Using stable water isotopes to distinguish aerosol chemistry from transport

Very often, changes in atmospheric constituents are strongly influenced by atmospheric transport and mixing processes, and it is difficult to separate out changes due to chemistry. Changes in particle number, size, and composition due to in situ nucleation and growth or evaporation may be difficult to differentiate from changes due strictly to advection. This can be especially problematic for Eulerian observation frameworks. The stable isotopic composition of water vapor is a powerful tracer of atmospheric mixing. During periods when distinct air masses exchange without undergoing condensation or evaporation, the inverse of total water vapor is linearly proportional to the δD value—the fraction of heavy-to-light vapor with respect to a reference standard. We present a new technique that exploits this relationship to account for the effect of mixing on observed changes in aerosol size distributions. We use simultaneous measurements of aerosols and vapor isotopologues, measured at the Mauna Loa Observatory with an Ultra-High Sensitivity Aerosol Spectrometer and a Picarro water vapor isotope analyzer. Using a two-member mixing model, the mixing exchange coefficient is constrained by the water isotopologue information, allowing aerosol changes due to advection to be singled out. The production rate is thus explicitly solved for, even during periods when atmospheric transport clearly dominates. Independent Observatory measurements of sulfur dioxide are used to estimate H2SO4 production, verifying the method and providing insight into the physical basis for the calculated aerosol source rate. Notably, during stable periods when mixing is negligible, we find little change in the aerosol size spectra. On the other hand, strong growth is found to follow periods when boundary layer and free tropospheric air interact, highlighting the importance of air mass mixing in promoting aerosol production.