Expand drought impact section on isoprene emissions

Added detailed information on the impact of drought on isoprene emissions, including mathematical formulations and references to recent studies.

Author: lkemmons

doc/source/tech_note/BVOCs/CLM50_Tech_Note_BVOCs.rst

@@ -24,3 +24,41 @@ where :math:`\gamma _{i}` is the emission activity factor accounting for respons
 where the LDF activity factor (:math:`\gamma _{P\_ LDF}` ) is specified as a function of PAR as in previous versions of MEGAN.
 
 The values for each emission factor :math:`\epsilon _{i,\, j}` are now available for each of the plant functional types in the CLM and each chemical compound. This information is distributed through an external file, allowing for more frequent and easier updates.
+
+The impact of drought on isoprene emissions is based on the theory proposed by Potosnak et al. (2014). Specifically, isoprene emissions are expected to increase under mild to moderate drought because drought raises leaf temperature, which stimulates isoprene emissions. Under severe drought, however, isoprene emissions are inhibited because substrate supply becomes constrained. Because the effect of leaf temperature is already represented by the leaf temperature module and its influence on isoprene emissions, only the inhibitory effect of severe drought is parameterized as (Wang et al., 2022):
+
+.. math::
+   :label: 28.2b)
+
+   \gamma _{sub} = 1 / \left(1 + b_{1} exp(a1 (beta-0.2)) \right)
+
+where a1 = -7.45 and b1=3.26 are empircal parameters.
+
+Compared with Guenther et al. (2012), updates have been made to represent isoprene emissions from high-latitude plants, specifically boreal broadleaf deciduous shrubs (BBDS) and C3 Arctic grass (C3AG), in order to account for acclimation processes. These updates are based on leaf-enclosure and in situ measurements conducted at Toolik Field Station in Alaska, USA (Wang et al., 2024a, 2024b).
+For BBDS, the isoprene emission factor is adjusted according to the mean temperature of the previous day as:
+
+.. math::
+   :label: 28.2c)
+
+Eopt_bbds = 7.9 exp(0.22 (T_{24}-297.15) )
+
+where T_{24} denotes the mean air temperature of the preceding day (Wang et al., 2024a).
+For C3AG, the isoprene emission factor responds over a longer timescale of 10 days and is parameterized as a function of the mean air temperature over the preceding 10 days (T240) as (Wang et al., 2024b):
+
+Eopt_C3AG = exp(0.22 (T_{240}-288.15) )
+
+In addition, a dynamic temperature response curve for C3AG depends on recent temperature history as:
+
+gamma_{T\_C3AG} = Eopt_C3AG exp((C_{C3AG}/R ( 1/303.15 - 1/Tleaf)))
+
+where Tleaf denotes the leaf temperature and C_{C3AG} is the parameter controlling the isoprene temperature response of C3AG and changes varies with T_{240} as:
+
+C_{C3AG} = 95 + 9.5 exp(0.53 (288.15-T_{240}))
+
+
+References
+
+Potosnak, M. J., LeStourgeon, L., Pallardy, S. G., Hosman, K. P., Gu, L., Karl, T., et al. (2014). Observed and modeled ecosystem isoprene fluxes from an oak-dominated temperate forest and the influence of drought stress. Atmospheric Environment, 84, 314–322. http://www.sciencedirect.com/science/article/pii/S1352231013009059
+Wang, H., Lu, X., Seco, R., Stavrakou, T., Karl, T., Jiang, X., et al. (2022). Modeling Isoprene Emission Response to Drought and Heatwaves Within MEGAN Using Evapotranspiration Data and by Coupling With the Community Land Model. Journal of Advances in Modeling Earth Systems, 14(12), e2022MS003174. https://doi.org/10.1029/2022MS003174. https://doi.org/10.1029/2022MS003174
+Wang, H., Welch, A., Nagalingam, S., Leong, C., Kittitananuvong, P., Barsanti, K. C., et al. (2024a). Arctic Heatwaves Could Significantly Influence the Isoprene Emissions From Shrubs. Geophysical Research Letters, 51(2), e2023GL107599. https://doi.org/10.1029/2023GL107599
+Wang, H., Welch, A. M., Nagalingam, S., Leong, C., Czimczik, C. I., Tang, J., et al. (2024b). High temperature sensitivity of Arctic isoprene emissions explained by sedges. Nature Communications, 15(1), 6144. https://doi.org/10.1038/s41467-024-49960-0