Atmospheric ammonia and its impacts on regional air quality over the megacity of Shanghai, China

Atmospheric ammonia (NH3) has great environmental implications due to its important role in ecosystem and global nitrogen cycle, as well as contribution to secondary particle formation. Here, we report long-term continuous measurements of NH3 at different locations (i.e. urban, industrial and rural) in Shanghai, China, which provide an unprecedented portrait of temporal and spatial characteristics of atmospheric NH3 in and around this megacity. In addition to point emission sources, air masses originated from or that have passed over ammonia rich areas, e.g. rural and industrial sites, increase the observed NH3 concentrations inside the urban area of Shanghai. Remarkable high-frequency NH3 variations were measured at the industrial site, indicating instantaneous nearby industrial emission peaks. Additionally, we observed strong positive exponential correlations between NH4+/(NH4++NH3) and sulfate-nitrate-ammonium (SNA) aerosols, PM2.5 mass concentrations, implying a considerable contribution of gas-to-particle conversion of ammonia to SNA aerosol formation. Lower temperature and higher humidity conditions were found to favor the conversion of gaseous ammonia to particle ammonium, particularly in autumn. Although NH3 is currently not included in China’s emission control policies of air pollution precursors, our results highlight the urgency and importance of monitoring gaseous ammonia and improving its emission inventory in and around Shanghai.


Description of measurement sites
The urban site is located on the campus of Fudan University, which is at the north-east of the urban area of Shanghai. The measurement site is close to a trunk road with heavy traffic and an expressway tunnel along with several branch roads around the campus. There are no industrial and agricultural emission sources nearby.
With about 14.03 km 2 , Jinshan Fine Chemical Industry Park is one of the largest chemical industrial zones in Shanghai, located in the south-west of Shanghai and northern coast of the Hangzhou Bay. Since the main activities in JSP are chemical production and related storage, industrial emissions are expected to be the main source of atmospheric ammonia there.
The rural site, at the east lakefront of Dianshan Lake, is about 65 km away from the urban center of Shanghai. Dianshan Lake, surrounded by agricultural and tourism scenic areas, is the largest freshwater lake in Shanghai with an area of 62 km 2 . The DSL site is also the regional air quality background station in Shanghai. Therein, volatilization of NH3-based fertilizer during the crops farming, vegetables and fruits cultivation, as well as wastes from the livestock and poultry industries, are responsible for the main portion of ammonia emissions. Table S1 summarizes the periods of atmospheric NH3 measurements and methods employed at each site.

Inter-comparability of NH 3 measurements by DOAS and MARGA
During this supplementary measurements at DSL, the averaged distance between DOAS light path and MAGRGA inlet is about 30 m, which was constrained by the instrument installation and cannot be even closer. Moreover, the discrepancies are inevitable mainly due to the totally different measuring principles between them 1-2 .
The DOAS data was the averaged concentration along the optical path whereas the MARGA result was the point concentration close to the sampling inlet.
Therefore, the correlation coefficient and biases for 1-h averages between MARGA and DOAS methods are reasonable and acceptable, compared to previous studies 1-2 .
In the view of inter-comparability, it is convincible to use long-term data series from DOAS and MARGA for inter-sites comparison in this study.   Nighttime was marked with shadow.
As an additional information to Fig. 2d, the possible reasons for NH3 peaked at early morning in summer were discussed following. To avoid the tough working at hot weather condition in later noontime, agricultural activities in summer, e.g. plow and fertilization, were performed 1-2 hours earlier than other seasons. Due to the impacts by agricultural sources in rural area, the atmosphere is heating up earlier in summer resulting in an earlier temperature-favored volatilization from agriculture sources. It also can be found from Fig. S5 that the start of ambient NH3 concentration increasing was consistent with the sunrise in seasons. So another potential photochemical process related to solar irradiance, releasing the ammonia-containing species from soils, could take effects in the NH3 diurnal pattern. It deserves further investigation but beyond the focus of this paper. To demonstrate the role of acid aerosol precursor gases in regulating NH3 concentration, we have presented the monthly averaged concentrations of NH3, SO2 and NOx at FDU site from July 2013 to September 2014. All the aerosol precursor gases were in high levels during the period of November to December of 2013, which further probably resulted in the frequent particle pollution events. However, unlike NH3, acid precursor gases of SO2 and NOx were in low levels in summer. This could be owing to that the subtropical monsoon climate brings clean air to Shanghai from northern continental inland 3 . Since there is no heating season in Shanghai, the high pollution levels in winter were probably due to the more pollutants transported from polluted upwind areas and the effect of wintertime shallow stable mixing layer.

Impacts of temperature and air mass transport on ambient NH 3 concentrations
Daily ambient temperature and correspond averaged NH3 concentrations at different locations were used to discussed the impacts of temperature on atmospheric NH3 levels, as shown in Fig. S7.