Urban Isoprene as a Precursor for Ozone Formation

Isoprene (C5H8) is the most important Biogenic Volatile Organic Compound (BVOC) on global scale and represents around 40 % of all BVOC emissions. As a result of its high reactivity, isoprene is a crucial ozone precursor and can play a key role in atmospheric chemistry even in urban environments with regularly low isoprene concentrations (Wagner 2013). Major source of urban isoprene are some species of trees and shrubs (e.g. oaks, poplars, plane trees). The plant emission rate depends on temperature and light conditions with a maximum release at leaf temperature > 30°C and radiation conditions around 1,000 µmol/(m-2•s-1) PAR. As a result, isoprene concentration in the vicinity of vegetation could be very high on hot and sunny days. Isoprene may also be of anthropogenic origin: By road traffic emissions with around 5 % of the benzene concentration (Duane et al. 2002), and by human breathing with 65 ppb – 99 ppb as a by-product of metabolic cholesterol synthesis (Kinoyama et al. 2008). Model studies showed that isoprene, cause a significant increase in ozone concentrations. It is not so much the average but the maximum ozone concentration that is boosted by BVOCs (Bao et al., 2010). In Essen (210 km2; 570,000 inh., 7°E, 51°N), Germany, the isoprene concentration was measured at various sites and in different seasons using two GC-PID systems (Wagner and Kuttler, submitted). The concentration measurements were compared with those of benzene and toluene which represent characteristic anthropogenic VOCs. In summer, the diurnal variation of isoprene was determined by the biogenic emissions in the city, with hourly mean concentrations between 0.13 ppb and 0.17 ppb appearing during midday/afternoon. At night, isoprene concentration values fell down to very low concentrations (< 0.01 ppb). In contrast to isoprene, hourly mean benzene and toluene concentrations reached their peak values during the morning rush hour and in the late evening hours (0.22 ppb and 0.78 ppb for benzene and toluene, respectively) and were at its lowest level in the afternoon (0.11 ppb and 0.20ppb, respectively). There are large diurnal differences in the isoprene/benzene ratio depending on weather. On a hot and sunny day in August 2012 isoprene/benzene ratio reached a maximum value of 5.86 whereas during cool days the maximum ratio was just 1.12. In relation to the ozone formation (using propylene equivalent concentrations), isoprene is an important precursor in the city area on sunny summer afternoons. The contribution of road traffic isoprene emissions shows very low isoprene/benzene ratios of around 0.02 at the kerbside of a busy road. In summer time isoprene/benzene ratios were often > 1, indicating the strong contribution of biogenic emissions to total isoprene concentration in urban environment. To investigate the impact of exhaled isoprene by human beings to ambient isoprene concentration, measurements in a Christmas market which was visited by a huge number of people, was performed in December 2011. Results show unexpected high isoprene concentrations and isoprene/benzene ratios up to 0.54 ppb and 1.34, respectively. Due to global climate change and a reduction of anthropogenic VOC emissions, isoprene is expected to become more important in urban environment for the formation of ozone.

Literature cited: Bao, H., Shrestha, K.L., Kondo, A., Kaga, A., Inoue, Y., 2010: Modeling the influence of biogenic volatile organic compound emissions on ozone concentration during summer season in the Kinki region of Japan. - Atmospheric Environment 44, 421-431. Duane, M., Poma, B., Rembges, D., Astorga, C., Larsen, B.R., 2002: Isoprene and its degradation products as strong ozone precursors in Insubria, Northern Italy.- Atmospheric Environment 36, 3867-3879. Wagner, P. (2013): Influence of isoprene on ozone formation in an urban environment. - IAUC Newsletter No. 47, p. 33-37 Wagner, P., Kuttler, W., 2013: Analysis of biogenic and anthropogenic isoprene in the near-surface urban atmosphere. (submitted to Atmospheric Environment)