Emissions of nitrous acid (HONO) and nitric oxide (NO) from soils and its impact on air quality in Shanghai
Authors: Wang Mengdi , Zhang Jingwei , An Junling , Zhou Feng , Zhang Xiuying , Wang Ruhai , Deng Lingling , Hou Lijun , Liu Min , Wu Dianming 
Published: March 2020

&lt;p&gt;Gaseous nitrous acid (HONO) and nitric oxide (NO) play a significant role in atmospheric chemistry through the contribution to the hydroxyl radical (OH) and influencing atmospheric oxidization capacity. Soil HONO emissions are considered as a major source of atmospheric HONO. However, soil HONO emissions and their contribution to air quality are rarely quantified. In this study, HONO and NO emissions from cropland, forest, urban green land, and grassland soils in Shanghai were measured by a dynamic chamber system under controlled laboratory conditions. HONO and NO emissions at the optimal water content (10 - 40% of water holding capacity) were highest from forest soil (50.3 &amp;#177; 30.1 and 70.4 &amp;#177; 43.9 ng m&lt;sup&gt;-2 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;; average &amp;#177; standard error, respectively), following by cropland soil (48.6 &amp;#177; 17.4 and 55.8 &amp;#177; 23.1 ng m&lt;sup&gt;-2 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;, respectively), urban green land soil (44.2 &amp;#177; 9.5 and 39.3 &amp;#177; 13.3ng m&lt;sup&gt;-2 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;, respectively), and grassland soil (27.7 &amp;#177; 15.6 and 18.4 &amp;#177; 6.9 ng m&lt;sup&gt;-2 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;, respectively). Correlation analysis showed that soil HONO and NO emissions were significantly related with nitrate, total nitrogen, and total carbon (P &lt; 0.01). The total soil emissions of HONO and NO in Shanghai were estimated based on &amp;#8220;wetting-drying method&amp;#8221;, and then upscaling to China and global. Results showed that global NO emissions from natural and fertilized soils were ~ 4.5 and 2.6 Tg N yr&lt;sup&gt;-1&lt;/sup&gt;, respectively, which are comparable with the results from IPCC report (2013). The estimated global HONO emissions from natural and fertilized soils were ~ 3.3 and 2.7 Tg N yr&lt;sup&gt;-1&lt;/sup&gt;, respectively, while those were 0.12 and 0.35 Tg N yr&lt;sup&gt;-1 &lt;/sup&gt;for China, and 0.01 and 0.33 Gg N yr&lt;sup&gt;-1 &lt;/sup&gt;for Shanghai, respectively.&lt;/p&gt;&lt;p&gt;The impact of soil HONO emissions on atmospheric oxidation capacity and O&lt;sub&gt;3&lt;/sub&gt; concentrations in Shanghai were evaluated using the WRF-Chem model in March of 2016. Daytime HONO concentrations were increased by 0.036 &amp;#177; 0.015 ppb after considering soil HONO emissions during typical wetting-drying days, and the contribution of HONO photolysis to OH radicals enhanced from 0.095 ppb h&lt;sup&gt;-1&lt;/sup&gt; to 0.22 ppb h&lt;sup&gt;-1&lt;/sup&gt; and was ~ 2 times the contribution of O&lt;sub&gt;3&lt;/sub&gt; photolysis (0.1 ppb h&lt;sup&gt;-1&lt;/sup&gt;), leading to 0.5 - 1.0 ppb enhancement of 8h-O&lt;sub&gt;3&lt;/sub&gt;. The sensitivity test showed that O&lt;sub&gt;3&lt;/sub&gt; enhancement caused by soil HONO emissions were larger (1.0-1.5 ppb) under low NO&lt;sub&gt;x&lt;/sub&gt; (cutting down 50%) conditions compared with the current conditions, implies that the importance of soil HONO emissions could be even larger in future considering the on-going NO&lt;sub&gt;x&lt;/sub&gt; reducing management in China.&lt;/p&gt;