Global and regional decreases in tropospheric oxidants from photochemical effects of aerosols

Abstract

[1] We evaluate the sensitivity of tropospheric OH, O 3 , and O 3 precursors to photochemical effects of aerosols not usually included in global models: (1) aerosol scattering and absorption of ultraviolet radiation and (2) reactive uptake of HO 2 , NO 2 , and NO 3 . Our approach is to couple a global 3-D model of tropospheric chemistry (GEOS-CHEM) with aerosol fields from a global 3-D aerosol model (GOCART). Reactive uptake by aerosols is computed using reaction probabilities from a recent review (g HO2 = 0.2, g NO2 = 10À4 , g NO3 = 10 À3 ). Aerosols decrease the O 3 ! O( 1 D) photolysis frequency by 5-20% at the surface throughout the Northern Hemisphere (largely due to mineral dust) and by a factor of 2 in biomass burning regions (largely due to black carbon). Aerosol uptake of HO 2 accounts for 10-40% of total HO x radical ( OH + peroxy) loss in the boundary layer over polluted continental regions (largely due to sulfate and organic carbon) and for more than 70% over tropical biomass burning regions (largely due to organic carbon). Uptake of NO 2 and NO 3 accounts for 10-20% of total HNO 3 production over biomass burning regions and less elsewhere. Annual mean OH concentrations decrease by 9% globally and by 5-35% in the boundary layer over the Northern Hemisphere. Simulated CO increases by 5-15 ppbv in the remote Northern Hemisphere, improving agreement with observations. Simulated boundary layer O 3 decreases by 15-45 ppbv over India during the biomass burning season in March and by 5-9 ppbv over northern Europe in August, again improving comparison with observations. We find that particulate matter controls would increase surface O 3 over Europe and other industrial regions.

Trace gas and aerosol interactions in the fully coupled model of aerosol‐chemistry‐climate ECHAM5‐HAMMOZ: 1. Model description and insights from the spring 2001 TRACE‐P experiment

“…HNO 3 is mainly reduced by uptake on mineral dust particles, which leads to a global mean reduction of 15% at the surface and of 22% at 725 hPa while the global burden decreases by 34% (Table 12). Martin et al [2003] found a decrease in OH surface concentration ranging from 5% to 25% over much of the Northern Hemisphere (but this includes also the photochemical effect of aerosols which is not accounted for in our sensitivity simulations NOHETDU and NOHET) which resulted in an increase in CO surface concentration of 5 -10% over much of the Northern Hemisphere.…”

“…Some papers also assessed the effect of aerosols on trace gases through the modification of photolysis rates. In general this effect on surface O 3 concentrations is found to be globally small [Bian and Zender, 2003;Tie et al, 2005], but can be significant in aerosol source regions such as eastern Asia, Europe, United States and Sahara [Tang et al, 2003;Martin et al, 2003]. Note that the comparison between these studies remains somewhat difficult as the representation of aerosols (e.g., bulk approach For relative humidity (RH) !…”

“…In general, those models include the same processes, but they differ substantially in terms of the aerosol parameterizations (resulting in varying aerosol number concentrations, size distributions, loads and spatial distributions), uptake coefficients, parameterizations of heterogeneous chemistry, etc. In particular, Dentener et al [1996], Bian and Zender [2003], Bauer et al [2004], Martin et al [2003], and Tie et al [2005] consider externally mixed particles while both internally and externally mixed particles are considered by Liao et al [2003Liao et al [ , 2004, and Liao and Seinfeld [2005]; the prognostic size distribution and mixing state of the aerosols is only described in HAM [Stier et al, 2005].…”

“…[57] Martin et al [2003] analyzed the effects of aerosols on tropospheric oxidants through the modification of the photolysis rates and the heterogeneous reactions of NO 2 , NO 3 , and HO 2 . They used the uptake coefficients recommended by Jacob [2000] for these later species as in the [2005] estimated somewhat larger effects that in the present study.…”

“…Martin et al [2003] evaluated the sensitivity of OH, O 3 , and its precursors to the photochemical effects of aerosols and to the reactive uptake of N 2 O 5 , HO 2 , NO 2 , and NO 3 on different aerosol types (including sulfate, organic and black carbon, mineral dust, and sea salt). They found no significant change in the global O 3 burden, but significant regional variations in surface O 3 concentrations due to heterogeneous reactions and to the aerosol photochemical effect (e.g., O 3 decrease by 10 -15% over northeastern China and Yellow Sea and by 10-40% over India).…”