Sensitivity of nitrate aerosols to ammonia emissions and to nitrate chemistry: implications for present and future nitrate optical depth

Abstract

Abstract. We update and evaluate the treatment of nitrate aerosols in the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric model (AM3). Accounting for the radiative effects of nitrate aerosols generally improves the simulated aerosol optical depth, although nitrate concentrations at the surface are biased high. This bias can be reduced by increasing the deposition of nitrate to account for the near-surface volatilization of ammonium nitrate or by neglecting the heterogeneous production of nitric acid to account for the inhibition of N2O5 reactive uptake at high nitrate concentrations. Globally, uncertainties in these processes can impact the simulated nitrate optical depth by up to 25 %, much more than the impact of uncertainties in the seasonality of ammonia emissions (6 %) or in the uptake of nitric acid on dust (13 %). Our best estimate for fine nitrate optical depth at 550 nm in 2010 is 0.006 (0.005–0.008). In wintertime, nitrate aerosols are simulated to account for over 30 % of the aerosol optical depth over western Europe and North America. Simulated nitrate optical depth increases by less than 30 % (0.0061–0.010) in response to projected changes in anthropogenic emissions from 2010 to 2050 (e.g., −40 % for SO2 and +38 % for ammonia). This increase is primarily driven by greater concentrations of nitrate in the free troposphere, while surface nitrate concentrations decrease in the midlatitudes following lower concentrations of nitric acid. With the projected increase of ammonia emissions, we show that better constraints on the vertical distribution of ammonia (e.g., convective transport and biomass burning injection) and on the sources and sinks of nitric acid (e.g., heterogeneous reaction on dust) are needed to improve estimates of future nitrate optical depth.

Comparing multiple model-derived aerosol optical properties to spatially collocated ground-based and satellite measurements

“…Nitrate is simulated in CM3 but is not radiatively active due to its small forcings, as found by previous studies (Naik et al, 2013). However, recent studies have shown that inclusion of nitrate in the radiation scheme improves model AOD (Paulot et al, 2016). Instead of using in-house emissions as in CM2.1 (Horowitz, 2006), the emissions in CM3 were provided by Lamarque et al (2010), an inventory that was compiled for the Climate Model Intercomparison Project Phase 5 (CMIP5) in support of the Intergovernmental Panel on Climate Change's Fifth Assessment Report (AR5).…”

“…5 (Yu et al, 2012). Previous studies have shown that inclusion of nitrate in CM3 considerably improves the seasonal cycle by reducing the contribution of sulfate to AOD in summer while increasing it in winter (Paulot et al, 2016).…”

First detection of ammonia (NH<sub>3</sub>) in the Asian summer monsoon upper troposphere

“…With regard to the predicted increase in NH 3 emissions and the possible compensating effect on aerosol radiative forcing, Paulot et al (2016) emphasize the need to also better constrain the vertical distribution of ammonia. However, there is very little information from measurements on NH 3 at mid-and upper-tropospheric levels.…”

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

“…Managed ecosystems, such as croplands and pasturelands, are the dominant source of ammonia emissions to the atmosphere through emissions from fertilizer and domesticated animals (Erisman et al, 2008). In combination with nitric acid formed from the oxidation of NO x , ammonia can produce ammonium nitrate, an increasingly important source of inorganic PM in regions where sulfur emissions controls substantially reduce sulfate (Paulot et al, 2016;Pinder et al, 2007). The terrestrial biosphere is also a sink of gases and particles.…”