Comparison of the RADM2 and RACM chemical mechanisms in O3 simulations: effect of the photolysis rate constant

Since the photolysis rate plays an important role in any photoreaction leading to compound sink and radical formation/destruction and eventually O3 formation, its impact on the simulated O3 concentration was evaluated in the present study. Both RADM2 and RACM were adopted with and without updated photolysis rate constants. The newly developed photolysis rates were determined based on two major absorption cross-section and quantum yield data sources. CMAQ in conjunction with meteorological MM5 and emission data retrieved from Taiwan and East Asia were employed to provide spatial and temporal O3 predictions over a one-week period in a three-level nested domain [from 81 km × 81 km in Domain 1 (East Asia) to 9 km × 9 km in Domain 3 (Taiwan)]. Four cases were analyzed, namely, RADM2, with the original photolysis rates applied in Case 1 as a reference case, RADM2, with the updated photolysis rates applied in Case 2, and RACM, with and without the updated photolysis rates applied in Cases 3 and 4, respectively. A comparison of the simulation and observed results indicates that both the application of updated photolysis rate constants and RACM instead of RADM2 enhanced all three error analysis indicators (unpaired peak prediction accuracy, mean normalized bias error and mean absolute normalized gross error). Specifically, RADM2 with the updated photolysis rates resulted in an increase of 12 ppb (10%) in the daily maximum O3 concentration in southwestern Taiwan, while RACM without the updated photolysis rates resulted in an increase of 20 ppb (17%) in the daily maximum O3 concentration in the same area. When RACM with the updated photolysis rate constants was applied in the air quality model, the difference in the daily maximum O3 concentration reached up to 30 ppb (25%). The implication of Case 4 (RACM with the updated photolysis rates) for the formation and degradation of α-pinene and d-limonene was examined.

Assessment of the impact of large changes in the ratio of photolysis rate of nitrate to that of gas nitric acid on HONO concentrations simulated by a 3-D chemical transport model

<p>Some studies show that photolysis of nitrate and deposited nitrate and gas nitric acid (HNO<sub>3</sub>) on the ground surface is much faster than that of HNO<sub>3</sub>. The former mechanism has been considered as a possible daytime HONO source and discussed in many laboratory and field studies. Although this mechanism is also coupled into some three-dimensional chemical transport models, the effect of large changes in the ratio of photolysis rate of nitrate to that of HNO<sub>3</sub> (RAT) on HONO concentrations has not been assessed and will be discussed here by using the updated WRF-Chem model. Simulations indicate that in the morning, this mechanism only resulted in a HONO increase of a few ppt, while the heterogeneous reaction of NO<sub>2</sub> enhanced HONO by about 150 ppt; in the afternoon, however, this mechanism led to a significant HONO increase, with its contribution to HONO concentrations being close to the contribution of the heterogeneous reaction of NO<sub>2</sub>. In some heavily nitrate-polluted areas, this mechanism contributed more than 80% of HONO concentrations during the afternoon. Large changes in RAT produced a substantial impact on HONO concentrations. When RAT was altered from 15 to 100, increase of HONO concentrations was enhanced by about 6 times. Our results suggest that more laboratory and field studies on the photolysis rates of nitrate and deposited nitrate and HNO<sub>3</sub> on the ground surface are still needed.</p>

What have we missed when studying the impact of aerosols on surface ozone via changing photolysis rates?

