMEAR II during the years 1997–2012. Additionally, we compared our simulated OH, NO3 and H2SO4 concentrations with proxies, which are commonly applied in case a limited number of parameters are measured and no detailed model simulations are available.Peer reviewe

The Synergistic Role of Sulfuric Acid, Bases, and Oxidized Organics Governing New-Particle Formation in Beijing

Intense and frequent new particle formation (NPF) events have been observed in polluted urban environments, yet the dominant mechanisms are still under debate. To understand the key species and governing processes of NPF in polluted urban environments, we conducted comprehensive measurements in downtown Beijing during January-March, 2018. We performed detailed analyses on sulfuric acid cluster composition and budget, as well as the chemical and physical properties of oxidized organic molecules (OOMs). Our results demonstrate that the fast clustering of sulfuric acid (H2SO4) and base molecules triggered the NPF events, and OOMs further helped grow the newly formed particles toward climate- and health-relevant sizes. This synergistic role of H2SO4, base species, and OOMs in NPF is likely representative of polluted urban environments where abundant H2SO4 and base species usually co-exist, and OOMs are with moderately low volatility when produced under high NOx concentrations.Peer reviewe

Measurement report : A multi-year study on the impacts of Chinese New Year celebrations on air quality in Beijing, China

This study investigates the influence of the Chinese New Year (CNY) celebrations on local air quality in Beijing from 2013 through 2019. CNY celebrations include burning of fireworks and firecrackers, which consequently has a significant short-term impact on local air quality. In this study, we bring together comprehensive observations at the newly constructed Aerosol and Haze Laboratory at Beijing University of Chemical Technology – West Campus (BUCT-AHL) and hourly measurements from 12 Chinese government air quality measurement stations throughout the Beijing metropolitan area. These datasets are used together to provide a detailed analysis of air quality during the CNY over multiple years, during which the city of Beijing prohibited the use of fireworks and firecrackers in an effort to reduce air pollution before CNY 2018. Datasets used in this study include particulate matter mass concentrations (PM2.5 and PM10), trace gases (NOx , SO2, O3, and CO), and meteorological variables for 2013–2019; aerosol particle size distributions; and concentrations of sulfuric acid and black carbon for 2018 and 2019. Studying the CNY over several years, which has rarely been done in previous studies, can show trends and effects of societal and policy changes over time, and the results can be applied to study problems and potential solutions of air pollution resulting from holiday celebrations. Our results show that during the 2018 CNY, air pollutant concentrations peaked during the CNY night (for example, PM2.5 reached a peak around midnight of over 250 µg cm−3 , compared to values of less than 50 µg cm−3 earlier in the day). The pollutants with the most notable spikes were sulfur dioxide, particulate matter, and black carbon, which are emitted in burning of fireworks and firecrackers. Sulfuric acid concentration followed the sulfur dioxide concentration and showed elevated overnight concentration. Analysis of aerosol particle number size distribution showed direct emissions of particles with diameters around 100 nm in relation to firework burning. During the 2019 CNY, the pollution levels were somewhat lower (PM2.5 peaking at around 150 µg cm−3 on CNY compared to values around 100 µg cm−3 earlier in the day), and only minor peaks related to firework burning were observed. During both CNYs 2018 and 2019 secondary aerosol formation in terms of particle growth was observed. Meteorological conditions were comparable between these 2 years, suggesting that CNY-related emissions were less in 2019 compared to 2018. During the 7-year study period, it appears that there has been a general decrease in CNY-related emissions since 2016. For example, the peak in PM2.5 in 2016 was over 600 µg cm−3, and in the years following, the peak was less each year, with a peak around 150 µg cm−3 in 2019. This is indicative of the restrictions and public awareness of the air quality issues having a positive effect on improving air quality during the CNY. Going into the future, long-term observations will offer confirmation for these trends

A new implementation of FLEXPART with Enviro-HIRLAM meteorological input, and a case study during a heavy air pollution event

We integrated Enviro-HIRLAM (Environment-High Resolution Limited Area Model) meteorological output into FLEXPART (FLEXible PARTicle dispersion model). A FLEXPART simulation requires meteorological input from a numerical weather prediction (NWP) model. The publicly available version of FLEXPART can utilize either ECMWF (European Centre for Medium-range Weather Forecasts) Integrated Forecast System (IFS) forecast or reanalysis NWP data, or NCEP (U.S. National Center for Environmental Prediction) Global Forecast System (GFS) forecast or reanalysis NWP data. The primary benefits of using Enviro-HIRLAM are that it runs at a higher resolution and accounts for aerosol effects in meteorological fields. We compared backward trajectories generated with FLEXPART using Enviro-HIRLAM (both with and without aerosol effects) to trajectories generated using NCEP GFS and ECMWF IFS meteorological inputs, for a case study of a heavy haze event which occurred in Beijing, China in November 2018. We found that results from FLEXPART were considerably different when using different meteorological inputs. When aerosol effects were included in the NWP, there was a small but noticeable difference in calculated trajectories. Moreover, when looking at potential emission sensitivity instead of simply expressing trajectories as lines, additional information, which may have been missed when looking only at trajectories as lines, can be inferred.</p