Evaluation of biogenic isoprene emissions and their contribution to ozone formation by ground-based measurements in Beijing, China

Highlights: • The annual, monthly and diurnal emissions of isoprene are estimated using a box model in Beijing. • The biggest difference of estimated isoprene emissions between box model and emission inventories was found in May. • The isoprene emissions contributed half of the total ozone formation potential (OFP) at 13:00 in August. This study employs a mass balance technique-box model to calculate the biogenic isoprene emissions based on the ground-level measurements between October 2009 and September 2010 in Beijing. The annual magnitude, monthly variations and diurnal patterns of isoprene emissions are estimated. The annual emissions of isoprene were estimated to be 23.2 Gg with an uncertainty of 120%. This falls within the range of previous emission inventories (EI; 3.8 Gg to 36.3 Gg between 1990 and 2010). Strong isoprene emissions were observed between May and September. The biggest difference was the isoprene emissions in May, with contributions of 23.3% to total annual emissions using box model estimates compared with 3.7% in EI. The diurnal profiles of isoprene emissions estimated in this study were generally similar to those in the EI, with the highest emissions occurring during mid-day (11:00–13:00). However, obvious differences were found for the growth rates and decreasing rates of isoprene emissions in the morning and afternoon respectively. Compared to anthropogenic volatile organic compounds (VOCs), the isoprene emissions contributed half (49.5%) of the total ozone formation potential (OFP) at 13:00 in August, which highlights the importance of isoprene in ozone formation. This study helps bound the isoprene emissions estimated by EI despite the inherent large uncertainty.