Experimental study of the sampling artifact of chloride depletion from collected sea salt aerosols

Sampling artifact of chloride depletion from collected sea salt particles w as studied, based on simultaneous measurements of size distribution measure ments by a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI) and of P M2.5 measurements by a Compact Porous Metal Denuder Sampler (PMDS) at a coa stal site in Hong Kong on May 7, 8, 9, 11, and 29, 1998. The ambient concen trations of SO2 HNO3, HNO2, and NH3 were also measured by the PMDS. PM2.5 m easurements by the PMDS, which is equipped with denuders and nylon back fil ters, are compared with the PM1.8 and PM3.1 measurements by the MOUDI. The percentages of chloride depletion from sea salt aerosols in PM1.8 and PM3.1 were 4-45% higher than that in PM2.5. This suggests that chloride evaporat ion in PM1.8 and PM3.1 collected on Teflon filters of the MOUDI during samp ling was present. From the sum of the contributions of particles on the Tef lon and nylon filters of the PMDS, nitrate formation almost completely acco unts for chloride depletion in PM2.5 prior to collection since the equivale nt ratio of [Na+] to ([NO3-] +[Cl-]) is close to the seawater ratio of 0.85 . However, it was found that 22-74% of nitrate and 45-86% of chloride in th e collected particles on the Teflon filter of the PMDS evaporated during sa mpling. Excess chloride depletion unexplained by NO3- and nss-SO42- was fou nd in the collected particles on the Teflon filter of the PMDS. Similarly, an amount of 3.7-27.2 nequiv m(-3) of excess depleted chloride (equivalent to 8-55% of total chloride depletion) was found in supermicron particles co llected by the MOUDI. In the 1.8-3.1 mum particles, the excess depleted chl oride is positively correlated to the chloride evaporated from the deposite d particles. Particle-particle interactions are proposed to explain the eva poration of nitrates and chlorides in the PMDS and MOUDI measurements. The observed chloride depletion from seasalt aerosols was partially attributed to sampling artifact due to particle-particle interactions.