[Chemical composition of n-alkanes in wheat straw and smoke]

Straw of six wheat species was burned under flaming and smoldering conditions. Chemical compositions of n-alkanes in the straw and its smoke were measured with GC/MS. The results showed, the main compounds in all samples were C16 to C33, and the total amounts of n-alkanes in the straw were between 19.6 and 62.3 mg x kg(-1), with a mean value of 36.0 mg x kg(-1). Moreover, the ratios of the contents of n-alkanes with low carbon number (< C23) to the total n-alkanes varied from 1.4% to 31.6%. The C27/C29 values in straws ranged from 20.2% to 47.4% with an average of 33.7%, and the C31/C29 values ranged from 22.9% to 48.1% with a mean value of 32.7%. In the flaming smoke, the total contents were 764.2 to 5 304.1 mg x kg(-1), with a mean value of 2 854.2 mg x kg(-1). And the contribution of the compounds with low carbon number to the total contents was 2.0% to 19.4%, the mean of which was 7.1%. Furthermore, the C27/C29 ratios changed from 10.0% to 31.6% with an average of 20.2%, and the C31/C29 values ranged from 47.1% to 98.9% with a mean of 71.2%. In the smoldering smoke, the total contents were in the range of 5 126 to 15 898 mg x kg(-1), and the mean value was 9 220.8 mg kg(-1). The ratios of the content of n-alkanes with low carbon number to the total n-alkanes varied from 1.9% to 13.0%, and the average value was 5.5%. In addition, the C27/C29 values ranged from 8.6% to 23.0% with a mean of 13.8%, and C31/C29 ratios varied from 42.5% to 90.2% with an average of 62.2%. In both the straw and the smoke, n-alkanes showed unimodal distribution with a peak at C29. The compounds with high carbon number (> C21) in all samples had evident odd to even carbon number predominance. The carbon predominance index (CPI) for the straw ranged from 4.0 to 33.6 with a mean value of 19. 2, and the corresponding average carbon chain length (ACL) varied from 25.2 to 29.0 with a mean of 28.0. The CPI for flaming smoke was between 4.4 and 27.1 with a mean value of 12.4, and the corresponding ACL varied from 27.6 to 29.4 with an average value of 28.8. The CPI values for smoldering smoke ranged from 5.5 to 29.6 with a mean of 18.0, and the ACL was from 28.0 to 29.5 with an average value of 28.9. In short, there were visible differentiations among the chemical compositions of n-alkanes in wheat straw and its smoke, which are helpful for identifying the organic contaminants in ambient aerosol from wheat straw burning.