Humidity effects on the mass spectra of single aerosol particles

On-line laser desorption ionization mass spectrometry has developed into a widely used method for chemical characterization of individual aerosol particles. In the present study, the spectra of laboratorygenerated particles were obtained as a function of relative humidity to elucidate potential artifacts associated with ambient measurements. Several anionic electrolytes typically found in marine aerosols were studied, including chloride, nitrate, sulfate and methanesulfonate. In most experiments, negative ion mass spectra were assessed. Particles were conditioned at humidities both above and below the respective deliquescence-relative humidities (DRH). The mass spectra of the conditioned particles could be affected by relative humidity in several ways: an abrupt transition between a “wet”- and “dry”-particle spectrum over a narrow humidity range, a gradual transition over a broad humidity range, or virtually no change with humidity. Particulate phase water influenced both the distribution of ions observed and their relative intensities. In most instances, water remained associated with the particle well below the DRH, yielding spectra similar to those of aqueous droplets. Particulate phase water below the DRH appeared to be the result of surface adsorption under ambient conditions rather than condensational growth in the mass spectrometer inlet. Signal quenching in multicomponent aerosols was more prevalent in the spectra of aqueous droplets than dry particles. For mixed composition droplets, only nitrate and chloride could be unambiguously identified from intense peaks in the spectra, while signals due to ions from methanesulfonate and sulfate were suppressed. In contrast, all components could be detected at low relative humidities.