Temperature and humidity dependence of secondary organic aerosol yield from the ozonolysis of β-pinene

Abstract

The temperature dependence of secondary organic aerosol (SOA) formationfrom ozonolysis of beta-pinene was studied in a flow reactor at 263K???303K and 1007 hPa under dry and humid conditions (0% and 26%???68%relative humidity, respectively). The observed SOA yields reachedmaximum values of 0.18???0.39 at high particle mass concentrations(Mo). Under dry conditions, the measurement data showed an overallincrease in SOA yield with inverse temperature, but significant oscillatorydeviations from the predicted linear increase with inverse temperature(up to 50% at high Mo) was observed. Under humid conditions the SOAyield exhibited a linear decrease with inverse temperature. For theatmospherically relevant concentration level of Mo=10??gm???3 andtemperature range 263 K???293 K, the results from humid experimentsin this study indicate that the SOA yield of beta-pinene ozonolysismay be well represented by an average value of 0.15 with an uncertaintyestimate of ??0.05. When fitting the measurement data with a twoproductmodel, both the partitioning coefficients (Kom,i ) and the stoichiometricyields (i ) of the low-volatile and semivolatile model species werefound to vary with temperature. The results indicate that not onlythe reaction product vapour pressures but also the relative contributionsof different gasphase or multiphase reaction channels are stronglydependent on temperature and the presence of water vapour. In fact,the oscillatory positive temperature dependence observed under dryconditions and the negative temperature dependence observed underhumid conditions indicate that the SOA yield is governed much moreby the temperature and humidity dependence of the involved chemicalreactions than by vapour pressure temperature dependencies. We suggestthat the elucidation and modelling of SOA formation need to takeinto account the effects of temperature and humidity on the pathwaysand kinetics of the involved chemical reactions as well as on thegas-particle partitioning of the reaction products.