Formation of secondary organic aerosol from sitka spruce emissions

Biogenic volatile organic compounds (BVOCs) emitted from a wide range of trees and plants undergo oxidation reactions in the atmosphere to produce secondary organic aerosol (SOA), which has direct and indirect effects on climate, and is accountable for a large proportion of climate uncertainties. The composition and yield of SOA is determined by the precursor BVOCs, which depends on the emission profile of the plant. In Ireland forestry is dominated by Sitka spruce (Picea Sitchensis), the emissions of which are not well characterised. The goal of this study is to identify the BVOC emissions from Sitka spruce, and to assess their SOA formation potential.

To characterise the emission profile of Sitka spruce, 4-year old trees were housed in a plant growth chamber under controlled environmental conditions and the emissions monitored on-line with a time-of-flight chemical ionisation mass spectrometer (ToF-CIMS), and off-line with thermal desorption gas chromatography mass spectrometry. The atmospheric reactions of the VOCs emitted by the Sitka spruce were investigated by oxidising them with hydroxyl (OH) radicals in an atmospheric simulation chamber. BVOC oxidation and gas-phase product formation was monitored by ToF-CIMS. A scanning mobility particle sizer (SMPS) was used to track particle formation and growth, and the SOA composition was determined with the use of a filter inlet for gas and aerosols (FIGAERO) fitted to the ToF-CIMS.

Over 60 different BVOCs were identified in the Sitka spruce emissions, with oxygenated species accounting for over 50% of them. The most abundant compound identified was piperitone, C₁₀H₁₆O an oxygenated monoterpene. Other compounds prevalent in the emissions included isoprene and monoterpenes, such as myrcene and β-phellandrene. During oxidation experiments the decay of the Sitka spruce emissions was observed with the ToF-CIMS in C₆H₆⁺ mode, while the formation of oxidised gas products was observed in I^- mode. The most prevalent gas-phase product was C₅H₆O₃. Analysis of the gas-phase oxidation products indicated that the oxidation of multiple BVOCs led to their formation. Particle formation and growth commenced quickly after the OH reaction was initiated. The composition of the SOA showed C₆H₈O₆ as the dominant species, but the majority of the products had formulas in the range #C_{7 – 15} and #O_{5 - 8}. Analysis of both gas and particle phase chemistry has been performed to determine the SOA formation potential of Sitka spruce BVOC emissions.