Continuous Measurements of Ice Nucleating Particles in Beijing

The generation of ice in mixed phase clouds has important impacts on atmospheric radiation and precipitation. The initiation of ice formation at cloud temperatures warmer than -38 C requires the presence of ice nucleating particles (INPs), which are subset of atmospheric PM2.5. Previously identified sources of INPs include mineral dust generated in desert regions, and biogenic particles originated from vegetated surfaces, croplands, and the ocean. The extent to which human activities affect atmospheric INP concentrations remains unclear, and if substantial would represent a potentially large atmospheric impact that needs to be understood and quantified. To address this, we installed and operated a continuous flow diffusion chamber (CFDC) capable of measuring deposition and condensation/immersion mode active INP at temperatures between -15 C and -30 C at the Beijing Cloud Laboratory and Observational Utilities Deployment Base. CFDCs have been used to measure INP on aircraft and in ground locations for short periods for several decades, but for the first time we present an extensive (weeks-to-months) series of measurements collected with an instrument modified for autonomous, continuous operation. We briefly describe the hardware modifications and instrument design changes needed to achieve long-term operation, then focus on describing INP concentrations as a function of temperature at the site, and relationships with meteorology and PM2.5 concentrations and source regions. We show a clear impact from transported dust, investigate other potential local sources of observed INP, and finally explore potential human impacts on INP using co-located measurements of a combustion tracer (black carbon), and aerosol size distribution measurements. We also compare the continuous INP measurements made by the CFDC with drop freezing measurements performed on extracts from filter samples, which also provide measurements at warmer temperatures.