Haze, a frequent and unwelcome visitor, has long been a subject of intense study and public concern. While the thick, gray air can seem like a simple case of pollution, the truth is far more complex. A recent study has uncovered a key player in the haze drama: nitrous acid (HONO), a seemingly small molecule with a big impact. This blog post will dive into how humidity, particularly during autumn, plays a crucial role in HONO formation and, consequently, in shaping the air quality in Beijing.
What is HONO and Why Does It Matter?
Nitrous acid (HONO) might not be a household name, but in the world of atmospheric chemistry, it’s a major actor. HONO is a precursor to hydroxyl radicals (OH), which are sometimes called the “detergent” of the atmosphere. These radicals play a crucial role in breaking down pollutants and driving the formation of secondary organic aerosols (SOA), a key component of haze.
The concentration of HONO in the atmosphere can significantly influence air quality. Higher levels of HONO lead to more OH radicals, which can result in increased formation of secondary pollutants like ozone and SOA. These pollutants contribute to the dense haze that blankets Beijing.
The Role of Relative Humidity in HONO Formation
The study focused on the autumn of 2021 in Beijing, a period for its persistent haze events. Researchers found that traditional atmospheric models often underestimate HONO concentrations, leading to inaccurate predictions about air quality. The missing piece of the puzzle, they discovered, is relative humidity (RH).
Relative humidity refers to the amount of moisture in the air compared to the maximum amount the air can hold at a given temperature. It turns out that as RH increases, so does the production of HONO, particularly at night. But how does this happen?
Relationship between the RH enhancement factor (EFRH) and RH at night.
The Nocturnal Chemistry of HONO
At night, when the sun is down and the temperature drops, the chemistry of the atmosphere shifts. The study found that the reaction of nitrogen dioxide (NO2) with surfaces like soil, roadways, and buildings becomes a dominant source of HONO during these hours. This process, known as a heterogeneous reaction, is significantly influenced by RH.
When RH is high, water molecules form a thin layer on surfaces, which can enhance the conversion of NO2 into HONO. This nocturnal source of HONO is particularly potent during autumn, when humidity levels tend to be higher in Beijing. As a result, HONO concentrations can rise significantly overnight, setting the stage for increased formation of OH radicals and secondary pollutants the next day.
Time series of RH, PM2.5 mass concentrations, observed and three simulated HONO concentrations.
Implications for Air Quality Management
The discovery that RH plays a critical role in HONO formation has significant implications for air quality management in Beijing and similar urban areas. Current atmospheric models often fail to account for this humidity-driven mechanism, leading to underestimation of HONO levels and, consequently, the severity of pollution events.
By incorporating RH effects into these models, we can improve the accuracy of air quality forecasts. This, in turn, can lead to better-informed public health advisories and more effective pollution control strategies. For instance, understanding the RH-driven formation of HONO could help in developing targeted interventions during critical periods, such as restricting traffic or industrial activities on nights when high humidity is forecasted.