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Reductions in anthropogenic emissions have drawn increasing attention to the role of the biosphere in O₃production chemistry in U.S. cities. We report the results of chemical transport model sensitivity simulations exploring the relative impacts of biogenic isoprene and soil nitrogen oxides (NOx) emissions on O₃and its temporal variability. We compare scenarios with high and low anthropogenic NOx emissions representing the reductions that have occurred in recent decades. As expected, summertime O₃concentrations become less sensitive to perturbations in biogenic isoprene emissions as anthropogenic NOx emissions decline. However, we demonstrate for the first time that across policy relevant O₃nonattainment areas of the United States, O₃becomes more sensitive to perturbations in soil NOx emissions than to identical perturbations in isoprene emissions. We show that interannual variability in soil NOx emissions may now have larger impacts on interannual O₃variability than isoprene emissions in many areas where the latter would have dominated in the recent past.

 

Dry deposition could partially explain the observed response in ambient ozone to extreme hot and dry episodes. We examine the response of ozone deposition to heat and dry anomalies using three long‐term co‐located ecosystem‐scale carbon dioxide, water vapor and ozone flux measurement records. We find that, as expected, canopy stomatal conductance generally decreases during days with dry air or soil. However, during hot days, concurrent increases in non‐stomatal conductance are inferred at all three sites, which may be related to several temperature‐sensitive processes not represented in the current generation of big‐leaf models. This may offset the reduction in stomatal conductance, leading to smaller net reduction, or even net increase, in total deposition velocity. We find the response of deposition velocity to soil dryness may be related to its impact on photosynthetic activity, though considerable variability exists. Our findings emphasize the need for better understanding and representation of non‐stomatal ozone deposition.

 

Abstract. Our work explores the impact of two important dimensions of landsystem changes, land use and land cover change (LULCC) as well as directagricultural reactive nitrogen (Nr) emissions from soils, on ozone(O3) and fine particulate matter (PM2.5) in terms of air quality overcontemporary (1992 to 2014) timescales. We account for LULCC andagricultural Nr emissions changes with consistent remote sensingproducts and new global emission inventories respectively estimating theirimpacts on global surface O3 and PM2.5 concentrations as well as Nrdeposition using the GEOS-Chem global chemical transport model. Over thistime period, our model results show that agricultural Nr emissionchanges cause a reduction of annual mean PM2.5 levels over Europe andnorthern Asia (up to −2.1 µg m−3) while increasing PM2.5 levels in India, China and the eastern US (up to +3.5 µg m−3). Land cover changes induce small reductions in PM2.5 (up to −0.7 µg m−3) over Amazonia, China and India due to reduced biogenic volatile organic compound (BVOC) emissions and enhanced deposition of aerosol precursor gases (e.g., NO2, SO2). Agricultural Nr emissionchanges only lead to minor changes (up to ±0.6 ppbv) in annual meansurface O3 levels, mainly over China, India and Myanmar. Meanwhile, ourmodel result suggests a stronger impact of LULCC on surface O3 over the time period across South America; the combination of changes in drydeposition and isoprene emissions results in −0.8 to +1.2 ppbv surfaceozone changes. The enhancement of dry deposition reduces the surface ozone level (up to −1 ppbv) over southern China, the eastern US and central Africa. The enhancement of soil NO emission due to crop expansion also contributes to surface ozone changes (up to +0.6 ppbv) over sub-Saharan Africa. Incertain regions, the combined effects of LULCC and agricultural Nr emission changes on O3 and PM2.5 air quality can be comparable (>20 %) to anthropogenic emission changes over the same time period. Finally, we calculate that the increase in global agricultural Nr emissions leads to a net increase in global land area (+3.67×106km2) that potentially faces exceedance of the critical Nr load (>5 kg N ha−1 yr−1). Our result demonstrates the impacts of contemporary LULCC and agricultural Nr emission changes on PM2.5 and O3 in terms of air quality, as well as the importanceof land system changes for air quality over multidecadal timescales. 

Abstract. Dry deposition is a major sink of tropospheric ozone.Increasing evidence has shown that ozone dry deposition actively linksmeteorology and hydrology with ozone air quality. However, there is littlesystematic investigation on the performance of different ozone drydeposition parameterizations at the global scale and how parameterizationchoice can impact surface ozone simulations. Here, we present the results ofthe first global, multidecadal modelling and evaluation of ozone drydeposition velocity (vd) using multiple ozone dry depositionparameterizations. We model ozone dry deposition velocities over 1982–2011using four ozone dry deposition parameterizations that are representative ofcurrent approaches in global ozone dry deposition modelling. We useconsistent assimilated meteorology, land cover, and satellite-derived leafarea index (LAI) across all four, such that the differences in simulatedvd are entirely due to differences in deposition model structures orassumptions about how land types are treated in each. In addition, we usethe surface ozone sensitivity to vd predicted by a chemical transportmodel to estimate the impact of mean and variability of ozone dry depositionvelocity on surface ozone. Our estimated vd values from four differentparameterizations are evaluated against field observations, and whileperformance varies considerably by land cover types, our results suggestthat none of the parameterizations are universally better than the others.Discrepancy in simulated mean vd among the parameterizations isestimated to cause 2 to 5 ppbv of discrepancy in surface ozone in theNorthern Hemisphere (NH) and up to 8 ppbv in tropical rainforests in July,and up to 8 ppbv in tropical rainforests and seasonally dry tropical forestsin Indochina in December. Parameterization-specific biases based onindividual land cover type and hydroclimate are found to be the two maindrivers of such discrepancies. We find statistically significant trends inthe multiannual time series of simulated July daytime vd in allparameterizations, driven by warming and drying (southern Amazonia, southernAfrican savannah, and Mongolia) or greening (high latitudes). The trend inJuly daytime vd is estimated to be 1 % yr−1 and leadsto up to 3 ppbv of surface ozone changes over 1982–2011. The interannual coefficient ofvariation (CV) of July daytime mean vd in NH is found to be5 %–15 %, with spatial distribution that varies with the dry depositionparameterization. Our sensitivity simulations suggest this can contributebetween 0.5 to 2 ppbv to interannual variability (IAV) in surface ozone, butall models tend to underestimate interannual CV when compared to long-termozone flux observations. We also find that IAV in some dry depositionparameterizations is more sensitive to LAI, while in others it is more sensitiveto climate. Comparisons with other published estimates of the IAV ofbackground ozone confirm that ozone dry deposition can be an important partof natural surface ozone variability. Our results demonstrate the importanceof ozone dry deposition parameterization choice on surface ozone modellingand the impact of IAV of vd on surface ozone, thus making a strong casefor further measurement, evaluation, and model–data integration of ozone drydeposition on different spatiotemporal scales.