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Modeling the Diurnal Variability of Agricultural Ammonia in Bakersfield, California during CalNex
Journal article   Open access   Peer reviewed

Modeling the Diurnal Variability of Agricultural Ammonia in Bakersfield, California during CalNex

C. R Lonsdale, J. D Hegarty, K Cady-Pereira, M. J Alvarado, D. K Henze, M. D Turner, S. L Capps, J. B Nowak, J. A Neuman, A. M Middlebrook, …
Atmospheric chemistry and physics discussions, pp 1-30
09 Mar 2016
DOI: https://doi.org/10.5194/acp-2016-44
url
https://doi.org/10.5194/acp-2016-44View
Published, Version of Record (VoR)CC BY V4.0,  Open

Abstract

NH 3 retrievals from the NASA Tropospheric Emission Spectrometer (TES), as well as surface and aircraft observations of NH 3(g) and submicron NH 4(p) , are to used to evaluate modelled concentrations of NH 3(g) and NH 4(p) from the Community Multiscale Air Quality (CMAQ) model in the San Joaquin Valley (SJV) during the California Research at the Nexus of Air Quality and Climate Change (CalNex) campaign. We find that simulations of NH 3 driven with the California Air Resources Board (CARB) CalNex emission inventory are qualitatively and spatially consistent with TES satellite observations, with a correlation coefficient (r 2 ) of 0.54. However, the surface observations at Bakersfield indicate a missing diurnal cycle in the model bias, with CMAQ overestimating surface NH 3 at night and underestimating it during the day. The surface, satellite, and aircraft observations all suggest that the afternoon NH 3 emissions in the CARB inventory are underestimated by at least a factor of two, while the night-time overestimate of NH 3(g) is likely due to a combination of overestimated NH 3 emissions, underestimated deposition, and insufficient vertical mixing in the WRF meteorological fields used to drive CMAQ. We used the surface observations at Bakersfield to derive an empirical diurnal cycle of NH 3 emissions in the SJV, in which night-time and midday emissions differed by about a factor of 4.5. Adding this diurnal profile to the CMAQ simulations while keeping the daily NH 3 emissions constant at the CARB values significantly improved the model performance at night, but sizable errors (up to 15 ppbv) in night-time NH 3 remain, likely due to remaining errors in vertical mixing at night. The model performance is slightly degraded during the afternoon when the diurnal cycle is adjusted, but this may reflect relatively small (~ 20 %) errors in the total NH 3 emissions rather than remaining errors in the diurnal cycle. Running CMAQv5.0.2 with bi-directional NH 3 flux also improves model performance on a similar scale, while combining bi-directional NH 3 fluxes and adjusted emissions significantly reduces the model bias at night.

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