The link between secondary organic aerosol and monoterpenes at a boreal forest site

Secondary organic aerosol (SOA) plays a significant role in climate change by altering the global atmospheric radiation budget. Biogenic volatile organic compounds (BVOCs) are major sources for the formation of SOA. Their relationship is critical to understand the continental biosphere-aerosol-cloud-climate (COBACC) feedback mechanism but receives less attention in previous studies. In this study, a long time-series dataset (2008-2014) of SOA and monoterpene (the dominant biogenic precursor at the site) concentrations measured at the SMEAR II station in Hyytiälä, Finland, was analyzed to quantify the correlation between SOA and monoterpene concentrations. Climate factors including wind direction, temperature and relative humidity are studied to investigate their effects on the correlation. Overall, there was a significant and moderate linear correlation between SOA and monoterpene concentration with the Pearson correlation coefficient R of 0.66. To rule out the anthropogenic influence, the dataset was filtered by selecting the data at the wind direction 270-360°, based on the air trajectory analysis. The correlation coefficient was improved from 0.66 to 0.68. Temperature has been found not only to affect monoterpene emission but also their oxidation and partition processes. Therefore, the correlation between SOA and monoterpene concentration was scrutinized from a number of temperature windows to study the temperature influence. The linear regression slopes at different temperature windows showed a strong temperature dependence (R=0.75), indicating an increasing SOA yield with temperature. Following the same method, a negative correlation was found between the regression slopes and relative humidity(R=-0.88), suggesting a decreasing SOA yield as relative humidity increased. However, this might be due to the anti-correlation between temperature and relative humidity. Finally, the correlations between the SOA concentration increase rates and the monoterpenes oxidation rates by O3 (R=0.78) and OH (R=0.36) were compared, suggesting BVOC-O3 was the dominant oxidation path in the night, comparing with OH. The general monoterpene oxidation level was also intense in the evening for the site.Aerosols affect the climate by scattering and absorbing light to change the global radiation balance. They can also take part in the cloud formation process to modify cloud properties and the earth’s hydrological cycle. A notable fraction of aerosols is consisted of secondary organic aerosols (SOA), which are predominately from organic compounds emitted by plants. It is suggested that the SOA-cloud interaction will cause a cooling effect under a warmer climate. Therefore, a better understanding of the link between SOA and these compounds would help us understand more about the mechanisms behind the SOA formation and the cooling impact. The aim of this study is to reveal the link between SOA and monoterpenes (the major organic compounds emitted by the trees at the site) behind the measurement data from 2008 to 2014 at a boreal forest site in Hyytiälä, Finland. How wind direction, temperature, relative humidity and oxidants affected the monoterpenes-SOA relation were also studied. In the results, we found a positive link between SOA and monoterpenes, implying that SOA concentration increased with monoterpene concentration. Temperature had a strong and positive impact on the rate at which SOA was formed from monoterpenes, suggesting a more efficient SOA formation at higher temperature. Humidity, on the contrary, exhibited a strong and negative impact on this rate. It was questionable if this negative influence was indirectly affected by temperature as well. Wind direction associated human pollutions showed very weak impact in this study and the night time oxidation was mainly driven by O3. Our findings suggested that in a warmer climate, SOA concentration is likely to increase and hence may mitigate the warming through the SOA-cloud interactions. Keywords: Physical Geography, Ecosystem Analysis, BVOC, Monoterpenes, SOA, Temperature, Relative humidity, Wind directio