Observations of Aerosol-Vapor Pressure Deficit-Evaporative Fraction coupling over India

Abstract. North India is a densely populated subtropical region with heavy aerosol loading, frequent heatwaves and strong atmosphere-biosphere coupling, making it ideal for studying the impacts of aerosols and temperature variation on latent heat flux (LH) and evaporative fraction (EF). Here, using in situ observations during the onset of the summer monsoon over a semi-natural grassland site in this region, we confirm that strong co-variability exists among aerosols, LH, air temperature (Tair) and vapor pressure deficit (VPD). Since the surface evapotranspiration is strongly controlled by both physical (available energy and moisture demand) and physiological (canopy and aerodynamic resistance) factors, we separately analyze our data for different combinations of aerosols and Tair/VPD changes. We find that aerosol loading and heatwave conditions both reduces SH. Further, we find that an increase in atmospheric VPD, tends to decrease the gross primary production (GPP) and thus LH, most likely as a response to stomatal closure of the dominant grasses at this location. In contrast, under heavy aerosol loading, LH is enhanced partly due to the physiological control exerted by the diffuse radiation fertilization effect (thus increasing EF). Moreover, LH and EF are positively associated with aerosol loading even under heatwave conditions, indicating a decoupling of plant’s response to VPD enhancement (stomatal closure) in presence of high aerosol conditions. With heat-stress, VPD and aerosols expected to increase in future India, our results warrant in-depth analysis of aerosol-plant-temperature-EF continuum and its impact on Indian monsoon dynamics and crop vulnerability.