| Aerosol samples were collected over 12h (daytime PM2.5 and nighttime PM2.5) and 24h (TSP and PM2.5) at four forest areas in Sichuan, i.e., Ya’an Baima Spring Scenic Area (BM), Panzhihua Cycas National Nature Reserve (PZ), Gongga Mountain National Nature Reserve (GG) and Wolong National Nature Reserve (WL), during the summers of 2010-2012. The mass concentrations of aerosols and of main chemical components in aerosols were measured. The average concentrations of 24 h PM2.5 were 72.42,104.9, 20.55 and 29.19μg/m3 at BM, PZ, GG and WL, respectively.Twelve water soluble ions in the forest aerosols were measured by ion chromatography. SO42- and NH4+ are the most abundant anion and cation respectively. They are shown a good linear correlation between each other. The sum concentrations of sulfate, nitrate and ammonium (SNA) were averaged in 16.34,18.22,4.43 and 8.89μg/m3, whereas the average concentrations of OC were 15.86,20.81,3.11 and 9.33μg/m3 in 24h PM2.5 aerosols from BM, PZ, GG and WL, respectively. Concentrations of SNA were comparable to those of OC. A better correlation between OC and EC was found at PZ, but not at the other three forest areas, suggesting that OC and EC have a common source at PZ, however, not at the other three forest areas. The concentrations of PM2.5 and TSP were all increased with temperature and decreased with relative humidity. As the top three components in PM2.5, OC, SO42- and NH4+ varied in the same trend as PM2.5.Based on the standard method established by USEPA (Compendium Method IO-4.1), we measured the concentrations of strong acidity H+([H+]str) in fine aerosols. The average concentrations of [H+]str were 6.26,2.9E-04,11.11 and 10.04 nmol/m3 in 24h PM2.5 aerosols from BM, PZ, GG and WL, respectively. Using the AIM-IV model with a system of H+-NH4+-Na+-SO42--NO3--Cl--H2O-T-NH3-H2C2O4, we also simulated in-situ acidity H+ ([H+]Ins) in PM2.5 aerosols. The average concentrations of [H+]Ins were 0.028,1.4E-05, 7.7E-04 and 0.0027 nmol/m3 for 24h PM2.5 samples from BM, PZ, GG and WL, respectively. [H+]str were significantly higher than [H+]Ins.Moreover, the in-situ pH values (pHIS) were 3.77,3.69,4.01 and 3.91 for 24h PM2.5 samples from BM, PZ, GG and WL respectively. Compared to pHIS reported for other areas in the world, pHIS values simulated in this study is relatively high. It is caused by high ambient concentrations of ammonia in the research areas. Furthermore, we found that oxalic acid can change the pHIS values of aerosol samples slightly. Using SPSS 21.0, a prediction equation was established for pHIS for the 24-h aerosol samples:pHIS=-0.038T+0.011RH+0.094NS+0.096RC/A+0.038NH3+ 0.051H2C2O4+13.715, R2=0.775 (n=42,p<0.001).The coefficients of divergences were all above 0.3 for any two of these forest areas, indicating that no significant common source can be found in the forest areas. A principal component analysis (PCA) was performed for the chemical components in forest aerosols. The result showed that main sources of BM aerosols included primary sources (soil dust, biomass burning, etc.) and long-distance transport aged aerosols; the main sources of PZ aerosols were aged aerosols and biomass burning, coming from the local area predominantly; the main source of GG included primary sources and aged aerosols from the local area and after long-distance transport. For WL, primary aerosols such as soil dust and biomass burning are important contributor to aerosols; the low photosynthetical active radiation (PAR) values in WL made the aerosols to be less aged. Additionally, the mass ratio of NO3- and SO42- were all less than 1 in this work, showing that stationary emissions are responsible for the dominant source at the four forest areas.Based on the PCA source apportionment method and the known OM/OC ratios for different sources, we estimated OM/OC ratios for forest aerosols. The ratios OM/OC ratios were 2.3,2.3,2.4 and 1.3 for BM, PZ, GG and WL, respectively. Mass closures of 24-h PM2.5 for the four forest areas were also obtained. OM and EC accounted for 38.0-49.3 and 2.0-5.7% of PM mass, while SNA (SO42-,NH4+ and NO3-) accounted for 23.0,17.4,22.1 and 30.5% of PM mass at BM, PZ, GG, and WL, respectively. Soil dust is an important source of PM2.5 aerosols in this study, which accounted for 6.3,17.0,10.4 and 19.1% of PM mass at BM, PZ, GG, and WL, respectively. In total, the reconstructed mass accounted for 75.9-102.0% of the measured PM mass in this study. |
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