How does the biosphere influence the fate of an urban plume? How do anthropogenic emissions and their photochemical products (e.g. ozone, PAN, nitric acid) interact with natural emissions from the biosphere? Predicting the consequences of future changes in climate and anthropogenic activities requires a detailed understanding of atmospheric chemistry at the urban-rural interface. To address the complex questions surrounding the coupling of anthropogenic and biogenic activities, we we are currently engaged in the Biosphere Effects on Aerosol and Photochemistry Experiment (BEARPEX), a collaborative field campaign taking place at the UC-Berkeley Blodgett Forest Research Station (BFRS) in August and September of 2007 and 2009. BFRS is an ideal location for investigating the interaction of urban and biogenic emissions, as the site regularly intercepts plumes from the Sacramento Valley.

THE CAMPAIGN

     In the summer of 2007, we deployed our chemical ionization mass spectrometer (CIMS) for 10 weeks to measure canopy-level fluxes and vertical gradients of a suite of acyl peroxy nitrates (APNs) above and within the forest canopy. With more than 20 instruments from 12 research groups, the campaign provided the comprehensive dataset necessary to test our understanding of the multifaceted mechanisms controlling atmospheric composition at this site. Our specific goals for this campaign include:


 - Inferring the relative contributions of surface deposition and
    within-canopy chemistry to the net vertical APN flux above the
    forest via direct eddy covariance measurements of APN fluxes
 - Exploring the unique physical and chemical processes occurring
    within the forest canopy via observation of APN gradients within
    and above the canopy
 - Characterizing the role of anthropogenic and biogenic VOC in the
    evolution and fate of the Sacramento urban plume with our
    speciated APN measurements

     BEARPEX 2007 was an exciting campaign for the Thornton group, as it represented both the first deployment of our new field-dedicated CIMS and our first use of the CIMS in an eddy covariance (EC) mode. EC is a powerful technique that utilizes fast simultaneous measurements of vertical wind speed and scalar properties (e.g. chemical concentrations). The vertical flux of the scalar is simply the covariance of these two measurements over the observations period (typically 30 minutes); thus, we obtain a direct observation of air-surface exchange processes. CIMS is an ideal technique for this application, as it is capable of making fast (>1 Hz), high-precision measurements under relative harsh field conditions.


RECENT RESULTS



The above figure illustrates the full time series of PAN measurements made at the flux measurement height (17.7m), as well as the calculated EC flux of PAN and the exchange velocity ( = flux/concentration). The daily arrival of the plume is prominent in the PAN concentrations, although larger timescale contributions to the variability are also evident. Diurnal cycles are present in the flux and exchange velocity series as well, although the processes controlling these variations (e.g. turbulence intensity and stomatal activity) are quite different from those influencing the concentrations (primarily plume advection and air temperature). Further analysis of this data is shown in the following figures.







BEARPEX 2009

While our focus in 2007 was primarily on APNs, the versatility of the CIMS technique presents the potential to detect many other species, including HNO₃, NO₃, N₂O₅ and organic acids. We intend to utilize these capabilities during the next field mission while continuing to improve our understanding of APN chemistry within and above the forest. Possible field experiments include:

 - Flux divergence measurements of APNs to investigate the potential chemical APN source in the canopy
 - Measurements of NO₃/N₂O₅ reactivity on atmospheric aerosol
 - Observations of gas and aerosol-phase organic acids via the MOVI-CIMS technique developed in our lab.

This material is based upon work supported by the National Science Foundation under Grant No. 0633897. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.