Degradation kinetics of levoglucosan initiated by hydroxyl radical under different environmental conditions

To understand the atmospheric stability of levoglucosan, which is a major molecular tracer used for source apportionment of biomass burning aerosols, degradation kinetics of levoglucosan by hydroxyl radical (OH) have been investigated using a flow reactor under different conditions. The second-order rate constant (k2) for the degradation of pure levoglucosan by OH is (9.17 ± 1.16) × 10−12 cm3 molecules−1 s−1 at 25 °C and 40% relative humidity (RH), while it depends on environmental conditions such as temperature, RH, and mixing state. At 25 °C, k2 of pure levoglucosan linearly decreases with increasing RH (k2 = (1.50 ± 0.04) × 10−11 − (1.31 ± 0.11) × 10−11RH), while it increases with increasing temperature and follows the Arrhenius equation k2 = (6.2 ± 5.6) × 10−9exp[(-1922.5 ± 268.2)/T] when the RH is 40%. At 25 °C and 40% RH, compared to pure levoglucosan, levoglucosan coated on (NH4)2SO4 or NaCl (levoglucosan@(NH4)2SO4 and levoglucosan@NaCl) shows larger k2 to OH with (9.53 ± 0.39) × 10−12 and (10.3 ± 0.45) × 10−12 cm3 molecules−1 s−1, respectively, whereas levoglucosan coated on soot (levoglucosan@soot) shows the smaller k2 of (4.04 ± 0.29) × 10−12 cm3 molecules−1 s−1. Either (NH4)2SO4 or NaCl internally mixed with levoglucosan ((NH4)2SO4@levoglucosan and NaCl@levoglucosan) prominently inhibits the degradation of levoglucosan. Based on the rate constants, atmospheric lifetimes of levoglucosan were estimated to be 1.2-3.9 days under different conditions. All the results indicate that the degradation of levoglucosan by OH is prominent during air mass aging, and it should have an important influence on the uncertainty of source apportionment if the contribution of degradation to levoglucosan concentration is not considered in source apportionment models.