You are correct in your assumption that CO2 levels will rise at night in summer, as they do in both day and night in the winter, for the same reason, a lack of photosynthesis.
CO2 + H2O + photons → [CH2O] + O2
the diurnal range will be in the region of 50ppm due to the photosynthetic effect, and as winter sets in that diurnal difference should reduce though the overall level will rise, (the famous NASA Earth Breathing Video)
https://www.youtube.com/watch?v=x1SgmFa0r04
as photosynthetic activity shuts down, (and people switch on their heating). However the range will not be in region most of us are reading from our sensors where the ranges are in 100s of ppm. day to night. (also note the CO emissions (not CO2) from bush fires in sub Saharan Africa in September. I use to live there, and when flying in September one would fly in through the relatively static inversion layer (hot air meeting cold) through about 50metres of smoke sitting in the atmosphere at about 3000 feet above ground level.)
https://books.google.co.uk/books?id=KaJH...g=PA294&dq
See page 295 of this book.
I am not sure if there is some other phenomena forcing this huge range, or perhaps it is the way the sensor is being read. You are also correct to say that the CO2 levels at Mauna Loa and Cape Grim are global readings, these stations are purposely located far from cities and in places where the prevailing wind comes across large expanses of ocean and is not affected by local pollution from citiies etc.
However despite all this if the sensor is outside and away from immediate sources of CO2 (boilers etc) the we should read between 400+ppm midday in summer to perhaps 500+ppm (maybe a little more) in winter at night. As can be seen from below wind has as much or more effect on CO2 diurnal ranges.
http://meteo.lcd.lu/papers/co2_patterns/...terns.html
But we still see diurnal ranges seldom exceeding 100ppm. and normally a lot less.
http://www.csiro.au/greenhouse-gases/
It seems the sensors are reading the correct pattern but perhaps most are reading the highs very high and lows about right.
CO2 + H2O + photons → [CH2O] + O2
the diurnal range will be in the region of 50ppm due to the photosynthetic effect, and as winter sets in that diurnal difference should reduce though the overall level will rise, (the famous NASA Earth Breathing Video)
https://www.youtube.com/watch?v=x1SgmFa0r04
as photosynthetic activity shuts down, (and people switch on their heating). However the range will not be in region most of us are reading from our sensors where the ranges are in 100s of ppm. day to night. (also note the CO emissions (not CO2) from bush fires in sub Saharan Africa in September. I use to live there, and when flying in September one would fly in through the relatively static inversion layer (hot air meeting cold) through about 50metres of smoke sitting in the atmosphere at about 3000 feet above ground level.)
https://books.google.co.uk/books?id=KaJH...g=PA294&dq
See page 295 of this book.
I am not sure if there is some other phenomena forcing this huge range, or perhaps it is the way the sensor is being read. You are also correct to say that the CO2 levels at Mauna Loa and Cape Grim are global readings, these stations are purposely located far from cities and in places where the prevailing wind comes across large expanses of ocean and is not affected by local pollution from citiies etc.
However despite all this if the sensor is outside and away from immediate sources of CO2 (boilers etc) the we should read between 400+ppm midday in summer to perhaps 500+ppm (maybe a little more) in winter at night. As can be seen from below wind has as much or more effect on CO2 diurnal ranges.
http://meteo.lcd.lu/papers/co2_patterns/...terns.html
But we still see diurnal ranges seldom exceeding 100ppm. and normally a lot less.
http://www.csiro.au/greenhouse-gases/
It seems the sensors are reading the correct pattern but perhaps most are reading the highs very high and lows about right.