Previous studies have emphasized that the decrease in photolysis rate at the surface induced by the light extinction of aerosols could weaken ozone photochemistry and then reduce surface ozone. However, quantitative studies have shown that weakened photochemistry leads to a much greater reduction in the net chemical production of ozone, which does not match the reduction in surface ozone. This suggests that in addition to photochemistry, some other physical processes related to the variation of ozone should also be considered. In this study, the Weather Research and Forecasting with Chemistry (WRF-Chem) model coupled with the ozone source apportionment method was applied to determine the mechanism of ozone reduction induced by aerosols over central East China (CEC). Our results showed that weakened ozone photochemistry led to a significant reduction in ozone net chemical production, which occurred not only at the surface but also within the lowest several hundred meters in the planetary boundary layer (PBL). Meanwhile, a larger ozone gradient was formed in the vertical direction, which led to the high concentrations of ozone aloft being entrained by turbulence from the top of the PBL to the surface and partly counteracting the reduction in surface ozone. In addition, contribution from dry deposition was weakened due to the decrease in surface ozone concentration. The reduction in the ozone's sink also slowed down the rate of the decrease in surface ozone. Ozone in the upper layer of the PBL was also reduced, which was induced by much ozone aloft being entrained downward. Therefore, by affecting the photolysis rate, the impact of aerosols was a reduction in ozone not only at the surface but also throughout the entire PBL during the daytime over CEC in this study. The ozone source apportionment results showed that 41.4 %–66.3 % of the reduction in surface ozone came from local and adjacent source regions, which suggested that the impact of aerosols on ozone from local and adjacent regions was more significant than that from long-distance regions. The results also suggested that while controlling the concentration of aerosols, simultaneously controlling ozone precursors from local and adjacent source regions is an effective way to suppress the increase in surface ozone over CEC at present.

What have we missed when studying the impact of aerosols on surface ozone via changing photolysis rates?

Previous studies have emphasized that the decrease in photolysis rate at the surface induced by the light extinction of aerosols could weaken ozone photochemistry and then reduce surface ozone. However, quantitative studies have shown that weakened photochemistry leads to a much greater reduction in the net chemical production of ozone, which does not match the reduction in surface ozone. This suggested that in addition to photochemistry, some other physical processes related to the variation of ozone should also be considered. In this study, the Weather Research and Forecasting with Chemistry (WRF-Chem) model coupled with the ozone source apportionment method was applied to determine the mechanism of ozone reduction induced by aerosols over Central East China (CEC). Our results showed that weakened ozone photochemistry led to a significant reduction in ozone net chemical production, which occurred not only at the surface but also within the lowest several hundred meters in the planetary boundary layer (PBL). Meanwhile, a larger ozone gradient was formed in vertical direction, which led to the high concentrations of ozone aloft being entrained by turbulence from the top of the PBL to the surface and partly counteracting the reduction in surface ozone. In addition, ozone in the upper layer of the PBL was also reduced, which was also induced by much ozone aloft being entrained downward. Therefore, by affecting the photolysis rate, the impact of aerosols was a reduction in ozone not only at the surface but also throughout the entire PBL during the daytime over the CEC in this study. The ozone source apportionment results showed that 41.4 %–66.3 % of the reduction in surface ozone came from local and adjacent source regions, which suggested that the impact of aerosols on ozone from local and adjacent regions was more significant than that from long-distance regions. The results also suggested that while controlling the concentration of aerosols, simultaneously controlling ozone precursors from local and adjacent source regions is an effective way to suppress the increase in surface ozone over CEC at present.

Carbon and nitrogen Isotope fractionation of the photo-degradation of herbicides in aqueous solution by nitrates under ultraviolet radiation

Background: Phenylurea herbicides are one of the most important and widely used pesticides in the world. Due to its potential persistence and toxicity in the aquatic environment, it poses certain risks to the ecological environment and human health. Studying the photochemical degradation behavior of herbicides is important for understanding the degradation and transformation fate in the environment.Results: This study evaluated the effectiveness of direct and indirect photo-degradation of the herbicides isopropiron (IUP) and methylamine (MN), investigating the influence of operational variables (initial herbicide concentration and light sources) and initial of induced nitrate concentration on these processes in aqueous solution. We also introduced a new technology of compound-specific isotope analysis (CSIA) to provide deeper information of the photochemical degradation mechanism. Results showed that the light source and the initial concentration have an important effect on the degradation of herbicides IUP and MN. The photolysis rate under the Hg lamp is higher than photolysis rate under Xe lamps. It is found that photolysis kinetics of herbicides were consistent with the quasi-first order model, and the photolysis rate decreases with the increasing of the initial concentration. In indirect photodegradation, the degradation rate increases with increasing NO 3 − concentration at low concentrations of pesticides (8 mg/L); while the degradation rate decreases with increasing NO 3 − concentration at high concentrations of pesticides (30 mg/L ). According to the isotope fractionation, photolysis of IUP exhibits normal carbon isotope fractionation with the degradation rate increases, and the stable isotope enrichment factors under different photolysis pathways are different. In the indirect photo-degradation process, no significant fractionation of nitrogen isotopes occurred, and stable nitrogen isotopes fractionation could not be fitted well in either of the two photodegradation pathways.Conclusion: Therefore, the structure and chemical characteristics of the molecules of herbicides play a determinant role in their photodegradation. The CSIA is useful both for a mechanism-based evaluation of experimental results and as a valuable tool to explore transformation pathways for organic pollutants in different environmental systems.

