Biogenic Emissions and Urban Air Quality

Abstract

Trees emit a species-specific mixture of biogenic volatile organic compounds (BVOCs) like isoprene, monoterpenes and sesquiterpenes. These highly reactive BVOCs are quickly degraded by OH- NO₃- and O₃ radicals and hence, alter the atmospheric composition. Under high NO_x conditions their chemical degradation causes the formation of ground-level ozone. Furthermore, due to progressing chemical reactions BVOCs become less volatile and form secondary organic aerosol (SOA), which is one of the main components of PM_2.5. In this way, BVOCs can have negative effects on air quality and thus on human health and the ecosystem through their influence on NO_x, O₃ and PM_2.5 concentrations. However, since BVOC emission is trees species-specific, this influence depends on the composition of the tree population. Air quality may even improve due to a selection of specific tree species. Studies with different land use datasets are performed with the model system COSMO-MUSCAT for Germany and May 2014. The consideration of isoprene, sesquiterpene, HOMs and the reaction of monoterpene with NO₃ results in a doubling of organic matter (OM) concentration compared to the original SORGAM mechanism at higher temperatures.

Questions and Answers

Questioner: Pius Lee

Question: Your study was focused on scenarios in May 2014 when the trees are in blooming stages. Your conclusions on SOA production and ozone concentrations were interesting but not quite the peak production for summers (July/August) where air quality is often worse than May. Do you have summer scenarios and/or campaigns?

Answer: The periode May 2014 was chosen for evaluation purposes as for this time periode there are AMS, ACSM and filter measurements available for the Melpitz field site. The results already show a great improvement with the extended SORGAM module especially for warmer temperatures. It is planned to further extend the simulations for the entire summer 2014 and to compare the simulations with another campaigne, the F-BEACH campaign at Waldstein in July 2014. However, this will require further changes in the chemistry and SOA mechanisms, which are currently under development. The new mechanisms will than be used for the urban setup of Leipzig and the rather hot and dry summer of 2018 and 2019 will be modeled and analyzed.

Questioner: Joachim Fallmann

Question: What efforts are planned to answer the question of what would be the best/right tree in terms of urban air quality (Resolution, urban scheme, model configuration …)?

Answer: As first step a new chemistry mechanism is beeing developed for better treatment of anthropogenic and biogenic VOCs, and associated therewith the SOA module. Therin the tree species specific monoterpene split will be addressed more detailed in terms of reaction constants. Simulations of test sceniarios are planed with diffent tree species under diverse environmental conditions (NO_x emissions, temperature, radiation, wind, …). For the urban setup of Leipzig a resolution of up to 200 m and a double-canyon effect parametrization has already been realized. For more details see “High-resolution air-quality modeling in urban areas—A case study for the City of Leipzig” from Bernd Heinold. The city of Leipzig provided data for forests (in percent of area), street and park trees (exact location but no area information) on tree species basis. In 2018, a Land use/Land cover dataset was published for Leipzig by object-based image analysis for 2012 which contains area information for artificial surfaces, agriculture, grass, trees (>5 m diameter) and young trees/shrubs (Banzhaf & Kollai, 2018). With these information a tree species based map for Leipzig will be generated and urban simulations will be performed. For air quality mitigation potential of the different tree species the findings of the test scenarios will be used for future air quality assessment, where the actual existing tree species distribution is used and additional trees will be added. The influence of the additional trees will then be further analyzed for different future scenarios (NO_x emissions, meteorological parameters).