Secondary organic aerosols (SOA) formed from common anthropogenic and biogenic volatile organic compounds (VOCs) account for a significant portion of organic particulate matter in the ambient atmosphere. The Pearl River Delta (PRD) in southern China, located in the subtropics and as a region with intensive manufacturing industries, has significant emissions of both anthropogenic and biogenic VOCs. Two recent SOA tracer-based measurement studies, one in Hong Kong (located at the mouth of the PRD) and the other at a site 20 to 50 km downwind of urban Guangzhou districts in the middle of the PRD, show a rather considerable difference in the relative SOA contributions from one group of two biogenic VOCs (isoprene and monoterpenes) and one group of anthropogenic VOCs, namely, toluene + xylenes. In Hong Kong, more SOA was formed from isoprene and monoterpenes than from toluene and xylenes, although the relative contributions of the two groups of VOCs were reversed at the site downwind of Guangzhou. An emission-based 0-D box model has been developed to investigate this issue. The emission inputs of major inorganic pollutants and VOCs are generated using the programs Sparse Matrix Operator Kernel Emissions and Model of Emissions of Gases and Aerosols from Nature for this region. Toluene/xylene emissions in Guangzhou are more than twice that in Hong Kong whereas isoprene and monoterpenes emissions were similar at the two locations. The model incorporates a CB05 chemical mechanism and gas–particle partitioning of condensable VOC oxidation products to simulate SOA formation from major VOCs including isoprene, monoterpenes, toluene, and xylenes. The model-simulated VOCs fall within the range of ambient observations, demonstrating reasonable representation of emissions and oxidation of VOCs. The model simulates the sum of the SOA formation from isoprene, monoterpenes, and toluene + xylenes. In Hong Kong, monoterpenes are the major contributor (up to 70%), followed by isoprene (14%–48%) and toluene + xylenes (15%–43%). In Guangzhou, toluene + xylenes contribute more to SOA than isoprene and monoterpenes (up to 76% from toluene + xylenes vs. 13%–44% from isoprene and 10%–45% from monoterpenes). The reasonable agreement between the simulated SOA for the target VOCs and the tracer-based measurements suggests that the significantly larger toluene + xylene emissions in Guangzhou could explain the substantial difference in relative SOA contributions by the two groups of VOCs in the two cities. This work has also identified a lack of good measurements of monoterpenes and their SOA tracers to be an important data deficiency in assessing the relative contributions of biogenic and anthropogenic VOCs to SOA in this region.