The Impact of Agroecosystems on Nitrous Acid (HONO) Emissions during Spring and Autumn in the North China Plain

Abstract

Solar radiation triggers atmospheric nitrous acid (HONO) photolysis, producing OH radicals, thereby accelerating photochemical reactions, leading to severe secondary pollution formation. Missing daytime sources were detected in the extensive HONO budget studies carried out in the past. In the rural North China Plain, some studies attributed those to soil emissions and more recent studies to dew evaporation. To investigate the contributions of these two processes to HONO temporal variations and unknown production rates in rural areas, HONO and related field observations obtained at the Gucheng Agricultural and Ecological Meteorological Station during spring and autumn were thoroughly analyzed. Morning peaks in HONO frequently occurred simultaneously with those of ammonia (NH₃) and water vapor both during spring and autumn, which were mostly caused by dew and guttation water evaporation. In spring, the unknown HONO production rate revealed pronounced afternoon peaks exceeding those in the morning. In autumn, however, the afternoon peak was barely detectable compared to the morning peak. The unknown afternoon HONO production rates were attributed to soil emissions due to their good relationship to soil temperatures, while NH₃ soil emissions were not as distinctive as dew emissions. Overall, the relative daytime contribution of dew emissions was higher during autumn, while soil emissions dominated during spring. Nevertheless, dew emission remained the most dominant contributor to morning time HONO emissions in both seasons, thus being responsible for the initiation of daytime OH radical formation and activation of photochemical reactions, while soil emissions further maintained HONO and associated OH radial formation rates at a high level, especially during spring. Future studies need to thoroughly investigate the influencing factors of dew and soil emissions and establish their relationship to HONO emission rates, form reasonable parameterizations for regional and global models, and improve current underestimations in modeled atmospheric oxidation capacity.

Keywords: HONO; NH3; atmospheric oxidation capacity; dew; guttation; soil emission.

Figure 1

Measurements of (a) HONO and NH₃, (b) soil temperature (T_soil) and RH, (c) O₃ and NO₂, (d) HONO/NO₂ and NO, (e) wind speed, direction (denoted by colors), and specific humidity (q) at GC Station in spring 2019. Grey shadings mark out nighttime periods (18:00 to 8:00) and red shadings denote periods with HONO morning peak occurrences.

Figure 2

Measurements of (a) HONO and NH₃, (b) soil temperature (T_soil) and RH, (c) O₃ and NO₂, (d) HONO/NO₂ and NO, (e) wind speed, direction (denoted by colors), and specific humidity (q) at GC Station in autumn 2021. Grey shadings mark out nighttime periods (18:00 to 8:00) and red shadings denote periods with HONO morning peak occurrences.

Figure 3

Diurnal variations in (a) HONO_c (solid line) and HONO (dashed line), (b) NH₃, (c) T_soil, (d) q, (e) dew temperature (T_d), and (f) HONO_c/NO₂ observed in GC during spring 2019 (red) and autumn 2021 (blue).

Figure 4

(a,d) Daytime OH production rates (P(OH)) of HONO and O₃, (b,e) relative contributions of HONO and O₃ to the total P(OH), and (c,f) photolysis rates of HONO and O₃ during (a–c) spring and (d–f) autumn.

Figure 5

The average diel variations of HONO_c (a,b), NH₃ (c,d) and q (e,f) in spring 2019 and autumn 2021 under different relative humidity conditions (g,h). Dark blue for high relative humidity conditions (RH_night > 90%), pale green for medium relative humidity conditions (75% < RH_night ≤ 90%), orange for low relative humidity conditions (RH_night < 75%).

Figure 6

Averaged diurnal variation in HONO budgets during (a) spring 2019 and (b) autumn 2021. Gray, green, red and pink bars represent vehicle HONO emissions, heterogeneous transformation of NO₂ to HONO on aerosol and ground surfaces, and HONO production during nitrate photolysis, respectively, while orange and yellow bars show dry deposition and photolysis removal of HONO.

Figure 7

The unknown variation rate of HONO (P_unknown) under different parameter assumptions. The solid black line represents the base case. The orange dashed and dotted lines represent P_unknown calculated using k = 0.18% and 2.1% as the lower and upper limits of vehicle emission coefficients obtained in previous tunnel experiments [13,56], respectively. The green dashed and dotted lines represent calculation results adopting γ_a = 1 × 10⁻⁹ and 7 × 10⁻⁶ as the lower and upper limits of aerosol surface uptake coefficients according to laboratory measurements summarized in Li, Su, Li, Ma, Pöschl and Cheng [67], respectively. The turquoise dashed and dotted lines represent those applying γ_g = 2 × 10⁻⁶ and 1.6 × 10⁻⁵ as the lower and upper estimates for ground surface uptake coefficients of NO₂ following the field measurement results of VandenBoer, et al. [66], respectively. The blue dashed and dotted lines represent the results obtained by adjusting the nitrate photolysis coefficient down or up by 20%, respectively. The red dashed and dotted lines represent the results obtained by multiplying [OH] by 2 and 0.5, respectively.

Figure 8

Diel profiles of P_unknown (red line) with calculated soil (brown line) and dew (dark blue line) HONO emissions during (a) spring and (b) autumn, as well as the corresponding changes in q and NH₃ during (c) spring and (d) autumn. The light blue shading denotes the time range from 6:00 to 10:00 LT, corresponding to the peak of P_dew, while the red shading denotes the time range from 12:00 to 15:00 LT, corresponding to the peak of P_soil.

Figure 9

The relative contribution of P_dew (blue) and P_soil (brown) to $P_{unknown}$.