Mechanistic representation of soil nitrogen emissions in the Community Multiscale Air Quality (CMAQ) model v 5.1

Quazi Z Rasool,Jesse O Bash,Daniel S Cohan

doi:10.5194/gmd-12-849-2019

Abstract

Abstract. Soils are important sources of emissions of nitrogen-containing (N-containing) gases such as nitric oxide (NO), nitrous acid (HONO), nitrous oxide (N2O), and ammonia (NH3). However, most contemporary air quality models lack a mechanistic representation of the biogeochemical processes that form these gases. They typically use heavily parameterized equations to simulate emissions of NO independently from NH3 and do not quantify emissions of HONO or N2O. This study introduces a mechanistic, process-oriented representation of soil emissions of N species (NO, HONO, N2O, and NH3) that we have recently implemented in the Community Multiscale Air Quality (CMAQ) model. The mechanistic scheme accounts for biogeochemical processes for soil N transformations such as mineralization, volatilization, nitrification, and denitrification. The rates of these processes are influenced by soil parameters, meteorology, land use, and mineral N availability. We account for spatial heterogeneity in soil conditions and biome types by using a global dataset for soil carbon (C) and N across terrestrial ecosystems to estimate daily mineral N availability in nonagricultural soils, which was not accounted for in earlier parameterizations for soil NO. Our mechanistic scheme also uses daily year-specific fertilizer use estimates from the Environmental Policy Integrated Climate (EPIC v0509) agricultural model. A soil map with sub-grid biome definitions was used to represent conditions over the continental United States. CMAQ modeling for May and July 2011 shows improvement in model performance in simulated NO2 columns compared to Ozone Monitoring Instrument (OMI) satellite retrievals for regions where soils are the dominant source of NO emissions. We also assess how the new scheme affects model performance for NOx (NO+NO2), fine nitrate (NO3) particulate matter, and ozone observed by various ground-based monitoring networks. Soil NO emissions in the new mechanistic scheme tend to fall between the magnitudes of the previous parametric schemes and display much more spatial heterogeneity. The new mechanistic scheme also accounts for soil HONO, which had been ignored by parametric schemes.

Highlights

Global food production and fertilizer use are projected to double in this half-century in order to meet the demand from growing populations (Frink et al, 1999; Tilman et al, 2001)
The mechanistic scheme updates the representation of the dependency of soil N on water filled pore space (WFPS) by utilizing parameters like water content at saturation, wilting point, and field capacity and their impact on gas diffusivity (Del Grosso et al, 2000; Parton et al, 2001)
The magnitudes of soil NOx emissions predicted by the mechanistic scheme are similar to those predicted by the YL parametric scheme and smaller than those predicted by the Berkeley–Dalhousie Soil NOx Parameterization (BDSNP) scheme

Summary

Introduction

Global food production and fertilizer use are projected to double in this half-century in order to meet the demand from growing populations (Frink et al, 1999; Tilman et al, 2001). Increasing nitrogen (N) fertilization to meet food demand has been accompanied by increasing soil N emissions across the globe, including in the United States (Davidson et al, 2012). US N fertilizer use increased from 0.28 to 9.54 g N m−2 yr−1 during 1940 to 2015. Hotspots of N fertilizer use have shifted from the southeastern and eastern US to the Midwest and the Great Plains comprising the Corn Belt region (Cao et al, 2017). Recent studies have pointed to soils as a significant source of NOx emissions, contributing ∼ 20 % to the total budget globally and larger fractions over heavily fertilized agricul-

Methods

Results

Conclusion