Heterogeneous Nitrate Production Mechanisms in Intense Haze Events in the North China Plain

Abstract

Studies of wintertime air quality in the North China Plain (NCP) show that particulate‐nitrate pollution persists despite rapid reduction in NOx emissions. This intriguing NOx‐nitrate relationship may originate from non‐linear nitrate‐formation chemistry, but it is unclear which feedback mechanisms dominate in NCP. In this study, we re‐interpret the wintertime observations of 17O excess of nitrate (∆17O(NO3−)) in Beijing using the GEOS‐Chem (GC) chemical transport model to estimate the importance of various nitrate‐production pathways and how their contributions change with the intensity of haze events. We also analyze the relationships between other metrics of NOy chemistry and [PM2.5] in observations and model simulations. We find that the model on average has a negative bias of −0.9‰ and −36% for ∆17O(NO3−) and [Ox,major] (≡ [O3] + [NO2] + [p‐NO3−]), respectively, while overestimating the nitrogen oxidation ratio ([NO3−]/([NO3−] + [NO2])) by +0.12 in intense haze. The discrepancies become larger in more intense haze. We attribute the model biases to an overestimate of NO2‐uptake on aerosols and an underestimate in wintertime O3 concentrations. Our findings highlight a need to address uncertainties related to heterogeneous chemistry of NO2 in air‐quality models. The combined assessment of observations and model results suggest that N2O5 uptake in aerosols and clouds is the dominant nitrate‐production pathway in wintertime Beijing, but its rate is limited by ozone under high‐NOx‐high‐PM2.5 conditions. Nitrate production rates may continue to increase as long as [O3] increases despite reduction in [NOx], creating a negative feedback that reduces the effectiveness of air pollution mitigation.

Important Role of NO3 Radical to Nitrate Formation Aloft in Urban Beijing: Insights from Triple Oxygen Isotopes Measured at the Tower

“…31−33 In general, quantifications of the relative contribution of N 2 O 5 hydrolysis to nitrate aerosol formation by model simulations exhibit high accuracy. Due to the different consumption of O atoms in different oxidation pathways, oxygen isotopes (δ 18 O and Δ 17 O) in nitrate would be distinct, and therefore oxygen isotopes have been used for differentiating the formation pathways of NO 3 − in the atmosphere. 26,34 Wang et al 26 found that the contributions of OH + NO 2 , NO 3 + HC/DMS, and N 2 O 5 hydrolysis pathways to PM 2.5 nitrate in Beijing were 32%, 34%, and 34%, respectively, on a yearly basis.…”

“…A recent study in Beijing winter suggested that the contribution of NO 2 uptake might be a weak source of nitrate on intensive haze days (eq ). However, some laboratory studies have indicated that the hydrolysis of NO 2 was not the main mechanism of HNO 3 formation due to its low reaction probability. − Other potential formation mechanisms of nitrate, which might be important in other regions (e.g., coastal area, rainforest areas such as Amazonia, etc. ), are shown in Figure S1.…”

“…The partitioning of NH 4 NO 3 favors the particle phase due to the neutral acidity and the excess ammonia in Beijing winter . Many studies have proved that the lower temperature and dark environment facilitates the occurrence of nocturnal chemistry during winter. ,,− A field measurement suggested that NO 3 was lost primarily to reactions with VOCs due to the large emissions of anthropogenic VOCs in industrial regions . Previous modeling studies showed that the N 2 O 5 heterogeneous pathway was comparable with that initiated from the OH + NO 2 pathway during the winter in Beijing. ,, Several studies suggested that N 2 O 5 uptake (N 2 O 5 + H 2 O/Cl – ) also contributes a majority of nitrate aerosols in the summer over northern China. ,, The efficiency of nitrate aerosol production by N 2 O 5 uptake was due to the differences in N 2 O 5 uptake coefficients, sunlight, and solar radiation.…”

“…Figure 1 − values could reflect the formation processes of atmospheric nitrate aerosols. 34,54 Due to the distinct δ 18 O variations in O sources such as water vapor, 55 the Δ 17 O-NO 3 − value was more appropriate than δ 18 O-NO 3 − in differentiating the relative contributions between OH + NO 2 and nocturnal pathways to nitrate formation. 54 3.2.…”

“…On this day, a significant increase in the OH/H 2 O + NO 2 contribution was observed at 120 m (37%) in comparison to that (∼7%) at the ground level. Chen et al 18 found that NO 2 hydrolysis was a weak source of nitrate in an intense haze. As shown in eq R10, the reaction of NO 2 and H 2 O produces HNO 3 and nitrous acid (HONO) simultaneously.…”