Importance of volatile organic compounds photochemistry over a forested area in central Greece

The impact of biogenic volatile organic compound (BVOC) on the chemical composition of the boundary layer in a valley-forested site of central Greece is investigated by using a chemical box model able to simulate α- and β-pinene and isoprene photochemistry in the troposphere. The model assimilates the meteorological conditions and mixing ratios of long-lived species observed during the AEROBIC field campaign in July–August 1997. Only 23–61% of the observed ozone (O3) mixing ratios can be attributed to the local photochemistry during the first part of the experiment, whereas this contribution increases to 80–96% during the second part of the campaign. The remaining part of O3 is reaching the boundary layer mainly from the free troposphere during the morning opening of the valley. The local net photochemical production of O3 is calculated to be up to 10 ppbv h−1, up to 60% of which is attributed to BVOC chemistry. BVOC oxidation is also shown to be an important source of carbon monoxide (CO) producing 1.5–2.5 ppbv CO h−1, carbonyl compounds and organic acids in particular contributing by about 1.5–4.3, 0.2–1.1 and 0.1–1 ppbv to the daytime ambient levels of formaldehyde, acetone and formic acid, respectively. BVOC oxidation is also able to produce about 1.3 μg m−3 (0.3–2.5 μg m−3) of secondary organic aerosol (SOA) that is 9–38% of the observed total organic aerosol levels.