Abstract
Coastal stations are critical for interpretation of continental‐scale CO2 exchanges although the impacts of land and sea breezes, local topography, katabatic winds, and CO2 transport from nearby terrestrial ecosystems are not well characterized. We applied a modeling framework that couples meteorological (MM5), land‐surface (LSM1), and tracer models to investigate the impact of these factors on coastal CO2 measurements. Model predictions compared well with measurements over 4 months at our case study site (Trinidad Head, California). We predicted that during midday and under strong onshore wind conditions, positive and negative CO2 anomalies from the assumed “background” marine layer air were sampled at the station. These anomalies resulted from two classes of mechanisms that couple transport and recent terrestrial ecosystem exchanges. First, and most important, are local and large‐scale recirculation of nighttime positive CO2 anomalies resulting from katabatic flows off the coastal mountain range. Second, negative anomalies generated by daytime net ecosystem uptake can be transported offshore in the residual layer and then entrained in the marine boundary layer. We predicted monthly averaged CO2 anomalies associated with terrestrial exchanges of 0.53, 0.34, 3.1, and 0.05 ppm during March, June, September, and December of 2002. Positive anomalies from nighttime ecosystem respiration were more likely to be sampled than are negative anomalies associated with daytime net ecosystem uptake. Current atmospheric models used in continental‐scale inverse studies do not resolve these two classes of mechanisms and therefore may infer incorrect CO2 exchange rates.
Highlights
As described below, samples are taken in the middle of the day when onshore winds are strong, and these conditions are assumed to result in air that has not recently been impacted by terrestrial ecosystems
The GLOBALVIEW-CO2 concentrations are broad zonal averages of the marine boundary layer, substantial deviations of these values from those measured at Trinidad Head may indicate some influence of local ecosystem CO2 exchange
Predicted cumulative net ecosystem CO2 exchange (NEE) are broadly consistent with measurements in Pacific Northwest regrowing forests [e.g., Law et al, 2004], the source strength in September may be an overestimate
Summary
[2] Evidence for a large Northern Hemisphere carbon sink depends largely on observations of the inter-hemispheric difference of atmospheric CO2 measured with flasks at surface stations distributed in many remote locations, including coastal sites on continents and on ocean islands [Tans et al, 1990; Gurney et al, 2002]. [6] Lalas et al [1983] studied, on two typical days in Athens, well-defined sea-breeze circulations and pollutant transport They concluded from ground-level trajectories and nighttime O3 measurements that pollutants can be recirculated by land and sea breezes, and that this feature must be included in atmospheric chemistry modeling. They observed multiple inversion layers, with different pollutant concentrations, in the air basin. We investigate impacts on coastal surface CO2 concentrations of land and sea breezes, katabatic flows, alongshore transport of CO2 from adjacent ecosystems north and south of Trinidad Head, and re-introduction of lofted negative CO2 anomalies into the marine boundary layer. As described below, samples are taken in the middle of the day when onshore winds are strong, and these conditions are assumed to result in air that has not recently been impacted by terrestrial ecosystems
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