Land cover change impacts on atmospheric chemistry: simulating projected large-scale tree mortality in the United States

Abstract

Abstract. Land use and land cover changes impact climate and air quality by altering the exchange of trace gases between the Earth's surface and atmosphere. Large-scale tree mortality that is projected to occur across the United States as a result of insect and disease may therefore have unexplored consequences for tropospheric chemistry. We develop a land use module for the GEOS-Chem global chemical transport model to facilitate simulations involving changes to the land surface, and to improve consistency across land-atmosphere exchange processes. The model is used to test the impact of projected national-scale tree mortality risk through 2027 estimated by the 2012 USDA Forest Service National Insect and Disease Risk Assessment. Changes in biogenic emissions alone decrease monthly mean O 3 by up to 0.4 ppb, but reductions in deposition velocity compensate or exceed the effects of emissions yielding a net increase in O 3 of more than 1 ppb in some areas. The O 3 response to the projected change in emissions is affected by the ratio of baseline NO x : VOC concentrations, suggesting that in addition to the degree of land cover change, tree mortality impacts depend on whether a region is NO x -limited or NO x -saturated. Consequently, air quality (as diagnosed by the number of days that 8 h average O 3 exceeds 70 ppb) improves in polluted environments where changes in emissions are more important than changes to dry deposition, but worsens in clean environments where changes to dry deposition are the more important term. The influence of changes in dry deposition demonstrated here underscores the need to evaluate treatments of this physical process in models. Biogenic secondary organic aerosol loadings are significantly affected across the US, decreasing by 5-10 % across many regions, and by more than 25 % locally. Tree mortality could therefore impact background aerosol loadings by between 0.5 and 2 µg m −3 . Changes to reactive nitrogen oxide abundance and partitioning are also locally important. The regional effects simulated here are similar in magnitude to other scenarios that consider future biofuel cropping or natural succession, further demonstrating that biosphere-atmosphere exchange should be considered when predicting future air quality and climate. We point to important uncertainties and further development that should be addressed for a more robust understanding of land cover change feedbacks.

The impact of historical land use change from 1850 to 2000 on secondary particulate matter and ozone

“…Tree mortality, for example associated with insect infestation or disease, can modulate biosphere-atmosphere exchange, generating transitory perturbations in air quality (Berg et al, 2013;Geddes et al, 2016). However, conversion of land cover, for example via clearing, can lead to longterm changes in surface properties and therefore atmospheric composition.…”

Responses of surface ozone air quality to anthropogenic nitrogen deposition in the Northern Hemisphere

“…The GEOS-Chem model has been applied in a number of studies to simulate atmospheric nitrogen deposition Ellis et al, 2013;Zhao et al, 2015Zhao et al, , 2017, surface ozone air quality (Zhang et al, , 2014, and recently impacts of land use changes on atmospheric composition through biosphere-atmosphere exchange processes (Fu and Tai, 2015;Fu et al, 2016;Geddes et al, 2016;Heald and Geddes, 2016). It includes a detailed simulation of tropospheric NO x -VOC-O 3 -aerosol chemistry (Park et al, 2004;Mao et al, 2010).…”

“…The canopy reduction and emission pulsing scalars from Hudman et al (2012) are applied to estimate the abovecanopy NO x emissions. Furthermore, we have mapped the 16 CLM PFTs to the deposition surface types of Wesely (1989) following Geddes et al (2016) to improve the consistency in dry deposition calculation.…”

“…We follow Geddes et al (2016) and use the 16 plant functional types (PFTs) from CLM in the GEOS-Chem land module. The biogenic VOC emissions in GEOS-Chem are calculated using the Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN v2.1) algorithm based on emission factors of the 16 PFTs and activity factors accounting for emission responses to soil and meteorological conditions, leaf age, and LAI (Guenther et al, 2012).…”

Not just carbon widgets

“…Just as forest disturbance had much larger effects on biodiversity than on forest cover in Brazil 1 , different forest processes may respond differently to external pressures. In North America, invasive herbivores and disease have changed forest community composition, leading to changes in volatile organic compound emissions from forests and knock-on effects on air quality 8 . Fragmentation of forest increases the risk of transmission of Lyme disease to human populations in suburban and periurban environments in the northeastern United States 9 and may be important for infectious disease emergence more generally 10 .…”