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PM2.5 aerosol samples were collected over 12 hr and 24 hr intervals in an inland background area,Gongga Mountain National Nature Reserve (hereafter shortened to Gongga),during the summer of 2011.Polar organic tracers,inorganic ions and meteorological data were measured.The purpose of this work was to investigate the variation patts,formation and sources of the secondary organic aerosol tracers in the studied atmosphere.The average concentrations of isoprene oxidation products,α-pinene oxidation products,β-caryophyllinic acid,sugars,sugar alcohols and anhydrosugars were 88.6 ± 106.1,3.6 ± 5.7,0.13 ± 0.30,13.6 ± 13.1,31.9 ± 31.4 and 14.8 ± 10.7 ng/m3 respectively in all aerosol samples.The aged a-pinene second organic aerosol (SOA) tracers (i.e.,3-hydroxyglutraric acid (3HGA),3-hydroxy-2,2-dimethylglutaric acid (HDMGA),3-acetylpentandioic acid (APDA) and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA)) correlated significantly with each other in the 24 hr PM2.5 aerosol samples,indicating that OH.is the major factor controlling the formation of these α-pinene SOA tracers.Using the positive matrix factorization (PMF) model and the tracer-based source apporttionment method,we calculated that isoprene oxidation products,δ-pinene oxidation products,sesquiterpene oxidation products,biomass bing fungi spores and anthropogenic SOA accounted for 21.9％ ± 5.5％,8.4％ ± 2.1％,3.0％ ± 0.7％,5.2％ ± 5.3％,5.0％ ± 6.2％ and 31.4％ ± 7.8％ of organic carbon respectively during the sampling period.