Effects of black carbon and boundary layer interaction on surface ozone in Nanjing, China

As an important solar radiation absorbing aerosol, the effect of black carbon (BC) on surface ozone, via reducing photolysis rate, has been widely discussed by “offline” model studies. However, BC–boundary layer (BL) interactions also influence surface ozone. Using the “online” model simulations and process analysis, we demonstrate the significant impact of BC–BL interaction on surface ozone in Nanjing. The absorbing effect of BC heats the air above the BL and suppresses and delays the development of the BL, which eventually leads to a change in surface ozone via a change in the contributions from chemical and physical processes (photochemistry, vertical mixing and advection). For chemical processes, the suppression of the BL leads to large amounts of ozone precursors being confined below the BL which has an increased effect on ozone chemical production and offsets the decrease caused by the reduction of the photolysis rate, thus enhancing ozone chemical formation from 10:00 to 12:00 LT. Furthermore, changes in physical processes, especially the vertical mixing process, show a more significant influence on surface ozone. The weakened turbulence, caused by the suppressed BL, entrains much less ozone aloft down to the surface. Finally, summing-up the changes in the processes, surface ozone reduces before noon and the maximum reduction reaches 16.4 ppb at 12:00 LT. In the afternoon, the changes in chemical process are small which inconspicuously influence surface ozone. However, change in the vertical mixing process still influences surface ozone significantly. Due to the delayed development of the BL, there are obvious ozone gradients around the top of BL. Therefore, high concentrations of ozone aloft can still be entrained down to the surface which offsets the reduction of surface ozone. Comparing the changes in the processes, the change in vertical mixing plays the most important role in impacting surface ozone. Our results highlight the great impacts BC–BL interactions have on surface ozone by influencing the ozone contribution from physical process. This suggests that more attention should be paid to the mechanism of aerosol–BL interactions when controlling ozone pollution.

Effects of black carbon and boundary layer interaction on surface ozone in Nanjing, China

As an important solar-radiation absorbing aerosol, the effect of black carbon (BC) on surface ozone, by influencing photolysis rate, has been widely discussed by offline model studies. However, BC-boundary layer (BL) interactions also influence surface ozone. Using the online model simulations and processes analysis, we demonstrate the significant impact of BC-BL interaction on surface ozone. The absorbing effect of BC heats the air above the BL and suppresses BL development, which eventually leads to changes in the contributions of ozone through chemical and physical processes (photochemistry, vertical mixing, and advection). Different from previous offline model studies, BL suppression leads large amounts of ozone precursors being confined below the BL which offsetting the influence from the reduction of photolysis rate, thus enhancing ozone photochemical formation before noon. Furthermore, the changes in physical process show a more significant influence on surface ozone. The weakened turbulence entrains much less ozone from the overlying ozone-rich air down to surface. As a result, the net contribution of ozone from physical and chemical processes leads to surface ozone reduction before noon. The maximum reduction reaches to 16.4 ppb at 12:00. In the afternoon, the changes in chemical process are small which influence inconspicuously to surface ozone. However, physical process still influences the surface ozone significantly. Due to the delayed development of the BL, less vertically mixed BL continues to show an obvious ozone gradient near the top of the BL. Therefore, more ozone aloft can be entrained down to the surface, offsetting the surface ozone reduction. Comparing all the changes in the contributions of processes, the change in the contribution of vertical mixing plays a more important role in impacting surface ozone. Our results show the great impacts of BC-BL interactions on surface ozone. And more attention should be paid on the mechanism of aerosol-BL interactions when we deal with the ozone pollution control in China.